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The Planetary Society Blog

By Emily Lakdawalla

LPSC, Wednesday lunch: Planning for the crash of SMART-1

Mar. 15, 2006 | 18:56 PST | Mar. 16 02:56 UTC

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Over lunch they had scheduled a special session to acquaint the community with the plans for the end of ESA's SMART-1 mission to the Moon. I ran and grabbed a pretty awful sandwich from the hotel lobby and sat down to listen to Project Scientist Bernard Foing talk about the status of SMART-1. Originally, their orbit would have had them crash on August 17 of this year on the far side of the Moon, where it wouldn't be visible from Earth. They've exhausted their xenon propellant (which is what they use in their ion engine), but they still have a couple of kilograms of hydrazine fuel left to try to change that.

Click to enlarge >

SMART-1
Credit: ESA

So, to make the impact happen on the near side instead of the far side, they are going to perform a maneuver beginning on June 23 that will give SMART-1 a delta-V of 12 meters per second, and delay its impact until September 3, 2006 at between 0:30 and 2:00 UT -- plus or minus 7 hours. I'll explain why that long uncertainty in a minute. The impact will occur on the near side, in the southern hemisphere, in highlands, in the dark, about 80 arcseconds from the first quarter terminator.

OK, so why don't they know the impact time to an accuracy of less than 7 hours? The problem is that when SMART-1 crashes, it will do so at a tiny, tiny, glancing angle -- it will be coming in at an angle of 1 degree over a landscape that has local slopes of up to around 10 degrees. Unfortunately, the topography of the Moon is only mapped at a horizontal resolution of about a kilometer. What that means is that they can't know the characteristics of the local topography underneath SMART-1's course well enough ahead of time to predict on which of three orbits SMART-1 will impact the surface. In other words, if there's a high hill in SMART-1's path, it will hit one orbit before the one they predict; if SMART-1 happens to slide through gaps between hills, it could hit one orbit after the one they predict. The orbits have about a 5-hour period. Thus the uncertainty. As a result, Foing has to go to a lot of telescopes on Earth and ask them to be ready to photograph a flash that they may end up not being able to see.

One mind-boggling fact he mentioned was that on the orbit before the crash, they could well be sailing along only 400 meters above the lunar surface. Yikes.

He offered some information about what kinds of effects the impact might produce. The spacecraft is about a meter cube and weighs 285 kilograms. Of that, 200 kg is aluminum, 3 kg will be leftover hydrazine, 0.26 kg is xenon, and then there are the 14-meter-wide carbon-fiber arrays. Based on the experience with the Hiten spacecraft, which crashed onto the Moon carrying only a kilogram of hydrazine, the flash from the hydrazine alone should be visible from Earth, at least in infrared wavelengths (around 2 microns). Another interesting possible effect is that although the impact will happen in the dark, if it manages to send off any ejecta with vertical velocities greater than 200 meters per second, that ejecta will reach an altitude of 24 kilometers, where it will be in sunlight, and should become visible. He estimated that the resultant crater should be 3 to 10 meters in diameter and be very elongated, and expressed hope that some of the future lunar orbiters planned by the US, India, and China might be able to spot it. One thing that I thought was amusing about this part of his discussion was that he kept on looking at Deep Impact Principal Investigator Mike A'Hearn as he was going through all these numbers. A'Hearn was sitting in the front row and nodding occasionally, so it seems he's been consulted on these predictions, and he should know about artificial impacts!

There was a brief question and answer period, and Chuck Wood stood up and asked when the imaging data from SMART-1 was going to be released. (He was sitting next to me earlier and had mentioned with some disgruntlement that they've only released 11 images so far.) Foing said that they are working very hard to clean up the data and release it on ESA's servers around the time of the impact. However, he said, if anyone in the room was interested in collaborating with the SMART-1 team in its waning months to work on the impact simulations or any other lunar science project, that they would certainly share the raw imaging data with those collaborating scientists.

I still have notes to write up from the afternoon, but I'm pretty weary; I'll have to have a go at it tomorrow, in between talks on Mars and the Galilean satellites, and before the second poster session. And I still have notes from Tuesday night's poster session to write up too. I may be approaching the end of my stamina for attending marathon talk sessions; I might just have to select a couple specific talks to go to tomorrow, and conserve my strength for the poster session in the evening. And I need to save a little for Hayabusa on Friday morning. But the Mars session opens tomorrow morning with someone arguing that the rover folks have got their interpretation of Meridiani Planum all wrong, so it seems like I'd better go to that just to see how it's received.