update 3: uhoh!

Reddit's amateurradio/comments/k4stjr/aerial_shot_of_the_antenna_collapse_at_arecibo/

Source: Reddit

update 2: Sadly, parts of the dish have collapsed, several cables have snapped, and the Arecibo radio telescope is now decommissioned and demolition has been recommended because it is now too unsafe to do anything else.

related in Space SE: How many total hours had Arecibo transmitted a signal?

update 1: Science November 9, 2020: Second cable breaks at Puerto Rico’s Arecibo telescope

The NASA News item Arecibo Observatory Returns from Tropical Storm Isaias Lockdown to Track Asteroid for NASA begins:

The National Science Foundation facility, managed by the University of Central Florida, determines that a newly discovered asteroid won’t threaten Earth

Of course right after this there was a serious failure, a support cable for the focal plane instrumentation broke and seriously damaged the dish as well.

Are we currently at elevated risk to getting hit by an asteroid now that Arecibo can not help in making precise range-rate measurements of asteroids? Optical tracking provides detection and some orbit reconstruction, but the precise range and radial velocity measurements that can be done by radar can provide very precise data that can help distinguish an impact trajectory from one that will miss.

Could the Five-hundred-meter Aperture Spherical Telescope fill in for Arecibo's role in asteroid astrometry?

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    $\begingroup$ It makes absolutely no difference. We currently have no means to deflect impactors. If they are gonna hit us, they will hit us. $\endgroup$ – Mick Aug 24 '20 at 2:18
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    $\begingroup$ @Mick I absolutely disagree! This kind of data can tell you exactly where the impact will happen and when. This allows for planning. Examples: if it's in the ocean, it may be possible to move populations away from areas that can experience subsequent flooding. If it's on land, people can move. Disaster planning and relief agencies can consider issues of food distribution, etc. $\endgroup$ – uhoh Aug 24 '20 at 2:21
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    $\begingroup$ I must admit that I'm sceptical, especially about the word "exactly". A prediction would have to be accurate enough (in time, location, trajectory and severity) for authorities to take effective action. Too wide an area or too severe an impact, and it becomes unmanageable. Too small an impact and it will just be ignored. I doubt if any nation on Earth has impactor/bolide contingency planning in place. $\endgroup$ – Mick Aug 24 '20 at 2:57
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    $\begingroup$ The hazard is exactly the same as before, but the vulnerability is higher, so the risk as well (risk = hazard $\times$ vulnerability). $\endgroup$ – Jean-Marie Prival Aug 24 '20 at 7:57
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    $\begingroup$ Oh what a pity! I didn't even know that it collapsed! This makes me sad; it's Ellie's observatory from the film 'Contact'. And it could perhaps have discovered Planet IX. Hopefully they'll get it back usable again. $\endgroup$ – Greenhorn Dec 2 '20 at 9:20

The linked article is copied from a university press release. Arecibo's article is more matter-of-fact but naturally also emphasizes the value of their own work.

NASA Goldstone can do planetary radar and remains operational. As this page says:

Arecibo has twice the range and can see three times the volume of Goldstone, while Goldstone, whose greater steerability provides twice the sky coverage and much longer tracking times, serves a complementary role.

FAST has no transmitter but presumably could take part in bistatic radar observations as GBT does.

Radar can measure distance and radial velocity ($r$, $\dot{r}$) in a single observation, especially useful when combined with astrometry ($\alpha$, $\delta$) since the two different kinds of data are orthogonal. Given enough observations over a long enough time, astronomers can determine asteroid orbits rather precisely through optical astrometry alone.

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    $\begingroup$ I found this interview with planetary scientist Edgard Rivera-Valentín The Arecibo Telescope Is Damaged — And That's A Big Deal interesting as well. $\endgroup$ – uhoh Sep 2 '20 at 4:11
  • $\begingroup$ I've added a new update to the question but I don't think it affects your answer at all. $\endgroup$ – uhoh Nov 20 '20 at 2:52

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