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NASA Ames Spacecraft to Smash into a Pole of the moon in Search of Ice
08.16.06
 
In the near vacuum of space there will be silence as a large NASA rocket smashes into one of the moon's polar regions in early 2009. There is no air to transmit sound waves where the rocket will strike, but the ground will shake. The 4,410-pound (2,000-kilogram) NASA rocket will be hurtling 1.56 miles per second (2.5 kilometers per second) towards the lunar surface. The Lunar CRater Observation and Sensing Satellite (LCROSS) will carry out this lunar collision mission and experiment.

As well as being soundless, some craters near the moon's poles are in permanent shadow and are so cold that ice could remain there for eons. When the LCROSS rocket's upper stage violently collides with the surface of a shadowed lunar crater, the massive impact will jolt up a huge cloud, or 'plume,' of lunar material - soil and maybe even water ice. Finding water ice is the main purpose of LCROSS. However, if LCROSS does not detect ice, that would not rule out ice at the lunar poles, according to scientists.

If there is enough of it, water ice would be as valuable as gold to astronauts on the moon because launching anything into space from Earth's surface costs as much as $10,000 per pound (0.45 kilogram.) Astronauts could drink life-sustaining moon water or make it into rocket fuel. If there is adequate water near one of the lunar poles that astronauts could use, that water could save NASA huge sums of money.

"Our objective is to detect and measure water in the lunar soil," said Tony Colaprete, the LCROSS principal investigator and a planetary scientist at NASA Ames Research Center, Moffett Field, Calif. "It's just like prospectors used to do when they were looking for precious metals. They would drill a hole in the side of a riverbank, stick a piece of dynamite in there and then blast a chunk of earth off. They would then sift through the debris - using a variety of methods - to detect ores," said Colaprete. "They'd wash the debris into the river and use slurry - a mix of water and debris - to separate gold from the rest of the dirt."

"We're doing the same thing. We're blasting a hole in the moon about half the size of an Olympic-size swimming pool. Instead of slurry and a tin pan, we're using a suite of instruments both at the moon and on Earth (to detect water and other materials)," he said. "We expect to excavate at least 220 tons (200 metric tons) of moon dirt," Colaprete noted. The impact will be visible to a number of lunar-orbiting satellites and possibly also to Earth-based telescopes.

Image left: The south pole of the Moon. Click on the image to download high resolution photo.

What triggered NASA's interest in locating possible water near the moon's poles? Both the Clementine (in 1994) and the Lunar Prospector (in 1998) spacecraft found indirect -- but not definitive proof -- that water ice may exist in the dark shadows of craters in the lunar south pole area - gloomy, extremely cold places that never see the light of day. Lunar Prospector found evidence of hydrogen, which along with oxygen, comprises water. Soon, LCROSS mission scientists hope to find solid proof of water.

LCROSS will be a 'secondary payload' when it is launched for its journey to the moon in October 2008. That is, LCROSS will be a hitchhiker. It will ride the same rocket as the Lunar Reconnaissance Orbiter (LRO), another NASA mission to the moon. The rocket, an Atlas V with a Centaur upper stage, will launch from Cape Canaveral Air Force Station, Florida.

The LCROSS spacecraft will arrive in the lunar vicinity independent of the LRO satellite. Instead of arriving at the moon in a few days like LRO, LCROSS will orbit Earth twice for about 80 days, and then will strike one of the lunar poles in January 2009.

The reason that the LCROSS spacecraft will take so long to arrive at the moon is that the spacecraft will use 'lunar gravity assist' to change the second stage Centaur rocket's trajectory so that the space vehicle will strike its target near one of the moon's poles. During a gravity assist maneuver, a spacecraft approaches a planet or a moon, and the spacecraft's orbit is affected, causing the probe to shift direction.

Because of lunar gravity assist, LCROSS will approach the moon's poles with more velocity -- 1.56 miles per second (2.5 kilometers per second) -- and will strike the lunar surface more squarely -- at 70 degrees to the moon's horizon -- a steeper impact angle that will produce a bigger plume. LCROSS also will take longer to reach the moon, and NASA will have more time to track the spacecraft, control it and precisely aim it at a crater.

On the way to the moon, the LCROSS spacecraft's two main parts, the Shepherding Spacecraft and the Centaur second stage, will remain coupled.

As the spacecraft nears one of the moon's poles, the upper Centaur stage will separate, and then will impact a crater in that region. A plume from the upper stage crash will develop as the Shepherding Spacecraft heads in toward the moon.

The Shepherding Spacecraft will fly through the impact plume, sending back real-time images and spectra of the plume material taken by infrared cameras and spectrometers. After sending back these data, the Shepherding Satellite will become a 1,543-pound (700-kilogram) 'impactor' as well. The second impact will provide another opportunity for lunar-orbiting satellites and Earth-based observatories to study the nature of the lunar soil in the second, smaller plume.

In 1988 during the Lunar Prospector lunar orbital mission, scientists estimated that as much as 6 billion metric tons of water ice could be under about 18 inches of lunar soil in the craters. However, Lunar Prospector did not provide proof positive of ice. Scientists now are determining how best to detect water - if any -- in the mammoth plumes of moon dust that will result from the two LCROSS impacts. Because the impacts will be so complicated, scientists need to understand them before they happen. Then, researchers can properly plan the science observations scientists would like to make.

time exposed image of impact "An impact is a very complex event," Colaprete observed. "There are a number of processes that occur one after the other, and some simultaneously, each of which contains clues about the moon's materials and the impact."

Image right: Time exposed image of simulated LCROSS lunar impact (without lights) taken slightly from the side. The heat created at impact will vaporize any water-ice near the surface, which LCROSS will then measure. Photo courtesy of LCROSS team member P.H. Schultz, Brown University, Providence, R.I. Click on the image to download high resolution photo.

Scientists must consider factors that include how long the lunar dust will linger above the lunar landscape, how bright the initial impact flash will be and how much material will fly up from the surface. To analyze the impact before it will happen has required a team of scientists -- the people who designed the mission.

The team is a dozen scientists strong. All of them are co-investigators on the LCROSS mission. They are from across the United States and work at NASA as well as Brown University, Providence, R.I.; University of California, Santa Cruz; Stanford University, Stanford, Calif.; Northrop Grumman, Redondo Beach, Calif.; and the SETI Institute, Mountain View, Calif.

The team includes impact specialists, astronomers, spectroscopists (scientists who analyze light and other radiation to determine material composition) and planetary scientists. Some team members are experts in two or more fields of study.

The team met at NASA Ames in the spring of 2006 to share their outlooks for the mission and to take a critical look at the mission design to make sure all objectives would be met.

"In the morning of the first day of our meeting, we heard from our impact specialists, including Peter Schultz of Brown University and Erik Asphaug and Don Korycansky both of the University of California, Santa Cruz," Colaprete said. "What we heard were their current expectations for the size and duration of the impact."

"We're going to be using the LCROSS as if we're kicking a divot out of the surface of the moon to expose what's below," Schultz said. "Every time you go into a sand trap on the golf course -- a place you don't want to be -- you pitch up sand with your sand wedge. If you watch your friend do this, you can (see) this cone of debris fly out of the trap. The formation of the cloud of sand is similar to what will (happen) during LCROSS."

Scientists used NASA Ames' Vertical Gun Range to simulate the lunar impacts by firing small pellets into materials that represented the lunar surface. "It's very cool," Schultz said. "In experiments -- just like in a science fiction movie -- you need to slow everything down to imagine what you would see at the scale of LCROSS. We slow it down by a factor of a thousand. We are using high-speed video and other special imaging," he explained.

What Schultz and others find out from the Vertical Gun tests may help them to refine their estimates of the impact flash, how hot the ejecta will become and how fast it will cool, ejecta trajectories and what the physical state of any water in the ejecta may be.

"We've watched all these complex processes every time we do an experiment at the Ames Gun Range. The (LCROSS) impact (on the moon) will be in the dark, and all we'll see at first is just a faint flash. And then we'll see this expanding ring of debris as it comes out of the darkness and is lit by sunlight," Schultz said.

"So, what we're really after is to bring material up into the sunlight for the very first time," he explained. "We want to know if this material contains ice. What we get to do is the same thing we do in the gun range at Ames (only) at a much larger scale."

According to Schultz, there will be a whole suite of instruments looking at the plume of lunar material above the moon. "The instruments we'll have on board will allow us to detect what makes up the surface and subsurface. We are going to try to get as much out of this one-second event when the crater is formed (as we can). But then we'll watch the material emerge into a cloud and a ring for about minute or more," Schultz continued.

The second impact must be observed from Earth and other spacecraft because the Shepherding Satellite carrying all the LCROSS instruments is the impactor for the follow-up collision with the moon.

simulated lunar ejecta During their meeting at Ames, scientists also considered the mission time line and how they will coordinate their observations of the impact from the Earth, and from satellites orbiting the moon. Astronomers Jennifer Heldmann of the SETI Institute and Diane Wooden of NASA Ames are leading the effort to coordinate and define Earth and moon-based observations of the impact.

Image left: A sequence "grab" from a high-speed video taken at the NASA Ames Vertical Gun Range. The images show the initial flash and the gradual appearance of the simulated lunar ejecta as they emerge into the 'sunlight'. Sequence courtesy of LCROSS team member P.H. Schultz, Brown University, Providence, R.I. Click on the image to download high resolution photo.

"We are encouraging the science community to observe the impacts from ground based telescopes, Earth-based telescopes and Earth and moon-orbiting satellites," said Heldmann. We also encourage amateur astronomers to observe the LCROSS impacts and plumes," Heldmann added.

Because scientists will use many instruments at various locations to observe the impacts, researchers will be able to combine independent measurements to potentially come to more concrete conclusions about the presence of water and other materials in lunar soil.

Large telescopes including the Keck II telescopic on Mauna Kea mountain in Hawaii will make observations of the LCROSS moon impacts. According to scientists, the Hubble Space Telescope may also be called upon to observe the impacts. In addition, LCROSS researchers plan to use three satellites orbiting the moon to study the impacts. One is NASA's Lunar Reconnaissance Orbiter (LRO); another is India's Chandrayaan-1 and Japan's SELenological and ENgineering Explorer (SELENE) - all of which have yet to be launched.

Mission planetary scientists include Colaprete and Heldmann. Schultz and Asphaug also are planetary scientists, but they will concentrate on impact studies for the LCROSS mission.

Spectroscopists include Wooden and Tony Ricco, a chemist from Stanford. They will search for clues of water and other minerals by looking at the initial impact flash of light and then the sunlight colors reflected from the dust cloud with special instruments on spacecraft and through telescopes. The instruments include special cameras and spectrometers.

Spectrometers are instruments that separate light beams into bands of color. Some of the color bands the spectrometer will see are invisible to human beings. Combinations of the bands are unique -- like fingerprints or barcodes. These color band combinations are 'signatures' that can identify virtually all materials.

According to Ricco, spectral signatures of water vapor, water ice and water bound in minerals show up in the infrared part of the spectrum. "These signatures are of keen interest, so the reflected light from the dust plume will be examined carefully for their presence using a unique spectrometer adapted for the LCROSS mission," Ricco said.

"The LCROSS mission will make two impacts separated by about 10 minutes," said Wooden. "The dust and water ice cloud lofted into sunlight will be observable from ground-based telescopes. Imagine an umbrella-shaped cloud that emerges from the limb (a heavenly body's outer disk edge) of the pole of the moon. If the cloud contains water ice, the ice will become water vapor, and the energetic ultraviolet solar photons (sunlight) will break it down further into hydroxyl (OH-), which can be detected for hours or even days," she explained.

"Also, dust grains will reveal their compositions when they are warmed by the sunlight. The ground-based observations are very important to the studying the longer (tens of minutes to hours) evolution of the cloud," Wooden added.

According to Wooden, it is just this kind of cloud that a comet impact would have created during the heavy bombardment period of the early solar system some four billion years ago. "If the water vapor in such a cloud was dispersed to the poles, it would have refrozen there - making the source of water ice that we now seek to find," said Wooden.

"NASA Ames will host a site selection workshop on Oct. 16 - Oct. 17, 2006, to ask the science community to give us their suggestions as to where the LCROSS impact should occur on the moon - at either the north or south pole," said Heldmann.

According to Colaprete, the LCROSS proposal named Shackleton Crater in the south pole area as an example around which scientists and engineers could develop a mission design. "The process for selecting the impact target will be modeled after the successful Mars Exploration Rover landing site selection method," Colaprete said.

The LCROSS prime contractor for the spacecraft and the spacecraft integration is Northrop Grumman.

For images related to the LCROSS mission, please visit:

http://www.nasa.gov/centers/ames/multimedia/images/2006/lunarorbiter.html

For information about the Clementine mission to the moon, please see:

http://science.nasa.gov/headlines/y2005/14apr_moonwater.htm

To read a fact sheet about the Lunar Prospector mission, please visit:

http://discovery.nasa.gov/prospector.html

 
 
John Bluck
NASA Ames Research Center, Moffett Field, Calif.
Phone: 650/604-5026
E-mail: jbluck@mail.arc.nasa.gov