That's no moon! Spacecraft mistaken for new natural satellite

Something looking awfully like a moon was detected in the sky today

For 13 hours today, Earth had a new moon – or so we thought. Now astronomers have realised that an apparent small asteroid orbiting our planet is actually the European Space Agency's Gaia space telescope.

The International Astronomical Union's Minor Planet Centre (MPC) in Cambridge, Massachusetts, keeps records of all the tiny space rocks in the sky and publishes new observations from around the world. This morning, MPC's Gareth Williams posted a description of 2015 HP116, seemingly an asteroid about a metre across that was spotted in a geocentric orbit by the Pan-STARRS telescope in Maui, Hawaii, last week.

Williams's analysis of the orbit suggested the object would remain bound to the Earth-moon system between October 2014 and March 2019, making it a temporary moon of our planet. That's not without precedent – simulations suggest hundreds of tiny moons could be orbiting Earth. One, called 2006 RH120, was spotted in orbit before drifting off a year later.

Near miss

There's just one problem. It turns out this object is actually Gaia, the European Space Agency telescope currently mapping a million stars in the Milky Way. Just 13 hours after announcing the discovery of the new moon 2015 HP116, the MPC issued a retraction. "These things do exist, this just isn't one of them unfortunately," says Williams.

The mysterious, temporary moon has turned out to be the Gaia space telescope (Image: ESA, image by C.Carreau)

Earth's orbital neighbourhood is littered with all kinds of space junk, from defunct satellites to leftover rocket boosters, so the MPC runs a number of checks to filter out sightings of artificial objects, but this time they failed. "For some reason, it didn't show up in the checks," says Williams. But after posting the notice on the MPC website, he reran the calculations, and out popped Gaia. The object was dimmer in Pan-STARRS's observations than Gaia normally is, which could account for the confusion.

If the story sounds familiar, it's because we've been here before. In 2007, the MPC issued a warning that an object called 2007 VN84 was heading for a near-miss with Earth. It actually turned out to be ESA's Rosetta spacecraft on a fly-by past Earth, building up enough speed for its historic rendezvous with comet 67P/Churyumov-Gerasimenko, which occurred last year.

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