The Time a Space Observatory Was Mistaken for a Second Moon

For 13 hours, the space observatory Gaia was misidentified as a natural satellite orbiting Earth

An artist's interpretation of the Gaia spacecraft -- mistaken for a small moon this week ESA

Last week, a tiny object was spotted by a telescope in Maui, Hawaii. It appeared to be about three-feet across and circling Earth in a geocentric orbit. Data about the object was sent to the International Astronomical Union’s Minor Planet Center (MPC), where all space rocks, including asteroids, comets and minor planets are tracked and identified. On Monday morning, Gareth Williams of the MPC published a description of the object, 2015 HP116.

For New Scientist, Jacob Aron reports:

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.

The new moon would have been classified with the likes of 3753 Cruithne, another natural satellite of Earth, and it was for 13 hours, if it wasn’t for one small detail. 2015 HP116 is no moon, it’s a space station observatory. It's the European Space Agency’s Gaia spacecraft, to be precise, at work cataloging about one billion stars in the Milky Way.

MPC quickly issued a retraction for 2015 HP116. The center usually is able to disregard all the man-made satellites and thousands of bits of space junk whizzing through the sky. But this time Gaia slipped through the filters. "For some reason, it didn't show up in the checks," Williams told New Scientist. When he was rerunning the calculations however, he realized that the object fit Gaia. Apparently Gaia showed up a little bit dimmer than usual in the Hawaiian Pan-STARRS telescope, which is what led researchers to think it could be a new moon.

A similar scenario played out in 2007 when the ESA’s Rosetta spacecraft, headed out to land on a comet, swooped close to the Earth in a gravity-assisted slingshot maneuver. The MPC at first thought it was an incoming asteroid and issued a warning. At least these minor blunders aren’t the other way around — mistaking an artificial satellite for a natural one is better than discounting a potential hazard as a man-made spacecraft.

"If it's going to hit, you need to work out when and where as quickly as possible," Mark Bailey, the director of Armagh Observatory in Northern Ireland told New Scientist during the Rosetta incident. "You don't want them sitting on the data wondering whether it's an asteroid or not."