[email protected] - a 5 GeV energy threshold array of imaging atmospheric Cherenkov telescopes at 5 km altitude
We discuss the concept and the performance of a powerful future ground-based astronomical instrument, [email protected] - a 5 GeV energy threshold stereoscopic array of several large imaging atmospheric Cherenkov telescopes installed at a very high mountain elevation of about 5 km a.s.l. - for the study of the -ray sky at energies from approximately 5 GeV to 100 GeV, where the capabilities of both the current space-based and ground-based -ray projects are quite limited. With its potential to detect the “standard” EGRET -ray sources with spectra extending beyond several GeV in exposure times from 1 to seconds, such a detector may serve as an ideal ”Gamma-Ray Timing Explorer” for the study of transient non-thermal phenomena like -radiation from AGN jets, synchrotron flares of microquasars, the high energy (GeV) counterparts of Gamma Ray Bursts, etc. [email protected] also would allow detailed -ray spectroscopy of persistent nonthermal sources like pulsars, supernova remnants, plerions, radiogalaxies, and others, with unprecedented for -ray astronomy photon statistics. The existing technological achievements in the design and construction of multi (1000) pixel, high resolution imagers, as well as of large, 20 m diameter class multi-mirror dishes with rather modest optical requirements, would allow the construction of such a detector in the foreseeable future, although in the longer terms from the point of view of ongoing projects of 100 GeV threshold IACT arrays like H.E.S.S. which is in the build-up phase. An ideal site for such an instrument could be a high-altitude, 5 km a.s.l or more, flat area with a linear scale of about 100 m in a very arid mountain region in the Atacama desert of Northern Chile.
keywords:Atmospheric imaging Cherenkov technique, GeV detector
Astroparticle Physics, accepted for publication
The high detection rate, the ability of effective separation of electromagnetic and hadronic showers, and the good accuracy of reconstruction of the direction of primary -rays are three remarkable features of the imaging atmospheric Cherenkov telescope (IACT) technique (see e.g. [1, 2, 3]). The recent detections of TeV -rays from several galactic and extragalactic objects (see e.g. ) provide the basis for the further development of ground-based gamma-ray astronomy.
Qualitative improvements of the IACT technique in the next few years will most probably be linked to stereoscopic observations of -rays . This approach not only allows an unambiguous determination of the energy and of the arrival direction of -ray primaries on an event-by-event basis, but also significantly improves the efficiency of rejection of hadronic showers produced by cosmic rays , as it was recently demonstrated by the HEGRA system of 5 imaging telescopes operating in the energy region from 500 GeV to 20 TeV [7, 8].
One of the important issues for future detectors is the choice of the energy region based on two principal arguments: (a) astrophysical significance (goals) and (b) the experimental feasibility/reliability (cost). If one limits the energy region from above to a relatively modest energy threshold around 100 GeV, then the performance of IACT arrays and their practical implementation can be predicted with confidence. In practice, an energy threshold of GeV can be achieved by a stereoscopic system of IACTs consisting of 10 m diameter class optical mirrors and equipped with high resolution cameras, based on conventional photo-multipliers (PMTs). Currently three such arrays are under development/construction in Australia (CANGAROO-3 ), in Namibia (H.E.S.S. ), and in Arizona (VERITAS ). With their superior angular resolution of several arcminutes and energy flux sensitivity close to , these projects perfectly suit to the energy range from 50 GeV to 10 TeV, which, from the point of view of scientific motivations and the potential astronomical targets, can be considered as a spectral domain in its own right.
On the other hand, it is expected that the next generation major satellite -ray mission GLAST (see e.g. [12, 13]), the successor of the EGRET instrument aboard the Compton Gamma Ray Observatory, will extend the exploration of the -ray sky up to 100 GeV. Thus the gap between space-based and ground-based -ray instruments will eventually disappear. It should be noticed, however, that in many cases this statement has a rather conditional or even symbolic character. Although at GeV energies GLAST will improve the EGRET sensitivity by almost two orders of magnitude, the capability of GLAST (and likely that of any post-GLAST space-based project) at energies well beyond 10 GeV will be quite limited because of the limited detection area.
This circumstance justifies recent activities to reduce the energy threshold of atmospheric Cherenkov detectors below 100 GeV. Currently two low threshold projects, CELESTE in France  and STACEE in USA , based on the concept of conversion of existing solar power plants into atmospheric Cherenkov telescopes, are in their final stage of realization. Although this technique allows in principle a reduction of the energy threshold down to 20-30 GeV (see e.g. Ref. ), the challenge remains to reach an adequate detection efficiency of -rays at such low energies. Another approach has been proposed by the MAGIC collaboration with a single imaging Cherenkov telescope having a large, 17 m diameter reflector. Equipped with a standard PMT-based high resolution camera this telescope is expected to allow effective detection of -rays at energies above 30 GeV .
With some exceptions, the GeV -ray sources () are expected to be quite different from TeV sources (). The proximity of the intermediate domain below 100 GeV to the energy range covered by EGRET suggests that many objects established as GeV emitters have a good chance to be detected also by the above mentioned ground-based instruments. This argument, however, cannot yet guarantee definite success. Indeed, although the two largest -ray source populations identified by EGRET, radiopulsars and distant AGN, do not show a significant steepening or cutoff up to 10 GeV, the theoretical studies of -ray production and absorption conditions in these objects, as well as rather general phenomenological considerations predict cutoffs in the energy spectra around 10 GeV or less. In addition, for any reasonable model of the diffuse extragalactic cosmic background radiation, we should expect sharp cutoffs in the spectra of distant extragalactic objects with redshift at energies as low as 50 GeV (see e.g. ). This implies that for the study of cosmologically distant sources, like the GeV blazars discovered by EGRET, or Gamma Ray Bursts (GRBs), the energy threshold of the detectors should be less than 10 GeV at which energy the Universe is most likely transparent up to at least . An instrument like GLAST, operating effectively in the 0.1 to 10 GeV energy region, nicely suits this task. In particular, it is expected that the number of AGN that GLAST will detect could exceed several thousands . At the same time, the relatively small detection area of GLAST, , limits the potential of this instrument for detailed studies of the temporal and spectral characteristics of highly variable -ray sources like blazars, which have variability timescales less than a few hours, or of solitary events like GRBs with a duration of to seconds. In this regard, GLAST can hardly match the performance of current X-ray detectors that have similar detection areas but operate in a regime of photon fluxes that exceed the fluxes of MeV/GeV -rays by many orders of magnitude.
The idea of a “Gamma-ray timing explorer” to study transient -ray phenomena with an adequate photon detection rate motivated, to a large extent, the present investigation. It concerns the possible extension of the domain of ground-based Cherenkov technique with its huge detection area of to down to energies of several GeV. We shall argue that such a goal could be best achieved by stereoscopic systems of several large, 20 m class imaging atmospheric Cherenkov telescopes located at very high, , mountain altitude.
2 Concept of an IACT array with 5 GeV threshold
The concept of stereo imaging is based on the simultaneous detection of a single air shower in different projections by at least two telescopes, separated at a distance comparable with the “effective radius” of the Cherenkov light pool. The stereoscopic approach allows (i) unambiguous and precise reconstruction of shower parameters on an event-by-event basis, (ii) superior rejection of hadronic showers, and (iii) effective suppression of the background light from different sources - the nigh sky background (N.S.B.), local muons, etc. . All these three advantages over single IACTs have been convincingly demonstrated at TeV energies by the HEGRA IACT system [7, 8].
Compared with single (“stand alone”) telescopes, which can adequately measure the shower inclination in the direction perpendicular to the plane containing the telescope axis, but poorly in the in-plane direction, the stereoscopic approach allows full reconstruction of the arrival direction of individual -ray showers. Apart from the good directional information, stereoscopic IACT systems make use of the fact that the Cherenkov images of a shower detected by several different, spatially separated telescopes, are only partially correlated. Therefore the stereoscopic measurements significantly improve the efficiency of rejection of hadronic (background) showers at both the hardware (trigger) and the software levels. The only disadvantage of the stereoscopic approach is a non-negligible loss in the detection rate because of the overlap of the shower detection areas of individual telescopes located from each other at distances . However, this loss of statistics is compensated, especially for steep spectra of primary -rays, by a significant reduction of the energy threshold of the telescopes operating in coincidence mode.
At -ray energies above 100 GeV stereoscopic IACT arrays do provide an excellent angular resolution of about or less, and a “gamma/hadron” separation efficiency (including the hadron rejection at the trigger level) of 1000:1. This improves the flux sensitivity dramatically compared with the sensitivity of single telescopes. The efficiency of the imaging technique is somewhat lower at energies below 100 GeV. In particular, the Cherenkov images become less elongated and less regular. In practice this introduces significant uncertainties in the reconstruction of image parameters. Even so, below it will be shown that the performance of the stereoscopic imaging remains adequately high even below 10 GeV.
The effective energy threshold of IACTs is basically determined by two conditions: (i) the number of photoelectrons in the image should be sufficient for an appropriate image analysis; typically, “good imaging” requires electrons111For example, in the case of the HEGRA stereoscopic IACT system the minimum number of photoelectrons corresponding to the showers classified as “high quality events”, i.e. the showers which are accepted for the further image analysis, is close to 40 electrons per telescope ., (ii) the accidental trigger rate introduced by the N.S.B. should not exceed the detection rate of -rays. For an ideal IACT the first condition in principle should dominate over the second, technical condition.
The number of photoelectrons detected by the imager - a multi-pixel camera placed in the focal plane of the mirror - depends on (i) the telescope photo-electron response (or aperture), (where is the geometrical area of the mirror, and is the photon-to-photoelectron conversion factor), and (ii) the density of optical Cherenkov photons produced at the typical distance from the axis of the shower initiated by a primary -ray of energy .
The density of the Cherenkov light at an elevation of km a.s.l. within 100 m of the shower core produced by a primary -ray photon of energy is close to . Correspondingly, the number of photoelectrons detected by a telescope with aperture is equal to . Thus, for reduction of the energy threshold down to 5 GeV, the telescope aperture should be as large as , assuming that the minimum number of electrons required for an image analysis . Detailed Monte-Carlo calculations presented below confirm that the energy threshold of such an instrument, determined as the energy at which the differential -ray detection rate reaches to its maximum, could indeed be as low as 5 GeV. For conventional aluminized optical mirrors and PMT-based cameras with typical conversion factor , would require a large optical reflector of approximately 20 m diameter. With the successful development of novel, fast (nsec) detectors of optical radiation with a quantum efficiency exceeding , the energy threshold of the telescopes could be pushed further down to 2 or 3 GeV, which is an absolute limit determined by the minimum energy of secondary electrons capable of producing Cherenkov light in the upper atmosphere.
The Cherenkov light density increases monotonically with elevation. Thus, for the given telescope configuration, the installation of an IACT array at very high altitude would allow a straightforward and unconditional reduction of the energy threshold. At the same time the increase of the telescope aperture leads not only to a proportional increase of number of photoelectrons registered per shower, but also to an increase of the accidental rate caused by the N.S.B. Thus, a significant gain from large aperture telescopes, with the ultimate goal to operate the telescopes in the linear regime, , can be achieved only through effective suppression of the N.S.B. Otherwise the reduction of the energy threshold would be rather slow, , and therefore would be difficult to justify economically.
In the imaging technique, the accidental events introduced by the N.S.B are suppressed by a trigger condition that requires signals above a threshold (photoelectrons) in adjacent pixels. A smaller pixel size not only reduces significantly the noise level due to the N.S.B., but it allows higher trigger multiplicity as well. At the same time the optimal size of the pixel is determined by the condition of “good imaging”, which implies that the area which is covered by the minimum number of pixels (typically 10) used in the image analysis, should not exceed the characteristic size of the image. At , the image area of electromagnetic showers is less than square degree. Therefore at such low energies high resolution cameras with a pixel size close to provide an adequate imaging quality. Although such small pixels allow also significant reduction of the N.S.B., it appears that for a single IACT with aperture , even a pixel size of is still not sufficient to operate the telescope at the minimum possible energy threshold, i.e. in the regime when the detection threshold is determined by the Cherenkov light amplitudes rather than by the N.S.B. noise. Further reduction of the pixel size makes the design of the imager with many thousands of channels technically very difficult. A solution of this dilemma is the stereoscopic mode of observations which offers a more feasible and economic approach. It requires simultaneous detection of a shower by at least two telescopes. Because of the flat lateral distribution of the Cherenkov radiation from electromagnetic showers, this requirement does not effect the -ray detection efficiency (if the distance between telescopes does not significantly exceed 100 m), but significantly reduces the accidental rate caused by the N.S.B. Below we will indeed show that the suppression of the N.S.B. by a stereoscopic system of IACTs with camera pixel size is sufficient to operate the aperture telescopes in the energy regime below 10 GeV.
The arrangement of an IACT array, in particular the number of telescopes, and the spacing between them, can be understood from the following simple considerations. The Cherenkov light pool on the ground produced by primary -rays of energies has a flat radial distribution with a pool radius of approximately 100 m. Hence the optimal spacing between the telescopes should be of the order of 100 m. A significantly smaller spacing reduces not only the detection area, but also the quality of images. A spacing of significantly more than 100 m reduces the coincidence rate dramatically, especially at low energies, and thus increases the energy threshold. The special investigation carried out for 100 GeV class telescopes [6, 23], generally confirms this simple conclusion. We expect that this should be the case also for the sub-10 GeV array, although detailed Monte-Carlo simulations are needed for optimization of the arrangement of reflectors in this array. Here we consider a reasonable arrangement with a spacing of 100 m. The question of optimization of the spacing will be discussed in a separate paper. The minimum number of telescopes is determined from the consideration that at least 3 stereoscopic views are needed to reconstruct the shower parameters reliably . For clarity, we assume here a square baseline with telescopes at the four corners, placed at an altitude of 5 km a.s.l. In order to have more homogeneous coverage of distances of impact parameters, especially for showers with cores outside the square, we assume in addition a further (5th) telescope in the center of the square. The main objective of this paper is to study the basic performance of a 5 GeV IACT array placed at very high altitude. The optimization of the arrangement of the array is outside of the scope of this paper, but we believe that the suggested layout is not far from the optimum design.
3 Basic characteristics of Cherenkov radiation induced by sub-10 GeV gamma-rays
The results discussed in this section have been obtained by using the ALTAI code  that simulates electromagnetic and hadronic showers and their Cherenkov radiation in the Earth’s atmosphere. This code has been used before for calculations of the performance of the HEGRA system of IACTs , as well as for studies of the expected characteristics of the new generation “100 GeV - threshold” IACT arrays  that are the basis of the H.E.S.S. (High Energy Stereoscopic System) project in Namibia . The predictions for the HEGRA IACT system have been thoroughly checked using the detected hadronic (background) showers produced by cosmic rays with a relatively well known energy spectrum and mass composition [8, 20]. More importantly, the calculations of the characteristics of the instrument for primary -rays, in particular the lateral distribution of the Cherenkov light, which determines the -ray detection area, the angular resolution, and the gamma/hadron separation efficiency, have been experimentally confirmed by the HEGRA collaboration using -ray data obtained during the active state of Mkn 501 in 1997 [21, 22]. This extraordinary high state of the source, with a duration of several months, resulted in approximately 40,000 -rays in the energy interval from 500 GeV to 20 TeV, detected by the HEGRA IACT system under almost background-free conditions .
In this paper we are interested in the possibility of detecting primary -rays over an interval of very low energies from several GeV to 100 GeV, by a system of IACTs located at a.s.l. Both conditions are rather extreme, and exhibit features which differ significantly from traditional studies of air showers. In particular, at energies of primary -rays below 10 GeV, we deal with Cherenkov radiation from only a handful of first generation electrons, while at TeV energies the Cherenkov radiation is contributed by a large number of electrons produced during the full cascade development. At such low energies we have therefore to expect more fluctuations of the parameters characterizing the showers and their Cherenkov radiation. In particular, the Cherenkov images of sub-10 GeV showers are expected to have a less regular shape, as well as to be more strongly affected by the geomagnetic field, as compared with the showers in the energy region. Also, both the very low energy domain and the very high elevation of the location of the suggested IACT array imply not only better transparency conditions for the Cherenkov radiation, but also a non-negligible reduction of the light produced above the telescopes from hadronic showers. All these effects add new features to the characteristics of the Cherenkov radiation of air showers.
3.1 Lateral and longitudinal development of showers
The lateral distribution of Cherenkov radiation from air showers, induced by -rays of energy 10 GeV and by protons of energy 100 GeV, are shown in Fig. 1 at 3 different observational levels - 2.2 km, 3.5 km and 5 km. It is seen that within the radius of 100 m from the shower core which typically determines the detection area of IACTs, the density of Cherenkov radiation from -ray induced showers has (i) a rather flat lateral distribution, and (ii) increases significantly with elevation above sea level. In particular, the rise from 2.2 km to 5 km results in an increase of the density of the Cherenkov light by a factor of 2 to 3 (Fig. 1a). This elevation effect is less pronounced for proton-induced showers ( per cent at ; see Fig. 1b). This implies that the choice of very high elevation for the operation of telescopes would allow a significant reduction of the energy threshold, by a factor of , as well as a noticeable improvement of the background rejection of hadronic showers.
The slow increase of the Cherenkov light density from hadronic showers with the rise of elevation is explained by the effect of deep penetration of the “-branches” (sub-showers) of the cascade into the atmosphere (Fig. 2), which results in the production of a non-negligible amount of Cherenkov light below the observation level at . This effect is quantitatively demonstrated in Fig. 3 where the longitudinal distributions of the Cherenkov light, produced by a 10 GeV -ray photon and a 100 GeV proton, are shown.
The low energy threshold itself already leads, in fact more strongly than the elevation effect, to a significant improvement of the gamma/hadron ratio. This effect, caused by the reduction of the yield of Cherenkov light by sub-100 GeV hadronic showers (see e.g. ) is demonstrated in Fig. 4 for 5 km a.s.l. The density of Cherenkov light in the electromagnetic showers is approximately proportional to the -ray energy down to , the dependence becoming a bit stronger below 10 GeV (Fig. 4a). At the same time, the density of
Cherenkov light from hadronic showers drops significantly faster with reduction of the primary energy (Fig. 4b). In Fig. 5 we show the ratio of densities of the Cherenkov light and produced by -rays and protons at 100 m distance from the shower core.
Dramatic rise of this ratio below 100 GeV implies a monotonic increase of the gap between the energies of electromagnetic and hadronic showers that produce the same amount of Cherenkov light. This effect is demonstrated in Fig. 6. The solid line corresponds to the relation between energies of -rays and protons defined from equation . For comparison, the line “” is also shown. It is seen, in particular, that the gap between and , which is close to at high energies (e.g. 1 TeV electromagnetic showers versus 2 TeV hadronic showers), becomes significantly larger at very low energies below 10 GeV (e.g. 2 GeV -ray events versus 30 GeV hadronic showers).
3.2 Field of view and pixel size
One of the principal parameters of the imaging camera is its field of view (FoV). At TeV energies the image centroids of showers detected at distances or beyond are shifted in the focal plane from the center of the camera by . Therefore an inner region with diameter provides high detection efficiency for -rays, and can be treated as optimal zone for the hardware trigger. On the other hand, the IACT technique requires that the camera should be larger than the trigger zone by about one degree, in order to avoid a distortion of the Cherenkov images because of a limited FoV; for lower -ray energies the shift of the image centroid decreases . This effect becomes significant especially at very low energies.
In particular at the shift does not exceed (Fig. 7). Thus in the energy range , the camera can be more compact compared with the conventional (FoV) cameras designed for high energies. In fact, a FoV seems a reasonable for point-like or even moderately extended () -ray sources. This is an important circumstance which keeps the number of the channels of the imager within reasonable () limits, especially taking into account that at such low energies the pixel size must be small, or less, dictated by two the equally important conditions: effective suppression of the N.S.B., and high image quality (see Sec. 2).
3.3 Trigger integration gate
Truncation of the Cherenkov pulse integration gate is another effective way to suppress the N.S.B. Ideally, the integration gate should be comparable with the duration of the time impulse of the Cherenkov light which for 10 GeV -rays, detected at 5 km a.s.l. altitude, is less than 5 ns (Fig. 8). The compression of the integration gate from conventional 20 ns to 5 ns reduces the average noise level - the number of photoelectrons produced by the N.S.B. - by a factor of 4, and correspondingly lowers the energy threshold significantly.
Note that both techniques, fast timing and small pixel size, have already been successfully employed by the CAT collaboration . A further suppression of the N.S.B. can be effectively achieved in the stereoscopic approach by requiring simultaneous detection of showers by two or more telescopes. This technique has been convincingly demonstrated by the HEGRA collaboration [7, 8]. Below we will show that, even for telescopes with an aperture as large as , the combination of all three techniques - small pixel size (), fast timing (), and stereoscopy - allows effective operation of the IACT system in the energy regime as low as several GeV.
3.4 Effect of geomagnetic field
An important issue concerning the quality of Cherenkov images at very low energies
is connected with the deflection of the secondary (cascade) electrons in the geomagnetic field (see e.g. Ref. ). Generally, our results agree well with the conclusion of
Patterson and Hillas  that down to -ray energies of 100 GeV the geomagnetic field has a rather small effect on the lateral distribution of the Cherenkov radiation as long as the perpendicular component of the magnetic field is less than 0.3 G. One cannot a priory exclude, however, that at very low energies the effect of the geomagnetic field is stronger. The main parameter characterizing the effect of the geomagnetic field is the average energy of the electrons radiating Cherenkov light at the shower maximum, rather than the energy of primary particles. In Fig. 9 we compare the parameter for showers produced by primary -rays at three energies - 10 GeV, 100 GeV, and 1 TeV. It is seen that, at any fixed depth, the average energy of electrons producing Cherenkov light is larger for cascades induced by higher energy -rays. However, the average electron energies become quite similar when we compare at the depths corresponding to the shower maxima for the given energy of the primary -ray photon. This interesting effect is explained by the fact that at low primary energies the maximum of an electromagnetic shower occurs at high altitudes where the density is low, and therefore the electrons need to be more energetic to produce Cherenkov light. In practice this implies that lowering the energy of -rays should not result in significant amplification of the geomagnetic effect. In Fig. 10 the density of the Cherenkov light from 10 GeV showers at the observation level are shown for three different values of the perpendicular component of the geomagnetic field - =0, 0.5 G, 1 G. The split of the patterns (azimuthal asymmetry) produced by electrons and positrons due to their deflections in opposite directions becomes noticeable only for a very large perpendicular component of the field, =1 G. More quantitatively, the effect of the geomagnetic field on the lateral distribution of Cherenkov photons from 10 GeV -rays is shown in Fig. 11. It is seen that for a reasonable field , the effect is less than 15 per cent.
4 Detection areas
For a given telescope configuration and arrangement of the IACT array, the energy threshold and the effective detection area of primary -rays are determined by the lateral and angular distributions of the Cherenkov light and the hardware trigger conditions.
The configuration of the IACT array assumed in this paper is the following: the system consisting of 5 IACTs is installed at the altitude a.s.l., four telescopes are located at the corners, and one in the center of a square with a linear size ; each telescope has an aperture , and is equipped with a -channel camera of individual pixel size . This comprises an effective field of view FoV. The hardware trigger is organized in a way that requires (1) signals above some critical threshold in adjacent pixels (“”) in each individual telescope (the “local trigger”); (2) detection of a shower by at least 2 telescopes (the “system trigger”). For calculations of the threshold we assume a 5 ns trigger gate, and require that the accidental rate caused by the N.S.B. is less than 1 Hz, i.e. less than 1 per cent of the detection rate of cosmic ray electrons (see below).
In Fig. 12 we show the detection areas for -rays from 1.5 GeV to 100 GeV calculated for 3 local trigger conditions: 6 ph.-e., 5 ph.-e. 5 ph.-e., assuming a standard N.S.B. flux (see e.g. ). It is seen that for a chosen pixel size of , a multiplicity greater than two () actually reduces the detection area, even though the requirement of higher multiplicity allows a lower trigger threshold . Therefore all calculations presented below were performed for .
In Fig. 13 the detection area of the IACT array for a somewhat higher threshold, ph.-e., is shown in order to demonstrate how sensitive the detection area is to the choice of , especially at low, sub-10 GeV energies (compare curves 1 in Fig. 12 and Fig. 13). For comparison, we show also the overall detection area of 5 telescopes if they would be located at large, distances from each other and operating independently in a stand alone mode, i.e. being not integrated in the system trigger. It is seen that at energies the array of independent telescopes has a larger detection area, but the difference compared with the array operating in the stereoscopic mode at energies does not exceed a factor of two. On the other hand, the analysis of images obtained in the stereoscopic mode provides much better suppression of the background caused by the cosmic ray protons and electrons which effectively results in a significant improvement of not only the quality of the data, but also of the flux sensitivity of the instrument.
At energies below 100 GeV, the cosmic ray background detected by the system of IACTs operating in the stereoscopic mode is essentially dominated by electromagnetic showers produced by cosmic ray electrons. This is seen in Fig. 14 where the detection rates of both cosmic ray protons and electrons are also shown. The strong dominance of the electronic component is explained by the combination of several effects, in particular (i) the large, up to a factor of 10, difference between the energies of electrons and protons producing the same amount of Cherenkov light, (ii) the high altitude of observations, (iii) the compact Cherenkov images of electromagnetic showers compared with hadronic showers, (ii) the noticeable increase (approximately ) of the electron-to-proton ratio of cosmic rays down to .
In Fig.14 we show also the detection rate of -rays from a point source with a power-law spectrum , and integral flux . The latter is somewhat larger than the fluxes of most of the EGRET sources . This implies that the -ray detection rate shown in Fig. 14 should be considered as an upper limit for “standard” EGRET sources. This curve still lies significantly below the rate of detection of cosmic ray electrons. However, for a point-like source the electron background can be reduced significantly if we select showers arriving from the direction of the -ray source.
The detection rates of electromagnetic showers within the detector’s energy-dependent Point Spread Function (PSF), given by the angle , which is the half-angle of the cone around the source direction containing of events at energy , are determined as follows:
for -rays and cosmic ray electrons, respectively, where is the differential flux of -rays from a point source, is the differential flux of cosmic ray electrons per solid angle, is the detection area for electromagnetic showers, , and is (by definition) the -ray acceptance.
For the chosen configuration of the IACT array, the Monte-Carlo calculations of the effective detection area (Fig. 15) in the interval from 1.5 GeV to 100 GeV can be presented in the following form
where E is the energy of a -ray or electron in units of GeV.
This presentation shows a strong energy-dependence of the detection area at energies below 10 GeV, , but at higher energies it gradually turns into a slow increase with energy, Close to 1 TeV the detection area actually becomes constant, in essence because of the limited field of the view () of the camera.
The stereoscopic approach allows the determination of the arrival direction of primary -rays on an event-by-event basis. The determination of the arrival direction of primary -rays is described in ref., and its practical implementation in the case of the HEGRA IACT system can be found in ref. [8, 28]. In Fig. 16 we show the Monte-Carlo calculations of the PSF which can be approximated in the simple form
Despite the small pixel size , the angular resolution of the 5 GeV IACT array at energies is significantly poorer than the resolution of “100 GeV” threshold instruments, which could be as good as , even for a larger pixel-size of about . Since for both categories of instruments the number of photoelectrons, or the so-called size of the image, from a detected -ray photon are comparable ( photoelectrons), the lower performance of the IACT technique is rather an intrinsic feature of Cherenkov images at very low energies (the images are less elongated and less regular). This effect can be seen also in the so-called alpha-distribution of images in a single telescope, where the alpha parameter is indicative of the orientation of the image in the camera (see e.g. ref.): while at TeV energies most of -rays from a point source have an angle alpha less than 5-8 deg , the alpha distribution is significantly broader in the energy region around 10 GeV (see Fig. 17). The suppression of the cosmic ray background at such low energies becomes correspondingly less effective. The characteristic values of the acceptance of the isotropic cosmic ray showers , and the point-source -ray showers , as well as the so-called Q-factor, , which characterizes the improvement of the signal-to-noise ratio after application of the image cuts, are shown in Table 1. It is seen that the best improvement of the signal-to-noise ratio is achieved for an alpha cut at which gives a rather modest Q-factor, . In contrast, at 10 GeV the stereoscopic measurements allow the determination of the shower direction with an accuracy of (Fig. 16). Therefore the -ray signal could be improved by a factor of , where the efficiency of the rejection showers from cosmic ray electrons is determined by the FoV of the imager, , and the angular resolution . Thus, despite the smaller (by a factor of two) detection area of the array operating in the stereoscopic mode compared with the overall area of 5 independent IACTs (see Fig. 13), the stereoscopic array would have at least by a factor of 2 better sensitivity, even disregarding other advantages of the stereoscopic approach, in particular, the complete removal of the hadronic background.
5 Detection rates and the energy threshold
The differential detection rates of -rays from a point source, calculated for the collection area given by Eq. (3) and assuming pure power-law spectrum in the form
are shown in Fig. 18a. Eq. (5) implies that, independent of the spectral index , the differential -ray flux is normalized at 1 GeV to . The latter corresponds to the typical flux of “standard” EGRET sources.
In Fig. 18b we show the differential detection rates of -rays with a hard power-law spectrum with and an exponential cutoff at :
for 4 different values of , and assuming the same absolute flux normalization at 1 GeV as in Fig. 18a.
It is seen from Figs.18a and 18b that for a large variety of -ray spectra the peak of the detection rate appears in a rather narrow band between 4 and 6 GeV. Now, defining the energy threshold as the energy at which the differential -ray detection rate reaches to its maximum, we may conclude that the suggested IACT array has an effective energy threshold of about 5 GeV.
In Fig. 18a,b we show also the differential detection rate of cosmic ray electrons within the cone limited by the PSF of the instrument given by Eq. (4). The energy spectrum of cosmic ray electrons is shown in Fig. 19. At energies above 10 GeV the differential spectrum is very steep with a power-law index . Below 10 GeV it becomes flatter. Within the uncertainties of the measured fluxes, the electron spectrum can be approximated in the entire region from several GeV to 1 TeV by the following function shown by the solid line in Fig. 19 :
In Fig. 20 we show integral detection rates for power-law -ray spectra represented by Eq.(5). It is seen that for a relatively flat -ray spectrum with and for an integral flux above 1 GeV of (this approximately corresponds to the total, i.e. pulsed plus unpulsed, flux from the Crab), the detection rate of -rays from the EGRET sources can be as high as 6 events per second, against the cosmic-ray background rate of about 25 events per sec caused by cosmic-ray electrons. This implies that an observation time of approximately 20-30 sec would be sufficient to detect a statistically significant signal from such a source. Remarkably, for the brightest persistent -ray source, the Vela pulsar with photon index from 100 MeV to 10 GeV, and the integral flux , the detection rates would exceed 100 events per 1 sec. Thus a statistically significant signal from the source could be obtained during an observation time less than 1 sec ! For the given normalization of the differential flux at 1 GeV, , the detection of sources with steep -ray spectra would require significantly longer exposure. Even so, the time needed for detection of sources with very steep power-law spectra with an index (like curve 4 in Fig. 18a), or with a sharp, e.g. exponential cutoff at a few GeV (like the curve 4 in Fig. 18b), does not significantly exceed 1 h.
6 Flux Sensitivity
The curves in Fig. 14 correspond to the detection rates before the image analysis. Remarkably, even after such effective rejection of hadronic showers at the trigger level, there still remains room for further suppression of the background from cosmic ray protons and nuclei by analyzing the shapes of the Cherenkov images of the detected showers. In Fig. 21 we show the so-called mean-scaled width parameter distribution of showers which have already passed the hardware trigger condition. This parameter represents the mean value of the width parameter measured by all telescopes and normalized to the impact distances and the image amplitudes . It is seen that the distributions of the electromagnetic and hadronic showers are rather well separated. The efficiencies of the acceptance of both type of showers for different mean scaled width cuts are presented in Table 2. We see that even very loose cuts at the level of provide suppression of the hadronic showers by a factor of 5, while the -ray acceptance can be as high as 90 per cent. This implies that after such a loose cut, which practically does not reduce the -ray (or CR electron) statistics, we may push the detection rates of cosmic ray protons and nuclei further down, and thus make -ray detection in the entire energy region below 100 GeV essentially free from hadronic background.
This makes the calculations for the differential flux sensitivity of the instrument straightforward and simple and, more importantly, there is no need to specify the spectrum of primary -rays. Indeed, because the showers produced by electrons are very similar to -ray showers222Actually there are some differences. In particular the primary electrons start to produce Cherenkov light earlier, but in this paper we will ignore these effects., the condition of detection of a -ray signal with statistical significance -sigma in the energy interval , , provided that the number of detected -rays , gives the minimum detectable differential flux for the observation time :
where it is assumed that , which corresponds to a rather conservative 25 per cent accuracy of reconstruction of the energy of the primary electron or -ray photon. The results of calculations for the so-called spectral energy distribution (SED), , based on Eqs.(3),(4) and (7) requiring 3-sigma detection () at each energy , are presented in Fig. 22 for 2 different observation times, 1 h and 25 h.
In the same figure we present also the power-low fluxes of -rays represented by Eq.(6). It is seen that 1 hour observations by the IACT array would be sufficient to detect a statistically significant signal from a “standard” EGRET source even at the presence of an exponential cutoff in the -ray spectrum as low as 3 GeV. In the case of a cutoff at 10 GeV or higher energies, the detection time () could be reduced to min.
For comparison in Fig. 22 we show the expected sensitivity of GLAST for an observation time . Since at energies above several GeV GLAST will operate at almost background free conditions (for point-like sources), the flux sensitivity is determined by the photon statistics, (see e.g. [12, 13]). Note that at energies above 5-10 GeV, 1 hour observation time by the IACT array could provide better sensitivity that the minimum detectable fluxes achievable by GLAST during 1 month of continuous observations. Moreover, even very short observations by the IACT array can give unusually rich (for -ray astronomical standards) photon statistics over the whole energy region from few GeV to 100 GeV; the number of detected -rays exceeds 100 at each energy interval (see Fig. 23). This would guarantee an appropriate -ray spectroscopy with energy resolution of about 20-25 per cent below 10 GeV, and better than 15 per cent at higher energies.
The flux sensitivity shown in Fig. 22 is obtained under a very robust and to a large extent non-standard condition which requires detection of a signal with at least 3-sigma significance in each energy band with width centered on , provided that the number of detected -rays in this band exceeds 10. Note that this definition of sensitivity does not require any knowledge about the shape of the spectrum of the primary -rays.
In Fig.24 we present the flux sensitivities determined in a more traditional way, namely requiring 5-sigma detection of -rays above the given energy, . This definition of the integral flux sensitivity obviously requires an assumption about the shape of the energy spectrum. The curves shown in Fig. 24 are calculated for observation time, assuming power-law spectra of -rays with photon indices 1.5 (dotted curve), 2 (dot-dashed curve), and 2.5 (dashed curve). The expected GLAST sensitivity shown by the solid curve corresponds to 1 year of continuous observations of the source.
Results presented in this paper show that a stereoscopic array of large, 20 m diameter class imaging Cherenkov telescopes installed at very high mountain altitudes could effectively enter into the domain of satellite-borne -ray astronomy. A 5 GeV energy threshold array of IACTs at 5 km a.s.l. - hereafter [email protected] - could provide a deeper probe of -ray sources compared with GLAST - the most powerful current satellite-borne -ray project. However, the scientific goals of these instruments are essentially different. While GLAST with its almost steradian field of view can provide very effective simultaneous monitoring of a very large number (hundreds or even thousands) quasi-stable -ray sources, as well as a study the galactic and extragalactic components of the diffuse -ray background radiation, [email protected] has an obvious advantage for the search and study of highly variable or transient -ray sources. The flux sensitivity of this instrument at 5 GeV of about (see Fig. 22) would allow the detection of any -ray flare with apparent luminosity , lasting only 1 h, where is the distance to the source. Of special interest are the gamma-ray blazars detected by EGRET (see e.g. ). A detailed study of the time structure of -radiation for these highly variable objects on timescales of several minutes by [email protected] would provide unique information about the relativistic non-thermal processes in astrophysical jets. An effective operation of this instrument in the sub-10 GeV regime guarantees detection of -rays arriving from cosmological distances up to or so, for which the intergalactic medium becomes almost transparent. The dynamic range from several GeV to 100 GeV would allow important cosmological measurements, in particular a study of the diffuse ultraviolet extragalactic background by detecting intergalactic absorption features in the spectra of -rays below 100 GeV. The confusion problem (spectral cutoff due to the internal or extragalactic absorption ?) at such redshifts can probably be overcome by simultaneous observations at optical and X-ray wavelengths.
[email protected] can also be effectively used for the study of galactic transient sources, in particular for the detection of short time () -ray activity expected during synchrotron radio flares of microquasars.
And finally, [email protected] can serve as a very powerful instrument for a study of the phenomenon of Gamma Ray Bursts (GRBs). If the spectra of GRBs extend to high energies without abrupt cutoffs up to several GeV, which is the case at least for some of GRBs , then the sensitivity of [email protected] would allow very detailed studies of the spectral and temporal features of GRBs in this extremely important energy region. In Fig. 25 we show the minimum time required for detection of GeV -ray flares with a given energy flux at 4 different -ray energies: 2 GeV, 5 GeV, 10 GeV, and 50 GeV. The calculations correspond to the 3-sigma signal at each energy within the interval , provided that the number of detected -rays exceeds 10. In the regime of low fluxes, typically , the -rays are detected in the presence of the heavy background induced by cosmic ray electrons. Therefore . For fluxes larger than , the detection occurs under almost background-free conditions, and therefore .
The results shown in Fig. 25 demonstrate the capability of [email protected] for detection of GeV counterparts of GRBs. The detection of GeV episodic events with typical GRB fluxes between and would require only observation time. Thus it would be possible to monitor the spectral evolution of the source with a typical GRB duration from several second to 100 seconds. Remarkably, even for fluxes as low as , the required exposure time does not exceed 100 sec. This implies that [email protected] could serve as a unique tool to study GRBs in the late stages of evolution, i.e. during the afterglows.
[email protected] is a detector with a small field of view. Therefore it requires special strategies for the search and study of multi-GeV -ray emitters. The proximity of this energy region to the energy range covered by EGRET suggests that almost all (more than 300) EGRET sources should also be detected by [email protected] The typical observation time for detection of a “standard” EGRET source would not exceed 1 hour, even if the spectrum of -rays cuts off at energies of several GeV. The full overlap of the energy range of this instrument with the energy domain of GLAST would make the latter a “the best guide” for developing a strategy for the study of persistent galactic and extragalactic objects. Generally, all sources seen by GLAST can be potential targets for observations with the [email protected] These observations with very large photon statistics - not achievable by GLAST - could provide detailed studies of the spectral and temporal features of -ray sources in the multi-GeV region. For highly variable objects like blazars or galactic sources with relativistic jets, a multi-wavelength approach including observations with radio, optical and X-ray detectors would be very important. These observations would not only inform about the pre-flaring or flaring states of the sources, but also would provide complementary information for understanding and comprehensive modeling of the physical processes in these objects.
A special strategy should be developed for the search for GeV radiation from GRBs during and after the main event. Apparently, prompt information (within 10 sec or so) from the new generation GRB detectors like SWIFT (and possibly also from GLAST) would be needed, containing the angular coordinates of an event with an accuracy better than . It would be very worthwhile to have also a nearby ground-based prompt optical telescope like ROTSE . In their turn, the telescopes of [email protected] should be rather fast in order to be directed to the source not later than 1 minute after receiving the alarm from these detectors.
Because of effective rejection of hadronic showers by [email protected], the cosmic-ray background below 100 GeV is dominated by the showers from cosmic-ray electrons. This component of electromagnetic showers remains a part of the background which can be hardly removed, and thus it is the most serious limiting factor of flux sensitivities, especially for extended sources. On the other hand, these electromagnetic showers with a known flux and spectrum of cosmic ray electrons, measured up to energies 1 TeV, can be used for absolute energy calibration of the instrument.
Energy calibration with the aid of the cosmic-ray electrons provides a unique tool for the continuous (on-line) control of the characteristics of the detector (e.g. the energy threshold, the detection area, etc.) during the observations. For example, a 10 min exposure will be enough for the detection of hundreds of electrons at any energy within in the entire dynamical region from few GeV to 100 GeV (see Fig. 23). It is difficult to overestimate the significance of such calibration and control, especially for the study of the spectral characteristics of highly variable -ray sources on sub-hour timescales.
The basic elements of the suggested [email protected] detector are the large optical reflectors and the multichannel high resolution cameras. Presently, 20 m diameter alt-azimuth mounts with the required precision of about 1 arcminute could be designed and built by many companies specialized in the construction of large radio dishes. The area of the optical reflector could be composed of several hundreds to thousand diameter glass mirrors with protective quartz coating, quite similar to the mirrors used in the current or planned imaging Cherenkov telescope projects. The general requirements on the imagers are a relatively large ( or so) field of view with a pixel size of about . This implies less than 1 thousand fast channels. Such a camera with similar parameters has been already built and successfully operated as part of the CAT imaging Cherenkov telescope .
The decrease of the energy threshold of the imaging atmospheric Cherenkov technique to several GeV would depend to a large extent on the availability of exceptional sites with a dry and transparent atmosphere at an altitude as high as 5 km. Nature does provide us with such an extraordinary site - The Llano de Chajnantor in the Atacama desert in Northern Chile. This site with its very arid atmosphere was recently chosen for the installation of one of the most powerful future astronomical instruments - the Atacama Large Millimeter Array (ALMA), a funded US-European project which will consist of 64 12-meter radio antennas with spacing from approximately 150 meters to 10 km (see http://www.alma.nrao.edu/). The large flat area on that site could certainly accommodate an additional Cherenkov telescope array as well which requires a relatively compact area with a radius of about 100 m. Another attractive feature of this site seems to be an adequate infrastructure which will be built up during the next several years for the ALMA project. The foreseen technological developments of ALMA, concerning in particular the construction of very large antennas operating in robotic or semi-robotic mode, could help very much in the design of the telescopes of [email protected] Moreover, the neighboring Cerro Toco site  offers suitable areas at even higher altitudes, and km a .s.l., with the same infrastructure advantages.
We thank the anonymous referee for her/his critical comments and remarks which helped us to improve the paper significantly. HQ is grateful for the award of a Presidential Chair in Science (Chile). His research is partially funded by FONDECYT Grant No. 8970009.
-  M.F. Cawley and T.C. Weekes, Exp. Astron. 6 (1996) 7.
-  A.M. Hillas, Space Sci. Rev. 75 (1996) 17.
-  D.J. Fegan, J. Phys. G: Nucl. Part. Phys. 23 (1998) 1013.
-  T.C. Weekes, in: Proc. GeV/TeV Gamma Ray Astrophysics Workshop, Eds. B. Dingus et al. (Snowbird, Utah), AIP Conf. Proc Ser. 515 (2000) 3.
-  F.A. Aharonian and C.W. Akerlof, Annu Rev. Nucl. Part Sci. 47 (1997) 324.
-  F.A. Aharonian, W. Hofmann, A.K. Konopelko, and H.J. Völk, Astropart. Phys. 6 (1997) 343.
-  A. Daum et al. (HEGRA collaboration), Astropart. Phys. 8 (1997) 1.
-  A.K. Konopelko et al. (HEGRA collaboration), Astropart. Phys. 10 (1997) 275.
-  M. Mori et al. in: Proc. GeV/TeV Gamma Ray Astrophysics Workshop, Eds. B. Dingus et al. (Snowbird, Utah), AIP Conf. Proc Ser. 515 (2000) 485.
-  W. Hofmann, in: Proc. GeV/TeV Gamma Ray Astrophysics Workshop, Eds. B. Dingus et al. (Snowbird, Utah), AIP Conf. Proc Ser. 515 (2000) 500.
-  F. Krennrich et al., in: Proc. GeV/TeV Gamma Ray Astrophysics Workshop, Eds. B. Dingus et al. (Snowbird, Utah), AIP Conf. Proc Ser. 515 (2000) 515.
-  E.D. Bloom, Space Sci. Rev. 75 (1996) 109.
-  N. Gehrels, P. Michelson, Astropart. Phys. 11 (1997) 277.
-  D. Smith, in: Proc. GeV/TeV Gamma Ray Astrophysics Workshop, Eds. B. Dingus et al. (Snowbird, Utah), AIP Conf. Proc Ser. 515 (2000) 416.
-  D. Ong, in: Proc. GeV/TeV Gamma Ray Astrophysics Workshop, Eds. B. Dingus et al. (Snowbird, Utah), AIP Conf. Proc Ser. 515 (2000) 401.
-  E. Pare, Space Sci. Rev. 75 (1996) 127.
-  E. Lorenz, GeV/TeV Gamma Ray Astrophysics Workshop, Eds. B. Dingus et al. (Snowbird, Utah), AIP Conf. Proc Ser. 515 (2000) 510.
-  J.R. Primack et al., Astropart. Phys. 11 (1999) 93.
-  A.K. Konopelko, A.V. Plyasheshnikov, Nucl. Inst. and Methods 450 (2000) 419.
-  F.A. Aharonian (HEGRA collaboration) Phys. Rev D 59 (1999) 092003.
-  F.A. Aharonian et al. (HEGRA collaboration), Astropart. Phys. 10 (1998) 21.
-  F.A. Aharonian et al. (HEGRA collaboration) Astron. Astrophys. 349 (1999) 11.
-  A.K. Konopelko Astropart. Phys. 11 (1999) 263.
-  A. Djannati-Atai et al. (CAT collaboration) Astron. Astrophys. 350 (1999) 17.
-  P. Chadwick et al. , J. Phys. G: Nucl. Part. Phys. 25 (1999) 1223.
-  J.R. Patterson, A.M. Hillas, J. Phys. G: Nucl. Phys. 9 (1983) 1433.
-  C.E. Fichtel et al., ApJ Suppl. 90 (1994) 917.
-  W. Hofmann, in: Proc. GeV/TeV Gamma Ray Astrophysics Workshop, Eds. B. Dingus et al. (Snowbird, Utah), AIP Conf. Proc Ser. 515 (2000) 318.
-  T. Taira et al., in: Proc 23rd ICRC (Calgary) 2 (1993) 128.
-  S.W. Barwick et al., ApJ 498 (1998) 779.
-  K. Hurley, Space Sci. Rev. 75 (1996) 43.
-  C. Akerlof et al., Nature 398 (1999) 400.
-  A.D. Miller et al. ApJ 524 (1999) L1.


Today we celebrate the Summer Solstice. The midsummer celebration. A celebration of energy, light and warmth. Exploring the spiritual significance, setting intensions and celebrating. This past week we have been counting down to this glorious day by focusing on different elements of the Summer Solstice that are worth embracing. We’ve focused on three focal elements that can be incorporated into your daily life, specifically energy, light and warmth.
The Summer Solstice is Thursday, June 21, the day of the longest light of the year in the northern hemisphere when the Sun ingresses tropically into Cancer. Venus is exactly opposite Mars, drawing the male and female principles of life together in tension. Passions run high, and love is in the air. The Moon forms an edgy square to both Mercury and Pluto. We could use these challenging aspects to confront issues through potentially difficult discussions. Mark this special day of the year with your own ritual.
With the Summer Solstice we have an opportunity to harness the energy, light and warmth given to us by the power of the Sun. Opposed to the Winter Solstice, the Summer Solstice feeds our lives and souls with a special kind of empowering energy that should be appreciated. The same goes for the light. Today is the longest day of the year. Meaning we get just a little bit extra sunshine than we got yesterday, but from today on the days will slowly become shorter and shorter. Embrace the light and rise with the Sun. Warmth. There is something magnificent and comforting when it comes to the feeling of warm sunshine on our skin. Take a moment today and as much as possible to step outside, feel the Sun on your skin and soak it all up. Let it fuel you.
The Summer Solstice is celebrated in many different ways by many different cultures. Some embrace it as a celebration of life and for some it has a deeper spiritual significance. Nevertheless it is a day worth celebrating. It is worth celebrating the wonder of the energy, light and warmth of the Sun. Celebrating the midsummer, the longest day of the year. Celebrating what the power of the Sun does for our beautiful Earth and for our beautiful souls and bodies. Take a moment for yourself today to meditate on the matter. Take a deep inhale and soak it all in and exhale out all the worry, stress and toxic energy that may be filling your body. Fill yourself with the good and release all of the bad.


Stuck Chinese Moon Rover Sends Back Lunar Insight
December 22nd, 2015, 3:38PMTwo years ago, China sent the Chang’e-3 lander and its rover, Yutu, to the moon. There was some bad news early on: A failure caused the Yutu rover to become immobile shortly after arriving, but it was still able to inspect some nearby lunar rocks and send back information to Earth. This is the first time scientists have been able to do close-up inspection of moon rocks since the U.S. Apollo and Russian Luna programs ended in the 1970s, and the new observations have led to some surprising results. The report published today in Nature Communications says that based on concentrations of titanium in the young lava bed where Chang’e-3 is stationed, the moon’s mantle is much less uniform than scientists initially thought based on Apollo and Luna samples. The research teams suspect that large impacts may have disrupted the mantle’s formation between 3-4 billion years ago. Additionally, the high concentration of olivine (a silicate common in lava rocks), could mean that when the magma-ocean was crystallizing early in the moon’s formation, heavy minerals began to sink at the boundary of the crust and mantle, and the olivine, which formed earlier, began to rise, mixing the minerals into a hybrid.


The Hubble Space Telescope's launch in 1990 sped humanity to one of its greatest advances in that journey. Hubble is a telescope that orbits Earth. Its position above the atmosphere, which distorts and blocks the light that reaches our planet, gives it a view of the universe that typically far surpasses that of ground-based telescopes.
Hubble is one of NASA's most successful and long-lasting science missions. It has beamed hundreds of thousands of images back to Earth, shedding light on many of the great mysteries of astronomy. Its gaze has helped determine the age of the universe, the identity of quasars, and the existence of dark energy.
- Ohio State Astronomy and Kepler: Caution, Science! - The Buckeye Battle Cry
- New Hubble Image Reveals True Shape of Ring Nebula - Sci-News.com
- UCSC Astronomy Lecture - Good Times
- Accurate distance measurement resolves major astronomical mystery - Astronomy Magazine
- Turbulence explains magnetic field misbehavior in solar flares - Astronomy Magazine


How big is our solar system? The short answer is it’s really, really big. The more technical answer is that it’s at least 50,000 Astronomical Units, or 7.4799 × 10^12 kilometers, or not quite a light-year to the Oort cloud on the fringes of the solar system.
The more fun answer is our solar system is bigger than five standard infographics, end-to-end. I’ll show you that [via, BBC] in a second, but first, here’s a video of the “Legonaut” portrayed in this graph (which was launched by Canadian teenagers Mathew Ho and Asad Muhammad this January).
The BBC reports that if you were to print this graphic, you’d need 27 pages of A4 paper (which is about 3/4″ longer than standard North American paper).
Are you still reading? Pretty cool, huh?


While at Mosport, enjoying some fast cars and racing, I also got to enjoy some astronomy. I set up the 'scope in the middle of the paddock!
Went Mercury chasing (I knew it was at a good elongation) but couldn't find it. Still, caught Venus at sunset. As it darkened, we checked out Jupiter.
I was very surprised to learn that Bill is a closet astronomer! He was very knowledgeable. And it seems I lit a fire. He said he was going to dust off his old telescope when he got home.


"While the SBAG committee finds that there is great scientific value in sample return missions from asteroids such as OSIRIS-Rex, ARRM has been defined as not being a science mission, nor is it a cost effective way to address science goals achievable through sample return. Candidate ARRM targets are limited and not well identified or characterized. Robotic sample return missions can return higher science value samples by selecting from a larger population of asteroids, and can be accomplished at significantly less cost (as evidenced by the OSIRIS-REx mission). Support of ARRM with planetary science resources is not appropriate."
Space & Planetary Science: July 2013 Archives
"In preparation for fiscal year 2014, a mission formulation review on Tuesday brought together NASA leaders from across the country to examine internal studies proposing multiple concepts and alternatives for each phase of the asteroid mission. The review assessed technical and programmatic aspects of the mission.
"At this meeting, we engaged in the critically important work of examining initial concepts to meet the goal of asteroid retrieval and exploration," said NASA Associate Administrator Robert Lightfoot, who chaired the review at the agency's headquarters in Washington. "The agency's science, technology and human exploration teams are working together to better understand near Earth asteroids, including ones potentially hazardous to our planet; demonstrate new technologies; and to send humans farther from home than ever before. I was extremely proud of the teams and the progress they have made so far. I look forward to integrating the inputs as we develop the mission concept further."
"Dear. Sr. Squyres:
Enclosed are NASA's responses to the nine recommendations from the NASA Advisory Council meeting held April 24-25, 2013, at NASA Headquarters. Please do not hesitate to contact me if the Council would like further background on the responses. I appreciate the Council's thoughtful consideration leading to the recommendations and welcome its continued findings, recommendations, and advice concerning the U.S. civil space program.
Charles F. Bolden, Jr. Administrator"
"For the first time since exoplanets, or planets around stars other than the sun, were discovered almost 20 years ago, X-ray observations have detected an exoplanet passing in front of its parent star.
An advantageous alignment of a planet and its parent star in the system HD 189733, which is 63 light-years from Earth, enabled NASA's Chandra X-ray Observatory and the European Space Agency's XMM Newton Observatory to observe a dip in X-ray intensity as the planet transited the star."
"NASA Administrator Charles Bolden has named planetary geologist Ellen Stofan the agency's chief scientist, effective Aug. 25.
The appointment marks Stofan's return to NASA. From 1991 through 2000, she held a number of senior scientist positions at NASA's Jet Propulsion Laboratory in Pasadena, Calif., including chief scientist for NASA's New Millennium Program, deputy project scientist for the Magellan Mission to Venus, and experiment scientist for SIR-C, an instrument that provided radar images of Earth on two shuttle flights in 1994."
"NASA Administrator Charles Bolden and the president of the Korea Aerospace Research Institute (KARI), Seung Jo Kim, met in Washington Monday to discuss collaboration in aeronautics research and space exploration, including KARI's robotic lunar mission and NASA's asteroid initiative.
Bolden and Kim also discussed NASA's plans for a new asteroid initiative, previously announced in President Obama's fiscal year 2014 budget proposal. Kim welcomed the chance to discuss opportunities for collaboration."
"The meeting will be open to the public up to the capacity of the room. This meeting is also available telephonically and by WebEx."
--2013 Science Plan
"NASA's Interface Region Imaging Spectrograph (IRIS) spacecraft has captured its first observations of a region of the sun that is now possible to observe in detail: the lowest layers of the sun's atmosphere.
The first images from IRIS show the solar interface region in unprecedented detail. They reveal dynamic magnetic structures and flows of material in the sun's atmosphere and hint at tremendous amounts of energy transfer through this little-understood region. These features may help power the sun's dynamic million-degree atmosphere and drive the solar wind that streams out to fill the entire solar system.
"With this grand opening of the telescope door and first observations from IRIS we've opened a new window into the energetics of the sun's atmosphere," said John Grunsfeld, associate administrator of the Science Mission Directorate at NASA Headquarters in Washington. "The mission is a great example of a successful partnership for science between government, industry, academia, and international institutions. We look forward to the new insights IRIS will provide."
"NASA will host a media teleconference at 1 p.m. EDT today to present the first images from NASA's Interface Region Imaging Spectrograph (IRIS), which was launched June 27 on a mission to study the sun."
The panelists for the briefing are:
-- John Grunsfeld, associate administrator, Science Mission Directorate, NASA Headquarters, Washington
-- S. Pete Worden, director, NASA's Ames Research Center, Moffett Field, Calif.
-- Alan Title, IRIS principal investigator, Lockheed Martin's Advanced Technology Center, Palo Alto, Calif.
-- Gary Kushner, IRIS project manager, Lockheed Martin's Advanced Technology Center, Palo Alto, Calif.
-- Bart DePontieu, IRIS science lead, Lockheed Martin's Advanced Technology Center, Palo Alto, Calif.
- Supporting information will be available online just before the briefing at: http://www.nasa.gov/sunearth
- You can listen to audio here.
"NASA's Hubble Space Telescope has discovered a new moon orbiting the distant blue-green planet Neptune, the 14th known to be circling the giant planet.
The moon, designated S/2004 N 1, is estimated to be no more than 12 miles across, making it the smallest known moon in the Neptunian system. It is so small and dim that it is roughly 100 million times fainter than the faintest star that can be seen with the naked eye. It even escaped detection by NASA's Voyager 2 spacecraft, which flew past Neptune in 1989 and surveyed the planet's system of moons and rings."
Hubble Spots Blue Planet [With Video], NASA/ESA/STSCI
"Astronomers using the NASA/ESA Hubble Space Telescope have, for the first time, determined the true color of a planet orbiting another star. If seen up close this planet, known as HD 189733b, would be a deep cobalt blue, reminiscent of Earth's color as seen from space.
But that's where the similarities end. This "deep blue dot" is a huge gas giant orbiting very close to its host star. The planet's atmosphere is scorching with a temperature of over 1000 degrees Celsius, and it rains glass, sideways, in howling 7000 kilometre-per-hour winds."
"Like a comet, the solar system has a tail. NASA's Interstellar Boundary Explorer (IBEX) has for the first time mapped out the structure of this tail, which is shaped like a four-leaf clover. Scientists describe the tail, called the heliotail, based on the first three years of IBEX imagery in a paper published in the July 10 edition of the Astrophysical Journal."
- Audio of today's teleconference (MP3)
"The Mars 2020 Science Definition Team (SDT) has outlined a mission concept for a science-focused, highly mobile rover to explore and investigate in detail a site on Mars that likely was once habitable. The SDT-preferred mission concept employs new in situ scientific instrumentation in order to seek signs of past life (had it been there), select and store a compelling suite of samples in a returnable cache, and demonstrate technology for future robotic and human exploration of Mars. The mission concept fully addresses the requirements specified by NASA in the SDT charter while also ensuring alignment with the recommendations of the National Academy of Sciences Decadal Survey for Planetary Science (Visions and Voyages, 2011).
Key features of the integrated science mission concept include:
- Broad and rigorous in situ science, including seeking biosignatures
- Acquiring a diverse set of samples intended to address a range of Mars science questions and storing them in a cache for potential return to Earth at a later time
- Improved landing technology to allow unprecedented access to scientifically compelling geological sites
- Collection of critical data needed to plan for eventual human missions to the martian surface
- Maximizing engineering heritage from NASA's successful Mars Science Laboratory (MSL) mission to constrain costs"
"NASA will host a media teleconference at 3 p.m. EDT Tuesday, July 9 to provide details about a report that will help define science objectives for the agency's next Mars rover.
The report, prepared by the Mars 2020 Science Definition Team (SDT) NASA appointed in January, is an early, crucial step in developing the mission and the rover's prime science objectives."
"NASA will host its first Google+ Hangout news briefing at 1 p.m. EDT Wednesday, July 10, on a new finding from the Interstellar Boundary Explorer (IBEX) mission.
The briefing will be shown live on YouTube, NASA Television and the agency's website. Journalists may participate in the briefing and ask questions by phone by contacting Steve Cole at 202-358-0918 or email@example.com with their affiliation by 10 a.m. July 10."
Marc's note: I'm all for holding a news briefing by Google+ Hangout but why not allow questions using the Hangout features?
"The OIG found that although NASA has made progress towards maximizing the research capabilities of the ISS, opportunities exist for increased utilization. NASA uses three main data points to assess utilization of ISS research capabilities: average weekly crew time dedicated to research activities, number of investigations, and use of allocated space for research. While no one measure provides a complete picture of the utilization rate, NASA has generally increased the level of activity for each metric since completion of ISS assembly in 2011.
Further progress in maximizing Station research capabilities largely hinges on two factors: the ability of CASIS to attract sufficient interest and funding from private users and the availability of reliable transportation to and from the Station for crew and cargo."
"For the Radar instrument, one of the most significant Titan fly-bys of the extended Solstice mission occurs July 10. Measurements from this flyby, combined with data from the previous flyby, will allow scientists to produce stereo images of lakes.
Inbound, the imaging science subsystem (ISS) will acquire a mosaic of high northern latitudes on Titan's leading hemisphere, approaching northern summer. This area of Titan's surface has only recently been well observed and each new flyby adds significantly to our data set."
"The engineering team has devised initial tests for the recovery attempt and is checking them on the spacecraft test bed at the Ball Aerospace facility in Boulder, Colo. The team anticipates that exploratory commanding of Kepler's reaction wheels will commence mid-to-late July. The Kepler spacecraft will remain in PRS until and during the tests.
Later this month, an update to the data processing pipeline software will be deployed. Called SOC 9.1, this enhancement has been underway for several months and is in the final stages of verification and validation. This software release provides additional refinements to better tease out small planet signatures from the four years of Kepler data. It will also decrease the frequency of false positives."
Related: NASA To Attempt To Revive Stricken Kepler Telescope in July, Space News
"I think the general feeling is that the odds are not good. We might see a wheel spin, but I suspect that it will not spin freely, that there will be noise on it -- vibrations -- which would not make the science happy," Charlie Sobeck, deputy project manager at NASA's Ames Research Center, told SpaceNews."
Marc's note: According to the Space News article they will work on wheel 4 first and then 2. Let's hope they beat the odds and some remote engineering does the trick.
"This July Fourth, the solar system is showing off some fireworks of its own. Superficially resembling a skyrocket, comet ISON is hurtling toward the sun presently at a whopping 48,000 mph.
Its swift motion is captured in this time-lapse movie made from a sequence of pictures taken May 8, 2013, by NASA's Hubble Space Telescope. At the time the images were taken, the comet was 403 million miles from Earth, between the orbits of Mars and Jupiter.
The movie shows a sequence of Hubble observations taken over a 43-minute span and compresses this into just five seconds. The comet travels 34,000 miles in this brief video, or 7 percent of the distance between Earth and the moon. The deep-space visitor streaks silently against the background stars."
"The sun lightens our world and enlightens our scientists as they look to our closest star for a better understanding of solar activity and what it means for our planet. Unique data from solar studies help researchers build on their knowledge of the Earth's atmosphere and climate change. June 30 marked the second time the International Space Station literally went out of its way to accommodate this research by providing a better viewing opportunity to meet Solar facility science objectives."


I don’t see or think about it nearly enough, but I’ve been fascinated by Lorser Feitelson’s 1936 collaged photo/painting Life Begins for years. 1936! LACMA acquired it in 1996.
Feitelson’s later, post-war, hard-edge abstraction gets much more attention than the 1930s “Post-surrealist” works, which generally makes sense. Something like Life Begins is just so unusual is almost doesn’t fit into that bucket, either. But seeing images of it again recently made me wonder just what is going on here. I can’t find almost anything written about it, except Steve Roden’s discussion of it in the LA Times a few years ago: “On some days it feels as hermetic as ‘outsider art,’ and on others it seems the most experimental painting he ever made. I’ve been visiting this work for 25 years, and I still don’t understand it. I really love that.”
Which, it’s nice to know it’s not just me who loves it, and who’s baffled by it.
The basics:Life Begins is oil and collage on a shaped masonite panel around two feet square. The painted elements are the blue space, which often gets called a sky, and a half peach and pit on a small plate. The collage elements are two black&white photographs, or close to it, of a doctor holding a newborn baby, cropped to preserve the caption, which gives the work its title; and an astronomical feature.
The “Life Begins” photo in Life Begins is easy enough to source: it’s the first photo printed in the first issue of LIFE Magazine, which began publication on November 23, 1936. [That means Life Begins was not in the Post-Surrealism show Feitelson organized for himself, his wife Helen Lundeberg, and other California-based artists at the Brooklyn Museum in May 1936. And it wasn’t among the Feitelsons included in Alfred Barr’s Fantastic Art, Dada & Surrealism show at The Museum of Modern Art in November 1936.]
The other element has taken more time to track down. When they describe it at all, most sources have called it a photo of a solar flare. But it’s not. While solar flares were being observed along with sunspots, on the face of the sun, there was no technology capable of photographing a solar flare like that in the 1930s. The only online source to identify the image correctly was a letter from an MD/amateur astrophotographer of the Journal of the American Medical Association, which had used Feitelson’s painting on their cover in 2004. It is a detail of the Western Veil Nebula (NGC 6960) in the constellation Cygnus.
We are very used to such images now, but in 1936, there were very few observatories capable of producing such a photo. The scientific understanding of nebulae, and of the universe itself, was in flux. It was only in 1924 that Edwin Hubble announced, first in the New York Times, that many of the objects called nebulae were actually galaxies, which existed far beyond our own Milky Way. As late as 1933, it was still a matter of speculation whether the Veil Nebula surrounded a star, and was actually the remnant of a supernova.
Let’s just say, thanks to Hubble the telescope, it’s difficult to search for historical astronomical images, which have been supplanted by higher resolution, full spectrum glitz. After a couple of evenings, though, I think I found Feitelson’s source. Wellesley astronomer John Charles Duncan, who published several articles on photographing nebulae, made a 7-hour exposure of NGC 6960 in 1921 using the largest telescope in the world, the Hooker Telescope at Mount Wilson Observatory. Duncan used the image as the frontispiece for Astronomy A Textbook, published in 1927. [above]
The image doesn’t match Feitelson’s in size, exposure, or cropping, obviously, but I suspect the artist either rephotographed the detail from the plate, or got access to the negative at Mount Wilson. Duncan later published a fainter, underexposed version of the image, which extrapolates to what an overexposed version like Feitelson’s would look like.
Helen Lundeberg, Red Planet (1934)
This image in Life Begins is not a one-off. Both Feitelson and his wife Helen Lundeberg included astrophotography in their Post-Surrealist paintings in the early 30’s. [Maybe Post-Surrealism feels a bit like Post-Internet: a way for artists to signal to lagging institutions they’ve incorporated something and are moving ahead.] Lundeberg’s Red Planet is a paradoxically lit interior featuring a red planet-looking orange hovering over a telescope mirror-looking tabletop, and a photograph of a comet leaning against a book titled, “Mars.”
Lorser Feitelson, Genesis #2, 1934, collection Smithsonian American Art Museum
Feitelson’s Genesis #2, also 1934, has a telescope pointing through the eyes of several aging masks and a skull, propped on books, toward a painting of what looks to me like a photograph of the Crab Nebula. There’s also a trompe l’oeil drawing of an Annunciation, a Picabian outline of a woman and her developing breast, a baby bottle, a conch, an eggshell, and a sliced melon and light bulb that immediately make me think of Matson Jones-era Johns. Which, any connection is impossible, I know, but still.
Genesis #2 combines scientific, religious, and metaphorical accounts of birth, which makes it feel closely related to Life Begins. Now the unusual shape of Life Begins feels related to the perspectival lines and sharp, flat planes Feitelson used to define his spaces. Which makes Life Begins a variation on a Genesis II theme; when Life Magazine launched with that photo, of all photos, Feitelson must have really felt like he was onto something big.
Life Begins [lacma.org]
Genesis #2 [americanart.si.edu]


Span classnews_dt12262009spannbsp018332credits to the original creators the best of eagle attacks 2018 most amazing moments of wild animal fights.
Rocket game online free build a rocket launch a rocket flying games for kids boys amp girls wonder rocket is a fun repeatplay online rocket maker good things happen daily free.
Here are some instructions for how to launch a rocket into space here are some instructions for how to launch a rocket into space made for launching orbiting and returning to earth space.
Play space rocket games online including popular games such as learn to fly fly hard into space and rocket toilet our fun rocket launching games see you try to get your character as high and as far as possible attached to a rocket or some other item.
Free vector icon download thousands of free icons of transport in svg psd png eps format or as icon font.
Getting started from kerbal space program wiki but provides a decent understanding of the basics as modifying and launching a rocket the rest will be explored with your intuition or guidance of another tutorials to learn the editor and the basics of rocket building or move to the tutorials page where you may find many secrets of ship.
The neptune escape rocket is a vehicle that is used to leave planet 4546b it consists of five constructable parts the neptune launch platform the neptune gantry neptune ion boosters neptune fuel reserve and neptune cockpit the neptune launch platform can be built using the mobile vehicle.


Venus is completely covered by a thick cloud layer, of which the upper part is composed of sulphuric acid and some unknown aerosols. The cloud tops are in fast retrograde rotation (super-rotation), but the factors responsible for this super-rotation are unknown. Here we report observations of Venus with the Venus Monitoring Camera on board the Venus Express spacecraft. We investigate both global and small-scale properties of the clouds, their temporal and latitudinal variations, and derive wind velocities. The southern polar region is highly variable and can change dramatically on timescales as short as one day, perhaps arising from the injection of SO2 into the mesosphere. The convective cells in the vicinity of the subsolar point are much smaller than previously inferred, which we interpret as indicating that they are confined to the upper cloud layer, contrary to previous conclusions, but consistent with more recent study.


With all the buzz about the Great American Eclipse of 2017, of course I had to post a playlist for the event. Hope you are all prepared with your special eclipse glasses and/or pinhole viewing apparatuses.
To further your viewing experience here are ten songs I plan to be playing on a loop throughout the afternoon as the moon casts its shadow across the country. So a quick list and then off to spend the day checking out all things solar.
- Cat Stevens’ Moonshadow Is there a more appropriate song for today’s eclipse? This isn’t the best recording, but I’ve had some trouble getting this song on You-Tube today. I suggest going through Spotify or pulling out those old CDs you’ve been holding onto. Now you know why you’ve been storing them. Be careful driving out there and don’t look directly at the eclipse without proper glasses. And if I ever lose my eyes, if my colours all run dry, Yes if I ever lose my eyes, Oh if I won’t have to cry no more.
- Eclipse from Pink Floyd’s Dark Side of the Moon Okay, Let me go on record here and say I’m adding the whole album, Dark Side of the Moon, to my playlist. I mean that just goes without saying, right? This is an album that was just made for a total eclipse.
- Ain’t No Sunshine Bill Withers I chose this one because there’s no need to be completely literal and this is a great song. I like to mix it up.
- Bad Moon Rising Creedence Clearwater Revival Love CCR, love this song, nuff said.
- Here Comes the Moon George Harrison I love the spacey quality of this song. One of Harrison’s most beautiful arrangements.
- Mountains of the Moon Grateful Dead This reminds of springtime in the San Francisco Peaks outside of Flagstaff, AZ.
- Howlin Wolf Any and all things Howlin Wolf. Here’s a full album.
- Elton John Bad Side of the Moon I used to listen to this for hours back in the day.
- Drunk on the Moon Tom Waitts I love Tom Waitts. Just right for eclipsing.
- The Moon’s a Harsh Mistress Jimmy Webb
There you go. Hope you enjoy the playlist and the eclipse. Have fun and stay safe. Also, be sure to share your favorite eclipse songs here on Tea Cakes and Whiskey.


Have a relaxing and fun-filled vacation on Venus
Since the atmosphere of Venus is made of mostly carbon dioxide, most of the activities we have are air-based. Venus's atmosphere also has many toxic chemicals in its air so, come at your own risk. Venus is 162 million miles away from Earth and is 3,760 miles long. That is good for lots of exploring. Lastly, Venus's one orbit lasts 243 days, lots of time to fit in all of your activities!


This is the monthly meeting of the Raleigh Astronomy Club
Details: Members share their holiday gear gifts or purchases! Afterwards, the club will discuss Astronomy Days, a signature event for the Raleigh Astronomy Club in conjunction with the NC Museum of Natural Sciences
Location: NCSU Crafts Center (directions)
Date: January 13, 2017
Synopsis: Did you receive any Astronomy Gear over the Holiday’s? If so bring it to the NCSU Crafts center and spend 5 – 10 Minutes describing what you got and how you plan on using it in your observing program.
After the meeting, we go out for a meal. Please plan to join us!


Gem Falls is an expansion to the Woodland village, released on June 29, 2016.
It must be accessed by the Woodland Hero Theia. She will focus to unlock the Shining Stones that guard the Sacred Passage. This takes her 1 minute.
According to Greek mythology, Theia is a Titaness said to have given birth to the Sun, the Moon, and the Dawn.


Eur. Phys. J. A 13, 383-397 (2002)
Neutrino-nucleon scattering rate in proto-neutron star matterL. Mornas1 and A. Pérez2
1 Departamento de Física, Universidad de Oviedo, Av.da Calvo Sotelo, E-33007 Oviedo (Asturias) Spain
2 Departamento de Física Teórica, Universidad de Valencia, E-46100 Burjassot (Valencia), Spain
(Received: 27 June 2001 / Revised version: 14 January 2002 Communicated by V. Vento)
We present a calculation of the neutrino-nucleon scattering cross-section which takes into account the nuclear correlations in the relativistic random phase approximation (RPA). Our approach is based on a quantum-hadrodynamics model with exchange of , , , and mesons. In view of applications to neutrino transport in the final stages of supernova explosion and proto-neutron star cooling, we study the evolution of the neutrino mean free path as a function of density, proton-neutron asymmetry and temperature. Special attention was paid to the issues of renormalization of the Dirac sea, residual interactions in the tensor channel, coupling to the delta-meson and meson mixing. In contrast with the results of other authors, we find that the neutral-current process is not sensitive to the strength g' of the residual contact interaction. As a consequence, it is found that RPA corrections with respect to the mean-field approximation amount to only 10% to 15% at high density.
26.60.+c - Nuclear matter aspects of neutron stars.
13.15.+g - Neutrino interactions.
24.10.Jv - Relativistic models.
© Società Italiana di Fisica, Springer-Verlag 2002


A mysterious, luminous object has appeared in the night sky over Seattle on Tuesday, leading some locals to question its origin.
“I swear i just saw a spacecraft over Seattle,” Twitter user @achlngbIush posted about 10:30 p.m. local time. “It was one straight string of lights soaring slowly over the sky what is happening.”
Other curious viewers were quick to reply, with one person asking: “Saw it too! What was that?”
“My neighbor saw it also when she was walking her dog,” another added.
One man suggested the mystery sighting could be the SpaceX Falcon 9 rocket which launched 60 Starlink internet satellites into orbit on Tuesday afternoon.
The rocket blasted off from NASA‘s Kennedy Space Center in Florida at 3:01 p.m. EDT. Due to the three-hour time difference between Florida and Washington State, it could be feasible that the rocket was seen over Seattle after or around 6 p.m.
“Targeting Tuesday, May 4 at 3:01 p.m. EDT for Falcon 9 launch of 60 Starlink satellites from LC-39A in Florida,” SpaceX tweeted on Monday.
The rocket stands at an impressive 70 meters tall and could account for the “straight string of lights” observed in the sky.
Last week, SpaceX launched 60 satellites into orbit in another step towards building its vast Starlink satellite internet network.
The satellites were packed into the payload section of a Falcon 9 rocket, which lifted off from the Cape Canaveral Space Force Station in Florida at 11:44 p.m. EDT Wednesday night on April 28.
SpaceX is known for its reusable space technology, and that particular Falcon 9 had flown six times before. It was also the fourth time that that rocket had delivered Starlink satellites into space.
The starlink satellites that were deployed on last week’s mission had a slightly different appearance to ones that had been delivered before them; they had been painted white for thermal reasons. That could make them appear brighter in the sky.
There are currently more than 1,395 Starlink satellites in orbit according to NasaSpaceFlight.
This sounds like a lot, but it is only the first “shell” of satellites in the Starlink network.
SpaceX is planning several more shells, each with hundreds or thousands of satellite units. The company eventually plans to put 12,000 Starlink satellites in orbit, though it has applied for permission to deploy 30,000.
SpaceX is now close to completing its first shell, which will consist of around 1,584 satellites. Once it’s done, Starlink will have coverage between 52 and minus 52 degrees latitude, or around 80 percent of the Earth’s surface, according to Everyday Astronaut. Subsequent shells will improve bandwidth and coverage.


FRANKFURT/NEW YORK CITY. High-profile scientists from around the globe are convening at the Science Summit around the United Nations General Assembly in New York City for the ninth time today to present and discuss contributions to achieving the Sustainable Development Goals (SDGs). This year, Professor Luciano Rezzolla from Goethe University Frankfurt’s Institute for Theoretical Physics is among them.
"It is a great honor and privilege to share my work in this context," says Rezzolla, who has been invited to offer insights into the creation of the first-ever image of a black hole and the first image of the black hole located at the center of our Milky Way. Images of these supermassive black holes, known as M87* and Sgr A*, are exciting not only to the astrophysical community, but have also generated significant media attention in recent years. What made these groundbreaking images possible was the global "Event Horizon Telescope" collaboration, which connects radio telescopes worldwide into a virtual telescope, the sheer size of which can be compared to that of Earth, and as part of which participating scientists and institutions process collected data, and conduct intricate theoretical calculations.
Establishing such international scientific collaborations is also a declared goal of the Science Summit, which aims to bring the realization of the SDGs closer – a goal that is also reflected in the title of the session that will feature Professor Rezzolla as the keynote speaker: "Expanding Scientific Horizons through International Collaboration and Networking." Emphasizing the importance of interdisciplinary cooperation, he says: "Unlike a century ago, many scientific breakthroughs today are made possible only through close and interdisciplinary collaboration among countless scientists worldwide. The Event Horizon Telescope Collaboration brought together more than 300 researchers from 80 different institutions and this has ultimately allowed what was till then thought to be impossible: take a photo of a black hole."
Interdisciplinary collaboration continues to play a central role in Rezzolla's daily research activities. He is the spokesperson of the ELEMENTS cluster project, which brings together more than 100 astrophysicists and nuclear physicists, who work collaboratively and employ diverse methods to investigate how heavy elements originated in the universe.
The 78th session of the United Nations General Assembly opened on September 5, 2023. Held concurrently from September 12-29, the 9th Science Summit brings together numerous stakeholders from the fields of science and politics to discuss the contributions of science to the implementation of the SDGs. The meeting also will provide input for the United Nations Summit of the Future, which will take place in September 2024.
Virtual participation in the 9th UN Science Summit is free of charge. Professor Luciano Rezzolla's talk will be broadcast on September 15, 2023, at 2:45 p.m. CEST. For further information, please visit https://sciencesummitunga.com.