Astronomers surprised by asteroid trailing Mars that's a 'dead-ringer' for the moon

An unusual asteroid trailing behind Mars has a lot in common with our moon.

Amanda Kooser
Freelance writer Amanda C. Kooser covers gadgets and tech news with a twist for CNET. When not wallowing in weird gear and iPad apps for cats, she can be found tinkering with her 1956 DeSoto.
Amanda Kooser
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An asteroid that follows Mars around sure looks an awful lot like parts of the moon.


Trojan asteroids share the same orbital path as certain planets, either leading ahead of the planet or trailing behind. Jupiter is famous for its many Trojans, but Mars has a few of them as well. One of these Martian companions -- asteroid (101429) 1998 VF31 -- could be a stunt double for our own moon.

A team led by researchers at the Armagh Observatory and Planetarium (AOP) in Northern Ireland took a close look at asteroid 101429's composition and drew some fascinating connections to our lunar neighbor in a study set for the January 2021 issue of the journal Icarus and published online August 1.

The planetary scientists used the X-shooter spectrograph on the European Southern Observatory's Very Large Telescope in Chile to look at how sunlight reflects off the asteroid's surface. They analyzed this data and compared it with other know space bodies.

Earlier observations of the asteroid, which trails behind Mars on the same orbital path, led the team to think it might have a similar composition to a common type of meteorite. "To their surprise, they found that the best spectral match was not with other small bodies but with our nearest neighbor, the moon," said lead author Apostolos Christou in an AOP statement on Tuesday.

Study co-author Galin Borisov with AOP described asteroid 101429 as a "dead-ringer for parts of the moon where there is exposed bedrock such as crater interiors and mountains."

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So what is the asteroid's origin story? The researchers proposed several options. It may actually be similar to common meteorites after all, but it's just been worn down by its space adventures over time. 

It could possibly be a chunk of the moon. Christou described the debris-filled early solar system back when the moon was getting smacked by large asteroids: "A shard from such a collision could have reached the orbit of Mars when the planet was still forming and was trapped in its Trojan clouds," he said. 

A third idea is that the asteroid was knocked off of Mars through a similar impact scenario. Christou described this concept as "perhaps more likely." 

The asteroid remains an open-ended mystery, but if the moon ever needs a stand-in for a few days, we know who to call.