30 years since the historic collision of comet Shoemaker-Levy 9 with Jupiter

Thirty years ago, Comet Shoemaker-Levy 9 collided with Jupiter, marking a historic moment in astronomy. This collision gave scientists a first-of-its-kind direct observation of an extraterrestrial collision within our Solar System, providing important information regarding Jupiter’s atmosphere and its role in protecting the inner planets from cosmic debris.

• Comet Shoemaker-Levy 9 was discovered on March 24, 1993, by Carolyn and Eugene Shoemaker and David Levy. It was dragged into Jupiter’s orbit in the 1920s. Jupiter’s strong gravitational forces caused the comet to split in 1992, resulting in a sequence of dramatic impacts that unleashed enormous energy and debris into Jupiter’s atmosphere.
• The impact, which occurred between July 16 and 22, 1994, left dark scars on Jupiter’s surface and revealed novel atmospheric chemicals like diatomic sulfur and carbon disulfide, offering important information about Jupiter’s atmospheric composition and dynamics.

Thirty years ago, Comet Shoemaker-Levy 9 (SL9) collided with Jupiter. This occurrence gave scientists and researchers a first-of-its-kind direct observation of an extraterrestrial collision within our Solar System, providing important information regarding Jupiter’s atmosphere and its role in protecting the inner planets from cosmic debris.

Astronomers Carolyn and Eugene Shoemaker, along with David Levy, found SL9 on March 24, 1993, using the Palomar Observatory’s 46 cm (18 inches) Schmidt telescope. Unlike ordinary comets that circle the Sun, SL9 was captured in orbit around Jupiter. Researchers inferred that the comet was pulled into Jupiter’s orbit by its gravitational force some 70 years ago around the 1920s.

Dr. Heidi Hammel who led visible-light observations of the comet with NASA’s Hubble Space Telescope and is currently serving as the Vice President of Science at the Association of Universities for Research in Astronomy (AURA), explained the unique nature of SL9, “Shoemaker-Levy 9 was a comet that was discovered by Eugene and Carolyn Shoemaker and David Levy, which is why it’s called Shoemaker-Levy 9. It was the ninth comet that the team had found. What was unusual about Shoemaker-Levy 9 was that it was in orbit around Jupiter.”

In July 1992, SL9 passed within Jupiter’s Roche limit which is the distance between one celestial body and another celestial body, held together by gravity where the second body will disintegrate because the first body’s tidal forces surpass the second body’s self-gravitation. This proximity caused the comet to fragment into multiple pieces, setting the stage for its eventual collision with Jupiter.

Between July 16 and 22, 21 SL9 fragments collided in the southern hemisphere of Jupiter at a speed of 60 km/s (50 miles/s). These collisions caused dark indentations in Jupiter’s atmosphere that became more evident than the Great Red Spot of Jupiter which lasted several months. Dr. Hammel explained the sequence of events, “It was a week-long occurrence. It took that long for all of the fragments of this fractured comet to strike Jupiter. It emitted a gigantic plume of material that extended for thousands of kilometres over Jupiter’s cloud tops. It took around 20 minutes to rise and then collapsed.”

The first impact from Fragment A took place on July 16, 1992 resulting in a fireball with a peak temperature of approximately 24 000 K. The plume from this fireball rose more than 3 000 km (1 864 miles) into Jupiter’s clouds, as witnessed by the Hubble Space Telescope. Over the next 6 days, 21 impacts were reported, the most devastating being from Fragment G, which released energy equivalent to 6 million megatons of TNT.

This event enabled astronomers to have a detailed study of Jupiter’s atmosphere. Spectroscopic studies identified new compounds such as diatomic sulfur (S₂) and carbon disulfide (CS₂) in Jupiter’s atmosphere for the first time. Heavy elements such as iron, magnesium and silicon were also identified in Jupiter’s atmosphere which provided a peek into the comet’s composition.

“The Hubble observations were fantastic,” Hammel said. “We saw massive explosions, we saw ripples in the atmosphere of Jupiter. We saw dark material processed by the intense heat of these explosions.”

Additionally, the collisions produced airwaves flowing at speeds of 450 m/s (1 476 ft/s) across Jupiter, and radio studies revealed a significant increase in continuum emission, most likely due to synchrotron radiation from relativistic electrons.

The dents in Jupiter’s atmosphere were noticeable for months. Spectroscopic studies revealed that ammonia and carbon disulfide stayed in the atmosphere for more than a year. Interestingly, air temperatures normalized faster at larger impact sites than at smaller ones.

Thirty years later, the Shoemaker-Levy 9 collision with Jupiter remains one of the most significant astronomical occurrences of the twentieth century. It offered a one-of-a-kind chance to investigate Jupiter’s atmospheric composition and behavior, as well as to emphasize the protective role of big planets in the Solar System. “It was just an amazing example of the power of collisions in our solar system,” Dr. Hammel stated.

This impact continues to influence knowledge of planetary science and extraterrestrial risks, emphasizing the active and occasionally violent nature of the Solar System.

Featured image credit: H. Hammel, MIT And NASA. Acquired on June 18, 1994