Extra ‘moon’ departs the Earth

Earth is saying goodbye this week to an extra moon that has been looping around us for seven years.

It is an asteroid that got caught up in Earth’s gravitational pull in 1999 and has been corkscrewing around us ever since, Nasa revealed yesterday.

The 20 yard wide space rock, called 2003 YN107, is too small to be seen without a powerful telescope. But experts have been keeping a close eye on its celestial dance. Nasa expert Paul Chodas says that on Saturday the asteroid dipped slightly closer to Earth than usual.

The close approach, at a distance of around two million miles, gave it the flip it needed to send it spiralling away into space again. But other asteroids are known that also become temporary extra moons – and YN107 will hook up with us once again in around 60 years time.

Astronomers call the objects “coorbitals” because they share Earth’s own orbit, going around the Sun in almost exactly one year. When one catches up with the Earth, or vice versa, the strange spiral dance begins and the asteroid slowly corkscrews around our planet.

Chodas said: “These asteroids are not truly captured by Earth’s gravity. But from our point of view, it looks like we have a new moon.” Chodas of NASA’s Near Earth Object Program at Pasadena, California, added: “We believe 2003 YN107 is one of a whole population of near-Earth asteroids that don’t just fly by Earth. They pause and corkscrew in our vicinity for years before moving along.”

Astronomers already know of at least four asteroids that perform the trick. The biggest, 2004 GU9, is around 200 yards wide and has been corkscrewing around Earth for 500 years.

Paul Sutherland

Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.
Paul Sutherland

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Paul Sutherland

I have been a professional journalist for nearly 40 years. I write regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy. I have also authored three books on astronomy and contributed to others.

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