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You hear a lot about how NASA, ESA and other space agencies are taking action to catalogue asteroids that might one day collide with us, but also figure out how to avert such a catastrophe if one day it were to be determined that in the absence of human intervention, an asteroid would collide with us at a future point in time.

This is important, and fortunately for us, at present, it doesn't seem like there any asteroids out there with Earth written on them; phjew!

However, by random happenstance, I found myself reading about Hale-Bop the other day, and found out that the nucleus of Hale-Bop was estimated to be a whopping 60 kilometers; crikey! Add to that the velocity at which it was traveling and a collision with Earth would pretty much have wiped the slate clean as far as macroscopic life is concerned.

While it didn't collide with us, it did show up in our neck of the woods, so to say, and it had me wondering... Are any space agencies thinking about how to deal with the threat posed by comets, and if so, what might be their plans? Is there anything that could be done given that we will only have a year's, or so, notice from the point it is discovered to its date with Earth?

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    $\begingroup$ I'm just curious; how close did the 60 km Hale-Bop comet approach? $\endgroup$ – uhoh Sep 20 '19 at 23:27
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    $\begingroup$ @uhoh: The closest approach to Earth was 1.315 AU, the closest to any planet in our solar system was 0.77 AU to Jupiter, which was close enough to significantly alter the comet's orbit from 4200 years to 2380 years. $\endgroup$ – Jörg W Mittag Sep 21 '19 at 8:14
  • $\begingroup$ @JörgWMittag okay thanks, so it wasn't a NEC (near-Earth comet) $\endgroup$ – uhoh Sep 21 '19 at 8:43
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    $\begingroup$ "...it doesn't seem like there any asteroids out there with Earth written on them..." I think that what things "seem like" isn't the right way to think about it. The best that you can say is "we don't know of any, but it's far from certain that there aren't any", cf. What is the “interesting story on the limitations of (NASA's) current (near Earth object) survey network”? $\endgroup$ – uhoh Sep 21 '19 at 11:48
  • $\begingroup$ Should probably have been more specific: there aren't any asteroids that could pose a global threat within the next thousand years. There might very well be a city killer out there that we've missed, but we have the orbits of all the big suckers nailed down pretty well. I'm sure there's an asteroid in the main belt, though, that through a collision or perturbation from a close encounter with another asteroid will enter into a resonance with Jupiter that will eventually set it on a collision course with Earth as its eccentricty increases, but that process takes millions of years to play out. $\endgroup$ – Happy Koala Sep 21 '19 at 12:11
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The threat from comets gets a lot less attention for several reasons:

  1. the threat from comets is about 1% of that from asteroids (executive summary of the 2017 Report of the Near-Earth Object Science Definition Team, page 8),
  2. the warning time is often much shorter than for asteroids,
  3. mitigation of a threat from a long period comet is much harder for a lot of the reasons you mentioned: larger object size on average, high impact velocity and also the possibility of disrupting the nucleus if you were to use kinetic impactors.

There were some papers discussing the threat from comets at the recent 2019 Planetary Defense Conference. In the PDC program:

  • Ramanjooloo et al. (IAA-PDC-19-02-16) discuss "Manx comets", comets on long-period orbits that have little or no tail. Based on synthetic orbits, they found that warning times could be as short as 5 days and impact energies were anywhere from 827 MegaTons (MT) of TNT-equivalent to 859,000 MT for 0.1 and 1.1 km diameter impactors respectively.
  • Eismont et al. (IAA-PDC-19-05-10) discuss a very preliminary plan for capturing a small asteroid or a boulder from a larger one (similar to the proposed and now cancelled NASA Asteroid Redirect Mission) and using an Earth gravity assist to redirect this towards deflecting a long-period comet.

Two additional talks, for which only the abstracts are available, also discuss the comet threat and potential mitigation:

  • Bauer et al. (IAA-PDC-19-02-P03) also surveyed the long-period comet hazard from the debiased NEOWISE survey and find it to be ~1000 lower than NEOs of the same size but also notes that discovery times are often less than 1 year from crossing 1 AU from the Sun (and thus potentially colliding with the Earth).
  • Melamed (IAA-PDC-19-05-P12) discusses mitigation of comet impact with kinetic impactors or nuclear standoff explosions but gives little details.
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