© Anthony Wesley, Broken Hill, Australia
A color composite image of the June 3rd Jupiter impact flash.
In a paper published today in the Astrophysical Journal Letters, a group of professional and amateur astronomers announced that Jupiter is getting hit surprisingly often by small asteroids, lighting up the giant planet's atmosphere with frequent fireballs.

"Jupiter is a big gravitational vacuum cleaner," says co-author and JPL astronomer Glenn Orton. "It is clear now that relatively small objects left over from the formation of the solar system 4.5 billion years ago still hit Jupiter frequently."

The impacts are bright enough to see through backyard telescopes on Earth. Indeed, amateur astronomers were the first to detect them, recording two fireballs in 2010 alone - one on June 3rd and another on August 20th.

Professional astronomers at NASA and elsewhere have followed up on the amateur observations, hoping to learn more about the impacting bodies. According to today's Letter, first-authored by Ricardo Hueso of the Universidad del País Vasco in Spain, the June 3rd fireball was caused by an object some 10 meters in diameter. When it hit Jupiter, the impact released about one thousand million million (10^15) Joules of energy. For comparison, that's five to ten times less energy than the "Tunguska event" of 1908, when a meteoroid exploded in Earth's atmosphere and leveled millions of trees in a remote area of Russia. Scientists continue to analyze the Aug. 20th fireball, but think it was comparable in scale to the June 3rd event.

© Aoki Kazuo
The Aug. 20th fireball recorded by Aoki Kazuo of Tokyo, Japan.
Before amateurs spotted these fireballs, scientists were unaware collisions so small could be observed. The first hint of their easy visibility came in July 2009 when Anthony Wesley, an amateur astronomer from Australia, discovered a dark spot on Jupiter. It was clearly the swirling debris of an impact event that he had only just missed. Next time, however, his luck would improve. On June 3, 2010, he caught a fireball in action.

"I was watching real-time video from my telescope when I saw a 2.5-second-long flash of light near the edge of Jupiter's disk," says Wesley. "It was clear to me straight away it had to be an event on Jupiter."

Another amateur astronomer, Christopher Go of the Philippines, confirmed that the flash also appeared in his recordings. Professional astronomers, alerted by email, looked for signs of the impact in images from larger telescopes, including NASA's Hubble Space Telescope, the European Southern Observatory's Very Large Telescope in Chile, and Gemini Observatory telescopes in Hawaii and Chile. Scientists saw no thermal disruptions or typical chemical signatures of debris, which allowed them to put a limit on the size of the object.