July 16, 2026

A Comet Broke Into a String of Pearls and Died on Jupiter

On July 16, 1994, the first shard of a comet ripped apart by Jupiter's own gravity slammed into the solar system's largest planet. Earth had a year's warning and a view it could barely use. The collision no one could see directly is the reason we now know how to nudge an asteroid off its course.

It began with a photograph that looked wrong. In March 1993, the astronomers Carolyn and Eugene Shoemaker and David Levy were reviewing plates from the 18-inch Schmidt telescope at Palomar when they found a comet that did not look like a comet. It was a dense, linear bar, "squashed," with no single bright core, and Brian Marsden at the International Astronomical Union noted it sat "some 4 deg from Jupiter." Within two months, orbit calculations delivered two stunners: the comet was not orbiting the Sun, it was orbiting Jupiter, and it was going to hit. The impact odds climbed from about 50% to near certainty in a single week that May.

The comet had been one body until July 1992, when it skimmed within 1.3 Jupiter radii of the planet's center and Jupiter's gravity tore it into 21 chunks, some over half a mile wide, strung out like glowing pearls on a thread. Astronomers labeled the fragments A through W and had a year to point every telescope they could at the largest planet in the solar system.

The first fragment, A, struck on July 16, 1994. There was a catch. The impacts were coming on Jupiter's far side, invisible from Earth. But NASA's Galileo spacecraft, already en route to Jupiter after its 1989 launch, sat 238 million kilometers out with a direct line of sight, and it filmed the night-side fireballs. From Earth orbit, the freshly repaired Hubble Space Telescope caught the plumes rising over Jupiter's limb and, as the planet turned, the great dark scars they left behind.

Over the following week, fragment after fragment hammered the gas giant. NASA estimates the combined force at roughly 300 million atomic bombs. Plumes climbed 1,200 to 1,900 miles high and heated the atmosphere to tens of thousands of degrees. Dark scars, thousands of kilometers across, wrapped around the planet and lingered for months before Jupiter's winds erased them.

The event was, as the astronomer Heidi Hammel put it, "a sort of punch in the gut." If Jupiter could get hit, so could Earth. Shoemaker-Levy 9 is what turned a fringe worry into NASA's planetary defense program. On September 26, 2022, that program's DART spacecraft deliberately crashed into a small asteroid called Dimorphos and shortened its orbit by 32 minutes, the first time humanity purposely changed the motion of a celestial body.

The comet's lead discoverer never saw that day. Eugene Shoemaker, the geologist who founded the field of astrogeology and trained the Apollo astronauts to read the Moon, was barred from flying there himself by Addison's disease. He died in 1997 in a car crash in the Australian outback while hunting impact craters. Two years later, some of his ashes were flown to the Moon aboard Lunar Prospector, and he remains the only person whose remains lie there. The man who found the comet that scarred Jupiter finally reached the world he most wanted to walk.

Also on this day

• In 1945, at 5:30 a.m. in the New Mexico desert, the Trinity test detonated the world's first atomic bomb and began the nuclear age; three weeks later a uranium bomb fell on Hiroshima.
• On the night of July 16-17, 1918, Bolsheviks under Yakov Yurovsky murdered Tsar Nicholas II and his family in a Yekaterinburg basement, ending three centuries of Romanov rule.
• In 1935, the world's first parking meter, the Park-O-Meter, went in on an Oklahoma City corner at a nickel an hour, denounced by critics as an un-American tax on cars.
• In 1973, in live Senate testimony, aide Alexander Butterfield revealed that a secret system had been recording every conversation in Richard Nixon's Oval Office.