NASA’s Lunar Reconnaissance Orbiter (LRO) has found the impacts made by its cousin(s), the Gravity Recovery and Interior Laboratory, or “GRAIL” spacecraft. Both of the mirror-image GRAIL spacecraft impacted the lunar surface Dec. 17, 2012, after it had completed its primary science mission.
The duo plummeted into the Moon’s north pole in hopes of squeezing one last tidbit of science out of them. LRO would review the plume ejected by the two spacecraft’s impacts in order to sniff out what the ejected material was comprised of. With less than a month prior to impact, this was no easy task for the LRO team.
“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. “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.”
During GRAIL’s impact, LRO was approximately 100 miles (about 160 kilometers) above the lunar surface. By all accounts it was a bumpy ride with gravitational anomalies giving the spacecraft a bit of a hard time.
LRO’s history seems to be made up of watching other spacecraft smack into the Moon’s surface. First, it was the Lunar CRater Observation and Sensing Satellite (LCROSS) which accompanied LRO to the Moon after they were launched in 2009. Now, GRAIL joins the list of the lost.
During the time of the impact, the site itself was cast in shadow, forcing LRO team members to wait until the plume ejected by GRAIL’s demise to rise high enough before they could begin making observations. LRO has a number of scientific equipment onboard the spacecraft; for this particular experiment the team used the Lyman Alpha Mapping Project, or “LAMP.” LAMP is an ultraviolet imaging spectrograph. It saw an increased amount of mercury and atomic hydrogen in the material ejected by GRAIL.
“The mercury observation is consistent with what the LRO team saw from the LCROSS impact in October 2009,” said Keller. “LCROSS 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.”
When searching for the impact craters left by GRAIL, once again LRO was brought into action. It was a harder task than it might seem due to the relatively small size of the craters left behind by the two craft. Each of the GRAIL spacecraft were about the size of a washing machine. Each weighed around 440 lbs (200 kilograms). When they impacted, the spacecraft were traveling at around 3,800 miles (6,100 kilometers) per hour.
“Both craters are relatively small, perhaps 4 to 6 meters (about 13 to 20 feet) in diameter, and both have faint, dark, ejecta patterns, which is unusual,” said Mark Robinson, LROC principal investigator. “Fresh impact craters on the Moon are typically bright, but these may be dark due to spacecraft material being mixed with the ejecta.”
LRO also measured how much residual heat was left over after the impact, using its Diviner lunar radiometer. The spacecraft used its Lunar Orbiter Laser Altimeter, or “LOLA,” to develop a precise map of the lunar surface, which now includes the features placed on the surface by the GRAIL impacts.
“Both impact sites lie on the southern slope of an unnamed massif [mountain] that lies south of the crater Mouchez and northeast of the crater Philolaus,” Robinson said. “The massif stands as much as 2,500 meters [about 8,202 feet] above the surrounding plains. The impact sites are at an elevation of about 700 meters [around 2,296 feet] and 1,000 meters [3,281 feet], respectively, about 500 to 800 meters [approximately 1,640 to 2,625 feet] below the summit. The two impact craters are about 2,200 meters [roughly 7,218 feet] apart. GRAIL B [renamed Flow] impacted about 30 seconds after GRAIL A [Ebb] at a site to the west and north of GRAIL A.”