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Last July, an amateur astronomer noticed that a mysterious dark bruise about the size of the Earth had suddenly appeared on the surface of Jupiter. Within hours, amateurs and professionals alike were training their instruments on the great planet to work out what had happened.

The consensus was that Jupiter had been hit by a comet or asteroid. But the surprise was that it had happened so soon after the Shoemaker-Levy comet strike observed in 1994. The worry was that this strike must have important implications for the likelihood of future impacts.

Today, Agustin Sánchez-Lavega from the University of the Basque Country in Bilbao and pals, publish their analysis of the impact and how it changes the probabilities of future impacts. They say the impactor was probably an icy object about 1 kilometre in diameter which came either from a group of main belt asteroids called Hilda asteroids or from a group of comets called the Jupiter Family.

Estimating the likelihood of such impacts is hard for a gas giant like Jupiter because the events leave no long-lasting scars on the surface. Jupiter’s bruise has already faded away.

So astronomers have to rely on historical records. Before last year’s impact, astronomers knew only of the Shoemaker-Levy impact and a possible impact observed by the Italian astronomer Giovanni Cassini in 1640. Together with other evidence such as crater counts on Jupiter’s large moons and various theoretical calculations, astronomers guessed that Jupiter was liable to a strike perhaps as rarely as once in every 350 years.

Sánchez-Lavega and co say that last year’s strike significantly changes these numbers. Seeing two strikes in 15 years means that that Jupiter may be liable to be hit as often as once a decade. The reason we haven’t seen impacts before is simple: digital cameras and image processing techniques have only become easily available to amateurs in the last ten years. (Before that, even professionals often had to rely on hand drawn pictures of the planets.)

What Sánchez-Lavega and co do not address are the implications for the likelihood of Earth impacts, which is strange given the huge importance and public interest in such an event. The Shoemaker-Levy impact on Jupiter changed the way astronomers think about possible impacts and generated huge interest.

Clearly Jupiter is at greater threat of future impacts than Earth: it is bigger and more massive by far and so is bound to attract more hits. But it can also send bodies our way.

The current thinking is that a 1-kilometer object ought to hit Earth every 500,000 years or so. Needless to say, such an event would change our civilisation beyond recognition.

If last year’s impact on Jupiter increased the probability of another strike by an order of magnitude, by how much does it increases the probability of a strike on Earth? The public deserves an answer to this question and the fact that this team are silent on the matter is worrying.

Let’s hope Sánchez-Lavega and his colleagues are working on an answer as a matter of urgency.

Ref: arxiv.org/abs/1005.2312: The Impact Of A Large Object With Jupiter In July 2009

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