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.