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.