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The tag has the current (my) awkwardly-written description.

related to the defense of a planet from a major destructive event such as an impact, or searches for or studies of objects (e.g. asteroids, comets or other near earth objects) that potentially pose such a planetary threat

In order to improve this it would be good to better define "defense".

Questions:

  1. What are all the activities related to defending planets against impacts from things like asteroids or comets? Are they only things like "identification", "tracking", and "deflection" or are there others?
  2. What are all the threats that can potentially be foreseen for which a defense might exist currently or in the foreseeable future? Are they only impacts of near Earth (or other planet) asteroids, or do comets or other near Earth objects count? Anything else?

notes:

  1. Including links to relevant Q&A here and in Astronomy SE is great.
  2. is something different.
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  • $\begingroup$ So "planetary defense" is this called. One could have told me when I confused it with planetary protection. $\endgroup$
    – Giovanni
    Commented Aug 27, 2020 at 16:24

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What are all the activities related to defending planets against impacts from things like asteroids or comets? Are they only things like "identification", "tracking", and "deflection" or are there others?

Mitigation. Suppose the Chelyabinsk meteor had been first observed a month prior to impact, and tracked well enough such that it was known that the object would impact near Chelyabinsk a couple of weeks prior to impact. Two weeks notice would have provided far too little time for any kind of deflection action. Two weeks notice would however have provided more than enough time to do things like boarding up windows and evacuating the city.


What are all the threats that can potentially be foreseen for which a defense might exist currently or in the foreseeable future? Are they only impacts of near Earth (or other planet) asteroids, or do comets or other near Earth objects count? Anything else?

Even long period comets can qualify as near Earth objects, if only briefly. Anything that comes within 50 million kilometers of the Earth is a near Earth object. A better term is perhaps "potentially hazardous objects".

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  • $\begingroup$ Thanks! "Mitigation" is the word I needed when writing this comment. I understand that comets can be NEO's, but would they be significant threats, or just melt and release some gravel before impact? I'm imagining the literal dirty snowballs I used to make when I was young, maybe that's not a good model for a comet. :-) $\endgroup$
    – uhoh
    Commented Aug 27, 2020 at 6:23
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    $\begingroup$ Melt??? You should reconsider. Think of Comet Shoemaker–Levy 9, which Jupiter's large gravity field tore apart. The biggest chunk tore a hole in Jupiter's atmosphere the diameter of the Earth. Imagine a comet and an asteroid of the same mass impacting the Earth. The comet would be an order of magnitude more destructive due to its much larger relative velocity. A dark comet (one that has vented most of its volatiles on previous perihelion passes) forms the basis of a nightmare scenario. A comet that still contains lots of volatiles would be just as deadly, but perhaps easier to see in advance. $\endgroup$
    – David Hammen
    Commented Aug 27, 2020 at 7:23
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    $\begingroup$ The primary reason that planetary protection focuses on asteroids is numbers. The number of potentially hazardous asteroids is estimated to be orders of magnitude larger than is the number of potentially hazardous comets. $\endgroup$
    – David Hammen
    Commented Aug 27, 2020 at 7:26
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    $\begingroup$ I'm agree with the answer, but for the example with Cheliabinsk meteorite - there are many transitional stages in precision of trajectory knowledge between "the asteroid will probably hit Earth in day X" and "it will hit Cheliabinsk city area". The mid-stages are like: "the asteroid will fall in day X somethere in North Eurasia", "the asteriod will hit Siberia somethere in ground track 3000 km long and 50 km wide, including several big cities". With such uncertainties it's impossible to manage evacuation. But probably radar ranging can narrow them down when the asteroid comes close enough. $\endgroup$
    – Heopps
    Commented Aug 27, 2020 at 8:58
  • $\begingroup$ @Heopps - It's impossible to evacuate a continent, or even a country (tiny countries the size of a city excluded). The type of mitigation I wrote about can only occur for impactors that aren't huge and for which the trajectory and predicted impact point are fairly precise. $\endgroup$
    – David Hammen
    Commented Aug 27, 2020 at 21:28

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