Collisions of Comets with
other Bodies

Since the orbits of comets sometimes cross the orbits of other bodies in the Solar System, collisions may occur. At one time it was thought that a collision of a comet with the Earth would not be a serious matter. We now believe differently. In fact, there is rather strong evidence that the Earth was struck by a comet or small asteroid early in this century, with devastating results.

The Tunguska Event

On the morning of June 30, 1908, in a remote region of central Siberia, a great blue-white fireball brighter than the Sun streaked through the sky and exploded while still in the air with a blinding flash and intense pulse of heat. The explosion was heard 1000 km away, and it flattened trees radially 30 kilometers out from a central point in the Stony Tunguska River valley. The resulting pulse of air pressure circled the Earth twice, and astronomers observed for several nights afterwards a glowing red haze in the upper atmosphere, though they were not aware at the time of the cause. It is estimated that the explosion had the force of a 10-20 megaton hydrogen bomb and detonated in the atmosphere about 6-8 km above the surface, which would explain why no crater has ever been found.

The region was so remote that there were few witnesses and presumably little loss of life. As a result, news of the event filtered only slowly to the outside world. Because of the remoteness and the political turmoil of the early part of this century, it was only 1927 when a scientific expedition finally went to investigate the event. Though various fantastic theories have been proposed (the crash of an alien spaceship), the simplest explanation that is consistent with all the data is that the Earth was struck by the head of a small comet or a small rocky asteroid maybe 100 meters in diameter that exploded before striking the ground.

Comet Shoemaker-Levy 9 Encounters Jupiter

We now have direct observations of a cometary impact on another body in the Solar System. In July of 1994, fragments of Comet Shoemaker-Levy 9 impacted the planet Jupiter. The points of impact were not directly visible from Earth but could be observed by the Galileo spacecraft, and the impact regions were quickly brought into Earth view by the rapid rotation of Jupiter.

The Orbit of Shoemaker-Levy 9

Shoemaker-Levy 9 was a comet with a somewhat elliptical orbit that partially intersected that of Jupiter, and went nearly as far out as the orbit of Saturn. The following sequence of images shows the orbit of Shoemaker-Levy 9 and its location relative to Jupiter in 1990, 1992, and 1994 (when it collided with the giant planet). The final image shows what would be the present location of Shoemaker-Levy 9 had it not impacted Jupiter in 1994.
  1. Shoemaker-Levy 9 on July 16, 1990.

  2. Shoemaker-Levy 9 on July 16, 1992.

  3. Shoemaker-Levy 9 on July 16, 1994 (fragments begin to strike Jupiter).

  4. Where Shoemaker-Levy 9 would be today, if it had not collided with Jupiter.

The Impact of Fragments from Shoemaker-Levy 9

Here is an animation illustrating Shoemaker-Levy fragments colliding with Jupiter. The following 4 images were taken by the Galileo spacecraft of the impact of fragment W from the ill-fated Comet Shoemaker- Levy 9.

Galileo images: Fragment W of Comet Shoemaker-Levy 9 Strikes Jupiter

These four images were taken at intervals of 2 1/3 seconds on July 22, 1994, with Galileo at a range of 238 million kilometers. The first image shows no impact. In the next three images, a point of light appears, brightens, and then fades, seven seconds after the first picture. Only Galileo was able to directly see the impact sites; from the Earth, the collision site was on Jupiter's back side. It is believed that the point of light in this picture shows the effects of the comet fragment entering Jupiter's atmosphere and is not related to the subsequent explosion and fireball (Ref).

The following images are Earth-based observations of the impact sites after they have rotated into view from the Earth.

IR from Mauna Kea

Visible Wavelengths from Hubble

UV from Hubble

The left image (Ref) is taken from the NASA Infrared Telescope Facility, Mauna Kea, Hawaii. The impact due to Fragment Q is to the far right and the R Fragment impact site shows up very brightly to its left. Additional impact sites form a chain of spots behind R (more info). The middle figure (Ref) shows a Hubble Space Telescope color image of the impact sites for fragments G and Q2. The right figure (Ref) shows an ultraviolet image of Jupiter taken by the Wide Field Camera of NASA's Hubble Space Telescope. The spots are very dark in the ultraviolet because a large quantity of dust is being deposited high in Jupiter's stratosphere and the dust absorbs sunlight (more info).