This story was updated at 11 p.m. ET.
Two fireballs from collisions with Jupiter in June and August provided a great show for the skywatchers who spotted them, packing a punch and suggesting the gas giant could be in for frequent punishment.
In both instances, amateur astronomers using backyard telescopes were the first to detect two small objects that burned up in Jupiter's atmosphere. Since then, the skywatchers teamed up with professional astronomers to study the fireballs, which were likely caused by rogue asteroids or comets.
"Jupiter is a big gravitational vacuum cleaner," said Glenn Orton, co-author of a study of the fireballs that appears in the Sept. 9 edition of the Astrophysical Journal Letters. "It is clear now that relatively small objects remnants of the formation of the solar system 4.5 billion years ago still hit Jupiter frequently. Scientists are trying to figure out just how frequently." Orton is an astronomer at NASA's Jet Propulsion Laboratory, in Pasadena, Calif. [ Video: Fireballs Light Up Jupiter ]
Fireballs on Jupiter
The object that caused the June 3 fireball was determined to be 30 to 40 feet wide (8 to 13 meters). In fact, it was comparable in size to 2010 RF12, the second of two asteroids that flew by Earth Wednesday, and it slightly larger than the asteroid 2008 TC3, which burned up above Sudan two years ago.
The energy released by the object as it plunged into Jupiter's atmosphere was estimated at 1 quadrillion to 4 quadrillion joules (300 million to 1 billion watt-hours).
Analysis is continuing on the Aug. 20 fireball, but scientists said the size of that object was comparable to the June 3 invader.
Though each fireball packed a wallop, the energy of the June 3 crash was still five to 10 times less than from the meteor or comet that entered Earth's atmosphere in 1908 and burst over a remote part of Russia, an explosion known as the Tunguska event that knocked over tens of millions of trees.
The June 3 fireball was spotted by skywatchers Anthony Wesley in Australia and Christopher Go in Cebu, Philippines. The amateur astronomers worked with a team of pros led by researcher Ricardo Hueso, of the Universidad del Pa Vasco, in Bilbao, Spain, to determine the size of the fireball.
Skywatchers like Wesley and Go who are able to detect such small-size impacts can help make significant contributions to wider astronomical studies, Hueso said.
"The discovery of optical flashes produced by objects of this size helps scientists understand how many of these objects are out there and the role they played in the formation of our solar system," he explained.
Three days after Wesley and Go detected the fireball, Hueso and his colleagues looked for signs of the impact in high-resolution images from larger telescopes, including NASA's Hubble Space Telescope, Gemini Observatory telescopes in Hawaii and Chile, the Keck telescope in Hawaii, the NASA Infrared Telescope Facility in Hawaii, and the European Southern Observatory's Very Large Telescope in Chile.
The scientists analyzed the images for thermal disruptions and chemical signatures, and compared them with those seen in previous images of Jupiter impacts. They saw no signs of debris from the June 3 event, which allowed them to limit their estimate of the size of the impactor.
Rogue asteroids and comets
Based on the images, the astronomers were able to agree that the flash probably came from a small comet or asteroid that burned up in Jupiter's atmosphere. They estimated the impactor had a mass of about 1 million to 4 million pounds (500 to 2,000 metric tons) or about 100,000 times less massive than the object from a July 19, 2009, collision that created a bruise on Jupiter the size of the Pacific Ocean. Scientists now think that spectacular crash involved an asteroid about 1,600 feet (500 meters) wide.
The Aug. 20 fireball was first observed by Japanese amateur astronomer Masayaki Tachikawa and later confirmed by Aoki Kazuo and Masayuki Ishimaru.
That fireball flashed for about 1.5 seconds. The Keck telescope, which observed the impact site less than a day later, found no debris remnants.
Other assaults on Jupiter have included the 1994 incident in which the comet Shoemaker-Levy 9 broke into more than 20 pieces and pelted the gas giant repeatedly. At the time, astronomers estimated such impacts could occur on Jupiter every 50 to 250 years.
With the recent collisions occurring less than a year after the July 2009 incident, researchers are rethinking the estimates of the frequency of such impacts.
"It is interesting to note that whereas Earth gets smacked by a 10-meter-sized object about every 10 years on average, it looks as though Jupiter gets hit with the same-sized object a few times each month," said Don Yeomans, manager of the Near-Earth Object Program Office at JPL, who was not involved in the paper. "The Jupiter impact rate is still being refined, and studies like this one help to do just that."