Dr. Colaprete: Well that gets back to my point regarding the importance of finding water in the form of water ice at the poles. There is hydrogen elsewhere on the Moon. Even in the equatorial region, there’s hydrogen bound in various mineral types. The difference is that the amount of hydrogen is much less than what has been observed at the poles by maybe a factor of a hundred. And it’s also important in that it’s in a different form; it’s found in mineral matrices and it takes much higher temperatures to liberate that hydrogen, to extract it. You’d have to scoop up a lot of dirt, put it in an oven, raise the temperature to about 800 degrees Celsius to drive off the hydrogen and oxygen, and make use of it. If it’s in the form of water ice, then it’s very easy to liberate the hydrogen and oxygen. It takes much, much less power – about a thousand times less power – and if it’s at the concentrations we expect – about 1-percent or 2-percent by weight – that’s a hundred times more, at least, than what’s in the equatorial region.
So you have two things working for you. You have a lot more of it, and it’s a lot easier to extract and utilize. The more difficult problem is that this water presumably exists in these permanently shadowed craters that are 200 degrees below zero Celsius. So you’d have to devise a mining operation that could operate at these extremely cold temperatures, and there are plenty of plans to do exactly that with large dozers and other apparatus that thermally mine the water. All you have to do is raise the regolith and the water will start to sublime out of it and you capture that and you can draw it back to a processing plant that’s in the sunlight, which is a little easier to cope with. But the first step is to just identify that the hydrogen we see is, indeed, water. That’s what LCROSS is all about.
There’s no other mission that will be able to do that. The LRO spacecraft will be able to refine our understanding of the hydrogen maps, but nothing on the LRO spacecraft, or any other international mission for that matter, will be able to unambiguously identify the form of that hydrogen. LCROSS will be able to do that.
NTB: This isn’t the first time NASA has tried this. In 1999 they crashed a spacecraft called the Lunar Prospector into Shoemaker Crater near the Moon’s South Pole and found nothing. So why do it again? What’s different about this attempt?
Dr. Colaprete: Well, Lunar Prospector was the spacecraft that started all of this. It’s the spacecraft that discovered this enhanced hydrogen.
The Lunar prospector spacecraft was an orbiter; its purpose was to map the mineralogy of the Moon. It was relatively small – about 160 kilograms or so – and it was in a lunar orbit. It was never conceived to be an impactor, but when it came to the end of its life it was going to impact the Moon anyway, so the controller said, “Let’s do an experiment. Let’s crash it into Shoemaker Crater.” That’s one of these permanently shadowed craters.
Lunar Prospector, being very light and in orbit, came in at a grazing angle, only at about 7 degrees or so above the surface. In doing so, a lot of its energy was imparted very near the surface; it might have even skipped across the surface. Therefore, it didn’t excavate very much material. Indeed, no material was seen being excavated. It wasn’t that we didn’t see any water; we didn’t see anything.
LCROSS, by more than a factor of 10, is more massive. The primary impactor is about 2200 kilograms, and it sends that impactor at a much steeper angle – greater than 60 degrees from the horizontal surface. That allows the energy of the impact to more efficiently couple with the surface and, therefore, excavates more material.