Comet Shoemaker-Levy 9

Hubble Telescope Image of the Comet Shoemaker-Levy 9 before collision with Jupiter

Image Credit:

H.A. Weaver, T. E. Smith (Space Telescope Science Institute), and NASA

 

In 1993, Comet Shoemaker-Levy 9 was discovered by Eugene and Caroline Shoemaker and David Levy in photographs taken of Jupiter. Sometime before its discovery, it passed within the Roche limit of Jupiter and ripped apart by tidal forces from Jupiter's gravity and broke up into 23 individual pieces.

 

Because of its discovery and by following the trajectories of the pieces, the orbits of the pieces could be calculated accurately. From observations, it was determined that the comet would impact Jupiter in July of 1994. This was the first impact that could be accurately predicted and be observed safely as it occured on another planet. The impact of Comet Shoemaker-Levy 9 was going to televised worldwide as this was before the invention of the internet.

 

Over the course of six days, the 23 different pieces of the comet fell on Jupiter. Unfortunately, the impact sites were on the side of Jupiter away from Earth, but luckily on the side that was rotating towards Earth. We might not have seen the impacts directly, but we could see the aftermath on the surface of Jupiter as well as plumes that could be seen over the horizon, if the pieces were large enough.

 

Galileo images of the impact sites

Dept of Physics and Astronomy, University of Tennessee at Knoxville

 

Image of Jupiter with 21 of the fragments

Image Credit:

NASA

MIRAC2 103 micron (one-millionth of a meter) Image of one of the impact

Image Credit:

MIRAC2/NASA/Harvard

 

Watching a comet fall onto Jupiter told us a lot about impacts in the Earth's distant past. It also helped us learn more about Jupiter's atmosphere, which will be addressed in the next post. Early in our history, Earth was continuously bombarded by asteroids and comets, which helped build our planet. In fact, it is believed that the majority of the water on our surface came from the impact of comets with Earth. However, we are not prone to impacts today because there are less asteroids and comets and because Jupiter captures many with its immense gravity. If there was no Jupiter and no large planets, we would be more likely to be impacted by comets and asteroids. If Comet Shoemaker-Levy 9 had hit us, life today might not exist, or at least the kind of life we live today. We should be thankful for Jupiter to vacuum up the majority of the small bodies wandering our Solar System, protecting our planet from catastrophe.

Jupiter

Jupiter: Note the Great Red Spot to the lower right and the shadow of Europa on the lower left

Image is from the Cassini Spacecraft

 

Jupiter is by far the largest planet in the solar system in terms of mass and size. However, it is still less then 99% of the Sun's mass. It has a semi-major axis distance of 5.2 AU which gives it an orbital period of about 11 years. It rotates once on its axis every 10 hours, making it one of the fastest rotating bodies in the Solar System.

Previously, we had talked about Galileo and how he had discovered the first moons around a planet other than our Earth. This discovery helped prove the geocentric theory of the solar system was incorrect (Objects can orbit something other than the Earth). Galileo originally wanted to call them the Medician moons after his sponsors, the Medicis, but it was eventually agreed to name them the Galilean moons and to name them after four of the lovers of Zeus; Io, Europa, Ganymede, and Callisto.

Saturn is no the only planet with rings. In fact, all four Jovian planets have a ring system, though not quite as magnificent as Saturn's rings. Jupiter's rings are not as extensive as the rings around Saturn and were not discovered until 1979 by Voyager 1.

Jupiter is also home to the largest storm in the Solar System. The Great Red Spot is at least 183 years old and may be older. It may have been first observed in 1665, but it isn't clear if it was observed again until 1831. It is between 24,000 to 40,000 km east-west and 12,000 to 14,000 km north-south, making it capable of holding 2 to 3 Earths.

Jupiter also shares its orbit with a two groups of asteroids known as the Greek and Trojan asteroids. These asteroids were discussed previously here. These asteroids, much like the Amor asteroids, will never impact Jupiter. However, Jupiter's gravity is strong enough to capture both asteroids and comets. Many of Jupiter's moons are possibly captured asteroids, and back in 1994, Comet Shoemaker-Levy 9 entered Jupiter's gravitational field, broke up, and collided with Jupiter.

Jupiter has a very thick atmosphere, as the majority of its volume are the many layers of gas. There is a rocky core, approximately Earth-sized, but is only a small fraction of the entire diameter of Jupiter. The combination of its large volume and the majority of the composition of the planet give Jupiter the largest mass of all the planets, but also a low density, just a little bit above that of water.