Tuesday, April 28, 2015

Gaia Spacecraft Mistaken for Earth's New Moon

Artist's impression of Gaia spacecraft: Credit: ESA–D. Ducros, 2013

Earth had a new moon on Monday for about 13 hours... or so we thought. On Monday morning, astronomer Gareth Williams from the International Astronomical Union's Minor Planet Centre (MPC) in Cambridge, Massachusetts, posted a description of 2015 HP116, seemingly an asteroid about a meter across that was spotted in a geocentric orbit by the Pan-STARRS telescope in Maui, Hawaii, last week. But the apparent near-Earth object orbiting our Earth ended up being something else. 

The small asteroid orbiting our planet is actually the European Space Agency's Gaia space telescope.

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.

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.

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.

Gaia is an ambitious mission to chart a three-dimensional map of our Galaxy, the Milky Way, in the process revealing the composition, formation and evolution of the Galaxy. Gaia will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group. This amounts to about 1 per cent of the Galactic stellar population.

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