After a nailbiting week when it appeared that Galileo's tape recorder may have failed, spacecraft engineers
were greatly relieved when test results over the weekend of October 21 showed that the device remains
functional. On October 24, a revised command sequence ordered the spacecraft to resume readouts of data from several science instruments as well as normal housekeeping duties and engineering operations such as
flushing of rocket thrusters.
The new command sequence replaced the one ground controllers stopped after the October 11 malfunction, when the data tape recorder failed to cease rewinding after recording an image of Jupiter. The tape recorder had remained in a standby mode until the October 20 test. Engineering data shows that the tape recorder can be unreliable under some operating conditions, project officials said. However, the problem appears to be manageable, and should not jeopardize return of the nearly 2000 images of the jovian system that are to be stored on the recorder for playback during Galileo's two-year mission.
New precautions included commands for the tape recorder to wind 25 extra times around a section of tape possibly weakened when the recorder was stuck in rewind mode with the tape immobilized for about 15 hours. This portion near the end of the tape reel has been declared off-limits for future recording. The approach image of Jupiter that Galileo took on October 11 is stored on this portion of tape and hence will not be played back.
The tape recorder is a key link in techniques developed to send images and data to Earth without Galileo's high-gain antenna, which is stuck, unusable, in a partially open position.
Since the tape recorder incident, Galileo project officials have decided to cancel imaging of Io and Europa on the day the spacecraft arrives at Jupiter in favor of devoting the tape recorder to gathering up to 75 minutes of data from Galileo's atmospheric probe as it descends into the atmosphere--the first-ever direct measurements of its chemistry and weather. Scientists are especially disappointed to lose imaging from the closest planned flyby of Io. "Our priorities are clear," said William O'Neil, Galileo Project Manager, "We have to get all the probe data."
A spacecraft designed to gather samples of the dust spewed from a comet and return it to Earth for analysis has been selected to become the fourth flight mission in NASA's Discovery program. Known as Stardust, the mission also will gather and return samples of interstellar dust that the spacecraft encounters during its trip through the solar system to fly by Comet Wild-2 in January 2004. Stardust was selected from among three Discovery proposals funded for further study last February.
"Stardust was rated highest in terms of scientific content and, when combined with its low cost and high probability of success, this translates into the best return on investment for the nation," said Wesley Huntress, NASA Associate Administrator for Space Science. "The Stardust team also did an excellent job of updating their plan to communicate the purpose and results of this exciting mission to educators and the public."
The Stardust mission team is led by Principal Investigator Donald Brownlee, University of Washington in Seattle, with Lockheed-Martin Astronautics, Denver, as the contractor building the spacecraft. Jet Propulsion Laboratory will provide project management.
Comet Wild-2 is known as a "fresh comet" because its orbit was deflected from much farther out in the solar system by the gravitational attraction of Jupiter in 1974. Stardust will approach as close as 100 kilometers (62 miles) to the comet's nucleus. "Space scientists are intensely interested in comets because we believe that most of them are well-preserved remnants from the earliest days of star and planetary formation," Huntress said.
Stardust will be launched on an expendable launch vehicle in February 1999 for a total mission cost of $199.6 million. The return capsule carrying the dust samples will parachute to Earth to land on a dry Utah lakebed in January 2006. Stardust will use an unusual material called aerogel to capture the dust samples. This porous, extremely-low-density material is somewhat like glass in that it is made of silica and it has about the same melting point. Although aerogel does not absorb moisture, the fluorescent substance can absorb large amounts of gas or particle matter because of its large internal surface area.
The spacecraft will also carry an optical camera that will return cometary images with 10 times the clarity of those taken of Comet Halley by previous space missions, as well as a mass spectrometer provided by Germany to analyze basic composition of the samples in-flight.
A 10-micron interplanetary dust particle collected in the stratosphere with U2 aircraft. This particle is
similar in elemental composition to primitive meteorites but differs in having higher carbon and volatile
element abundance. The particle is composed of glass, carbon, and many types of silicate mineral grains. It
is a sample of either an asteroid or a comet. The porosity and unusual mineralogical composition suggests
that it may be of cometary origin. In the first hour of examination of the returned Stardust samples it will
be possible to determine whether this particle or any other type of meteoritic material is similar or related
to comets.
, and a confirmatory image from the Hubble Space Telescope. The collaborative
effort involved astronomers at the California Institute of Technology and the Johns Hopkins
University.<P>
The brown dwarf, Gliese 229B (GL229B), is a small companion to the cool red star Gliese 229, located
19 light-years from Earth in the constellation Lepus. At 20 to 50 times the mass of Jupiter, GL229B is too
massive and hot to be classified as a planet, but is too small and cool to shine like a star. At least 100,000
times dimmer than the Sun, the brown dwarf is the faintest object ever seen orbiting another star.<P>
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The Hubble data show that the object is far dimmer, cooler (no more than 1300 degrees Fahrenheit), and
less massive than previously reported brown dwarf candidates, which are all near the theoretical limit
(eight percent the mass of our Sun) at which a star has enough mass to sustain nuclear fusion.
Brown dwarfs form the same way stars do, by condensing from a cloud of hydrogen gas, but they do not accumulate enough mass to generate the high temperatures needed to sustain nuclear fusion at their core, which is what makes stars shine. Instead brown dwarfs radiate energy as Jupiter does, through gravitational contraction. In fact, the chemical composition of GL229B's atmosphere looks remarkably like that of Jupiter.
The discovery is an important first step in the search for planetary systems beyond the solar system because it will help astronomers distinguish between massive Jupiter-like planets and brown dwarfs orbiting other stars. Advances in ground- and space-based astronomy are allowing astronomers to further probe the "twilight zone" between larger planets and small stars as they search for substellar objects, and eventually, other planetary systems.
Caltech astronomers Kulkarni, Tadashi Nakajima, Keith Matthews, and Ben Oppenheimer and Johns Hopkins scientists Sam Durrance and David Golimowski first discovered the object in October 1994. Follow-up observations a year later were needed to confirm that it is actually a companion to Gliese 229. The discovery was made with a 60-inch reflecting telescope at Palomar Observatory, using an image- sharpening device called the Adaptive Optics Coronagraph, designed and built at the Johns Hopkins University. The scientists teamed up with Chris Burrows of the Space Telescope Science Institute to use Hubble's Wide Field Planetary Camera-2 for follow-up observations on November 17. Another Hubble observation six months from now will yield an exact distance to GL229B.
The astronomers suspect that the brown dwarf developed during the normal star-formation process as one of two members of a binary system. "All our observations are consistent with brown dwarf theory," Durrance said. However, the astronomers say they cannot yet fully rule out the possibility that the object formed out of dust and gas in a circumstellar disk as a "super-planet."
The difference between planets and brown dwarfs is how they formed. Planets in the solar system are believed to have formed out of a primeval disk of dust around the newborn Sun: Their orbits are nearly circular and lie almost in the same plane. Brown dwarfs, like full-fledged stars, would have fragmented and gravitationally collapsed out of a large cloud of hydrogen but were not massive enough to sustain fusion reactions at their cores.
The orbit of GL229B will provide clues to its origin. If the orbit is nearly circular then it may have formed out of a dust disk, where viscous forces in the dense disk would keep objects at about the same distance from the star they orbit. If the dwarf formed as a binary companion, its orbit probably would be far more elliptical, as seen with most binary stars. The Hubble observations will provide initial data for calculating the orbit. However, because the orbital motion is so slow, it will take many decades of telescopic observations before a true orbit can be calculated. GL229B is at least four billion miles from its companion star, roughly the distance between Pluto and the Sun.
These two false-color telescope images reveal the faintest object ever seen around a star beyond our Sun, and the first unambiguous detection of a brown dwarf. The brown dwarf, called GL229B, orbits the red dwarf star Gliese 229, located approximately 18 light-years away in the constellation Lepus. The brown dwarf is about 20 to 50 times the mass of Jupiter, but is so dense it is about the same diameter as Jupiter (80,000 miles).
Brown dwarfs are a mysterious class of long-sought objects that form the same way stars do, by condensing out of a cloud of hydrogen gas. However, they do not accumulate enough mass to sustain nuclear fusion at their core, which makes stars shine.
[left] The brown dwarf (center) was first observed in far red light October 27, 1994, using the adaptive optics device and a 60-inch reflecting telescope on Palomar Mountain in California. Another year was required to confirm that the object was actually gravitationally bound to the companion star. GL229B is at least four billion miles from its companion star, roughly the separation between the planet Pluto and our Sun. Even though a cornograph on the detector masked most of the light from the star, which is off the left edge of the image, it is so bright relative to the brown dwarf the glare floods the detector.
[right] This image of the GL229B (center) was taken with Hubble Space Telescope's Wide Field Planetary Camera-2, in far red light, on November 17, 1995. The Hubble observations will be used to accurately measure the brown dwarf's distance from Earth, and yield preliminary data on its orbital period, which may eventually offer clues to the dwarf's origin. Though the star Gliese 229 is off the edge of the image, it is so bright it floods the Hubble detector. The diagonal line is a diffraction spike produced by the telescope's optical system.