Lunar Prospector Over The Moon

Water ice probably exists at both the Moon's north and south poles, according to data sent back to Earth by NASA's Lunar Prospector.

The robot probe's neutron spectrometer spotted the lunar water ice, allowing scientists on Earth to estimate its volume and location. The Moon's water ice is not concentrated in polar ice sheets. Rather, it seems to be in very low amounts distributed across a significant number of craters in the polar regions. The water probably is only 0.3 percent to 1 percent of the Moon's rocky soil.

How Much Lunar Water Ice Is There?

Assuming a water ice depth of about a foot and a half -- the depth to which the neutron spectrometer's signal was able to penetrate -- there may be as much as 11 million to 330 million tons of lunar water ice dispersed over 3,600 to 18,000 square miles around the north pole, and 1,800 to 7,200 square miles around the south pole. Twice as much water ice was detected in the north as in the south.

Scientists assume most water ice on the Moon must be a result of meteors and comets striking the lunar surface. The amount of soil that could have been "gardened" by all meteor and comet impacts over the last 2 billion or so years extends down to a depth of about 6.5 feet. Researchers caution that their estimates of the amount of water could be off by a factor of ten in either direction.

An earlier Defense Department-NASA mission to the Moon known as Clementine used a radar-based technique to detect ice deposits in permanently shadowed regions of the lunar south pole. Unfortunately, those results can't be compared directly with the Lunar Prospector results because of the different sensors, measurement "footprints," and analysis techniques. The Clementine science team concluded that its radar signal detected from 110 million to 1.1 billion tons of water ice, over 5,500 square miles of south pole terrain.

How Valuable Is This Water?

NASA is calculating the economic value of the lunar water as a resource for human exploration of the Moon. For instance, the space agency has estimated the cost of transporting the same volume of water from Earth to orbit. Today, it costs about $10,000 to put one pound of material into orbit, but NASA is working to reduce that to only $1,000 per pound. It would cost $60 trillion to transport 33 million tons of water to space, plus an unknown additional cost to transport it to the Moon's surface.

From another perspective, a typical person on Earth consumes an estimated 100 gallons of water per day for drinking, food preparation, bathing and washing. At that rate, 33 million tons of water -- which is 7.2 billion gallons of water -- could support a community of 1,000 two-person households for well over a century on the lunar surface, without recycling. Of course, a cheap way to mine the water crystals from the lunar soil would have to be developed for the water to become a useful resource for drinking or rocket fuel to support human explorers.

Lunar Gravity Maps

Before Lunar Prospector, tracking data from old-time NASA Lunar Orbiter and Apollo missions had suggested the lunar gravity field is not uniform. It was effected by variations in the concentrations of mass on the Moon caused by lava which had flowed into the Moon's huge craters eons ago. Maps of those mass concentrations only covered the equatorial region of the nearside of the Moon. Lunar Prospector improved that situation dramatically.

Telemetry data from Lunar Prospector was used to produce a gravity map of both the near and far sides of the Moon. Researchers found two new mass concentrations on the Moon's nearside. This new first-ever engineering-quality gravity map of the entire Moon will make future lunar missions safe and fuel-efficient.

Lunar Prospector is scheduled to continue gathering data at an altitude of 62 miles above the Moon throughout 1998. In 1999, the spacecraft will be moved into an orbit as low as six miles above the lunar surface so its instruments can collect close-up data. Meanwhile, the probe's gamma ray spectrometer will measure the Moon's surface composition and structure.
A Long Time Between Visits

When NASA launched Lunar Prospector from Earth on January 6, 1998, it was the first time in 25 years the space agency had flown a research spacecraft to Earth's natural satellite. After blast off on a Lockheed Martin Athena II rocket from Spaceport Florida's pad 46 -- part of a new commercial launch complex at Cape Canaveral, Florida -- Lunar Prospector cruised for 110 hours to the Moon where it was inserted into its circular, polar orbit. It was the third launch in NASA's Discovery program of low-cost highly-focused planetary science missions.

In its search for water ice, Lunar Prospector has not landed on the surface. Instead, it travels around the Moon -- from pole to pole -- 62 miles above the surface. Each orbit takes 118 minutes -- about two hours. After a year, Lunar Prospector will fly at altitudes as low as six miles above the lunar surface.

The 660-lb. spacecraft is shorter than five feet tall and controlled remotely from Earth. It is a graphite epoxy drum, 4.5 ft. in diameter and 4 ft. high with three eight-foot-long instrument booms. Five instruments attached to those boom arms look for signs of iron, aluminum, uranium and calcium on the surface. Lunar Prospector is spin-stabilized and controlled by six hydrazine monopropellant 22-Newton thrusters.

For electrical power, the probe has 202 watts of solar cells storing energy in a set of 15-ampere-hour NiH batteries. Oddly, there is no computer aboard Lunar Prospector. Ground command is through a 3.6 kbps telemetry link using two S-band transponders and a slotted, phased-array medium gain antenna and omnidirectional low-gain antenna.

Data from the one-to-three-year mission will allow construction of a detailed map of the surface composition of the Moon, and will improve our understanding of the origin, evolution, current state, and resources of the Moon. Lunar Prospector will map the surface composition, measuring magnetic and gravity fields, studying lunar outgassing events, and prospecting for minerals. Then, humans can go back to the Moon to build a base and learn to live off the land. Data from Lunar Prospector will tell those pioneers where to get the necessary resources.

The first data returned will answer a question raised in the 1970s and again in 1994 by the Clementine mission. Is there water held as ice in some polar craters? Because previous missions spotted evidence of hydrogen in the polar regions of the Moon, suggesting traces of water, a neutron spectrometer aboard the spacecraft will look for hydrogen.

Instruments also look for signs of gases spewing from the interior of the Moon and will measure the lunar magnetic field.

Scientists are looking for similarities between Earth and the Moon. They hope to prove or disprove a theory that an asteroid slammed into Earth billions of years ago, shooting debris into space which clumped together to form the Moon. By mapping the surface of the Moon, they will learn about the bulk composition of the Moon, which will allow them to compare it to the bulk composition of the Earth.
Ashes of a "Great Founder" On Board

NASA placed an ounce of the cremated remains of a man NASA scientists called a "great founder" of planetary science aboard Lunar Prospector. The ashes are the remains of Eugene Shoemaker who was a co-discoverer of Shoemaker-Levy 9, a comet that crashed into Jupiter in 1995. The crash, captured by the lenses of the Hubble Space Telescope, established Shoemaker as "one of the great founders of planetary science." OTHER ASHES ORBIT EARTH

Lunar Prospector's science instruments were provided by the Los Alamos National Laboratory, the University of California Berkeley Space Science Laboratory, the University of Arizona at Tucson, NASA's Goddard Space Flight Center at Greenbelt, Maryland, and NASA's Jet Propulsion Laboratory at the California Institute of technology at Pasadena, California. Science work is at the Lunar Research Institute, Gilroy, California, while the flight is managed at NASA's Ames Research Center, Moffett Field, California.

Ancient Lava Flows on the Moon

The false-color photograph of the Moon, at left, is a composite of 15 images of the Moon taken through three color filters by the Galileo spacecraft's solid- state imaging system on December 8, 1992, as the deep space probe was just 262,000 miles from the Moon and 43,000 miles from Earth.

The false colors help researchers at
NASA JPL interpret the composition of the Moon's surface soil. Red areas are lunar highlands, while shades of blue and orange are the ancient volcanic lava flow of a mare, or lunar sea

Bluer mare areas contain more titanium than do the orange regions. For instance, the deep blue patch on the right, Mare Tranquillitatis, is richer in titanium than Mare Serenitatis, which is the slightly smaller circular area immediately adjacent to the upper left of Mare Tranquillitatis.

The blue and orange areas covering much of the left side of the Moon represent many separate lava flows in Oceanus Procellarum.

The small purple areas near the center are pyroclastic deposits formed by explosive volcanic eruptions. The fresh crater Tycho, with a diameter of 53 miles, is prominent at the bottom of the photograph, where part of the Moon's disk is missing.

The Moon Has a Molten Interior

Love numbers for the relationship between Earth and its Moon seem to indicate the Moon has molten slush surrounding its core, according to scientists at NASA's Jet Propulsion Laboratory at Pasadena, California.

Love numbers. Love numbers are named after Oxford mathematician Augustus E.H. Love who worked out mathematical theories of elasticity and waves in the late 1800s and early 1900s. The numbers are a measure of how much a planet's surface and interior move in response to the gravitational pull of other bodies nearby in space.

Researchers calculated the Love numbers from data gathered over the years by the Lunar Laser Ranging Experiment, which uses retroreflectors left on the Moon's surface by several U.S. and Russian missions in the 1960s and 1970s.

Retroreflectors. The retroreflectors on the Moon are small mirrors called "corner cube retroreflector arrays." Each array is about the size of a suitcase. Because they can be seen from Earth, and used by scientists on Earth, the human-constructed retroreflectors often are cited as proof that Apollo astronauts actually visited the Moon.

Retroreflectors were conceived in the 1960s by Montana State University physicist Kenneth Nordtvedt who suggested to NASA that a series of reflectors on the surface of the Moon could be helpful in determining the exact distance between Earth and the Moon.

Laser ranging. Nordtvedt saw that light from a laser fired from Earth could hit a reflector on the Moon and bounce back to Earth. He understood that the time light took to travel to the reflector and back would reveal the distance. That information, in turn, would reveal a great deal of information about the lunar orbit, which could be used to test some of Albert Einstein's theories.

NASA sent three arrays of 100 to 300 prisms to the Moon during three flights in the Apollo Moon-landing program. The first retroreflector was positioned there in 1969 by the Apollo 11 astronauts. Two other arrays from the Soviet Union and France were delivered to the Moon aboard unmanned Lunakhod missions launched from the Soviet Union.

Since retroreflectors require no electrical power, they continue to operate decades after Neil Armstrong set foot on the Moon. Scientists around our planet regularly fire off laser pulses to bounce off of the distant reflectors -- to better understand the Moon's rotation, Earth's tides, and Einstein's theory of relativity.

The prisms reflect light back to its point of origin, allowing scientists to pin down the distance to the Moon to within about 10 inches by the early 1970s, and now to less than an inch.

Because Lunakhod 2 was not manned, its retroreflector was not placed as carefully on the lunar surface as when the Apollo astronauts were able to aim their retroreflectors toward Earth. As a result, its bigger mirror reflects a weaker laser echo than the smaller Apollo reflectors.

The laser beam. How powerful does the light beam have to be? One laser generator in use with a 3.5-meter telescope operated by the Astrophysical Research Consortium at Apache Point, near Sunspot, New Mexico, generates a peak power of a one billion watts (1 gigawatt) for a short time, but just long enough to fire off a one-inch bullet of light aimed through the telescope at the lunar surface.

The distance the light travels is calculated by measuring the light pulse's round-trip travel time and multiplying that figure by the speed of light.

Earth's atmosphere distorts the beam so that it is expanded out to 1.25 miles in diameter when it hits the Moon. Only one in 30 million of the original photons in the beam actually will hit the retroreflector. By the time the light makes it back to Earth, the beam will have expanded to 9.3 miles in diameter. Of the returning photons, only one in 30 million will hit the telescope on Earth.

Distance to the Moon. The Moon is in an elliptical 28-day orbit ranging from 220,000 to about 252,000 miles from Earth. On average, the center of the Moon is about 238,700 miles from the center of Earth. Laser ranging techniques have allowed astronomers to narrow the exact measurement to within less than an inch.

Learn more about lunar exploration: Learn more about the Moon:

Inner Planets: Mercury Venus Earth Mars
Outer Planets: Jupiter Saturn Uranus Neptune Pluto
Other Bodies: Moons Asteroids Comets The Voyagers

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