Twenty years ago, Jupiter took one for the team as it was pummeled by fragments of Comet Shoemaker-Levy 9
Wednesday, July 16, 2014, 1:41 PM - On July 16, 1994, the people of Earth were treated to a very rare and spectacular sight, as nearly two dozen icy fragments from Comet Shoemaker-Levy 9 plunged into the atmosphere of Jupiter, producing dark scars that persisted for months. This event not only gave us a frightening look at the power of these impacts, but also revealed how our 'big brother' planet has been protecting us for billions of years.
At the time of the impacts, astronomers from around the world were keenly focused on the event. Every telescope that could spare the time, including the Hubble Space Telescope, was pointed at Jupiter to capture as much information about the comet and the planet - before, during and after the comet hit. One of the best parts about this was that it wasn't just going to be one impact. Even as the comet was discovered, in 1993, it was found to be a 'train' of objects flying through space rather than one large object, as it had been torn apart by Jupiter's gravity over time, strung out into what they called a 'string of pearls'.
"Shoemaker-Levy 9 on 1994-05-17" by NASA, ESA, and H. Weaver and E. Smith (STScI) - http://hubblesite.org/newscenter/archive/releases/1994/26/image/c/ (direct link). Licensed under Public domain via Wikimedia Commons.
This infrared image of Jupiter shows the incredibly bright flash of light that resulted from the impact of fragment G from the comet, one of the larger fragments from Shoemaker-Levy 9, estimated at roughly 1 kilometre across.
While this image - a composite from the Hubble Space Telescope - shows off the dark scars created by these impacts.
The video below, from Discovery TV, talks about the comet, the impacts, and even what would happen if there was such an impact here on Earth:
This is, apparently, a service that Jupiter has been providing for us for billions of years. The incredible gravity of this massive planet has been diverting asteroids and comets, sometimes capturing them and pulling them in for impacts, acting as a silent protector for Earth and the other planets of the inner solar system. There has even been more recent examples of this since Shoemaker-Levy 9 - four, in fact - as similar impacts were spotted. The Gemini North Telescope, on Hawaii's Mauna Kea volcano, captured one impact in Jupiter's southern hemisphere on July 22, 2009. Two amateur astronomers, Anthony Wesley and Christopher Go, independently spotted an impact in early June 2010, and then another was seen just over two months later, first reported by another amateur astronomer, Masayuki Tachikawa. The fourth happened on September 10, 2012, which was again spotted first by amateur astronomers (this time Dan Peterson from Wisconsin).
However, Jupiter's gravity is something of a double-edged sword, as the influence it exerts on objects can propel them in our direction as easily as it can divert them away.
From the New York Times, right after the June 2009 impact on Jupiter, Dr. Brian Marsden of the Harvard-Smithsonian Center for Astrophysics discussed one particular case where Jupiter's 'help' could have caused quite a bit of trouble here.
Take, for example, Comet Lexell, named after the Swedish astronomer Anders Lexell. In 1770 it whizzed only a million miles from the Earth, missing us by a cosmic whisker, Dr. Marsden said. That comet had come streaking in from the outer solar system three years earlier and passed close to Jupiter, which diverted it into a new orbit and straight toward Earth. The comet made two passes around the Sun and in 1779 again passed very close to Jupiter, which then threw it back out of the solar system.
“It was as if Jupiter aimed at us and missed,” said Dr. Marsden, who complained that the comet would never have come anywhere near the Earth if Jupiter hadn't thrown it at us in the first place.
So, that's only one near-miss for us (one million miles, or 1.6 million kms, is only a little over 4 times the distance to the Moon) in the past 250 years, compared to five hits that Jupiter has taken just in the past 20 years, and at least Jupiter got rid of Lexell fairly shortly thereafter. Still, it only takes one poorly timed encounter with the massive planet to send something tumbling our way. Hopefully we can continue to enjoy Jupiter's 'protector' role for many years to come.