June 3rd a comet or asteroid hit Jupiter. This would normally create a disturbance in the gas-giant's atmosphere, but it seems as if the planet has just swallowed it!

On June 3rd, 2010, something hit Jupiter. A comet or asteroid descended from the black of space, struck the planet's cloudtops, and disintegrated, producing a flash of light so bright it was visible in backyard telescopes on Earth. Soon, observers around the world were training their optics on the impact site, waiting to monitor the cindery cloud of debris which always seems to accompany a strike of this kind.

They're still waiting.

"It's as if Jupiter just swallowed the thing whole," says Anthony Wesley of Australia, one of two amateur astronomers who recorded the initial flash. The other, Christopher Go of the Philippines, says "it was thrilling to see the impact, but the absence of any visible debris has got us scratching our head

Indeed, it is a bit of a puzzle. "We've seen things hit Jupiter before," says planetary scientist Glenn Orton of JPL, "and the flash of impact has always been followed by some kind of debris."

For instance, when fragments of Comet Shoemaker-Levy 9 hit Jupiter in 1994, each major flash observed by NASA's Galileo spacecraft produced a "bruise," a murky mixture of incinerated comet dust and chemically altered Jovian gas twisting and swirling among the native clouds. Just last year, in July 2009, Wesley discovered a similar mark thought to be debris from a rogue asteroid crashing into the planet.

So where is the debris this time?

A possibility offered by some observers is that the flash wasn't an impact at all. Maybe Go and Wesley witnessed a giant Jovian lightning bolt.

"I consider that very, very unlikely," says Orton. "NASA spacecraft have seen lightning on Jupiter many times before, but only on the planet's nightside. This dayside event would have to be unimaginably more powerful than any previous bolt we've seen. Even Jupiter doesn't produce lightning that big."

Nor could it be a flash of lightning in Earth's atmosphere fortuitously happening in front of Jupiter. Simultaneous observations of the same flash from widely-spaced observatories in Australia and the Philippines rule that out. For the same reason, it couldn't be, say, a terrestrial meteor or any other phenomenon in the atmosphere of Earth.

In short, the flash really happened at Jupiter.

Curiously, the impactor struck right in the middle of Jupiter's South Equatorial Belt (SEB), one of the two broad stripes that girdle the planet. This is "curious" because the SEB itself vanished earlier this year. Orton has proposed that the missing belt still exists, it's just temporarily hidden underneath some high-altitude cirrus clouds.

Astronomers around the world suspected that something significant must have hit the giant planet to unleash a flash of energy bright enough to be seen here on Earth, about 770 million kilometres away. But they didn’t know how just how big it was or how deeply it had penetrated into the atmosphere. Over the past two weeks there have been ongoing searches for the “black-eye” pattern of a deep direct hit like those left by former impactors.

The sharp vision and ultraviolet sensitivity of the Wide Field Camera 3 aboard the NASA/ESA Hubble Space Telescope were used to seek out any trace evidence of the aftermath of the cosmic collision. Images taken on 7 June — just over three days after the flash was sighted — show no sign of debris above Jupiter’s cloud tops. This means that the object didn’t descend beneath the clouds and explode as a fireball. If it had done, then dark sooty blast debris would have been ejected and would have rained down onto the clouds.

Instead the flash is thought to have come from a giant meteor burning up high above Jupiter’s cloud tops, which did not plunge deep enough into the atmosphere to explode and leave behind any telltale cloud of debris, as seen in previous Jupiter collisions.

“The cloud tops and the impact site would have appeared dark in the ultraviolet and visible images due to debris from an explosion,” says team member Heidi Hammel of the Space Science Institute in Boulder, Colorado, USA. "We can see no feature that has those distinguishing characteristics in the known vicinity of the impact, suggesting there was no major explosion and no ‘fireball’.”

Dark smudges marred Jupiter’s atmosphere when a series of fragments of Comet Shoemaker-Levy 9 hit Jupiter in July 1994. A similar phenomenon occurred in July 2009 when a suspected asteroid slammed into Jupiter. The latest intruder is estimated to be only a fraction of the size of these previous impactors and is thought to have been a meteor.

Sources: NASA and the ESA Hubble homepage