Dubbed the most spectacular show of the century, 15 years ago today the first piece of Comet Shoemaker–Levy 9 slammed into the hydrogen-rich cloud tops of Jupiter, heralding a six-day long assault on the giant planet as cometary fragments rained down, causing unprecedented scars that marked Jupiter’s atmosphere for up to fourteen months afterwards. Despite the passing of time, the immense power of this catastrophic event has not diminished in our memories, and is a stark reminder of the danger posed to Earth by rocky or icy bodies from outer space.

When the comet-discovering trio of husband and wife team Eugene and Carolyn Shoemaker and their colleague David Levy first spotted the comet on 24 March 1993, they couldn’t quite believe what they were seeing. They’d been imaging the sky with the 0.4-metre Schmidt Telescope at the famous Palomar Observatory in California when they discovered a bizarre looking object. It was a comet for sure, but it was stretched out, and along its 50-arcsecond length were bright knots, like pearls on a string. Furthermore, it appeared to be close to Jupiter, and by backtracking with precise orbital calculations it soon became apparent that the comet had been captured by Jupiter in 1930, entering a loose orbit around the planet. This in itself was unusual – comets orbit the Sun on highly elliptical orbits, they don’t orbit planets!

The calculations also showed that in 1992 the comet had passed extremely close to Jupiter, inside the orbit of innermost moon Metis and just 21,000 kilometres above the planet’s cloud tops (1.3 Jovian radii from the centre of the planet), which is well inside what we call the Roche limit. Beyond this invisible boundary the tidal forces of Jupiter’s (or any planet’s) gravity is sufficiently strong to rip a comet, or even a moon, to shreds.

Comet Shoemaker–Levy 9 had been torn into at least 21 fragments, each one developing its own coma and tail, like a chain of mini-comets. That wasn’t the most exciting news, not by far – taking the orbital calculations into the future, it was shown that on their next orbit of Jupiter, the fragments’ closest point to the planet (called perijove) would actually fall inside the atmosphere of Jupiter. In other words, they were going to hit.

D-day was marked on the calendar for 16 July 1994. With advance warning, every major observatory on Earth, and thousands of amateurs, planned ahead to watch the first ever major impact event in the Solar System observable to humans. No one really new what to expect. Some thought that the cometary fragments, the largest of which was two kilometres across and most of the others being less than a kilometre in diameter, would be swallowed up with little fanfare by the 142,984-kilometre wide Jupiter. Others thought we might see the fireballs, but they would leave little mark on the planet. In the end, Shoemaker–Levy 9 certainly didn’t disappoint, exceeding most people’s expectations in the most dramatic way possible.

Impact!
The first fragment, A (they were unimaginatively, but understandably, named after letters of the alphabet), struck Jupiter at 7:13pm BST (18:13 UT) on 16 July. As bad luck would have it, every fragment ended up hitting Jupiter on the far side of the planet, ranging between three and nine degrees beyond the eastern limb. But NASA’s Galileo probe, still around 240 million kilometres from Jupiter and en route to the planet for rendezvous in 1995, had a direct line of sight, and it wasn’t long before the impact regions rotated into view for observers on Earth. As fragments peppered Jupiter, incandescent fireballs rose above the limb, and as the impact sites spun into view dark, giant bruises were evident in the atmosphere, some as large as Earth itself.

The best observed impact was that of fragment R, which hit on the morning of 21 July. Infrared observations made at Palomar Observatory and the newly-built (at the time) Keck Telescope on Mauna Kea in Hawaii detected three flashes as the fragment smashed through the atmosphere. The first flash coincided with its fiery journey into the depths of Jupiter, lasting 40 seconds. This was followed by a second blast seen at a wavelength of 4.5 micrometres, lasting for three minutes, and a third flash at a shorter wavelength of 2.5 micrometres (coinciding with the methane absorption band) that died away after 30 seconds. It’s though that the final blast came from the heat of the rising fireball, spreading up to 200 kilometres across in the atmosphere and climbing upwards at a rate of 13 kilometres per second through the wake left by the entry of the fragment, cooling as it rose.

The Hubble Space Telescope imaged the resulting debris plume towering 3,200 kilometres above the limb of the planet, which was incredibly bright at infrared wavelengths as debris dropped back into the atmosphere, shock-heating the surrounding gas even more as temperatures peaked above 10,000 degrees Celsius. After each impact, seismic waves rang through Jupiter’s atmosphere, moving at 450 kilometres per second. After fragment K hit, it even sparked an aurora in the magnetosphere above it as the upward moving blast wave accelerated charged particles through Jupiter’s immense magnetic field.

The largest of the impacts belonged to fragment G on 18 July, which left a bruise 12,000 kilometres across, unleashing the equivalent of six million megatons of TNT on poor, battered Jupiter. That’s 600 times more powerful than the world’s entire nuclear arsenal. It just shows that no matter how destructive we humans can be, mother nature can always put us in the shade.

Battered and bruised
Those bruises lingered on Jupiter for many months afterwards, and were even more noticeable to telescopes on Earth than the Great Red Spot. They all had the same general structure – a brown central spot surrounded by a black ring expanding at 0.5 kilometres per second, with a crescent-shaped smudge to the south-west where debris spilled back down on the planet (indicating the angle at which the fragments smashed into Jupiter). And what was in that debris anyway? Its darkness came from the carbonaceous materials within it; in other words, soot. Astronomers measured, spectroscopically, the signature of molecular (diatomic) sulphur – only the second ever detection of this in space – carbon disulphide, hydrogen sulphide, increased concentrations of ammonia (already present in Jupiter’s atmosphere), and iron, magnesium and silicon from the comet itself.

Much of the material, particularly the sulphur, was dredged up from deep within Jupiter, giving us a unique view into the materials hundreds of kilometres below the cloud tops. Oddly enough, oxygen-bearing compounds were noticeable by their absence, and whilst some water vapour was detected, perhaps from the cometary fragments, there wasn’t as much as there was expected to be. Jupiter has a layer of water vapour within it, but the shortfall measured either means that this layer is deeper than we thought, or that the fragments didn’t penetrate as deep into Jupiter was we think they did.

The final fragment, W, impacted on 22 July, bringing to an end a frantic and brutal six-day period. We have never observed anything like it, before or since, but its legacy is a constant reminder that, sometimes, planets find themselves in the wrong place at the wrong time. We’ve seen it happen to Jupiter. We know it has happened before on Earth. It’s only a matter of time before it happens again.