Jupiter Scores Another Hit

Amateurs saw Jupiter get struck by something again on June 3.  Last year, an asteroid also hit the giant planet.  Good thing Jupiter caught it and not Earth.  The asteroid, believed to be about 500 meters across, left a scar as big across as the Pacific Ocean.  National Geographic and the BBC News have photos of the scar left by the 2009 impact.  Had something like that hit our planet, “it would be a catastrophe,” one astronomer said.  Yesterday’s impactor is unknown, since only the fireball flash was observed for a few seconds (a movie clip can be seen on the BBC News article), but astronomers will be monitoring Jupiter for clues in the aftermath.
    This is the third time humans have seen impacts on Jupiter: in 1994 (Comet Shoemaker-Levy 9), last year’s asteroid, and now the June 3 fireball.  Astronomers are puzzled, because they assumed impact events were rare.  Jupiter does have the largest gravity well of any planet and acts like a giant vacuum cleaner of the solar system.  Still, many objects lie outside its sphere of influence – or they can be accelerated by Jupiter outward like a slingshot.  A lesson in both articles was how little scientists know compared to what they thought they knew.  “It’s back to the drawing board on our understanding of the statistics of impacting bodies,” said Heidi Hammel of the Space Science Institute in Boulder, Colorado.
Update 06/18/2010: According to Space.com, the object observed by the amateurs is believed to have been a giant fireball or bolide – a meteor that burned up high in the atmosphere of Jupiter before it reached the cloud tops.  This explains why it did not create a visible scar days afterward as seen by Comet Shoemaker-Levy 9 fragments in 1994.  The statement was made by Heidi Hammel based on Hubble Space Telescope observations.

Does Jupiter absorb impacts that might hit Earth?  Assuredly.  Does it also fling some toward the Earth?  Probably.  Between those extremes is a vast playground of unknowns for theorists to dabble in models, most of which are probably wrong.  Are impacts rare on Jupiter?  No; it would be ridiculous to claim that; certainly Jupiter is pulling in stuff every day.  Observable impacts from Earth are rare – but that draws human beings and planet Earth into the equation.  Clarity is important.  The answer to a question like that depends on where you draw the line.  Impacts are not rare on Earth at all, if you draw the line to include micrometeoritic dust, that rains like manna from heaven all the time.  Every night the darkness is punctuated by meteors.  Occasionally some strike the ground or the oceans as meteorites.  Flashes on the less-massive moon are not that uncommon (05/21/2008).
    Certainly Jupiter, with 318 times the mass of Earth, is capable of drawing in much more material than Earth – assuming one knows the amount of impacting material available in its neighborhood, and each impactor’s position and momentum.  One must also factor in unknowns like the material coming in from the distant reaches of the solar system, where data are increasingly difficult to obtain, or from outside the solar system altogether.  Many of these factors scientists have little or no way of knowing.  Then there are observational unknowns, such as impacts hitting the far side of Jupiter, or those hitting Jupiter during the day, when we can’t see them.  Observing three impacts within 16 years is well within the probability range of a random distribution.  It’s nothing to be all that concerned about.  Announcing a model about which one can have any confidence, though is another matter.  Consider an attempt at gathering empirical data with a dust collecting instrument on a spacecraft.  If you capture dust particles on one rare and expensive flight to Saturn with such an instrument, how justified can you be in extrapolating that data over the entire azimuth of the solar system, the entire altitude of the solar system, and the entire lifetime of the solar system, for an entire range of particle sizes, compositions and velocities, without making numerous assumptions that are profoundly theory-laden and untestable?
    In the limit, planetary science becomes a version of the Stuff Happens Law.  Something happens, and planetologists tell us it happened.  That’s about as useful as Eyewitness Weather.  “Yes, ladies and gentlemen, the sun is up!  You can see outside your window, oops – I mean, it’s raining!  Yes, it is raining right now!”  Thank you very much.  No planetary scientists predicted or expected an impact on Jupiter.  How could they?  There are way too many particles, forces, and unobservables in play.  Even a 3-body problem is devilishly difficult to solve.  Something like this happens, the planetologists look stunned, and they rush to their drawing boards to try to understand it, because it didn’t fit their earlier statistics of impacting bodies.  Others spin it according to Finagle’s Second Law #2c, that it happened according to their pet theory.
    You notice the discovery was made by amateurs – citizen scientists, God bless them.  The only advantage the pros have is more money and better equipment.  They write the textbooks, they make a living at it, they get on TV once in awhile, they’re good at math, they get to go to conferences, they write papers (or have their grad students do it).  Once in awhile they discover something, some of them are nice people (and some aren’t), they can usually bring clarity to complex things and consider factors that many would leave out, but they don’t know everything.  They’re mortals like the rest of us.  They have biases, favorite foods, political affiliations, social preferences, hangups, and hangouts.  Don’t assume that the evident progress in data collection correlates with progress in understanding.  It should not be surprising when they have to go back to the drawing board when stuff happens in a flash, in n-body problems, amidst swirling clouds of unknowns.