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Space Topics: Lunar Reconnaissance Orbiter (LRO)

LRO & LCROSS : One-on-One Chat with LCROSS Scientist Kimberly Ennico

by Ken Kremer

June 23, 2009

Ken Kremer
Ken Kremer

LRO and LCROSS have now entered orbit around the Moon following the thunderous June 18 launch to comprehensively investigate the Moon with 21st Century technology in ways here to fore unmatched.  Searching for water, resources and landing sites.

Prior to launch, I was very fortunate to chat one-on-one with Dr. Kimberly Ennico, Payload Scientist for LCROSS at NASA Ames Research Center (ARC).  I met Dr. Ennico at the Kennedy Space Center press site during the STS-125 Hubble Repair mission.  We discussed key aspects about LCROSS & the Centaur and she kindly provided me some LCROSS graphics herein especially for The Planetary Society.  She is an astrophysicist and joined LCROSS in June 2006.  LCROSS is helmed by Principal Investigator Dr. Anthony Colaprete. 

The purpose of LCROSS is to unlock one of the Moon's hidden sweet secrets and determine if there are significant caches of water ice preserved inside permanently shadowed craters at the lunar poles.   To accomplish this, LCROSS will direct the Centaur upper stage to smash into the Moon at high speed and act as a kamikaze-like projectile “to blast lunar crater soil upwards about 10 km and create an artificial debris plume” while LCROSS searches the plume for signatures of water ice and organics with cameras, spectrometers and a photometer”, said Ennico.

The Centaur thus serves a dual role.  First in its conventional use as an upper stage to propel LRO & LCROSS to our Moon and second as a man made impactor in a manner for which it has never been designed or been used before.

LCROSS Mission conversation between Kim Ennico (NASA Ames) and Ken Kremer (The Planetary Society)

Ken Kremer: How much power is required to run the LCROSS spacecraft and science instruments?

Kim Ennico: Power to onboard systems is provided by a fixed 600-watt peak power solar array and a Li-ion battery. A star tracker assembly and 10 coarse sun sensors maintain orientation to the sun.

The power allotment to the science payload aboard the LCROSS spacecraft is 100W, although most instrument modes required 70W or less, leaving good power margin.

Ken: How is the data transmitted?

Kim: Data return is all direct to earth using the Deep Space Network. It’s not through LRO. LRO and LCROSS are two completely independent missions which are just launching on the same rocket. We are totally separated from LRO. LRO is its own mission. We just sit underneath LRO at launch. We are very benign to them.

Dr. Kimberly Ennico
Dr. Kimberly Ennico
describing components of LCROSS shepherding satellite attached to Centaur upper stage at an LRO/LCROSS press briefing at Kennedy Space Center. Credit: Ken Kremer

Ken: Please describe the testing of the science instruments and collection of science data during the approximately 4 months in transit?

Kim: Calibration of the LCROSS science payload is performed during key times during the first week of the mission to measure the post-launch bore-sight and check health and status of the instrument performance. There is a baseline of eight activations of the LCROSS science payload during the 4-month trajectory.

Following a ‘quick-look’ and ‘star field’ calibration, the entire instrument suite is configured in an operations mode similar to that required for observing the impact event, for a lunar swing-by on day 5 of the mission. Here, the LCROSS spacecraft still attached to the Centaur upper stage, flies within 8000 km of the lunar surface to get into its Lunar Gravity-Assist, Lunar Return Orbit (LGALRO). This approach was chosen to allow LRO time to complete its two-month commissioning phase and conduct nearly a month of science data collection of polar crater measurements. This LCROSS lunar swing-by also provides a target to test alignment and radiometric calibrations of all the instruments.

In the next few months prior to impact, the LCROSS spacecraft directs the payload bore-sight to observe the Earth and the Moon (depending on best maximum phase and largest angular diameter for a sufficient communication angle and bandwidth to Earth) for health and alignment checks. The science payload is also turned on, after a 180-degree maneuver to observe the Centaur's separation trajectory at approximately 9 hours, 40 minutes prior to impact.

Ken: Can you describe these mission phases in more detail please?

Kim: On Day 5 of the mission we swing by the Moon, turn on all our cameras and spectrometers and take pictures. We’ll do some alignments on the lunar limb and look at the terminator. We’ll be only about 8000 km away.

Then we have 3 opportunities for an ‘earth-Moon look’ where we turn on our instruments and take a few pictures. We are in a earth orbit which does not swing past the Moon or we’d get caught up in the lunar gravity. For the ‘earth Moon look’ on July 31 we’ll be 700,000 km from the Moon and 350,000 km from the earth. So on this day we’ll be looking back at the earth as though we were looking from the Moon (see trajectory graphic).

So LCROSS will be in a giant looping orbit for all three ‘earth-Moon looks’. It’s a very fuel efficient, benign orbit to accomplish several objectives; buys time for LRO to collect data, empty the fuel out of the Centaur and allow us to target the Moon at a particular time and place.

LCROSS trajectory in Earth-Moon system
LCROSS trajectory in Earth-Moon system
following nominal launch during first window (P 10) on June 17. Provided courtesy of K. Ennico/A. Colaprete/NASA. Credit: NASA/ LCROSS mission

Ken: Let’s discuss the critical sequence of payload activations in the first 5 days after launch.

Kim: We turn the payload on for the first time on Day 2 to evaluate the health and status of every instrument and making sure we survived launch. We do an alignment calibration looking at Canopus on Day 4.

Then we do the lunar swing-by on Day 5 where we look at 3 distinct targets on the Moon’s surface: the maria, the terminator and a dark area. We have chosen three well studied places so we can gauge how well our instruments are performing. So we do 3 payload activations in the first 5 days to assess instrument performance.

Ken: What was the effect of the two week launch (link) delay from early June to mid June ?

Kim: By moving the launch window to mid June from early June, the impact angle changes from 65 degrees to 85 degrees and leads to an expected 14% increase in total ejecta mass. So our later June windows give us better targeting angles for impact. Again, that impact angle being steep helps kick a lot of regolith up high into the lunar sky to see the sun.

Ken: When do you turn the science instruments on for the impact phase?

Kim: For the final impact phase, the science instruments are powered for 55 minutes prior to the predicted time of the Centaur impact, followed by the remaining ~4 minutes of observing the impact event and evolving ejecta before the LCROSS spacecraft itself impacts the Moon.

The descent speed for the LCROSS spacecraft is 2.5 km/s, therefore at 55 minutes prior to the Centaur impact, the LCROSS spacecraft is thus at 8,250 km lunar altitude. At the moment of the Centaur impact, the LCROSS spacecraft views the flash event at 600 km lunar altitude. The expected curtain ejecta is observed by LCROSS as it descends down to 150 km altitude, after which the remnant new crater formed by the impact is observed until LCROSS impacts the Moon.

Ken: How and where will the Centaur impact and at what angle?

Kim: For the LCROSS experiment, a requirement of an impact angle greater than 75 degrees for latitudes pole ward of 70 degrees, needs to be met in order to excavate sufficient mass of regolith high enough so as to be able to be illuminated by the sun. The actual impact angle is intimately tied to the mission design and crater location. For the nominal June 17, 2009 launch, the impact on October 8, 2009, at 10:30 UTC has an impact angle of 85 degrees for a crater within 85-90 degrees south latitude.

The LCROSS spacecraft separates itself from the Centaur upper stage at approximately 9 hours 40 minutes prior to the Centaur's lunar impact event. The Centaur upper stage descends towards the lunar surface under the influence of the Moon's gravity.

The Centaur is not actively controlled as it falls to the surface. It gets pulled in by the Moons gravity. There is no trajectory correction. At that point the Centaur is just ‘dead junk’.

Ken: Will the Centaur be tumbling? Does that matter? Can you induce tumbling?

Kim: It could be tumbling. We prefer it to do a belly flop rather than just go straight in. No, we can’t induce tumbling. But we have a test. After we separate we rotate LCROSS 180 degrees to take images of the Centaur as it is falling away. We can maybe get an idea of whether it is tumbling or not. We use pyros at the separation and we’ll see maybe with the images what that will do to the Centaur.

At that point after the rotation, LCROSS is looking at the Moon. The Centaur is in the lead. LCROSS will do a breaking burn to slow itself down and space itself out from the Centaur so its following 4 minutes behind as it is approaching impact.

Ken: Kimberly, Thank you for your generosity! And as well to the entire LRO/LCROSS teams. Good luck on this fantastic mission in search of water.

Check out my earlier reports: LRO & LCROSS Up-Close Tour, LRO & LCROSS Up Close Tour: Part 2, Hunting for Lunar Water, LRO & LCROSS Up Close Tour: Poised on Atlas V Rocket at Launch Complex 41, LRO & LCROSS: 1 Day Launch Delay, LRO & LCROSS: Ready to Roll to Pad 41 and LRO & LCROSS: Launch!

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