The Moon isn't the only traveling buddy keeping the Earth company on its journey around the Sun. Occasionally an asteroid will get caught in a gravitational dance with our home for a few years or so, before disappearing into space again. Last year astronomers discovered one of the most stable of these "quasi-satellites", and now they've peered closer to get a better understanding of what it is and where it might have come from.

Measuring less than 100 m (330 ft) wide, asteroid 2016 HO3 was first spotted last year by Pan-STARRS 1, a telescope used to keep an eye on near-Earth objects (NEOs) that might one day pose a threat to the planet. Although 2016 HO3 technically orbits the Sun and not the Earth, our planet's gravity has just enough sway over the little space rock to keep it from drifting too close or too far away and cause it to loop around us in a dance that's estimated to have been going on for close to 100 years – and will continue for several centuries still to come.

That means that HO3 is the most stable quasi-satellite of the five that have been discovered so far. It never gets closer than 38 times the distance between the Earth and the Moon, but never drifts further than 100 lunar distances away either.

But not everything about HO3 was immediately apparent from that initial observation. In fact, there was a chance that our companion was nothing more than a piece of space junk, like a discarded rocket booster. To find answers to some questions about the quasi-satellite, astronomers from the University of Arizona (UA) aimed the Large Binocular Telescope at the space rock earlier this year.

"In an effort to constrain its rotation period and surface composition, we observed 2016 HO3 on April 14 and 18 with the Large Binocular Telescope and the Discovery Channel Telescope," says Vishnu Reddy, a UA astronomer on the project. "The derived rotation period and the spectrum of emitted light are not uncommon among small NEOs, suggesting that 2016 HO3 is a natural object of similar provenance to other small NEOs."

The observations found that HO3 rotates once every 28 minutes, and its light signature suggests it's made up of materials similar to other asteroids, meaning it's most likely a leftover fragment from the formation of the Solar System. That, coupled with its close proximity to Earth, makes it a prime target for exploratory and mining missions, like NASA's ambitious Asteroid Redirect MIssion, which is aiming to drag a space rock into orbit around the Moon for closer study.

Reddy and colleagues presented their findings at the 49th annual Division for Planetary Sciences meeting in Utah this week.

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