杏吧原创

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

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

The International Astronomical Union鈥檚 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鈥檚 Gareth Williams posted a description of , 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鈥檚 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鈥檚 not without precedent 鈥� 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鈥檚 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聽hours after announcing the discovery of the new moon 2015 HP116, the MPC issued a . 鈥淭hese things do exist, this just isn鈥檛 one of them unfortunately,鈥� says Williams.

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

Earth鈥檚 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. 鈥淔or some reason, it didn鈥檛 show up in the checks,鈥� 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鈥檚 observations than Gaia normally is, which could account for the confusion.

If the story sounds familiar, it鈥檚 because we鈥檝e 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鈥檚 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.