Another Giant Leap

July 19th was a good day for cosmic coincidence. Something hit Jupiter, 15 years after Comet Shoemaker-Levy 9 (S-L 9) left its famous string of impact scars across the giant planet. Coming the day before the 40th anniversary of the Apollo 11 landing, this felt like a sardonic wink at our halting progress. Though we are still for the most part stuck here on Earth, our knowledge of solar system events has been taking giant leaps.

Our awareness that big things can still hit planets exploded in the 1980s when we learned that an impact drastically re-directed evolution on Earth 65 million years ago. This helped start a serious movement to develop the capacity to find and deflect dangerous Earth-crossing objects, something the dinosaurs never achieved. If we needed a violent reminder to maintain vigilance, the S-L 9 impact provided one from a safe distance.

Unlike S-L 9, the July 2009 Jupiter impact came without warning. It tested our ability to respond quickly to a transient event whose photogenic and scientifically valuable effects would soon fade into the turbulent cauldron of Jupiter’s stormy atmosphere.

When sharp-eyed amateur Anthony Wesley reported his observation of a new dark spot on Jupiter, professionals paid attention (page 34). Once the word was out, many of Earth’s multitude of telescopes slewed toward Jupiter in a westward wave as night fell around the world and Jupiter rose into evening skies.

Professional astronomers, acting alone, might never have looked. They don’t generally monitor planets for changing phenomena. To justify the use of the biggest telescopes, they make proposals to look at specific things at specific times. Fortunately, Earth has a growing network of well-equipped and knowledgeable amateur observers, all instantaneously web-connected with one another and the professional community. 

After the S-L 9 impact, astronomers estimated that such events should occur with a frequency ranging from hundreds to thousands of years. So how lucky are we that another one was seen so soon? Are there more small objects out there than we thought, and what does this imply for Earth’s safety? Jupiter is a much bigger and more-massive target than Earth, and the population of objects out near Jupiter is quite different from that near Earth.

I don’t lose sleep over the impact threat. Rather, I find it encouraging that we are taking the first steps to understand and mitigate it. Our ability to pay attention to long-term global threats is going to be one of the qualities that determines whether humanity will be just a brief, colorful experiment in proto-intelligence, or something more.

Jupiter has surely been clobbered plenty of times while humans went about their daily business unaware of events elsewhere in our planetary system. Now, as observing technology has improved and become more widespread and the globe has become interconnected, we have crossed a threshold and entered an age when earthlings continuously monitor distant events (as also evidenced by amateur Frank Melillo’s discovery of a new bright spot on Venus). The global realm of intelligent activity has sprouted vigilant compound telescopic eyes and grown a central nervous system. Never again will something like this go unnoticed.

This article originally appeared in print in the November 2009 issue of Sky & Telescope. Subscribe to Sky & Telescope.