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Confirming the Presence of Water on the Moon

Anthony Colaprete, Principal investigator of the LCROSS missionThe Lunar CRater Observation and Sensing Satellite (LCROSS) is a NASA mission to look for traces of water in a permanently shadowed crater at the Moon’s South Pole. LCROSS launched with the Lunar Reconnaissance Orbiter (LRO) in June 2009, and impacted the Cabeus crater four months later, creating a plume of debris. A study of that debris has revealed evidence of the presence of water on the Moon. Dr. Anthony Colaprete is the LCROSS mission’s Principal Investigator.

Anthony Colaprete
Planetary Atmospheric Scientist, NASA Ames Research Center
Dr. Anthony Colaprete is a planetary climatologist who was Principal Investigator on the Lunar CRater Observation and Sensing Satellite (LCROSS) mission, which impacted and observed the Moon in 2009.
Dr. Colaprete received a Ph.D. in Astrophysical, Planetary and Atmospheric Science in 2000 from the University of Colorado. From 1990 to 2000, he worked on design, fabrication, calibration and flight analysis of instrumentation on shuttle and satellite missions at the Colorado Space Grant Consortium. From 1992 to 2000, he worked on aerosol modeling on the Mars Pathfinder and Mars Global Surveyor missions at the Laboratory for Atmospheric and Space Physics. From 2000 to 2003, he was a National Research Council Associate at the NASA Ames Research Center. In 2003, he was a principal investigator at the SETI Institute. He has been in his current position at NASA Ames since August 2003. Currently, he is working on several future NASA missions, including the Lunar Atmosphere and Dust Environment Explorer (LADEE).

A Close-Up View of a Lunar Impact

Q. How did the idea come about for the LCROSS mission?

LCROSS thermal vacuum test (Courtesy of NASA, Northrop Grumman)
LCROSS thermal vacuum test (Courtesy of NASA, Northrop Grumman)

The idea for the LCROSS mission came about when NASA moved the Lunar Reconnaissance Orbiter (LRO) to a larger launch vehicle. The larger launch vehicle allowed an extra thousand kilograms of mass to go to the Moon, and rather than let this thousand kilograms go to waste, NASA put out a call for proposals for what to do with this extra capacity. LCROSS was one of 19 proposals submitted, and one of the four that were ultimately selected for final evaluation. After that final evaluation, LCROSS was selected for flight.
The idea came from a number of people, and it wasn’t really unique. Impacts have been used to study objects in the past, such as the comet Tempel 1 in the Deep Impact mission. During the Apollo era, a number of booster rockets were crashed into the moon to do seismic experiments. There were seismometers set up, and they used the large Saturn IV booster (third stage of the Saturn V rocket) to create little moonquakes so they could make measurements of the Moon. So the idea wasn’t particularly novel.
What we did do, though, was we used the extra capacity of the launch vehicle to send up what we call a shepherding spacecraft. Rather than just impact the Moon and rely on earth-based or LRO observations, we impacted the Moon and then watched it from a very unique vantage point straight overhead, with a suite of instruments. That’s what, I think, set the LCROSS mission apart from a number of other impact missions that were also proposed for this opportunity.

Q. How did you choose a location on the Moon?

LCROSS was launched with the Lunar Reconnaissance Orbiter aboard an Atlas V rocket in June 2009 (Courtesy of NASA)
LCROSS was launched with the Lunar Reconnaissance Orbiter aboard an Atlas V rocket in June 2009 (Courtesy of NASA)

That was an ongoing process, because we were a secondary mission, a secondary payload on the Lunar Reconnaissance Orbiter, so we had to go when they were ready to go.
We had some specific requirements with regards to where and when we needed to impact. The most important requirement was the ability of ejecta, or debris, from our impact to reach sunlight, so that really prescribed the depth of the crater and the season that we impacted. If we impacted on the night side of the Moon, deep in shadows, of course, it would be much harder for the ejecta to reach sunlight. So that really dictated where we could impact, but, of course, the arrival time depended on our launch time, which was controlled by the LRO project. We had to evaluate targets on an ongoing basis, as launch dates moved and the seasons progressed on the Moon. It was an ongoing process of optimizing our impact location against the launch time and illumination conditions on the Moon. The ultimate launch date was what determined what Quadrant of the moon we could hit. With a June launch, the Cabeus region in the South Pole was our prime target area. That was the sunny side of the Moon, where ejecta would have to travel the least distance to make it to sunlight.
In the Cabeus region, there were three primary targets we were considering, because of their depth, illumination and association with hydrogen measurements made by Lunar Prospector. Those craters were Cabeus proper, Cabeus A and Cabeus B. We originally had picked Cabeus A because of its superb view from Earth. It was at the lowest latitude, where the impact would have been, potentially, directly in view of Earth. But after LRO went into orbit, our measurements increasingly placed doubt on exactly where in Cabeus A we would impact. The LRO observations supported Cabeus proper as a place of very enhanced hydrogen concentrations, so ultimately, just two weeks before impact, we moved our target to Cabeus. And we did our final burn maneuver to Cabeus about two weeks prior to impact, which is very, very late. We were really lucky that we were able to go that late, actually.

One-Shot Opportunity

Q. No matter how much research you do in advance, you never know what the inside of a crater is like until you really see it. What were your concerns just before impact?

Impact target of Centaur rocket in the Cabeus Crater (Courtesy of NASA)
Impact target of Centaur rocket in the Cabeus Crater (Courtesy of NASA)

We had considered everything we knew, and made best decision we could. It was a very difficult few weeks when we were trying to take in as much data, incomplete data, as we could. I was on the phone or in meetings constantly with the principal investigators of the Lunar Reconnaissance Orbiter.
Cabeus had a significant drawback - a large hill called M1, which would block most of the view from Earth. So we really had to understand and appreciate that limitation, so that we could properly present it to the program office or headquarters. At the same time we were coordinating with the Lunar Reconnaissance Orbiter, which at that time, one week prior to impact, was adjusting its orbit to time its pass of Cabeus with our impact. And at the same time, we were trying to time our impact with the Hubble space telescope, which was in Earth orbit and coming around to observe the impact. So it was a very, very busy time. We spent a lot of time really thinking where to impact and what to do. Once the decision was made, it was, "let’s make it happen and make sure we’ve crossed our T’s and dotted our I’s." It’s an incredible dance with all the astronomers on the ground, and the LRO, etc. We didn’t have much time to think about it, although there was a lot of concern about what we didn’t know.

Q. What was the mood of the mission team during the impact event?

LCROSS mission team celebrating the successful impact (Courtesy of Eric James / NASA Ames)
LCROSS mission team celebrating the successful impact (Courtesy of Eric James / NASA Ames)

We had conducted so many models, theoretical studies and experiments trying to ascertain what we would see and how we would see it, and the fundamental conclusion was that there were a lot of possible outcomes and it wasn’t obvious what we would see. That’s how we designed our entire observation campaign - we had instruments of all sorts, in all directions, of all wavelengths, different colors, cameras, spectrometers, whatever. We went into it with our eyes wide open - we weren’t going to focus on a specific measurement but rather try to cover as much as possible. And I’m very glad we did that because there were a lot of surprises in what we saw.
This was a lesson from Deep Impact, the impact mission at Tempel 1: there were a hundred models of what would occur, and none of them was quite right. When you impact something like this, it’s very difficult to control the experiment, and if we were to do it again we would be much better at it. But I certainly wouldn’t think we would necessarily see the same thing again. So much of it really depends on what it is you impact into, and how you impact into it. Those kinds of variables are very hard to control. Something unexpected always happens with planetary explorations, but impact missions are particularly difficult to predict and are very unique.

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