Feature

Shy moon gives up its secrets in a plume

The first image of lunar material raised by the impact of NASA's LCROSS mission has been released, a week after the impact occurred. It was taken by a spacecraft trailing behind the impactor, whose bird's-eye view allowed it to see what has so far eluded the best telescopes on Earth and in Earth's orbit.

Researchers are still studying the faint plume of material to try to identify its composition and search for signs of water.

On 9 October, the LCROSS mission used a "shepherding" spacecraft to send the 2-tonne upper stage of its launch rocket into a permanently shadowed crater at the moon's south pole. The shepherding spacecraft observed the impact before crashing into the moon itself 4 minutes later.

Scientists had hoped that dust and vapour ejected by the impact would climb high enough to catch sunlight, allowing telescopes to hunt for traces of lunar water in the ejecta. But no obvious plume of ejected material was seen by any observers on the ground or even by the Hubble Space Telescope.

Now, scientists report that a faint plume of ejecta was imaged by the shepherding spacecraft. "I think we are the only ones that have images," LCROSS principal investigator Anthony Colaprete told New Scientist.

Other instruments, such as LAMP on the Lunar Reconnaissance Orbiter probe in orbit around the moon, caught spectroscopic signs of a plume at an altitude of about 10 or 15 kilometres above the lunar surface. But the ejected material was too thin there to be visible in an image, says Colaprete.


Vantage point

Ejecta would have had to rise at least 2 km above the surface to be seen from Earth, so the lack of a clear detection from ground-based telescopes suggests most of the ejecta stayed below that altitude.

By contrast, the LCROSS shepherding spacecraft was flying right behind the rocket stage. So it was able to peer down into the crater from overhead and see ejecta that did not get lofted very high. "The ejecta had to only come out and get into the sunlight a little more than a kilometre [high] for us to see it," he says. "It only had to rise half as high."

Before the impact, mission members said they expected the plume to reach no higher than about 10 km. But projectile experiments carried out on Earth weeks before the impact suggested the plume might reach far lower altitudes.


Crumpled rocket

That's because the rocket stage was hollow, giving it a low density, and the surface of the moon slightly spongy, or compressible, due to pores between particles of soil.

In such a situation, "a lot of the energy [of impact] goes into the crumpling of the low-density object [the rocket] and the compaction of the soil instead of being transferred into vertical velocity," Colaprete told New Scientist. "An analogy is what we do to make ourselves safe in car crashes – when a car crashes into something now, the frame is meant to crumple."

So was using a hollow impactor instead of a dense 'cannonball' design a good idea? Colaprete says that even though hollow impactors may throw up less material at high angles – where it is more easily observed – than dense ones, they create wider, shallower craters. "What we've been able to get with this is a nice, broad area at relatively shallow depth," he told New Scientist.

"That's kind of nice because we're interested in stuff a metre or 70 centimetres deep," he says, pointing out that hydrogen – and thus possibly water – has been detected in the top 70 cm of soil near the lunar poles by neutron spectrometers on spacecraft.


Spectral data

The researchers are analysing the images to try to determine the plume's extent, which will allow them to estimate the total mass that was kicked up in the impact.

And they are scrutinising spectral observations of the impact "flash" – created on the surface at the time of impact, the crater's heat and the ejected material to try to measure the composition of the material at the impact site.

"Our spectrometers worked very well and we got data from beginning to end," says Colaprete. "It's a matter of analysing it now – you have to be careful because you're looking for small [spectral] signatures."

Did they see any sign of water? "Stay tuned," says Colaprete, who aims to have an analysis of the data done by mid-November.


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This was first published in October 2009

 

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