Amateur astronomers' passion, vigilance and free time have led to some amazing discoveries. They include new comets, asteroids and supernovae and this month, an impact flash observed at Jupiter. Amateur astronomy can be as simple as learning the constellations to looking for new extrasolar planets.

Often, people will specialize in a particular celestial object or class of objects. For instance, a dedicated group of Jupiter-watchers routinely takes video of the gas giant through their telescopes, hoping to record rare flashes of light from the impacts of comets and asteroids. This month, at least eight of them struck pay dirt.

Jupiter's powerful gravitational pull can capture these objects and bend their orbital paths towards to the planet itself, where they ultimately collide with the atmosphere and self-annihilate. A flash occurs when the air heats the object to tens of thousands of degrees and causes it to explode. Flares are usually brief, lasting just a second or two, and most go undetected not only because of their brevity, but we rarely know in advance when and where to look.

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That's where amateurs come in. Unlike professionals, they have unlimited telescope time and can continually video-monitor Jupiter every clear night. When finished, they examine their take with a software tool called DeTeCt, which alerts the user to any unusual, short-lived events at Jupiter. If a suspect flare pops up, the observer studies the frames to make sure the flicker wasn't just a passing satellite or airplane. Then it's sent out to groups dedicated to monitoring Jupiter or individual Jupiter observers for confirmation. If one or more amateurs recorded the impact at the same time and location, it's a lock!

That's exactly what happened on Sept. 13 at 5:30:39 p.m. Central Time. José Luis Pereira of Brazil, along with at least seven other European amateurs, captured images of a bright point of light that appeared out of nowhere, rapidly brightened and then just as quickly faded out — there and gone in about two seconds. The flash appeared in Jupiter's equatorial region. Based on brightness and duration, it's estimated that an object between 65 and 165 feet (20-50 meters) across impacted the planet. Work is still ongoing to determine an accurate size.

That's really, really small compared to the enormity of Jupiter's globe, which spans 88,846 miles (142,984 km) along its equator. Several previous impacts left obvious dark "scars" that lingered in the atmosphere for days and weeks. I, along with many eager Jupiter-watchers, were hopeful we'd see an inky spot form in the wake of the collision, but to date, no one, not even the professionals, have discerned any changes in Jupiter's clouds at or near the entry point.

The Hubble Space Telescope took this photo of Shoemaker-Levy 9 on May 17, 1994, and recorded its train of 21 icy fragments stretched across 710,000 miles (1.1 million km). Jupiter's gravity caused the comet to crumble into pieces, each of which blossomed into a separate "mini-comet."  Before it shattered, SL-9 was about 1.1 miles (1.8 km) across. 
Contributed / NASA, ESA, and H. Weaver and E. Smith (STScI)
The Hubble Space Telescope took this photo of Shoemaker-Levy 9 on May 17, 1994, and recorded its train of 21 icy fragments stretched across 710,000 miles (1.1 million km). Jupiter's gravity caused the comet to crumble into pieces, each of which blossomed into a separate "mini-comet." Before it shattered, SL-9 was about 1.1 miles (1.8 km) across. Contributed / NASA, ESA, and H. Weaver and E. Smith (STScI)

The first confirmed impact at Jupiter hit it out of the park. That happened in July 1994 when 21 fragments of comet D/1993 Shoemaker-Levy 9 (SL-9) rained down on the spinning planet over a period of six days. After discovery in 1993, astronomers tracked the comet's motion and discovered it had passed so close to Jupiter in 1992 that the planet's gravity had broken it into pieces. Even more exciting, orbital calculations showed that the comet — piece by piece — would soon collide with its nemesis!

Dark spots mark impact sites in Jupiter's southern hemisphere where fragments of Comet Shoemaker-Levy 9 crashed into the planet and burned up. Contributed / Hubble Space Telescope Comet Team and NASA
Dark spots mark impact sites in Jupiter's southern hemisphere where fragments of Comet Shoemaker-Levy 9 crashed into the planet and burned up. Contributed / Hubble Space Telescope Comet Team and NASA

Some of the fragments were quite large. The biggest, called "G", may have been about 1,000 feet (0.3 km) across. It and several others left prominent dark impact scars that were visible in even smaller telescopes. The sight of these sooty wounds remains the most remarkable thing I've ever seen in my telescope.

Since SL-9 eight additional impacts have been seen or recorded, most by amateurs. Asteroid and comet strikes likely happen all the time, but many are missed due to lack of coverage, poor weather and simply because they're too small to produce bright enough flashes to see at Earth. Yet however few we see, each reminds us of Earth's vulnerability. Cosmic impacts have affected life's progress (witness the demise of the dinosaurs) in the past and will do so again in the future.

"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.