Astronomers have captured video evidence of a collision between Jupiter and a small celestial object, likely a comet or asteroid. Though it looks like a small blip of light, the resulting explosion was unusually powerful. As Phil Plait of Bad Astronomy reports, the collision occurred on March 17, but confirmation of the event only emerged this week. An amateur Austrian astronomer used a 20cm telescope to chronicle the unexpected event, but it could've been some kind of visual artefact.
A second video taken at the same time with a 28cm telescope in Ireland has now confirmed it as an actual impact.
Plait says that the asteroid or comet wasn't very large, probably measuring only a several dozen metres in diameter. But when it comes to celestial collisions, it's not the size of the impactor that counts. Owing to Jupiter's huge mass, the object must've have been accelerating rapidly, releasing a tremendous amount of kinetic energy on impact. Plait explains:
On average (and ignoring orbital velocity), an object will hit Jupiter with roughly five times the velocity it hits Earth, so the impact energy is 25 times as high. The asteroid that burned up over Chelyabinsk, Russia, in 2013 was 19 meters across, and it exploded with the energy of 500,000 tons of TNT.
Now multiply that by 25, and you can see how it doesn't take all that big a rock to hit Jupiter for us to be able to see it from Earth.
Incidentally, at these huge speeds, hitting the atmosphere is like slamming into a wall. A lot of people get understandably confused how an asteroid can explode due to air, but the pressures involved as it rams through the atmosphere at these speeds are ridiculously huge. The air and rock heat up, the rock starts to fall apart, and each chunk then gets hot, and so on, creating a very rapid cascade that releases the energy of motion in just a second or two.
The result, says Plait, was a "very, very big bang".
This is not the first time we've seen Jupiter get struck by an object. Back in 1994 it was hit by cometary fragments from Shoemaker-Levy 9, and again in 2010 and 2012. Plait says the gas giant gets hit by something big enough to see from Earth about once a year.