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Science News, Jan 3, 1998 by Ron Cowen
In space, small is in. The majority of missions scheduled for launch this year are downright diminutive, and most carry only a handful of instruments. Even the $1.4 billion Advanced X-ray Astrophysics Facility, the last of NASA's great observatories, is a scaled-back version of its former self.
Three of the missions this year rely on the services of a small launch vehicle called the Pegasus XL, and the space agency is hoping that problems with the vehicle are a thing of the past (SN: 1/18/97, p. 42). Early last year, NASA put a moratorium on Pegasus flights after several mishaps. The last four launches with Pegasus have been successful, however.
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The following is a tentative schedule of spacecraft launches. Given the vicissitudes of equipment and weather, it is subject to change.
January
* This week, NASA heads back to the moon. The tiny Lunar Prospector, small enough to fit in the backseat of a car, may determine within its first month of operation whether the permanently shadowed craters at the moon's poles contain water ice. Radar measurements made with a previous lunar-orbiting craft, Clementine, found hints of frozen water, but the finding remains controversial.
Like Clementine, Prospector can't look for water directly. However, its neutron detector can infer the abundance of hydrogen. Cosmic rays striking atoms on the moon's surface create a steady stream of neutrons, which are efficiently scattered into space by hydrogen. If Prospector finds an unexpectedly large amount of hydrogen, it's likely that the excess is tied up in water.
Prospector is expected to orbit the moon at an altitude of 100 kilometers for 1 year. If enough fuel remains, the craft will ultimately swoop down to an orbit just 10 km above the lunar surface. Prospector's gamma-ray spectrometer will determine the abundance of radioactive trace elements and more common elements, such as iron and silicon.
A magnetometer will measure the strength of magnetic fields at orbiting altitude, while an electron reflectometer measures fields at the surface. These readings will help pinpoint the size of the moon's core and its electrical conductivity, providing a test of the most popular theory of the moon's formation. According to the theory, a body at least as big as Mars slammed into Earth early in its history, and some of the debris from that collision coalesced to form the moon. In this picture, the abundance of magnetic materials in the moon, including iron, should not: differ significantly from the amounts in Earth's mantle.
To determine the geologic activity of the moon, an alpha particle spectrometer will measure the rate at which radioactive gases are released from the surface. By measuring where and when gases are vented, scientists hope to determine the frequency of moonquakes and other disturbances. These gases--believed to consist mostly of nitrogen, carbon monoxide, and carbon dioxide--could become key resources if a lunar base is established.
By monitoring shifts in the frequency of radio signals emitted by the craft, ground controllers can track changes in Prospector's velocity as it orbits the moon. The information will allow scientists to construct a detailed gravity map, revealing concentrations of mass beneath the surface.
The public can view data from Prospector at the same time that mission scientists receive it by visiting the NASA Web site at http://lunar.arc.nasa.gov.
March
* TRACE (Transition Region and Coronal Explorer) will explore the link between small-scale magnetic fields rooted in the surface of the sun and the structure of hot gases in the sun's upper layers. The craft will accomplish this task by taking simultaneous high-resolution images of three regions: the sun's visible surface; the extended layer of gas above it, known as the transition region; and the outer atmosphere, or corona.
Carried aloft by a Pegasus XL, TRACE will enter into an orbit that will keep it on the sunlit side of Earth and enable it to stare continuously at the sun for 8 months of its 1-year mission. Its ultraviolet detectors will image magnetic field structures on the solar surface as small as 750 km across and record changes in those structures on a scale of seconds.
In concert with the SOHO craft, launched 2 years ago, TRACE will track the magnetic fields from the base of the convection zone, deep within the sun, all the way up to the corona. It will also record the disturbances triggered by these fields, such as the blobs of ionized gas ejected from the corona.
July
* NASA plans to launch the first craft in its new millennium program, a fleet of low-cost, rapidly designed space explorers. Deep Space 1, the program's first deep-space craft, will visit a near-Earth asteroid, fly past the moon, and encounter a comet--all while testing several new technologies. Deep Space 1 will be the first craft of any kind to rely on an onboard, autonomous navigation system, which will enable it to modify its orbit and encounter celestial bodies with minimal assistance from ground controllers. Instead of being weighed down with fuel, the craft will, in part, employ solar power to propel itself.
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