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Evidence For 200-Year-Old Comet Impact On Neptune
astroengine writes "Astronomers using ESA's Herschel space observatory have spotted evidence of a cometary impact in Neptune's upper atmosphere (publication, PDF). Whereas impact craters on rocky planetary bodies can remain for billions of years, an impact in the dynamic atmospheres of gas giants aren't obvious, especially if long periods of time have elapsed. This ultimate 'cold case' tracked the unusual distribution of carbon monoxide in Neptune's stratosphere, a sure sign it was deposited there by an external source. Once they realized they were looking at a comet impact, researchers were able to deduce when the impact occurred: 200 years ago."
Note that this hypothesis is more plausible than it might seem at first glance since we've seen comets impact gas giants before. Most famously, in 1994 Shoemaker-Levy 9 was observed directly impacting on Jupiter http://en.wikipedia.org/wiki/Comet_Shoemaker-Levy_9. This also isn't the first time this sort of technique has been used to detect historic comet impacts. As TFA notes, this technique was previously used to show that a similar event likely occurred around 230 years ago on Saturn.
Although comets hit the outer planets frequently, this is due to a variety of issues including the large size of the planets and the exact orbit of Jupiter (which makes Jupiter very effective at clearing interplanetary debris). Thus, this sort of situation doesn't pose much of a risk for Earth. However, even a single such comet colliding with Earth would be an extinction level event. The asteroid that caused the Chicxulub crater in the Yucatan is generally estimated to be around 10 km diameter and most comets are generally larger than that (Halley's Comet has a mean diameter of 11 km, and many have larger mean diameters). Comets are also much easier to spot generally than asteroids and so we have a better idea about their orbits and are more likely to have a lot of warning before a potential impact event on Earth. Asteroids are much harder to see and pose much more of a threat even though they are smaller objects (with the exception of a handful such as Ceres).
We are rapidly learning more about the cometary impact rate on Jupiter, and now Neptune. It should be possible to extrapolate from this to calculate the impact rate on Earth.
Better extrapolation method: look at historic impacts on Earth. See Chapman's 1994 paper in Nature "Impacts on the Earth by asteroids and comets: assessing the hazard" v. 367, Issue 6458, pg. 33-40. This paper gives a good summary of the literature at the time (my impression is that this hasn't changed much since then but this is far from my area of expertise).
We seem to be getting a handle on the risk from asteroids, but a comet can come our way without warning.
Not exactly. Comets that are anywhere near the inner system become visible very quickly due to their outgassing. In contrast asteroids are much harder to spot. On the other hand, asteroids stay where they are supposed to and don't have wildly elliptic orbits so they are much easier to track in the long run and tag. So there's a mix here, but overall asteroids are more likely to strike without warning. Comets will likely give us at least a few days to have a giant orgy.
So there's a mix here, but overall asteroids are more likely to strike without warning.
...until we catalog them. We can do that from low earth orbit with infrared telescopes. The wise mission has massively increased the rate of discovery, which is why I think the uncertainty about impacts will come from comets in the future.
http://michaelsmith.id.au
If that was 200 earth years ago that the comet hit, then Neptune has made less than 1.5 orbits around the sun since then.
Damn_registrars has no butt-hole. Damn_registrars has no use for a butt-hole.
When the comet Shoemaker-Levy 9 hit Jupiter sixteen years ago, scientists all over the world were prepared: instruments on board the space probes Voyager 2, Galileo and Ulysses documented every detail of this rare incident.
That's funny, because my back-of-the-napkin estimate is that at the time, Voyager 2 was 3 billion miles further away from Jupiter than the Earth is. Wonder what they thought they were gonna see with 15-year-old technology that they weren't going to see with, say, the Hubble telescope, new ground-based instruments, or hell, even the naked eye that was 3 billion miles closer to the event.