# Jupiter gets rocked by an impact again

By Phil Plait | September 17, 2012 10:00 am

By now you’ve probably heard that on September 10, Jupiter got whacked by an asteroid or comet again. It was seen directly by amateur astronomer Dan Peterson, and video was taken by George Hall, who kindly posted a fairly awesome a four-second clip on Flickr:

I tweeted about this shortly after it happened (I was returning from the UK and didn’t have time to write about it here), and got a lot of people wondering how big the flash was. On the video it looks like it’s thousands of kilometers across! But that’s not real; the size you see is due mostly to the optics of the telescope and video camera Hall used (plus other factors like atmospheric blurring). Given the brightness I guessed the object was a kilometer or so across, but I heard a radio interview the other day (I missed the guest’s name; sorry) where an astronomer said it may have been only 10 meters across – the size of a large truck.

I was surprised at first when he said that, but then realized something critical: Jupiter is a lot bigger than Earth.

The energy released by an asteroid upon impact depends primarily on two things: how big it is and how fast it’s moving. For a given asteroid, that means it’ll explode with far more energy if it hits Jupiter, because Jupiter’s gravity is much stronger than Earth’s, and pulls the rock in faster. Simply because of this, impacts on Jupiter can be greater than 20 times more energetic than on Earth. There are other factors (like orbital speeds, what direction the asteroid was moving, and so on), but in general and pound for pound Jupiter impacts are bigger than on Earth.

A 10-meter rock hitting the Earth will release roughly as much energy as a 0.1 megaton bomb, whereas on Jupiter that same rock will release about 2 megatons. A rock twice that size will have 8 times the mass (volume increases with the cube of the radius) so even if it were 20 meters across, the explosion could’ve been in the 15 – 20 megaton range, which is starting to get to the size of the largest nuclear weapons ever detonated on Earth.

So yeah, that’s a lot of power. It doesn’t take a big rock to make a bright flash when you’ve got Jupiter pulling the strings.

And the big planet gets hit a lot. The last one seen was on August 20, 2010, and it got whacked in June 2010 as well as in July 2009 (not to mention the ferocious series of impacts in 1994 from comet Shoemaker-Levy 9). That’s pretty close to one biggish impact seen per year, and remember we only see half of Jupiter at a time, and it’s not observed constantly! So the real rate is probably far higher.

I also got a lot of people asking why we call it an impact when Jupiter has no solid surface. That’s because the rock will still explode as it rams through Jupiter’s dense atmosphere; I wrote an explanation of this for an earlier impact. The 1908 Tunguska event here on Earth was an air blast, as well.

It’s amazing we can see these planetary at all, but that’s due to the digital revolution: amateur astronomers take video of Jupiter and other objects to maximize the number of frames they get, which they then can combine into amazing images. A nice side-effect from this is the collection of rapidly-taken data providing long coverage of the planets, which means significantly increasing the odds of seeing something like this. That is precisely why we’re seeing more impacts now than ever before. They’ve always been happening, we’re just a whole lot better at seeing them.

Which is a sobering thought. The Universe is worth investigating, if only for our own self-interest.

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1. By Jove that’s amazing footage! 8)

Brief but still. Wow. ðŸ˜®

Wish we had a spacecraft orbiting there now – wonder if Juno was watching en route?

I also got a lot of people asking why we call it an impact when Jupiter has no solid surface.

Wonders if these same individuals talk of “hitting” the water if they dive off the high board or a cliff or something too? ðŸ˜‰

2. Aaron Riley

Are there any pictures or videos of impacts on Saturn, Uranus or Neptune?

3. Ryan the Biologist

So, based on the numbers you provided, Phil, the 10-meter object would have exploded with only 2 megatons in Jupiter’s atmosphere? Looking at the video, where you see a boom roughly the diameter of planet Earth, even if that flash is being exaggerated, I just can’t see it being only 10-meters.

Were a 2-megaton bomb set off on the Earth’s surface, I’ve got doubts you’d be able to even see it from the distance of Jupiter, much less see a flash that encompasses the entire planet, optical effects/atmospheric blurring or not! Granted I’m not an expert in these matters, but that just seems impossible to me.

I can’t help but think your estimate of 1km diameter is more accurate. The energy released by something like that on Earth would be in the 9000-10,000-megaton range, and so would be in the 180,000-200,000 megaton range on Jupiter. This seems much more capable of causing that kind of flash, at least to me, but like I said, I’m no expert.

4. Daniel J. Andrews

Rather remarkable a small handful of people have captured impacts on Jupiter, but a few hundred million people with digital cameras haven’t yet captured a UFO here on earth.

5. Ryan,
If you pause the video during the explosion, you can see that the there are some rings etc around the brightest spot. These are parts of an Airy disk, typical when a telescope is observing something below its resolution limit: http://en.wikipedia.org/wiki/Airy_disk, so the actual impact could have been essentially a point source.

Also, George doesn’t mention his exposure time, but it might be as long as 1/60s or 1/30s , so it could be integrating quite a bit of light into each frame. The fact that it isn’t saturating the imager completely ( ie, bleeding into nearby pixels) is a give-away that it really isn’t *that* bright.

6. Ryan the Biologist

Bah, my previous numbers were off by 1 order of magnitude too small. But still, my point stands. I just can’t imagine the object being only 10-meters across.

Plus, the frequency seems odd. The 5-10m objects hit Earth about once a year. With an object the size of Jupiter, those frequencies should be considerably more common. I understand that we aren’t always watching Jupiter, but it is observed often enough that you would think these types of big bangs would occur much more frequently than the 1 per year we seem to be observing them, if they really were only 10m across.

7. Magrathea

@Ryan the B

The problem with 1km objects is that they would get deeper atmospheric penetration and then most likely leave a blemish or mark, so there is indeed an upper limit to how deep the object can plow. I don’t think it’s larger than 10-20 meters but heh, I’m a toxicologist working on back of the envelope calculations

8. ” I understand that we arenâ€™t always watching Jupiter, but it is observed often enough that you would think these types of big bangs would occur much more frequently than the 1 per year we seem to be observing them, if they really were only 10m across.”

I think you are overestimating just how often Jupiter is observed. There are a lot of eyes and cameras pointed to the sky, but there is hardly 24/7 observation of Jupiter. I was imaging Jupiter on Sunday morning basically non-stop from 3am till 7am. Many of the elite imagers aren’t like me on a typical imaging night. Many of them check periodically for pockets of good seeing and that time may be a small fraction of the time they have their scope pointing to the skies.

If you take Phil’s “pretty close to one biggish impact seen per year”, you can immediately double that because we can’t see Jupiter’s far side. You can probably double that again because there is about 1/2 of the year where Jupiter is either getting close to the sun or is only above the horizon in daylight and it is not observed. Right there you have about 4 impacts per year….I think you can *AT LEAST* double that because hours observed/total hours available to observe Jupiter is probably pretty low.

I would guestimate that Jupiter gets hit at least 10 times a year which shows an observable impact, and it wouldn’t surprise me if it wasn’t in the range of 20-40.

What surprises me about this latest impact is that it seems to be a lot brighter than the previously recorded events, and it *still* did not leave an impact mark. I really would like to know just how bright the imapact was that left the scar back in 2009….it must have been a doozy!

9. Ryan the Biologist

OK, let’s just take this list of observed objects from the JPL’s NEO program:

http://neo.jpl.nasa.gov/risk/

In 2013 alone, there are 10 objects that would have a near 100% chance of hitting the Earth if it were the size of Jupiter, (based on the width per sigma of each object in Earth radii) and all of them are roughly 10m across or larger.

Now factor in all the objects that didn’t make this list because there is no chance of them hitting an Earth-sized object, but would hit a Jupiter-sized object.

Now factor in Jupiter’s immense gravitational pull on these objects.

Now factor in Jupiter’s place in the outer solar system where large rocks are more frequent.

Personally, I can’t imagine Jupiter only getting hit by 20-40 of the small 10m objects each year. It’s just too darn big. Even if we only counted Jupiter’s facing compared to the Earth (not including gravity or position), Jupiter should statistically see ~123 times more 10m objects than the Earth, and the Earth sees about 1 per year. I’m just not seeing it.

10. Pete Jackson

@Ryan the Biologist: Here are my â€˜back of the envelopeâ€™ calculations. Assume a 1 Mt nuke has a fireball of 2 km diameter. Assuming that the fireball has the same effective temperature as the Sun (diameter 1.4 million km), then the nuke puts out (2/1400000)**2 as much light as the sun, i.e. 2 x 10**-12. So if the sun has an apparent magnitude -26.9 as seen from the Earth, then the nuke would have an apparent magnitude [m1-m2 = 2.5 log L2/L1] of -26.9 + 29.2 = +2.3 if it were at the sunâ€™s distance (1 AU) from Earth. Now, Jupiter is 5.2 AU away on average, so the apparent magnitude as seen from Jupiter would be 2.5 log 5.2**2 or 3.6 magnitudes fainter, or +5.9, which is similar to the brightness of Jupiterâ€™s Galilean satellites. That is probably also about the brightness we see in the video.

A megaton is 4.2 x 10**15 joules. Assuming an impact speed v of 45000 m/sec, the mass of a one megaton impactor can be calculated from KE = Â½ mv**2, and which gives a mass of 4.2 million kg. or 42000 tons. Assuming a density of 2 tons per cubic meter, we get a volume of 21000 cubic meters, giving a diameter of 34 meters.

Given all the approximations in the calculations, I would consider this to be consistent with BAâ€™s figures.

11. David W

Shouldn’t the extra mass of Jupiter have an effect other than additional velocity? Gravity always goes both ways, and in addition to the asteroid hitting Jupiter, Jupiter is hitting the asteroid. As an analogy, think of a motorcycle hitting a minivan head on vs hitting a tractor trailer head on. A smaller body’s own momentum will change more after an impact, which means less of the energy of the collision is redirected back on the object. A larger body’s momentum is changed much less, which means more of the collision’s energy is redirected back on the object. So, a bigger explosion.

12. Ryan the Biologist

@Pete- I’m not sure that it is necessarily true that the ~6000K temps of a 1MT bomb will extend as far as 2km, but if all of your calculations are correct, it would mean that a mere 1MT bomb would be visible to the naked eye from the average distance to Jupiter. From the distance to the Sun, it would be quite visible, even in an urban area. Does that sound right to you?

Looking at this another way, the amount of visible radiation that would reach you at 150,000,000km from the blast is ~10^-13 the amount that would reach you from 50km away (far enough to be safe from the explosion and thermal effects). I just can’t imagine being able to see that.

Now, I have another comment in moderation due to the link I included, but to summarize it, I basically said that 10m objects should statistically hit Jupiter ~123 times more often than the Earth on the basis of size alone, not even counting its gravitational pull. Maths:

(pi*71,0000^2)/(pi*6,400^2) ~=~ 123 times the Earth’s target area

These objects hit the Earth about once a year, so objects that big hitting Jupiter should be an event that occurs multiple times per week, at least once per 3 days. Even counting that we are not always watching and that half the planet is out of sight, that is a lot more impacts than we’ve been observing. This, to me, can only mean that this object was considerably larger than 10m across, and closer to the 1km that Phil originally estimated, for it to be this bright.

These objects are already traveling at a very high speed; will the planetary attraction in the last days really have a significant effect on the overall momentum?

14. kevbo

All these “I’m not a expert” comments… What does Richard Hoagland say about it?

15. Tony

Isn’t the outer solar system also a lot sparser than the inner solar system? Both less primordial material out there, and it gets constantly swept by the huge gas giants, ejecting them from the solar system or sending them in. That could undo some of the size and gravity advantage Jupiter has in impact frequency.

16. @2. Aaron Riley asked : “Are there any pictures or videos of impacts on Saturn, Uranus or Neptune?”

Not as far as I’m aware – those gas giants are much further off – twice as far away for Saturn double that again for Ouranos and more again for Neptune – and therefore have much smaller apparent diameters esp. those last two.

Indeed, I cannot recall hearing about people capturing *any* impact on any planet other than Jupiter . (Please let me know that I’m mistaken here if I’m wrong – would be awesome if I am!)

However, we *have* captured images of meteorite impacts on the Moon such as the “Lunar meteor impact on video” post linked to my name here that the BA posted back in 2008 on March 19th at 8:30 AM.

17. Tom

Jupiter is like the magnet on the vacuum cleaner that grabs the staples, pins and paperclips before they get sucked into the hose. Jupiter is grabbing the flotsam and jetsam of the solar system before it reaches us.

18. Just amazing!!!
I’m a bit surprised that the flash was so short, though. You’d think that something that big would leave a glowing trail of hot gas. For instance, in a nuclear explosion on Earth, though the nuclear reaction itself is over in a split second, the fireball continues to glow for many seconds as it radiates all that energy that it absorbed.

19. @17 Tom: I’m not sure what the current consensus is among astronomers, but I’ve heard it hypothesized that one of the prerequisites for extrasolar life would be (along with the usual “Goldilocks zone”, liquid water etc stuff) a huge Jovian world, not too far out, to absorb bolides and keep the hypothetical biosphere from getting disrupted too often by impacts.

@Messier#16: Maybe you mean any planet other than earth?

21. @Joseph G #17
I’ve heard that too, though I’m not sure I buy it. On Earth, life flourished after mass extinctions, with plants and animals rapidly evolving to fill newly-vacated niches. The Cambrian Explosion is one example. The ascent of mammals after the K-T extinction (and to a lesser extent the PETM extinction) are a few others. I think a bolide impact now and then is good for a life-bearing planet. It may be that Jupiter’s holding us back…

22. reidh

I dare say Tom. That the rate for overall impacts around the whole solar system is way above what those who say that an earth impact is only once every 100,000 or more years. I like the amazing Randi would like to bet a million dollars that the earth has a major impact within 25 years.

23. @21 The Math Skeptic: Yeah, there’s the whole “evolutionary pump” idea, too. I guess it’s hard to say. There’s probably some sort of optimum time period between mass extinction events to maximize the rate of change while allowing for some some ecological equilibrium to be restored in between. Hard to say where we fit on that scale.

@22 reidh: Depends what you mean by “impact”? For instance, at shallow angles especially, it’s possible for surprisingly large stony objects to explode high in the atmosphere, when a similarly sized metallic object at a different anglemight easily make it to the ground (see the Tunguska event). And stuff impacts Earth (or our atmosphere) every day, it’s just that most of it is tiny. It’d be more accurate to say that the Earth gets hit by an object of X size every Y hours, with Y getting greater as the cube of X. Or something like that. ðŸ˜›

24. KAE

I saw this video when it first came out and immediately thought it was a fake because it hadn’t been mentioned on BA. Thanks for the update!
I originally read the last line as “The Universe is worth investigating, if only for *it’s* own self-interest.” – Either version is true I suppose. ðŸ˜‰

25. Matt B.

“…which means significantly increasing the odds of seeing something like this.”

And yet we still have no solid UFO evidence. I wonder why.! (Note: I use an exclamation point after other sentential punctuation as a sarcasm mark.)

26. Joseph G

@25 Matt: Heh, we really do need a new punctuation mark for that. It’d keep so many from getting caught in a sarchasm*.

*The gulf of understanding between the ironic statement and the writer’s actual intent.

27. Nigel Depledge

Ryan the biologist (12) said:

@Pete- Iâ€™m not sure that it is necessarily true that the ~6000K temps of a 1MT bomb will extend as far as 2km, but if all of your calculations are correct, it would mean that a mere 1MT bomb would be visible to the naked eye from the average distance to Jupiter.

Wait, can you see the Galilean satellites with the naked eye?

While it is technically true that mag +6 is (roughly) the limit of what a human eye can see under ideal seeing conditions, you forget that the impact flash will be on top of the fourth-brightest object in the sky.

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