Last year, in July, something smacked Jupiter. Hard.

It was discovered when an amateur astronomer found a black spot marring Jupiter’s cloud tops. Followup observations saw the spot glowing in the infrared, meaning it was hot, and therefore was not just a storm (which are common). For real and for sure, something impacted Jupiter and exploded – and I mean exploded, releasing the equivalent of hundreds of thousands of nuclear weapons.

But what was it?

A new paper just published indicates that it was an asteroid that hit Jupiter. However, since it wasn’t seen beforehand, how do we know?

Because we’ve seen this sort of thing before. In 1994, the big planet was hit repeatedly by the comet Shoemaker-Levy 9. The comet had broken up into dozens of pieces, including several chunks a kilometer or so across, and they slammed into Jupiter one after another over the course of a week. That was one of the most well-observed astronomical events in history; every telescope on the planet was focused on Jupiter at the time.

And telescopes off the planet were too: Hubble took a lot of data, and found two key differences between the 1994 and 2009 events.

One is that there was a halo of lighter debris around the comet impact points in 1994 (as seen in this image on the right), but no such halo around the 2009 impact site. That indicates that the impacting objects were different.

Also, Hubble observed Jupiter both times in the ultraviolet. Images like that show where small, lighter-weight particles fell from the impact because those particles absorb UV light, leaving dark spots. Even nearly two weeks later, dark spots on Jupiter were evident around the 1994 impact sites. Since comets have a lot of ices in them — what you might call frozen gases like ammonia, methane, and so on — that’s expected. The lighter particles floated around in Jupiter’s clouds, absorbing the UV for quite some time.

But observations from last year’s impact don’t show that behavior! As you can see in big the image above, the dark spot fades rapidly. That’s most likely due to the impact debris (made up of vaporized impactor plus material from Jupiter’s atmosphere that got heated and chemically altered) to sink beneath the clouds. That indicates the particles were heavier than the SL9 impact in 1994, pointing toward the impacting object being an asteroid, not a comet. This is also consistent with the lack of a halo as mentioned above; halos are also made by finer particles. The 2009 impact site lacking a halo means the particles were heavier, as you’d expect from an asteroid.

So even though we didn’t see the object before it hit, it left — haha! — a smoking gun pointing toward its origin.

And here’s a funny thing: this impact occurred 15 years to the week after the SL9 onslaught. I remember getting a lot of email asking me if that meant they were related, but if you think about it you’ll realize it has to be a coincidence: after all, why would Earth’s orbital period have to do with anything hitting Jupiter? And now we see that the object that hit was not comet-like, proving the point.

Asteroid and comet impacts are a real threat to us on Earth. Jupiter, being so much more massive than the Earth, is a bigger target; its gravity draws in more debris. By observing it we can get a better idea of just how much stuff is out there in the solar system, waiting to put the hurt on a planet, including our planet. Just in case you have any lingering doubts, astronomy is important. It is no exaggeration at all to say that learning about astronomy and astronomical events may very well save the human race one day.

Image credits: NASA, ESA, M. H. Wong (University of California, Berkeley), H. B. Hammel (Space Science Institute, Boulder, Colo.), I. de Pater (University of California, Berkeley), and the Jupiter Impact Team; HST Comet Team and NASA