[meteorite-list] Novel Lunar Impactor Cleansed of Water (LCROSS)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 10 Aug 2009 17:01:32 -0700 (PDT)
Message-ID: <200908110001.n7B01WM6028884_at_zagami.jpl.nasa.gov>

http://www.spaceflightnow.com/lcross/090810bakeout/

Novel lunar impactor cleansed of water
BY STEPHEN CLARK
SPACEFLIGHT NOW
August 10, 2009

Nearly halfway through its crash course with the moon, NASA's lunar
impact mission is being scrubbed of Earth water that could throw the
probe off course and pollute potential ice on the moon.

The mission will pass the halfway mark this week on its way to a
pinpoint collision with the moon on Oct. 9. Debris from the impact plume
will be analyzed in search of water molecules scientists believe could
exist inside craters at the lunar poles.

The Lunar Crater Observation and Sensing Satellite, or LCROSS, is
tugging a two-and-a-half ton Centaur rocket coated in foam that
collected water from humid air at the mission's Florida launch site.

"When we're sitting on the pad, the Centaur, which we're impacting on
the moon, is essentially a giant thermos bottle," said Tony Colaprete,
the project's chief scientist at NASA's Ames Research Center in Moffett
Field, Calif.

The Centaur consists of two propellant tanks holding chilled liquid
hydrogen and liquid oxygen.

Foam applied to the outer shell of the rocket helps insulate the cold
fuels from warm outside temperatures. But it can also attract and
condense water from rain and humidity, common conditions at the Atlas 5
rocket's oceanfront launch pad at Cape Canaveral, Fla.

"The foam absorbs quite a bit of water white it's sitting there, even
for the short amount of time that it's in the elements in Florida on the
launch pad," Colaprete said.

Valves on the Centaur also build ice during fueling.

"If you've seen the launch video from the rocketcams during the launch,
looking down the Centaur, you can see icicles literally hanging off near
the fill and drain valves lower down on the Atlas 5. It is that
accumulated ice that we want to get rid of," Colaprete said.

If the ice survived during the spacecraft's circuitous four-month voyage
to the moon, the probe could be pushed off course and the scientific
results of the $79 million mission could be skewed.

"Our principal objective is to measure in situ water on the moon. We
don't want to confuse our measurements by having some terrestrial water
on the Centaur," Colaprete said.

"The interesting thing is even though space is a vacuum, the water can
sustain itself there if it's cold enough," said Paul Tompkins, LCROSS
flight director.

After propelling LCROSS and the Lunar Reconnaissance Orbiter toward the
moon, the Centaur safed itself and vented leftover fuel from its tanks.

United Launch Alliance, builder of the Atlas 5 rocket, assured NASA the
Centaur would contain less than 220 pounds of residual water, hydrogen
and oxygen at the end of its mission.

Colaprete said the Centaur easily met that requirement, with the extra
mass totaling approximately 143 pounds, according to engineers' best
estimates.

Instruments that will sense lunar debris thrown up by the Centaur's
impact shouldn't detect such small amounts of Earth water, according to
Colaprete.

But Colaprete noticed evidence of a water cloud trailing the spacecraft
during instrument testing in the early days of the mission.

The water cloud was enough for the LCROSS navigation team to recommend a
series of maneuvers called cold side bakeouts that turn the back side of
the Centaur toward the sun. Such techniques had been discussed before
launch, but were not in any mission plans.

Because the LCROSS shepherding spacecraft has a fixed solar panel, the
stack must stay in the same orientation during most of the mission to
generate electricity. That means the other side of the Centaur remains
very cold.

"It's so cold that the ice just sits there and doesn't sublime very
well, and certainly doesn't migrate out of the foam," Colaprete said.

To remedy the situation, engineers sent commands for LCROSS to rotate
the spacecraft 180 degrees.

"We rotate the entire spacecraft around, go off of solar panel power and
run on battery power, and just let that back side of the Centaur warm
up," Colaprete said.

Like ice on the Centaur side normally warmed by the sun, frozen water
embedded in the cold part of the rocket will slowly turn to gas and
float into space.

"Once it gets exposed to that sunlight, it will warm up and those water
constituents will start to bake out of the surface of the Centaur,"
Tompkins said.

LCROSS navigators were concerned the uncontrolled Centaur could turn its
cold side toward the sun after separating from the shepherding spacecraft.

Boiling off water creates a small propulsive impulse that could push the
Centaur off course right before impact. Scientists want to precisely aim
the impactor to a specific point inside the target crater, so any errors
could affect science results.

"Those little molecules coming off and subliming actually impart their
exit velocity to the Centaur and actually push it off target," Colaprete
said.

Now cruising nearly 300,000 miles from Earth, LCROSS has already
completed two cold side bakeouts. At least one more maneuver is planned
before impact.

The first bakeout produced a velocity change of more than one-tenth of a
foot per second, enough force to cause a "significant targeting error,"
Colaprete said.

Several pounds of water boiling off could push the Centaur nearly two
miles off course, according to Colaprete.

Officials waited a month after the mission's June 18 launch to begin the
bakeouts. The first few days were spent turning on the spacecraft and
preparing for a swing past the moon.

"The first week was just maddening for us on LCROSS. We had to
essentially get all our trajectories right, get the swingby right and
commission the payload for swingby calibrations. Once we did all that
and we could actually catch our breath and catch up on our sleep, then
we started saying, 'OK, let's assess our situation,'" Colaprete said.

The swingby used lunar gravity to send LCROSS into a distant Earth orbit
to set up for the October impact.

"We swung from the Earth all the way out to lunar distance, and just as
the moon goes by, it threw us into this super-high inclination, highly
elliptical orbit around the Earth, but at about lunar distance. What
that allowed us to do is phase our orbit such that in three months, we
come back on the moon again and have a direct impact in the south pole,"
Tompkins said.

LCROSS will separate from the Centaur less than 10 hours before getting
back to the moon. The probe will slow itself down, allowing the empty
rocket to pull ahead and strike the moon about four minutes before the
shepherding spacecraft.

The tightly choreographed impact sequence will occur around 1130 GMT
(7:30 a.m. EDT) on Oct. 9, excavating more than a million pounds of
lunar material, including potential water ice.

Scientists still have not picked a target for the impact, but it will
probably be one of eight permanently shadowed craters near the moon's
south pole, according to Colaprete.

Those eight craters were selected before launch based on the best
available data from international lunar orbiters and ground-based radar.

Officials are busily studying early results from LRO's mission to narrow
down the list of finalists. A science meeting at Ames next week will
include extensive discussion of potential impact sites.

"Out of that meeting, I'll come away with hopefully a much more narrow
list, from eight or so, which it is now, to maybe one or two," Colaprete
said.

Colaprete will make the final decision in early September, in time for
LCROSS to fire its engines and tweak its trajectory to line up with its
destination.

NASA tentatively plans to announce the target crater around Sept. 10.
Received on Mon 10 Aug 2009 08:01:32 PM PDT