July 21, 2009 -- If you live in the southern hemisphere of Jupiter, you probably had a rough day Sunday.
Something -- possibly a comet or meteoroid undetected by earthlings -- went slamming into the planet, was swallowed up in its thick atmosphere of ammonia and methane, and left the "scar" you see in the image opposite from NASA's Infrared Telescope Facility atop Mauna Kea in Hawaii.
It was first spotted by an amateur Australian astronomer, Anthony Wesley, who realized he was seeing something unusual, and reported it to NASA in the United States.
"I suspect the impact scar itself should remain visible for a few days, maybe a week," said Leigh Fletcher, a scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., who monitored the aftermath of the impact.
The mark is roughly the size of Earth, Fletcher said. Having missed the actual impact, astrophysycists say they may never be able to know the size of the object that hit Jupiter.
"We were extremely lucky to be seeing Jupiter at exactly the right time, the right hour, the right side of Jupiter to witness the event. We couldn't have planned it better," said Glenn Orton of the Jet Propulsion Lab.
Jupiter will heal. In the 4.6 billion years since the solar system formed, its powerful gravity has drawn in a disproportionate share of the debris that wanders through space between planets. Since Jupiter is mostly a mix of thick gases, it will absorb this latest impact, much as a lake settles down after you toss a stone in the water.
Some scientists, including Donald Brownlee and Douglas Ward in their book "Rare Earth," argue that Jupiter's presence has done a lot to allow the rise and endurance of life on Earth.
Click here for our slide show, 'Photos from the Final Frontier.'
Even though Jupiter is half a billion miles away, on average, its powerful gravity does Earth a favor. It diverts a fair amount of the random stuff -- comets, asteroids, meteoroids and the like -- that might otherwise have hit Earth.
Jupiter Hit by Comet?
"Jupiter is 318 times more massive than Earth, and it exerts enormous gravitational influence," they write. "Its gravitational interactions very efficiently scatter bodies that approach it, and it has largely cleaned out stray bodies from a large volume of the solar system."
This was especially true 4 billion years ago, when the solar system was young, and a lot more crowded. Back then, the rise of life may well have been delayed by such violent interruptions. But they became less and less frequent, so scientists say when life took hold, it stuck.
"The very fact that we have a beautiful planet just right for intelligent life may be something we owe to Jupiter," Fletcher said.
Earth still takes a major hit from something more than, say, 10 kilometers (6.2 miles) in diameter -- once every 100 million years or so -- and some of the past impacts are believed to have wiped out much of the life on the planet at the time.
NASA has a program under way to scan the sky for so-called Near Earth Objects that could still hit Earth, and various schemes -- plus a few Hollywood films like "Armageddon" and "Deep Impact" -- have been hatched to divert anything that comes this way, if we have warning time.
But Jupiter has done Earth a favor. If not for its presence, say Brownlee and Ward, Earth might have been hit much more often.
Fifteen years ago this month, earthly telescopes saw a comet, Shoemaker-Levy 9, break up and go plowing into Jupiter's atmosphere, leaving a string of dark spots in the cloud tops. If there are any Jovians (not very likely), they must feel rather put-upon.