LCrOSS to impact the Moon, look for water

Contributed by
Apr 10, 2006

NASA is pretty serious about putting people back on the Moon. But you can't just build a rocket, stick some folks on it, and ship them off. You need to have a good understanding of the lunar surface, which means high-resolution maps, excellent knowledge of the topography, and even mineralogy of the surface. All these allow you to pick the best landing sites, and not slam into a mountain on the way down.

NASA did mapping of the Moon before Apollo, and they'll be doing it again before this next round of exploration. The Lunar Reconnaissance Orbiter (LRO) is the first step in this return to the Moon. It will have high-res cameras, an altimeter, and other instruments to help map the Moon. It's scheduled to launch in 2008.

A few years ago, it was found that there might be water ice on the Moon, locked away in the permanently dark regions of deep craters at the lunar south pole. In reality, hydrogen was detected by earlier spacecraft, and the easiest way we know to have a repository of hydrogen is to lock it up in water (H2O, right?), which is common in the solar system (for example, comet impacts might deposit water in the polar craters). The water doesn't melt/evaporate there because some craters are very deep, and sunlight in these regions never reaches the crater bottoms. Studies have shown that water could stay in the bottom of these craters for many hundreds of millions of years. This water would make it a lot easier to place laboratories and colonies on the Moon; water is heavy and hard to transport via rockets, so finding some already there would save a huge amount of effort and, of course, cost. The problem is that the water was not directly detected, only hydrogen was. Confirming the existence of this water is a pretty big deal.

It's so important that NASA decided to take a more pro-active course in finding it. As it happens, the LRO spacecraft is launching on a big rocket, so big that they room left over to add more hardware to the original spacecraft. So NASA got an idea: send a secondary spacecraft along with LRO. While LRO goes into orbit around the Moon, the second spacecraft can do something to try to detect that water.

NASA opened up this idea to the scientific/engineering community, and 19 proposals submitted. The winner was just announced in a press conference today. It's called LCrOSS: the Lunar Crater Observation and Sensing Satellite. LCrOSS is basically two pieces: one piece is a hunk of metal with steering rockets on it. That part will impact the Moon, sending up a plume of material that will hopefully contain water. The second piece of LCroSS has infrared and visual cameras on it which can detect the impact and the plume, determining if it indeed has water in it.

The impactor will hit the Moon at 2.5 kilometers per second, at an angle of 75 degrees (nearly vertical). The impactor weighs 2000 kilograms (two tons!), so the energy of impact is about equal to exploding more than a ton of TNT on the surface of the Moon. Bang!

The "follower" spacecraft with the detectors will observe the impact, and then 15 minutes later it will pass right through the plume, and then it too will impact the Moon-- the course is set to be as close as possible to the impact site to get a good look at it, and the best way to do this is to have it hit the Moon as well.

This is all scheduled to happen well after LRO itself gets to the moon. LRO will get to the Moon first, get into its orbit, and go through the usual shake-down procedures to get it into shape. In the meantime, LCrOSS will be on a long orbit that takes it around both the Earth and the Moon. The orbit is 43 days long, and it will orbit twice (for a total of nearly three months) before the impact. This should be plenty of time to get LRO ready to observe the effects of impact as well.

The crater should be about 100 feet across and 16 feet deep, and the plume may reach heights of 30-40 miles above the lunar surface. It will be heavily observed by telescopes here on Earth, and engineers expect that the plume itself may be visible by amateur astronomers with big enough telescopes (I'll be watching with mine: count on it!). Incidentally, the target crater is named Shackleton, which is appropriate enough: he was an explorer of the Antarctic, at the Earth's south pole.

I'll note that Lunar Prospector tried this same thing in 1999, but it hit at a very low impact angle, so the material was shot out sideways. Plus, the spacecraft wasn't very massive, so it didn't create a large plume. Nothing was seen, even by Hubble (much to my chagrin; I was on that project to get the data from Hubble). The difference now is that the LCrOSS impactor hits at a steep angle, and is much more massive, so the plume should be much larger and easier to see.

So what's my opinion of this? At first I was skeptical; it seems pretty late to add something like this onto LRO. However, the LCrOSS spacecraft is separate from LRO. They launch on the same rocket, but they separate early on. The way it was described in the press conference, there will be minimal impact (har har) on LRO, so the added risk is small. They have a budget cap of $80 million, which is really not very much for a space mission, and it does add a lot of value to LRO, even if no water is detected.

Look: at some point, we need to find out if this water exists or not. A dedicated mission for this would cost a lot. Adding a special detector to a subsequent mission would be nice, and maybe even necessary, but this is perhaps the quickest and easiest way to look for the water. Sending up a plume makes it possible for telescopes on Earth to look for the water as well, and that provides a lot of backup.

I like the idea of doing this, since it's not terribly expensive as these things go, has a high probability of working (we've done it before under more difficult circumstances), and can be done without affecting the main LRO mission. If -- if -- all this can be done, then I think this is a good idea.

Also, I think going back to the Moon is a good idea, and I know it won't fly without public support. Doing something splashy like this (again, har har) will get a lot of attention, and is something people can actually go out and see with their own eyes (well, through a telescope). And it's not just a stunt; real science may come out of this, science that has a direct impact on future exploration of the Moon. I think this has a pretty good chance of sparking interest in the public.

Based on what I've heard, I support this mission, and I'm excited by it! I'll be very interested to see what happens next.'

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