Selected Articles from the
January 1999 Odyssey
Editor: Norm Cook
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
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,
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.
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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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