Lunar Prospector Facts

The Lunar Prospector Space Craft SCIENCE Kit Fact Sheet

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Highlights of NASA's Lunar Prospector Mission

Lunar Prospector, a NASA Discovery Mission, was launched January 6, 1998. After a 5-day journey to the Moon, it used on-board rocket thrusters to establish itself in a nearly circular, north-south polar orbit about 100 km above the lunar surface. From this vantage it maps the entire lunar surface, which drifts by as the Moon rotates below Prospector's orbit. Prospector is equipped with five sophisticated on-board science instruments, the next generation of devices flown aboard Apollo and other missions. In addition, its radio communications system is used to measure tiny variations in the spacecraft's speed, yielding data on the Moon's uneven gravity field.

Water on the Moon! The Lunar Prospector Mission team announced in a historic press conference March 5, 1998, that the tiny, low budget ($65 million) craft had found the answer to one of the most hotly debated questions in lunar science. Prospector found somewhere between 10 to 300 million tons of water-ice scattered inside the perpetually shadowed craters of the lunar poles. Not only was ice found, as expected, in the Aitken Basin of the lunar South Pole, but also in the craters of the North. Prospector detected nearly 50% more water ice in the North than in the South. How did Prospector's Neutron Spectrometer confirm water? Read on.

Lunar Prospector is the first competitively selected, and third to launch in a series of missions in NASA's recently implemented Discovery program. This program was developed to produce frequent, low-cost missions to explore the Solar System. Prospector is a simple and reliable spin-stabilized spacecraft. It rotates around its own central axis in order to control its orientation. Prospector is small. With full propellant tanks, the spacecraft's mass was only 295 kg. Like all Discovery missions (Mars Pathfinder was another), Prospector progressed rapidly, from development to completion and testing phases, in a period of only 22 months. Where feasible, the spacecraft was manufactured from "off-the-shelf," flight-proven hardware. From an engineering perspective, a spin-stabilized spacecraft like Prospector is inexpensive to design and simple to operate.

After orbiting at 100 km for a year to identify the Moon's natural resources, mission control will drop Prospector to an altitude of 10 km, to examine the surface, and the gravity field, in much greater detail.

The spacecraft's spin axis is approximately aligned with the ecliptic north pole, but it is tilted slightly by mission control as needed for thermal management, and to reduce boom shadowing on the solar arrays. As the Earth moves around the sun, the spin axis must be adjusted. At mission's end, the craft's orbit will be adjusted to impact the lunar surface, to prevent it from becoming an orbiting hazard.

Lunar Prospector's Science Instruments:

The Gamma Ray Spectrometer collects data to help understand the chemical composition of the lunar surface. A gamma ray is a more energetic photon than those of visible light ray or X-ray. The instrument detects them from two sources: "natural" ones are emitted spontaneously by radioactive elements like thorium and uranium. "Induced" gamma rays are emitted by elements like iron, silicon, and oxygen on the Moon's surface when they are bombarded by cosmic rays. The energy of a gamma ray, which the instrument reads, serves as a signature of the atom that emits it.

The Magnetometer uses a "fluxgate," a coil through which the field passes, to measure the Moon's magnetic field. It also senses Earth's field, and the field carried from the Sun by the solar wind. The magnetic field at the lunar surface is also affected by local deposits of magnetic material, and those are examined in more detail by a different instrument.

The Electron Reflectometer, unlike the magnetometer, is a remote instrument: it measures the magnetic field at the surface of the Moon. Here's how it works: Electrons from the Sun are absorbed on the lunar surface, unless there is magnetic material there to reflect some electrons back into space. The instrument measures their pitch angle, to use as data to study the surface fields.

The Neutron Spectrometer is the instrument that discovered the water. Since water molecules each contain two atoms of hydrogen, they're a good marker for water. Traces of hydrogen can also be implanted in the Moon's surface by the solar wind, but the amount locked up in water molecules dwarfs those traces. To remotely detect hydrogen, the instrument identifies "cool" neutrons, those that have bounced off a hydrogen atom somewhere on the lunar surface. When cosmic rays collide with atoms in the crust, they violently dislodge neutrons and other subatomic particles, such as gamma rays. Some of the neutrons escape directly to space, as hot or "fast" neutrons. Other neutrons shoot off into the crust, where they collide with other atoms, bouncing around like pinballs. If they only run into heavy atoms, they do not lose very much energy in the collisions, and are still traveling at close to their original speed when they finally bounce off into outer space. They are still "warm" when they reach Lunar Prospector.

On the other hand, neutrons which have collided with something their own mass cause the instrument to detect these "cool" neutrons, and a dropoff in the number of "warm" ones. "Cool" neutrons loose energy sort of like the way a marble stops after it collides with another marble, as opposed bouncing energetically off a big rock.

The Alpha Particle Spectrometer detects alpha particles (helium nuclei) emitted by radioactive gases, such as radon and polonium, leaking out of the lunar interior. Alpha particles escape from radioactive elements as part of their natural decay process. They are emitted with a precise energy -- which is read by the instrument -- a fingerprint for the atom from which they came. While the Moon currently lacks volcanoes, it does appear to vent gases such as radon , nitrogen, and carbon dioxide. Any detection of gases depends very much on whether any outgassing events occur while Lunar Prospector is in orbit, and how many there are.

The Doppler Gravity Experiment does not have a dedicated instrument aboard the spacecraft, but rather it uses the end-to-end communications system to measure tiny variations in the spacecraft's orbital speed. Variations in the way mass is concentrated at and below the lunar surface cause an orbiting spacecraft to speed up or slow down as it flies overhead. By tracking the velocity of the spacecraft over time new maps of the Moon's gravity field will be created, enhancing current understanding, as well as potentially enabling more fuel-efficient journeys to the Moon in the future.

Further information Don't miss the wealth of information which the Lunar Prospector project keeps updated on the world-wide web. Also, to keep up with discoveries from Lunar Prospector, refer to publications such as Science News, Sky & Telescope, and Astronomy, which publish popular articles and images covering discoveries made by missions such as Lunar Prospector.

Your Lunar Prospector kit could not have been produced without the kind permission and valuable technical assistance from the NASA's Lunar Prospector Project management at the Ames Research Laboratory. Copyright (c) 1998-2007 SCI Space Craft International, Pasadena, CA U.S.A. Note: NASA does not endorse this, or any product.

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