# Radiation/Airborne

Astronomy that benefits from using either an airborne observatory or such a telescope or detector system is airborne astronomy.

## Airborne observatory

An airborne observatory is an airplane or balloon with an astronomical telescope. By carrying the telescope high, the telescope can avoid cloud cover, pollution, and carry out observations in the infrared spectrum, above water vapor in the atmosphere which absorbs infrared radiation.

## Observation posts

An observation post, temporary or fixed, is any pre-selected position from which observations are to be made - this may include very temporary installations or even an airborne aircraft.

## Magnetometric surveys

"A detailed airborne magnetometric survey [such as by the helicopters center and on the left] indicated the structure of the area was followed by a west northwest striking domal feature and the lithologies are probably siliceous, clastic sediments. The southern margin of the antiform has been transected by a west northwest striking shear, whilst the western part of the dome has undergone a later stage folding regime and the intrusion of a granitoid. [Broken Hill Type Ag-Pb-Zn (BHT) mineralisation] BHT mineralisation is predominantly hosted at a major stratigraphic break, and remobilised or offset into both the hangingwall and footwall."
Magnetometric surveys readily detect magnetic iron minerals in red or black bands within banded iron formations such as in the image on the right.
"This high-resolution map [second image down on the right] of magnetic field variations measured over northwestern Minnesota shows many details of the 2.7- to 2.0-billion-year-old geology underlying the cover of glacial deposits."
"The helicopter flies back and forth over the [the area], tracing small fluctuations in Earth’s magnetic field with a magnetometer at the end of its wandlike “stinger” attachment and marking the locations with GPS coordinates. Later, scientists scour the magnetic field data for clues to mineral and petroleum deposits, faults, buried lava flows, water resources, and even pipelines and landfills buried beneath the surface of the ground."
"High-quality aeromagnetic data are profoundly useful—they help geologists characterize features on the surface and below, providing information directly relevant to geologic mapping and structure, mineral resources, groundwater, earthquake hazards, volcanic hazards, and petroleum resources. Moreover, aeromagnetic data are inexpensive to acquire compared with other types of geoscientific data."
The "lower the terrain clearance of the aircraft (i.e., the closer the magnetometer is to the ground surface) is, the greater the resolution of the data in the direction of flight is, meaning that subtle anomalies related to finer details of the geology are more likely to be captured [...]."

## Electromagnetic surveys

"Airborne electromagnetic surveys using a grounded electric dipole source and magnetic surveys were conducted to delineate resistivity and magnetization structures".
"Airborne Electromagnetic (AEM) data [such as collected by the TEMPEST system shown on the right, the transient electromagnetic (TEM) SkyTEM system shown on the left, or the Versatile Time Domain Electromagnetics (VTEM) system in the center] are one form of the geophysical data acquired by Geoscience Australia. The data are gathered by transmitting an electromagnetic signal from a system attached to a plane or helicopter. The signal induces eddy currents in the ground which are detected by receiver coils towed below and behind the aircraft in a device called a bird. Depending on the system used and the subsurface conditions, AEM techniques can detect variations in the conductivity of the ground to a depth of several hundred metres, [sometimes up to 2000 metres in particularly favourable conditions]. The conductivity response in the ground is commonly caused by the presence of electrically conductive materials such as salt or saline water, graphite, clays and sulfide minerals."
"Since 2006, Geoscience Australia and its State and Territory partners have been collecting AEM data over large areas at broad line spacing (1000-6000 metres) to more fully survey Australia. AEM surveys also require complex processing to allow interpretation and, therefore, are usually designed to detect particular subsurface targets which are based on a perceived conductivity contrast, for example:
* the spatial extent of geological features, such as a clay-rich unit in a sedimentary sequence or a graphite-bearing unit in a metamorphic complex
* the depth of an unconformity between sedimentary cover and the underlying basement rock
* the location of groundwater resources, such as fresh or saline aquifers."

## Conductivity surveys

Conductivity measurements of the distribution of electrical conductivity in the ground is made aerially with a sensor suspended from the helicopter such as in the image on the right map changes in the ground water.

## Hyperspectral imaging systems

"[Airborne Real-time Cueing Hyperspectral Enhanced Reconnaissance (ARCHER)] is essentially something used by the geosciences. It's pretty sophisticated stuff … beyond what the human eye can generally see."
"It might see boulders, it might see trees, it might see mountains, sagebrush, whatever, but it goes 'not that' or 'yes, that'. The amazing part of this is that it can see as little as 10 per cent of the target, and extrapolate from there."

## Aerial induced polarization

The airplane imaged on the right is equipped with an induced polarization/resistivity device for use in time and frequency modes. Induced polarization is a reliable technique for detecting disseminated sulphides associated with base metal and gold deposits.

## Aerial magnetotellurics

"Magnetotellurics (MT) is an electromagnetic method of imaging the earth's subsurface [conducted both aerially portrayed in the image on the left and through ground contact]. It uses natural variations in the earth's magnetic field to map contrasts in the electrical resistivity of the subsurface. These data [as in the image on the right] are used to image changes in the electrical resistivity over a large range of depths: from the top of the crust to the mantle. Such resistivity models are then interpreted geologically in terms of the fluid, thermal and structural evolution of the lithosphere."

## Meteors

Starting on November 16, 1935, at 1:00 A.M., the American Airlines Douglas aircraft took off from Chicago and rose to 8,000 feet. "I had a view of the sky between altitudes 20° and 65° extending over 200° in azimuth through the windows of the pilots' compartment. Leo was in sight all of the time. [...] The pilots, Mr. Hiram W. Sheridan and Mr. William H. Records, and I saw only three meteors on the Chicago-Detroit leg of the journey. [...] After a delay of about one hour in Detroit [the aircraft took off again and rose] to about 7500 feet. [...] we saw not a single meteor. [On the return flight from Newark, New Jersey, directly to Chicago the aircraft maintained an altitude of 11,000 feet.] Only three meteors were seen during the flight from Buffalo to Chicago. Two of these were moderately bright Leonids which flashed across Ursa Major."
"Two photographs, taken 1939 April 29, by Mr. John H. Spikes (photographer [about 10:20 A.M. EST]) and Capt. D. Z. Zimmerman (pilot [a few minutes later]), from an altitude of 9,300 feet about 20 miles east of Salem, Alabama, show what the writer interprets as a meteor train. The train is about 13° long on the first picture and 8° on the second."
In the second photograph, there are "marked changes, especially apparent doubling and twisting."
"A skydiver may have captured the first film [image is second down on the right] ever of a meteorite plunging down at terminal velocity, also known as its “dark flight” stage."
"The footage was captured in 2012 by a helmet cam worn by Anders Helstrup as he and other members of the Oslo Parachute Club jumped from a small plane that took off from an airport in Hedmark, Norway."
“It can’t be anything else. The shape is typical of meteorites -- a fresh fracture surface on one side, while the other side is rounded.”
“It has never happened before that a meteorite has been filmed during dark flight; this is the first time in world history.”
"Having the rock in hand would certainly help. But despite triangulations and analyses, Helstrup and his recruits still haven’t found it."

## Cosmic rays

"By the total exposure of 5865.7 m$^2$·hour·str on DC-8 [airplane at 260 gm/cm$^2$ altitude], we have obtained hadronic and gamma-ray family spectra [from cosmic rays]."

## Neutrons

The "effect of time-variations in galactic cosmic rays on the rate of production of neutrons in the atmosphere [was studied using] a series of balloon and airplane observations of the [fast neutron] flux and spectrum of 1-10 MeV neutrons, in flights at high geomagnetic latitude, during [quiet times as well as during Forbush decreases, which are rapid decreases in the observed galactic cosmic rays following a coronal mass ejection (CME), and solar particle events for] the period of increasing solar modulation, 1965-1969. It also included latitude surveys in 1964-1965 and in 1968."
In the image on the right for Forbush decreases, data include GOES-15 X-rays, energetic particles, and magnetometer. Cosmic Rays from the Moscow station show a Forbush Decrease.

## Gamma rays

Airborne gamma-ray spectrometry is now the accepted leading technique for uranium prospecting with worldwide applications for geological mapping, mineral exploration & environmental monitoring.
On June 19, 1988, from Birigüi (50° 20' W 21° 20' S) at 10:15 UTC a balloon launch occurred which carried two NaI(Tl) detectors (600 cm$^2$ total area) to an air pressure altitude of 5.5 mb for a total observation time of 6 hr. The supernova SN1987A in the Large Magellanic Cloud (LMC) was discovered on February 23, 1987, and its progenitor is a blue supergiant (Sk -69 202) with luminosity of 2-5 x 10$^{38}$ erg/s. The 847 keV and 1238 keV gamma-ray lines from $^{56}$Co decay have been detected.
"Gamma rays at energies of 0.3 to 8 megaelectron volts (MeV) were detected on 15 April 1988 from four nuclear-powered satellites including Cosmos 1900 and Cosmos 1932 as they flew over a [balloon-borne] double Compton gamma-ray telescope."

## Infrareds

The Gerard P. Kuiper Airborne Observatory (KAO) was a national facility operated by NASA to support research in infrared astronomy. The observation platform was a highly modified C-141A jet transport aircraft with a range of 6,000 nautical miles (11,000 km), capable of conducting research operations up to 48,000 feet (14 km). The KAO was based at the Ames Research Center, NAS Moffett Field, in Sunnyvale, California. It began operation in 1974 as a replacement for an earlier aircraft, the Galileo Observatory, a converted Convair CV-990 (N711NA).
The "Stratospheric Observatory for Infrared Astronomy [(SOFIA) is] mounted onboard a Boeing 747SP. [...] SOFIA’s 2.7 m mirror and optimized telescope system combines the highest available spatial resolution with excellent sensitivity. SOFIA will operate in both celestial hemispheres for the next two decades."
It has an operating altitude of 12-14 km, 39,000-45,000 ft and a spatial resolution of 1-3" for 0.3 < λ < 15 µm, and λ/10" for λ > 15 µm.

## Radars

Numerous airborne and spacecraft radars have mapped the entire planet, for various purposes. One example is the Shuttle Radar Topography Mission, which mapped the entire Earth at 30 m resolution.
Per the image on the right: "In 2015 I was looking at a new map of the bedrock below the Greenland Ice Sheet and discovered a large circular feature under the Hiawatha glacier in northwest Greenland."
"You can see the rounded structure at the edge of the ice sheet, especially when flying high enough."
"For the most part the crater isn’t visible out the airplane window. It’s funny that until now nobody thought, ‘Hey, what’s that semicircular feature there?’ From the airplane it is subtle and hard to see unless you already know it’s there. Using satellite imagery taken at a low sun angle that accentuates hills and valleys in the ice sheet’s terrain—you can really see the circle of the whole crater in these images."
"It is correct that the crater is not well dated but there’s good evidence that it is geologically young, that is, it formed within the last 2 to 3 million years, and most likely it is as young as the last Ice Age [which ended around 12,000 years ago],” Larsen explained to Gizmodo. “We are currently trying to come up with ideas on how to date the impact. One idea is to drill through the ice and get bedrock samples that can be used for numerical dating."
On the left is an image of bed "topography based on airborne radar sounding from 1997 to 2014 NASA data and 2016 Alfred Wegener Institute (AWI) data. Black triangles represent elevated rim picks from the radargrams, and the dark purple circles represent peaks in the central uplift. Hatched red lines are field measurements of the strike of ice-marginal bedrock structures. Black circles show location of the three glaciofluvial sediment samples".
"Glaciofluvial sediment from the largest river draining the crater contains shocked quartz and other impact-related grains. Geochemical analysis of this sediment indicates that the impactor was a fractionated iron asteroid, which must have been more than a kilometer wide to produce the identified crater. Radiostratigraphy of the ice in the crater shows that the Holocene ice is continuous and conformable, but all deeper and older ice appears to be debris rich or heavily disturbed."

## Aerial gravity gradiometry

The aircraft imaged on the right carried-out aerial high-resolution gravity gradiometry system in combination with LIDAR digital terrain mapping, electromagnetics, digital video, and gamma-ray spectrometry over "onshore areas along the South-Eastern Tanzanian Coastal Basin and the eastern arm of the East African Rift."

## Craters

* Buried craters can be identified through drill coring, aerial electromagnetic resistivity imaging, and airborne gravity gradiometry.
At right is a "[r]ecent airborne geophysical surveys near Decorah, Iowa [which is] providing an unprecedented look at a 470- million-year-old meteorite crater concealed beneath bedrock and sediments."
"Capturing images of an ancient meteorite impact was a huge bonus," said Dr. Paul Bedrosian, a USGS geophysicist in Denver who is leading the effort to model the recently acquired geophysical data. "These findings highlight the range of applications that these geophysical methods can address."
"In 2008-09, geologists from the Iowa Department of Natural Resources' (Iowa DNR) Iowa Geological and Water Survey hypothesized what has become known as the Decorah Impact Structure. The scientists examined water well drill-cuttings and recognized a unique shale unit preserved only beneath and near the city of Decorah. The extent of the shale, which was deposited after the impact by an ancient seaway, defines a "nice circular basin" of 5.5 km width, according to Robert McKay, a geologist at the Iowa Geological Survey."
"Bevan French, a scientist the Smithsonian's National Museum of Natural History, subsequently identified shocked quartz - considered strong evidence of an extra-terrestrial impact - in samples of sub-shale breccia from within the crater."
"The recent geophysical surveys include an airborne electromagnetic system, which is sensitive to how well rocks conduct electricity, and airborne gravity gradiometry, which measures subtle changes in rock density. The surveys both confirm the earlier work and provide a new view of the Decorah Impact Structure. Models of the electromagnetic data show a crater filled with electrically conductive shale and the underlying breccia, which is rock composed of broken fragments of rock cemented together by a fine-grained matrix."
"The shale is an ideal target and provides the electrical contrast that allows us to clearly image the geometry and internal structure of the crater," Bedrosian said.

## Earth

In the image at left is an aerial view of the Barringer Meteor Crater about 69 km east of Flagstaff, Arizona USA. Although similar to the aerial view of the Soudan crater, the Barringer Meteor Crater appears angular at the farthest ends rather than round.

## Balloons

Balloons provide a long-duration platform to study any atmosphere, the universe, the Sun, and the near-Earth and space environment above as much as 99.7 % of the Earth's atmosphere. Unlike a rocket where data are collected during a brief few minutes, balloons are able to stay aloft for much longer. Balloons for astronomy offer a low-cost, quick-response method for conducting scientific investigations. They are mobile, meaning they can be launched where the scientist needs to conduct the experiment, in as little as six months.

## Infrared telescopes

"SOFIA’s primary mirror, located near the bottom of the telescope, is 2.7 meters (almost 9 feet) across. The front surface, which is highly polished and then coated with Aluminum to ensure maximum reflectivity, is deeply concave (dished inward). Incoming light rays bounce off the curved surface and are all deflected inward at the same time they are reflected back up toward the front of the telescope."
"Before the light reaches the telescope’s front end, however, it is intercepted by a small secondary mirror (about .4 meters across), which sends the light back down toward the center of the main mirror. About a meter above the center of the main mirror, a third mirror sends the light out through the side of the telescope, down a long tube which projects through the main aircraft bulkhead into the interior of the SOFIA aircraft. There, at the telescope’s focal point, the light will be recorded and analyzed by one of several different instruments."
"Astronomers tend to compare telescopes based on the diameter of their primary mirrors. SOFIA’s telescope is usually referred to as a 2.5-meter meter telescope, rather than 2.7 meters, because the optical design requires that only about 90% of the mirror’s reflecting surface (called the "effective aperture") can be used at any one time. Although SOFIA’s telescope is by far the largest ever to be placed in an aircraft, compared to normal ground-based research observatories it is only medium-sized (the world’s largest single-mirror telescope, the Subaru, is 8.2 meters across)."

## Hypotheses

* Extremely high altitude powered flight may allow observation at a lower cost than a satellite.