Yes, I know I just posted on new entries from RNAA (Research Notes of the AAS). The newest Notes include a report of the James Webb Space Telescope getting time to spot asteroid 2024 YR4, which was initially tracked to a potential Earth impact in 2032. (Now we know it will not.)
Rivkin, A. S. Muller, T. MacLennan, E. et al. JWST Observations of Potentially Hazardous Asteroid 2024 YR4 70 adc6f0
Ordinarily, time on JWST is scheduled quarters to years in advance; however, there is a percentage set aside for high-priority targets, called “Discretionary Time.” If the Time Allocation Committee agrees that your proposal for such an observation is, in fact, worthy and date-specific, then you get some Discretionary Time. Rivkin et al. wrote such a proposal, with such an argument.
JWST is, unlike many ground telescopes, sensitive enough for something like 2024 YR4. Aside from simply having a big aperture (6.4 meter equivalent), the observatory doesn’t have to deal with Earth’s atmosphere. It can then view into the ‘solar direction’ (dawn and dusk, to us) where ground telescopes might work, but would be confronted with lots of airmass in the way. All that air is a noise source in much of the infrared, drowning out the asteroid signal. JWST, then, observed with the (European-built) MIRI instrument. First things first, then, merely spotting 2024 YR4 at this time extends the observing arc- the total length of time we have been tracking it. The longer the observing time, the lower the error on our track. Using JWST then adds arc time, where pretty much all ground telescopes cannot.
Secondly, viewing in thermal infrared gives data that visible-light telescopes do not. A spectra was taken with ground telescopes, indicating 2024 YR4 is likely rocky (S-complex). Fore-armed with that knowledge, rocky asteroids likely have an albedo (reflectivity) of ~18 percent. (That is, of the incoming sunlight, 18% or so of the light is reflected, the rest absorbed.) Given this moderate albedo (more than 4%, but less than 40%), YR4‘s temperature should also be moderate, compared to brighter or darker asteroids. This would show up as mid-infrared brightness or darkness. Sure enough, JWST found that the temperature was as expected. 2024 YR4 is thus ~60 meters in diameter.
The upshot: 2024 YR4‘s chances of impacting Earth in its 2032 pass are within a rounding error of zero- no impact. And, even should it impact, the size is just not that threatening. We already observed that the range of potential impact sites in 2032 (the uncertainty zone) included much ocean and jungle; now with a size number, we can say the impact would have caused damage over a quite local spot of that wilderness, roughly the size of a city (that isn’t there). 60 meters is not something to dismiss, sure, but not a threat to the wider world outside that one spot.
2024 YR4 will still still STILL not kill you.