March 1983:JPL space report

STARPROBE

Unlike ISPM, which will observe the Sun from a
distance, Starprobe will travel to our nearest star to
observe it at close range. Starprobe is an advanced-
technology mission that, if financed, is being planned
within NASA's Office of Space Science and Applications
to approach the Sun as close as 2 million km sometime in the 1990's. The Starprobe study, begun in
1978, is being conducted for NASA by JPL.

Starprobe's scientific experiments can be grouped
into three sets: gravitation and relativity, fields and
particles, and optical sciences.

To achieve the high accuracies necessary for measuring the Sun's oblateness (the flattening of the sphere,
with a bulge around the equator), and relativistic parameters for the gravitational experiment, a ''drag-compensation" system will be carried. This is a new device
consisting of a spherical mass free to move inside the
satellite in a protective cavity. This means that the
mass is unaffected by, for example, light pressure from
the Sun. The rest of Starprobe, however, will be
affected and measuring the movement of the mass
inside its cavity will allow scientists to separate the
effects of gravity and those of other forces. The gravitational field around the Sun is affected by the solar
oblateness so accurate measurement of the former will
allow calculation of the latter. The system is very
sensitive. For example, the ”J2 oblateness factor" will
be determined to one part in one hundred million.
However, the system will also respond to such onboard
activity as sloshing of fuel in the tanks. These must be
recorded so that their effects can be accounted for in
the data analysis. A new concept in radio tracking with
an onboard "doppler extractor" will also be needed.

The fields and particles experiments will, like ISPM,
study the solar wind and magnetic fields, but the
measurements will take place within the corona itself.

The optical experiments will study the structure and
dynamical behaviour of the photosphere, chromosphere
and corona at a resolution of better than 10 km.

Two other major technological problems must be
solved in the Starprobe design: construction of a heat
shield and the development of a communications system that will make itself heard above the hiss and
crackle of the solar corona. The conical heat shield will
dissipate heat by allowing its material to vaporise at
the rate of 9 g/hr at perihelion. Testing of a shield
prototype, made from carbon-carbon, is presently under
way at Odeillo-Font Romeu in France at the world's
largest solar furnace.
It is not easy to lose energy to drop in close to the
Sun, so once again the familiar trek out to the kinetic
energy dealer of the Solar System, Jupiter, will have
to be made. The inclination of the trajectory will be
close to 90 degrees, resulting in a polar pass of the
Sun in order to observe the coronal holes. The pass
also yields the best gravity-measurement results.

Thirty days from perihelion the observatory phase
of the mission will begin, switching to the far encounter
phase at ten days or 0.5 AU (the closest approach to
the Sun, to date, was accomplished by the US-West
German Helios at 0.3 AU), with heavy emphasis on
fields and particles measurements. The near-encounter
phase starts at four days, with optical observation of
coronal holes. Finally, passage from pole to pole of the
Sun takes place in only 14 hours.

The Starprobe study is being managed by James E.
Randolph of JPL.

EXTREME ULTRAVIOLET EXPLORER

The visible portion of the electromagnetic spectrum
extends from about 4000 to 7000 A (the angstrom
unit A is equal to 10 -8 metres) with the infrared and
radio portions lying on the long side of the visible, and
the ultraviolet, extreme ultraviolet, X-ray, and gamma
ray portions lying on the short side. The Extreme
Ultraviolet Explorer (EUVE) satellite is planned by NASA
to conduct an all-sky survey in the-EUV spectral region
from 100 to 1000 A. The phrase "extreme ultraviolet"
refers to the fact that the location’of that region is at
the extreme end of the ultraviolet, as measured from
the visible portion of the spectrum. EUVE differs from
the presently-operating International Ultraviolet
Explorer (IUE) in that the latter satellite covers the
range from 1100 to 3200 A and is used to observe
specific astronomical sources as opposed to providing
an all-sky survey.

EUVE has been assigned to JPL by NASA's Office
of Space Science and Applications for Phase B studies
after which a formal project start is planned. Astronomical instrumentation for the satellite will be provided
by the University of California at Berkeley, under the
direction of the Principal Investigator, Professor Stuart
Bowyer. It was the Berkeley group that conceived and
executed the successful Apollo-Soyuz experiment.

Current plans call for a launch in 1988, but this date
is dependent upon funds available from the Explorer
Program of NASA. Since the Explorer budget is based
on the principle of allocating a constant number of
dollars per year to the entire programme, the funds
consumed by an ongoing Explorer project, such as IRAS
(Infrared Astronomical Satellite) or COBE (Cosmic Background Explorer), can affect the funds available for a
later Explorer project, such as EUVE. On the whole,
this method of funding provides a highly stable fiscal
environment and allows NASA to conduct a planned
programme of scientific research using Earth satellites.

The satellite will be spin stabilised with the spin axis
electromagnetically torqued to continually point toward
the Sun. Then the three telescopes, which point radially
outwards from the spin axis, will complete coverage
of the celestial sphere after six months of operations.
Each point of the sky will be observed many times
since the satellite will be spinning at several rpm. Three
telescopes will be used for the sky survey in order to
obtain information about three subspectral regions of
the EUV. The 20 in Wolter-Schwarzchild telescopes
are tailored for these spectral regions by the use of
very thin filters over their apertures.

The astronomical data are to be stored on tape
recorders and relayed to Earth when one of the two
relay satellites is in view of the EUVE antenna.

The manager of the EUVE project is John Paulson,
who also directs the Solar Mesosphere Explorer (SME)
now in orbit and collecting data relevant to ozone
balance and dynamics in the Earth's atmosphere.

WIDE FIELD/PLANETARY CAMERA

When the Space Telescope (ST) is launched in 1985,
one of the five science instruments it will carry is the
Wide Field and Planetary Camera system designed and
built by JPL and Caltech.

The system consists of two cameras of different
focal lengths sharing the same housing and electronics.
The wide-field camera is an f/12.9 instrument with a
total field-of-view of 160X 160 arcseconds, while the
planetary camera is f/30 and 68.7X68.7 arcseconds.
The cameras each cover the very broad spectral range
from the ultraviolet through the near infrared: 1200 to

12,000 A.
Eight charge-coupled devices (CCDs), four for each
camera system, are used to record the images. The
four images will be computer processed on the ground
and recombined in a mosaic. Each CCD is a solid-state
array of silicon detectors and has 800 by 800 picture
elements. The use of CCDs in space astronomy was
pioneered by JPL and Texas Instruments in conjunction
with the Galileo mission to Jupiter. The virtue of
the CCD is that it is an extremely sensitive device with
a quantum efficiency well in excess of the eye, photographic film or photomultiplier tubes over a broad
spectral range from the ultraviolet to the near infrared.
Earth-based telescopes that use CCDs have observed
objects fainter than 25th magnitude. A recent, dramatic
demonstration of the power of CCDs occurred with the
recovery of Halley's Comet by Palomar Observatory
last October at apparent magnitude 24.3.

Testing and calibration of the system will be completed by this summer, and then the Wide
Field/Planetary Camera will be shipped to the Goddard
Space Flight Center for integration with the data system
and the other scientific instruments on ST. Following
this, the instrument will be tested in ST by the Lockheed
Missile and Space Company in California.
In addition to high resolution studies of objects such
as galactic centres, the system will be used in an
attempt to detect perturbations of nearby stars in a
search for Jupiter-sized planets that might be in orbit
about them.

The other instruments on ST are: the Faint Object
Camera, provided by the European Space Agency; the
High-Speed Photometer, built b\p- the University of
Wisconsin; the Faint-Object Spectrograph, built by
Martin Marietta for the University of California at San
Diego; and the High-Resolution Spectrograph, built by
Ball Brothers for the Goddard Space Flight Center.

Raymond L. Heacock of JPL is the Acting Manager
of the Wide Field/Planetary Camera project, and Professor James A. Westphal of Caltech is the Principal
Investigator. Heacock has also served as manager of
the Voyager Project and of the new, low-cost Mariner
Mark II project which is designing planetary missions
for the next decade.