Friday, October 09, 2009

Did NASA's Moon impact Fail?


Palomar Observatory's image of the LCROSS impact site
Image Credit: Palomar Observatory / Caltech


No. At least, not yet, and I don't think it will.

For a few months now, NASA has been hyping this morning's impact of the LCROSS spacecraft with a shadowed crater on the Moon.  Many websites (including this blog yesterday) passed on information on how people with 10-inch or larger telescopes could watch the event, yet professional observatories with 200-inch diameter telescopes didn't see anything obvious.  Because of this, many websites, bloggers, and news casts have painted the LCROSS mission as a fizzle and a failure.  This is as unfair and exaggerated as the predictions of spectacular fireworks were.

I do believe now that NASA overhyped the LCROSS impact, at least in terms of what we might be able to see from the Earth.  But I do not believe the mission has failed.  And that's because science is not about getting the answer we want, it's about getting the answer right.

The LCROSS spacecraft's main mission was to look for water on the Moon, specifically water frozen as ice in the permanently-shadowed craters at the Moon's poles.  The methodology that the scientists who lead the mission chose was to impact a spent rocket engine into a crater, and then fly a spacecraft through the plume of debris tossed into space.  The LCROSS spacecraft would analyze the content and structure of the plume before crashing into the Moon itself, sending up a second plume of debris.  With luck, these plumes would also have risen high enough for Earth-based telescopes to see; these telescopes could then provide even more data.


All of this worked except some of the last part.  The spent rocket engine collided with the Moon, LCROSS flew through the debris plume, made measurements, and sent those to Earth before colliding with the Moon itself.  LCROSS saw a flash from the first crash and did indeed detect debris.  Some satellites, including LCROSS's sister spacecraft, the Lunar Reconnaissance Orbiter (LRO), did detect the debris plume.  But, at first glance, many telescopes did not.  Palomar did not, Hubble did not.

What does this mean?  It means the debris plume (which we know was created, because both LCROSS and LRO saw it) was not as big or substantial as mission scientists thought they might see.  This means the Moon is different than we thought, at least in these polar craters.  This means we're learning something unexpected.  That's science!  In fact, it's more interesting than if we'd seen a big plume of water like we hoped, because it means we scientists were wrong and that there's something we still need to learn.  That's not failure, it's success.

Now, it still may be possible to snatch failure from the jaws of victory.  Maybe we'll learn that some human error was responsible for the lack of plumes.  Maybe some computer made a mistake on the size of the debris plume.  Maybe the trajectory of the impactors was miscalculated.  Maybe the wrong model of lunar ice and soil was used.  Maybe the Hubble was pointing just the tiniest fraction of a degree in the wrong direction.  As the scientists involved in the LCROSS team analyze their data, they'll be checking all of these things, even as they look for signatures of water and the other components of the debris plumes.  But my gut feeling is that everything went right.

So, what happened?  Where was the towering pillar of water rising from craters that haven't seen the sun in two billion years?  I'm just guessing, but I suspect that big plume we were all expecting to see was one of the best-case scenarios.  NASA probably felt comfortable predicting the best-case scenario after the spectacular fireworks of the Deep Impact mission, which used the same basic idea to probe the comet Temple 1.  Certainly the claims of fireworks got the mission a lot of attention, but when the best-case scenario failed to materialize, it led to a lot of disappointment.  I wonder what would have happened if NASA said, "You probably won't see anything, but there's a small chance you'll see fireworks on the moon."  Perhaps fewer people would have been watching, but at least NASA wouldn't be the butt of jokes this evening.

Whatever problems they've been handed in public outreach, the LCROSS team has put together an ambitious mission that has, so far, worked.  Time will tell how much we'll learn from the mission.  How much time?  Having worked with tons of data myself, we're talking months.  The team will look at the data, study it, poke it, prod it, and push the data to its limits.  Once they are sure of what they've found, they'll tell us.  So, we just need to wait.  I hate waiting.

4 comments:

  1. Anonymous11:43 AM

    Any way you slice it, any reasonable person would expect a 2-ton projectile hitting the surface of the moon at 5,000 mph to cause a visually observable impact. There was NOTHING! It is no wonder the public was so skeptical. This was a super high-energy impact. The most basic thing that NASA could assume was that this would create a huge plume of dust and debris. That did not happen at all. What happened, then?

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  2. Although the impact is high energy, the moon is big and a quarter million miles away. The crater where the impact occured (in the picture above)is 60 miles in diameter, at least a couple of miles deep, and permanently shadowed. The LCROSS impact inside this crater therefore needed to loft debris at least a mile or two into the air to get it into the sunshine. The impact was probably sufficient to do that. But suppose the debris cloud was a couple miles high and a couple of miles across. That would be 1/30 of the diameter of the crater in the picture above -- relatively small as seen from Earth.

    NASA's hope was that the impact would release a lot of water. That water would instantly freeze in the vacuum of space and reflect a lot of light back toward Earth. That should have been visible, despite the small size.

    However, if the plume was mostly lunar dust, the plume would be no brighter than the Moon's surface (which is as black as asphalt). Trying to pick out a small black cloud of dust against a background of black rock is extraordinarily difficult. And my guess (and that's all it is) is that the plume was mostly lunar dust, with little if any water. From a quarter million miles away, it's just not going to be visible without careful image processing.

    We know the Centaur rocket stage hit, because LCROSS saw a flash in infrared light (http://antwrp.gsfc.nasa.gov/apod/ap091010.html), and we know LCROSS hit the Moon because all of its transmissions ceased right at the predicted instant of impact.

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  3. I knew that they shouldn't have used disposable cameras!

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  4. Anonymous11:20 AM

    If our beloved country could do thid it would do it in a way that everyone in the world could see it. It would boost respect, it would boost the economy, it would tell the world we are "leading" in science... But like the moon landing, all this seems to be just a political stunt to create the illusion that we can make it. In my opinion; it never happened, not the moon landing, not this.

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