Selected Articles from the
January 1999 Odyssey
Editor: Norm Cook
A Year at the Moon: Lunar Prospector Continues Mission of Discovery
NASA's Lunar Prospector spacecraft has marked one year in orbit around the Moon and continues to provide high quality data to scientists.
"Lunar Prospector has performed flawlessly over the past year," said Dr. Alan Binder, Lunar Prospector principal investigator and director of the Lunar Research Institute in Gilroy, Calif. "The quality of the data we've gathered is, in some cases, a factor of ten better than that which we promised to NASA at the outset, and we fulfilled all of our science objectives long before this first anniversary."
"Lunar Prospector has been an extraordinarily successful mission," said Scott Hubbard, NASA Mission Manager at the Ames Research Center in Mountain View, Calif. "This little spacecraft has returned wonderful science and has proved the concept of 'faster, better, cheaper'."
On December 19, 1998, the spacecraft was commanded into a 40 kilometer (25 mile) lunar polar orbit, down from its 100 kilometer (63 mile) mapping orbit, signaling the transition to the extended mission. The spacecraft will remain in the new 40-km orbit for about 4 weeks, and then be commanded to an even closer 25-30 km (15 to 19 mile) orbital path later this month. These actions will officially complete the end of the very successful primary mission, which began January 1998.
The extended mission is expected to continue through June 1999, during which time the five instruments onboard will gather additional science data at significantly higher resolutions. These higher resolutions will enable scientists to continue to refine their estimates concerning the concentration and form of hydrogen detected at the north and south lunar poles, which mission scientists interpret as deposits of water ice. Mapping of the Moon's magnetic and gravity fields will also benefit greatly from the lower orbit. Additionally, initial global maps of the Moon's elements will be confirmed with the close-up data.
Lunar Prospector was launched on Jan. 6, 1998, aboard a Lockheed Martin Athena 2 rocket and entered lunar orbit on Jan. 11, 1998. The Lunar Prospector mission is a joint effort of Lockheed Martin Missiles & Space, NASA Ames Research Center, and the Lunar Research Institute. Additional important contributions came from Los Alamos National Laboratory, the U.C. Berkeley Space Science Laboratory, and the NASA Goddard Space Flight Center. The $63 million mission is managed by the Ames Research Center.
Lunar Prospector Findings
Water Ice at the Poles
The north and south poles of the Moon may contain up to six billion metric tons of water ice, a more than ten-fold increase over previous estimates, according to scientists working with data from NASA's Lunar Prospector mission.
Growing evidence now suggests that water ice deposits of relatively high concentration are trapped beneath the soil in the permanently shadowed craters of both lunar polar regions. The researchers believe that alternative explanations, such as concentrations of hydrogen from the solar wind, are unlikely.
In March of 1998, mission scientists reported a water signal with a minimum abundance of one percent by weight of water ice in rocky lunar soil (regolith) corresponding to an estimated total of 300 million metric tons of ice at the Moon's poles. "We based those earlier, conscientiously conservative estimates on graphs of neutron spectrometer data, which showed distinctive dips over the lunar polar regions," said Binder. "This indicated significant hydrogen enrichment, a telltale signature of the presence of water ice.
"Subsequent analysis, combined with improved lunar models, shows conclusively that there is hydrogen at the Moon's poles," Binder said. "Though other explanations are possible, we interpret the data to mean that significant quantities of water ice are located in permanently shadowed craters in both lunar polar regions.
"The data do not tell us definitively the form of the water ice," Binder added. "However, if the main source is cometary impacts, as most scientists believe, our expectation is that we have areas at both poles with layers of near-pure water ice." In fact, the new analysis "indicates the presence of discrete, confined, near-pure water ice deposits buried beneath as much as 18 inches (40 centimeters) of dry regolith, with the water signature being 15 percent stronger at the Moon's north pole than at the south."
How much water do scientists believe they have found? "It is difficult to develop a numerical estimate," said Dr. William Feldman, co-investigator and spectrometer specialist at the Department of Energy's Los Alamos National Laboratory, NM. "However, we calculate that each polar region may contain as much as three billion metric tons of water ice."
Elemental Composition Maps
In other results, data from Lunar Prospector's gamma ray spectrometer have been used to develop the first global maps of the Moon's elemental composition. The maps delineate large compositional variations of thorium, potassium, and iron over the lunar surface, providing insights into the Moon's crust as it was formed. The distribution of thorium and potassium on the Moon's near side supports the idea that some portion of materials rich in these trace elements was scattered over a large area as a result of ejection by asteroid and comet impacts.
Lunar Magnetic Fields
Mission scientists also report the detection of strong, localized magnetic fields. While the magnetic fields are relatively weak and not global in nature like those of most planets, the Moon does contain magnetized rocks on its upper surface, according to data from Lunar Prospector's magnetometer and electron reflectometer. The resultant strong, local magnetic fields create the two smallest known magnetospheres in the Solar System.
These mini-magnetospheres are located diametrically opposite to large impact basins on the lunar surface, leading scientists to conclude that the magnetic regions formed as the result of these titanic impacts. One theory is that these impacts produced a cloud of electrically charged gas that expanded around the Moon in about five minutes, compressing and amplifying the pre-existing, primitive ambient magnetic field on the opposite side. This field was then "frozen" into the surface crust and retained as the Moon's then-molten core solidified and the global field vanished.
Gravity Map of the Moon
Using data from Prospector's Doppler gravity experiment, scientists have developed the first precise gravity map of the entire lunar surface. In the process, they have discovered seven previously unknown mass concentrations, lava-filled craters on the lunar surface known to cause gravitational anomalies. Three are located on the Moon's near side and four on its far side. This new, high-quality information will help engineers determine the long-term, altitude-related behavior of lunar-orbiting spacecraft, and more accurately assess fuel needs for possible future Moon missions.
Iron Lunar Core
Finally, Lunar Prospector data suggests that the Moon has a small, iron-rich core approximately 186 miles (300 kilometers) in radius, which is toward the smaller end of the range predicted by most current theories. "This theory seems to best fit the available data and models, but it is not a unique fit," cautioned Binder. "We will be able to say much more about this when we get magnetic data related to core size later in the mission." Ultimately, a precise figure for the core size will help constrain models of how the Moon originally formed.
Stardust Mission Set to Bring Back a Piece of a Comet
NASA's Stardust mission, scheduled for launch February 6, 1999 from Cape Canaveral, FL, will send a spacecraft flying through the cloud of dust that surrounds the nucleus of a comet--and, for the first time ever, bring cometary material back to Earth.
Comets are thought to hold many of the original ingredients of the recipe that created the planets and brought plentiful water to Earth. They are also rich in organic material, which provided our planet with many of the ready-to-mix molecules that could give rise to life. They may be the oldest, most primitive bodies in the solar system, a preserved record of the original nebula that formed the Sun and the planets.
"Scientists have long sought a sample directly from a known comet because of the unique chemical and physical information these bodies contain about the earliest history of the solar system," said Dr. Edward Weiler, NASA's associate administrator for space science. "Locked within comet molecules and atoms could be the record of the formation of the planets and the materials from which they were made."
Stardust is the first U.S. mission dedicated solely to a comet and will be the first to return extraterrestrial material from outside the orbit of the Moon. Stardust's main objective is to capture a sample from a well-preserved comet called Wild-2 (pronounced "Vilt-2").
The spacecraft will also collect interstellar dust from a recently discovered flow of particles that passes through our solar system from interstellar space. As in the proverbial "from dust to dust," this interstellar dust represents the ultimate in recycled material; it is the stuff from which all solid objects in the universe are made, and the state to which everything eventually returns. Scientists want to discover the composition of this "stardust" to determine the history, chemistry, physics, and mineralogy of nature's most fundamental building blocks.
Because it would be virtually impossible to equip a spacecraft with the most sophisticated lab instrumentation needed to analyze such material in space, the Stardust spacecraft is more of a robotic lab assistant whose job it is pick up and deliver a sample to scientists back on Earth. The spacecraft will, however, radio some on-the-spot analytical observations of the comet and interstellar dust.
"The samples we will collect are extremely small, less than a micron, or 1/25,000th of an inch, in size, and can only be adequately studied in laboratories with sophisticated analytical instruments," said Dr. Donald C. Brownlee of the University of Washington, principal investigator for the Stardust mission.
"Even if a ton of sample were returned, the main information in the solids would still be recorded at the micron level, and the analyses would still be done a single grain at a time." Stardust will meet up with Comet Wild-2 on January 2, 2004. A gravity assist flyby of Earth will put Stardust on a trajectory that will allow it to capture cometary dust intact at a low relative speed of 6.1 kilometers per second (about 13,600 miles per hour). An onboard camera will aid in navigating the spacecraft as close as about 150 kilometers (100 miles) from the comet's nucleus, permitting the capture of the freshest samples from the heart of the comet.
Dressed for survival behind armored shields, Stardust will document its 10-hour passage through the hailstorm of comet debris with scientific instruments and the navigation camera. On approach to the dust cloud, or "coma," the spacecraft will flip open a tennis-racket-shaped particle catcher filled with a smoke-colored glass foam called aerogel to capture the comet particles. Aerogel, the lowest-density material in the world, has enough "give" in it to slow and stop particles without altering them too much. After the sample has been collected, the aerogel capturing device will fold down into a return capsule, which closes like a clamshell to enclose the sample for its safe delivery to Earth.
In addition, a particle impact mass spectrometer will obtain in-flight data on the composition of both cometary and interstellar dust, especially very fine particles. The optical navigation camera should provide excellent images of the dark mass of the comet's nucleus. Other equipment will reveal the distribution in both time and space of coma dust, and could give an estimate of the comet's mass.
On January 15, 2006, a parachute will set the capsule gently onto the salt flats of the Utah desert for retrieval. The scientifically precious samples can be studied for decades into the future with ever-improving techniques and analysis technologies, limited only by the number of atoms and molecules of the sample material available. Many types of analyses now performed on lunar samples, for example, were not even conceived at the time of the Apollo missions to the Moon.
Extensive information on the Stardust mission is available on the Stardust home page: http://stardust.jpl.nasa.gov.
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