NASA Lunar Orbiter Captures Stunning Images Of Twin GRAIL Impacts
March 21, 2013

NASA Lunar Orbiter Captures Stunning Images Of Twin GRAIL Impacts


Lawrence LeBlond for - Your Universe Online

After spending nearly a year in lunar orbit studying the moon´s interior, two NASA spacecraft, known as GRAIL A (Ebb) and GRAIL B (Flow), were ordered to embark on a final mission that would hopefully give scientists one last bit of data about the moon´s composition. The twin spacecraft, which were launched in September 2011 and arrived in lunar orbit in late December 2011 and early January 2012, were crash-landed into a mountain near the moon´s northern pole on December 17, 2012 after a successful mission.

The GRAIL (Gravity Recovery and Interior Laboratory) twins had accomplished what they were programmed to do and while most spacecraft wind up floating aimlessly through space after their missions have ended, it only seemed fitting to give the twins a proper burial — all in the name of science.

By crashing Ebb and Flow into the moon, scientists knew that a cloud of dust and gas would be kicked up over the lunar surface, giving them a good chance to study more of the moon´s makeup; however, they knew it would be difficult to observe the crash plumes from more than 236,000 miles away.


Fortunately, the GRAIL twins were not the only spacecraft in lunar orbit.

NASA´s Lunar Reconnaissance Orbiter (LRO), which has been studying the moon since 2009, was in a prime position to give the GRAIL team a closer look of the impact (see image). But with only three weeks´ notice of the blaze of glory descent, the LRO team had to scramble to get their spacecraft in the right place at the right time to witness the impact.

"We were informed by the GRAIL team about three weeks prior to the impact exactly where the impact site would be," said LRO Project Scientist John Keller of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The GRAIL team's focus was on obtaining the highest resolution gravity measurements possible from the last few orbits of the GRAIL spacecraft, which led to uncertainty in the ultimate impact site until relatively late."

Because LRO is in a low lunar orbit, around 100 miles above the surface, the team has to continually compensate for gravitational forces from larger lunar features such as mountains.

"We had planned a station-keeping maneuver — a periodic adjustment to the orbit to prevent the spacecraft from hitting the lunar surface — a few days before the GRAIL impact," Keller said. "I asked the Flight Dynamics folks here at Goddard if they could combine the station-keeping maneuver with a phasing maneuver — firing the engines to slightly speed up or slow down the spacecraft so it is in the right place at the right time to see the impact. They said it didn´t really work; we'd have to do another station-keeping maneuver to compensate.”

“Based on this, we were leaning against observing this impact because we were going to observe another lunar impact to end the European Space Agency's successful Herschel mission. That impact would have created a much larger plume because the Herschel spacecraft is more massive than GRAIL. However, ESA decided against the collision, so we went with the impact we had,” Keller explained.

He said the LRO team had lost nearly a week trying to figure out which spacecraft to follow, and whenever “we fire the engines on LRO, mission safety requires us to schedule communications coverage from NASA's Deep Space Network (DSN)."

Since so many spacecraft rely on the DSN, it's not easy to schedule with little notice, he noted.

However, since the team was already in communication with the DSN, Keller said they decided to postpone station keeping until a later date and turn the mission into a phasing maneuver and go witness the GRAIL impact.


Making the observation was not an easy process however. The site was in shadow at the time of impact, so the LRO team had to wait until the plumes grew high enough to be in sunlight to make an observation. The team utilized the Lyman Alpha Mapping Project (LAMP) ultraviolet imaging spectrograph on the spacecraft to grab the images. They detected mercury and atomic hydrogen in the plume.

"The mercury observation is consistent with what the LRO team saw from the LCROSS impact in October 2009," said Keller. "LCROSS (Lunar CRater Observation and Sensing Satellite) saw significant amounts of mercury, but the LCROSS site was at the bottom of the moon's Cabeus crater which hasn't seen sunlight for more than a billion years and is therefore extremely cold."

Since mercury is volatile and vaporizes easily, scientists suggest that it accumulates in the cold, permanently shadowed craters of the moon, such as in the crater where LCROSS impacted. But it surprised the team to see it in the plume of the GRAIL collisions, since the impact zone sees regular sunlight.

Keller explained that the moon, much like the Earth, has an abundance of mercury. However, it was reasonably believed that most mercury would be stored away deep within the lunar crust or at least in permanently darkened and cold areas on the moon´s surface. It doesn´t seem plausible that an area that is exposed to heat from the sun, radiation from space and meteoric impacts would be a likely safe haven for mercury to exist.

However, it was evident in the crash plume.

"These new results help us continue to understand the nature of volatiles near the lunar poles," said Kurt Retherford, LAMP principal investigator at Southwest Research Institute, San Antonio, Texas. "In the last four years we have begun to understand that the amount of water ice near the polar regions is higher than previously thought. In addition to direct measurements of water from the LCROSS impact plume there were several other volatile species detected in the Cabeus crater cold-trapping region, including mercury atoms and hydrogen (H2) molecules detected with the LAMP instrument.”

“While our results are still very new, our thinking is that the mercury detected by LAMP from the GRAIL site might be related to an enhancement at the poles caused by mercury atoms generally hopping across the surface and eventually migrating toward the colder polar regions. The detection of hydrogen atoms from the GRAIL impact plumes compared with H2 molecules in the LCROSS impact plume might tell us more about hydrogen and/or water near the poles, but this is a work in progress," explained Retherford.

Keller added that this insight into how volatiles move around the lunar surface “gives us a data point that helps constrain models of volatile transport, especially for models that describe how volatile material can get transported from warm to cold areas on the moon."


After the dust and gas plumes settled, LRO´s camera (LROC) was able to grab images of the craters left by Ebb and Flow despite their relatively small size.

The GRAIL twins, each about the size of a washing machine and weighing roughly 440 lbs., were traveling at 3,771 mph at the time of impact, according to Mark Robinson, LROC principal investigator at Arizona State University´s School of Earth and Space Sciences in Tempe.

Robinson said the GRAIL craters were quite small, roughly 13 to 20 feet in diameter and leaving behind faint, dark, ejecta patterns, which he found unusual. “Fresh impact craters on the moon are typically bright, but these may be dark due to spacecraft material being mixed with the ejecta.”

Robinson said the impact craters “lie on the southern slope of an unnamed massif (mountain) that lies south of the crater Mouchez and northeast of the crater Philolaus.”

The summit of the massif stands at about 4,900 feet and the impact sites are at about 2,296 feet for Ebb and 3,281 feet for Flow, according to Robinson. He noted that Flow impacted about 30 seconds after Ebb at about 7,218 feet (1.4 miles) to the west and north.

"The LRO spacecraft team, with much help and input from the GRAIL navigation team, did an excellent job of tailoring the timing of the LRO spacecraft's passage nearest the impact site to coincide with the impact events and needed delays for the plumes to rise up into sunlight," said Retherford in a statement. "Our two spacecraft teams communicated well with one another, which was crucial to making this coordinated observation a success."

Image Below: An after shot from LRO's LROC of the twin GRAIL impacts. Credit: NASA/GSFC/Arizona State University