'Shrapnel' risk to future moon surface missions

The “shrapnel” generated by small space rocks that periodically hit the moon may pose a larger risk to lunar missions than was previously believed.

A number of countries and private consortia have stated their plans to send robotic and crewed missions to the lunar surface in the coming decades. A relatively small impact on the moon last year hurled hundreds of pieces of rocky debris out of the crater. Many were travelling at the speed of a shotgun blast.

The meteoroid strike sprayed small rocks up to 30 km from the initial impact site, said Professor Mark Robinson from Arizona State University.

Along with colleagues, he used the LROC imaging instrument aboard NASA’s Lunar Reconnaissance Orbiter spacecraft to follow up on observations from March 17, 2013, of an apparent collision on the moon’s surface. The orbiter took pictures of the area that corresponded to coordinates for the impact flash.

Robinson and his team found a fresh 18 meter-wide crater, punched by a 0.3-1.3 meter-wide space rock. The crater is surrounded by typical “ejecta” deposits – the continuous blanket of rock and soil heaved out when the meteoroid thumped into the lunar surface. However, they also saw 248 small “splotches” extending up to 30 km from the primary crater. This was further than the typical extent for continuous ejecta deposits from a lunar crater.

Robinson interprets these surface splotches as relatively low velocity, secondary impacts into the lunar soil by material flung out in different directions by the primary impact.

“Since they’re spread out at great distances, we really need to start thinking that ‘secondaries’ from small craters pose possibly a larger risk to future long-lived surface assets than the actual primary craters themselves,” he told an audience at the LPSC.

He said that the LROC team had discovered hundreds of similar splotches around other lunar craters and that some of these were “directional.” Most of the 33 tons of rock that hits the Earth every day burns up high in the atmosphere, never making it to the ground. However, the moon has only the thinnest of atmospheres, so there’s nothing to stop meteoroids from hitting the surface.

There are uncertainties over the rates at which meteoroids of different sizes hit the moon, but the Lunar Impact Monitoring Programme has recorded more than 300 flashes, including this one since 2005.

The lunar surface remains a high-priority target for exploration by the space agencies of several countries, including not just the U.S., but China and India. So a better understanding of the risks posed by the lunar environment could be crucial to the success of those missions.

In late 2013, China landed its first robotic rover, Yutu on the moon. Not long after landing, the robot suffered a failure, the exact nature of which has not been fully elucidated by the Chinese authorities.

China has also stated its aim of mounting the first human missions to the Moon since the U.S. Apollo program – something scientists think the country could achieve by the 2020s. Robinson said the team would soon begin analysing images from Lunar Reconnaissance Orbiter of the area on the Moon where a record-breaking impact flash was observed by Spanish astronomers in September last year.