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An impression of the two GRAIL probes in orbit before the crash. Credit: NASA An impression of the two GRAIL probes in orbit before the crash. Credit: NASA

Double Moon crash caught on camera

Sen— The dramatic demise of two space probes that were deliberately crashed into the Moon has been captured by another spacecraft in lunar orbit. Its images have just been revealed.

NASA's Lunar Reconnaissance Orbiter (LRO) was steered into position to get a grandstand view when the twin probes Ebb and Flow bit the dust in December.

The two spacecraft on NASA's GRAIL mission - it stands for Gravity Recovery and Interior Laboratory - had spent 12 months mapping variations in the Moon's gravity field to tell scientists more about our neighbouring world's interior.

They had been launched in September 2011 to begin a three-month journey to the Moon. NASA decided to crash them into a 2.4km high mountain near the lunar north pole to find out more about the Moon's makeup.

The plumes of dust thrown out by each impact would provide these clues, but they would have been very difficult to observe from Earth. Instead a special request was put in to the LRO team - with just three weeks notice of the GRAIL probes' impact location.

GRAIL A impactThe LRO images show Ebb's crater plus dark ejecta. Credit: NASA/GSFC/ASU

LRO offered an ideal observing platform from a height of 160km. Its camera was already designed to provide high-resolution images to help map the lunar surface.

LRO Project Scientist John Keller of NASA's Goddard Space Flight Center in Maryland, said: "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."

The GRAIL probes were not particularly big. Each was about the size of a washing machine. But travelling at around 6,100 kilometres per hour, they threw up plenty of dust on impact.

LRO scientist Mark Robinson said: "Both craters are relatively small, perhaps 4 to 6 metres in diameter and both have faint, dark, ejecta patterns, which is unusual. Fresh impact craters on the moon are typically bright, but these may be dark due to spacecraft material being mixed with the ejecta."

The GRAIL B crash siteDramatic evidence of where Flow hit the Moon. Credit:NASA/GSFC/ASU

As well as taking photos, LRO carried out observations to provide information about the impacts. LRO's Diviner lunar radiometer observed the impact site and confirmed that the amount of heating of the surface there by the relatively small GRAIL spacecraft was within expectations.

And LRO's Lunar Orbiter Laser Altimeter (LOLA) instrument bounced pulses off the surface to build up a detailed map of the local terrain, including mountains and craters.

Keller said: "Combining the LRO LOLA topography map with GRAIL's gravity map yields some very interesting results. You expect that areas with mountains will have a little stronger gravity, while features like craters will have a little less.

"However, when you subtract out the topography, you get another map that reveals gravity differences that are not tied to the surface. It gives insight into structures deeper in the Moon's interior."

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