Shoemaker–Levy 9 hits Jupiter
21 comet fragments slammed into Jupiter over a week, leaving Earth-sized scars — the first planetary impact ever witnessed.
The moment of Shoemaker–Levy 9 hits Jupiter: https://nowsky.org/e/sl9-jupiter — opens at the right place and time, replayable, no login.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
From 16 to 22 July 1994, twenty-one fragments of the shattered comet Shoemaker–Levy 9 slammed one after another into Jupiter's southern hemisphere at 210,000 km/h. It was the first collision between solar-system bodies that humanity had predicted in advance and watched as it happened. The comet had been discovered in March 1993 by Carolyn and Eugene Shoemaker and David Levy at Palomar.
When found it was already a "string of pearls", torn apart by Jupiter's tides during a pass within 20,000 km of the planet in July 1992. Orbit calculations pinned the impacts sixteen months ahead to the hour. The impact sites were on the far side, but Jupiter's ten-hour rotation brought the scars into view within minutes.
The largest fragment, G, struck with the energy of six trillion tonnes of TNT — six hundred times the world's nuclear arsenal — and left a dark scar wider than Earth. The Galileo spacecraft, en route to Jupiter, saw the flashes directly, and Hubble tracked the scars spreading and fading over the following months.
It was vivid proof that Jupiter's gravity sweeps up comets falling toward the inner solar system — the "vacuum cleaner" of the planets. The event triggered serious near-Earth-object surveys worldwide and is the starting point of planetary defence, the line that leads to DART and Hera. Move the orbit view to Jupiter in July 1994 to see the arrangement at the time of impact.
Why it matters
SL9 was the first real-time evidence that impacts are not a thing of the past. Lunar craters and the dinosaur-extinction impact were known, but after watching scars larger than Earth appear in front of them, the US Congress in 1998 directed NASA to find 90 percent of near-Earth objects larger than 1 km.
The impacts opened a cross-section of Jupiter's atmosphere. Material dredged up by the plunging fragments revealed sulphur, ammonia and hydrogen sulphide, and the speed of the shock waves spreading through the air gave the structure of the stratosphere — data hard to obtain without a probe.
It was also a case of amateurs contributing to professional science. The scars were visible in small telescopes, and records from amateurs worldwide were used to string together their evolution over time. The Jupiter impact marks of 2009 and 2010 were likewise first spotted by amateurs.
How to watch
The scars had dispersed in Jupiter's winds by the end of 1994. Jupiter today shows no trace, but the brown belts and the Great Red Spot visible in a small telescope give a sense of the southern latitude band (44° S) where the scars formed.
Follow Jupiter in the orbit view and move the time to 16 July 1994 to reproduce the arrangement of Jupiter, Earth and the Sun at the time of impact. The fragments themselves are not in the orbit data and are not drawn.
Select Jupiter in the sky view for its altitude, bearing and rise time from your location. At magnitude −2 it is among the brightest planets in the night sky, and binoculars show the four Galilean moons.
See this for yourself — replay the moment in the orbit view
Background
The comet is thought to have been captured by Jupiter in the 1970s and to have orbited the planet with a two-year period. The 1992 pass fell inside the Roche limit and broke the nucleus, whose original diameter is estimated at 1.5–2 km.
There was scepticism about the forecast — some predicted the fragments were too small and would be swallowed by Jupiter's atmosphere without a trace. The dark spot Hubble photographed when fragment A hit ended the debate.
On 19 July 2009 the Australian amateur Anthony Wesley found a fresh impact scar on Jupiter, and in June 2010 an impact flash was filmed live. Jupiter is estimated to be struck thousands of times more often than Earth.
FAQ
How many fragments were there?
Twenty-one, striking in sequence from 16 to 22 July 1994; the largest was fragment G.
Were the impacts seen directly from Earth?
The sites were on the far side, but Jupiter's ten-hour rotation brought the scars into view quickly, and the Galileo spacecraft saw the flashes directly.
How big were the scars?
The scar from fragment G was wider than Earth; all had dispersed in the winds by the end of 1994.
What does it have to do with planetary defence?
It showed impacts still happen, which launched serious near-Earth-object surveys and led to the DART and Hera missions.
Related
- DART — humanity moves an asteroid
- Hera arrives at Didymos
- Hubble Space Telescope launch
- Jupiter — object index
- Hubble Space Telescope — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.