Hot spot created by impact on Jupiter, taken by NASA's Infrared Telescope Facility in Hawaii. Picture credit, NASA. An Earth-sized hole on Jupiter! the email alerts, websites, and finally news channels were saying on Monday, July 20th. At Chabot, we were polled by at least two local news channels asking what had happened. So, what happened?

Evidently, the aftermath of some kind of collision on Jupiter was spotted by an amateur astronomer in Australia that Monday morning. He spotted a dark marking near the planet's South Pole, and alerted NASA. NASA in turn turned its large infrared telescope in Hawaii onto the scene of the crash.

There glowed the thermal footprint of the likely impact, the affected area roughly the size of the Earth. Had this impact taken place on Earth instead, the results would have been catastrophic. Fortunately this was Jupiter, half a billion miles away and large enough to absorb the impact without lasting effects. (And, owing to the fact that Jupiter is a gaseous planet with no solid surface, it would quickly heal from the trauma, not unlike that liquid-metal Terminator from the second movie of the same name.)

A significant event? Yes, in fact. But that's not all…

Rewind 15 years to July 20th, 1994, the middle of the week during which twenty-something fragments of the broken comet Shoemaker-Levy 9 were in fact colliding with Jupiter… An amazing coincidence? Yes; the two events likely have nothing to do with each other. So, then, a common event, if we're seeing two of them in the span of only 15 years? Well… not really.

When the string of fragments of Shoemaker-Levy 9 hailed down on Jupiter, it was the first time in history that humans had observed actual impacts on a Solar System body (other than perhaps the Sun–but as it turns out comets hitting that huge target are not uncommon). The Shoemaker-Levy 9 impacts, and the one on July 20th this year, left highly visible marks that lasted for days. The amateur astronomer who discovered the recent scar did so with a relatively small 14.5″ backyard telescope! So, if this sort of impact were a common event, even if the impacting comets or asteroids were never seen, the gashes they leave in Jupiter's atmosphere ought to be spotted from time to time.

Impacts—on Jupiter, Earth, and all the bodies of the Solar System—do occur, and the smaller the impacting object, the more frequently they happen. For a planet like Earth, on average a chunk of rock a few meters across enters our atmosphere about once a year, and often burns up completely or explodes before hitting the ground. A 50 meter object, again on average, is likely to strike Earth once in a century. A one-kilometer object impact averages every few hundred thousand years, and a multi-kilometer sized asteroid or comet similar to the one that wiped out the dinosaurs and which would cause global catastrophe—well, the last one of that size struck ground 65 million years ago.

As for Jupiter, being a larger target than Earth, having a much stronger gravitational pull, and being close to the asteroid belt—well, Jupiter's impact statistics should probably involve higher frequencies than Earth.
In fact, impacts like the one on July 20th are happy events for us; every time Jupiter is hit by a large object, that's one less object in the Solar System that could potentially hit the Earth in the future. So, on July 20th, Jupiter took another bullet for us.

Saturn's moon Epimetheus from the Cassini spacecraft.
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA
and APOD.

On the bus in Denali National Park a few years ago, I found myself sitting next a couple from the East Bay. If you’ve ever been on the Denali bus, you know that it’s a long ride and it was just a matter of time before we struck up a conversation. As often happens, we wound up talking about work and then about astronomy research. Both of them were very interested in the field but were unsure of where to find good information on the web. At the time, I hadn’t really thought about that and wasn’t much help.

Now that I’m writing for QUEST, I am much better suited to answer them. I spend a lot of time surfing the web for images and links to websites to provide the full details for readers who want to follow up on my posts. Over the course of a year or so, I’ve discovered quite a few resources and have settled on a few favorites. Of course, being a Berkeley and Cornell grad, I have a few biases…

First of all, it is common for a university astronomy department to organize a public outreach campaign. I won’t bother with the obvious disclaimers and instead will just say that two of my favorites are “Ask an Astronomer” at Cornell University and the Berkeley Center for Cosmological Physics.

These two sites are quite different. As the name implies, the Cornell site encourages questions and suggestions from readers. The content of the site is therefore governed by the public, covering a wide variety of topics in fairly brief, straightforward language. The Berkeley site is much more structured. They cover the history of cosmology and outline the history of our universe with all the appropriate links (scroll down to see the links). This provides a very detailed and organized explanation of a specific field of astronomy.

In addition to universities, there are quite a few NASA missions that maintain excellent public relations. Almost everyone knows the Hubble Space Telescope and Mars Rovers. Both sites are updated almost daily with galleries, discoveries, and recent news. NASA also has several other large missions at other wavelengths that are probably not as well known. Three examples are the Chandra X-ray observatory, the WMAP mission, and the Spitzer infrared observatory. Like the Hubble and Rover sites, these space-based observatories perform ground-breaking science and do an excellent job explaining their discoveries to the public.

Besides QUEST, there are also quite a few other excellent blogs out there. Each site has a different approach and finds its own balance between astronomy coverage, opinion, and discussion of general science. One of the most popular is the Bad Astro site–we even have a link on the right hand side of the QUEST blog web page. You can also check out About.com's top ten space and astronomy blogs.

Of course, one obvious place to learn about astronomy is from journalists. Two websites that do a very good job of covering the field are Space.com and New Scientist (some content requires subscription).

Finally, if you enjoy beautiful images of the sky, a great place to look is the “Astronomy Picture of the Day." This is where I got my image for today. If you look tomorrow you’re guaranteed to find something just as exciting!

Kyle S. Dawson is engaged in post-doctorate studies of distant supernovae and development of a proposed space-based telescope at Lawrence Berkeley National Laboratory.

latitude: 37.8768, longitude: -122.251