Is it Rare, or Not?

The planet Jupiter got smacked again by an object this past week.  Recall that in 1994, pieces of Comet Shoemaker-Levy 9 blasted their way into the Jovian cloud tops.  At the time, people called it a once-in-a-lifetime event. That is, they did until something else plowed into Jupiter on July 19, 2009.

In an amazing coincidence, a major research paper came out this week, explaining just what it was that impacted Jupiter in 2009.  In another coincidence to THAT coincidence, the same observer who first chronicled the 2009 impact — Australian amateur observer Anthony Wesley — also saw the one that occurred a few days ago.  Christopher Go of the Phillippines captured a video of the event that sent a flash of light out from the Jovian cloud tops. Both are posted at Spaceweather.com, along with frequent updates.

You might be tempted to ask, doesn’t it seem like Jupiter’s getting hit a lot more lately?  The answer is: probably not.  In fact, Jupiter maybe getting less whacked NOW than it was in the past, back when the solar system had a LOT more debris flying around in orbit around the Sun and amongst the planets.  The difference is that we have people watching Jupiter all the time — well-equipped and qualified amateurs — and they’re bound to see these things happening, simply as a consequence of having more people pay attention. And, Jupiter being what it is — massive, strong gravity, and surrounded by debris in various orbits — it’s bound to get whacked pretty frequently.  We just haven’t always had the chance to see it happen.

Think of it this way: if trees fall in the forest on an average of once a day and nobody’s there to see it happening, we could come to the conclusion that trees falling in the forest are rare things. But that would be only because we aren’t looking. It’s an erroneous assumption, of course.  And, once we start putting people in the forest to watch the trees fall, we’d then find out that they’re falling once a day.  But, that doesn’t mean that more trees are falling. It just means we’re watching more. It’s probably the same with Jupiter. We’re watching it more and catching more of these events that could be very commonplace out in the Jovian neighborhood.

Jupiter has a long history of sucking up and sweeping in debris (comets, asteroids, etc.) throughout the life of the solar system.  In the beginning, it probably saved Earth’s skin more than once, by intercepting larger bodies that could have whacked the young Earth and changed it into something that might not be the planet we know and love today.  It’s still picking off debris today, and we’re fortunate enough to now have the technology to see it happen.

Now, about that paper explaining the 2009 collision.  When the impact occurred last year, scientists immediately turned their attention to the aftermath of the collision. They used a whole host of observatories on (and orbiting above) Earth. They mapped the thermal changes in the impact site (in other words, the temperature changes) and searched out spectral evidence of chemical elements that were part of the impactor (that is, they examined the planet in different wavelengths of light to tease out the fingerprints of chemical elements given off during the collision).  It now turns out that the evidence points toward an asteroid plowing into the cloudtops of Jupiter.  Critical examination of the impact debris suggests that the asteroid came from a family of bodies called the Hildas — a secondary asteroid belt that orbits near Jupiter and has about 1,100 members. Hildas are mainly rocky bodies that may also contain some ice.

This past February, when I was out at NASA Jet Propulsion Laboratory for a meeting, I had dinner with an old friend — Padma Yanamandra-Fisher — who was part of the team working on the data analysis of the impact debris. We talked about the event and I asked her if the impact could have been an asteroid. At that time, she was just finishing analysis, but she thought that it sure looked like an asteroid had been the culprit in the collision.

What about the impactor last week?  What’s it made of? It’s too soon to tell. Scientists are busily studying the impact site. There’s not much of an impact cloud to study, but they can take infrared measurements of the zone where the event occurred and get some idea of what the incoming debris could have been.  As with all ongoing science, stay tuned!