# Fastest recorded apparent motion of a comet or asteroid seen from Earth (degrees/day)?

If I've done my maths correctly in this answer comet 2I/Borisov (C/2019 Q4) was seen to move about 0.2 degrees in 7 hours, which is almost 0.7 degrees per day.

I'm assuming this is pretty rapid motion for a comet, but I'm wondering if comets (or asteroids) have been observed to move even faster than this relative to the celestial sphere.

Yes I know the speed of apparent motion depends on incidentals like how close the comet happens to pass to the Earth. I'm just wondering if any particular comet stands out as been particularly fast, possibly even difficult to track because perhaps some software didn't allow such a large offset in tracking relative to the stars.

Question: What is the fastest recorded apparent motion of a comet or asteroid, seen from Earth (degrees/day) relative to the celestial sphere?

note: @JamesK's comment reminds me that I would like to exclude meteor tracks. For the purposes of this question the recorded tracks should be produced reflected light, either of the Sun or of radar illumination, and not be something burning up in our atmosphere.

• According to Wikipedia, Comet IRAS-Araki-Alcock "swept across the sky at an incredible speed of some 30 degrees per day". I'm not sure if this is the fastest or not. – JohnHoltz Oct 17 '19 at 12:27
• @JohnHoltz Wow! In a telescope that might have been noticeable fairly quickly. – uhoh Oct 17 '19 at 12:30
• Well there was a very small asteroid visible in Chelyabinsk that covered up to 180 degrees in a few seconds..... – James K Oct 17 '19 at 21:29

## 1 Answer

This is more of a recent example than a record. On 2019-07-25, asteroid 2019 OK passed about 65000 km from the Earth at a relative speed of 24.5 km/s. The Minor Planet Center lists multiple observations from sites in Italy and Armenia an hour before closest approach. Using pairs of these (streak endpoints?) by the ISON-Castelgrande observatory, I compute motions of 29.8° per hour at 00:23 UT and 31.7° per hour at 00:26 UT.

The MPC ephemeris service gives sky motions of 30.6°/hr and 32.5°/hr from site L28 at those times. If someone had observed the asteroid from (25°S, 85°E) in the Indian Ocean at 01:22 UT, they would have seen it move 76° per hour.

Update: On 2020-08-16, tiny (H=29.8) asteroid 2020 QG passed by at 12.3 km/s, about 9320 km from the Earth's center or 2950 km from the surface. A NASA news article said this was a new record for close approach distance. An observer in the right place at 04:09 UT would have seen a maximum sky motion of $$\mathrm{\frac{12.3~km/s}{2950~km} = 0.0042~radian/s = 860^\circ/hr}$$ From pairs of actual observations reported to MPC, ZTF Palomar (I41) saw a sky motion of 566°/hr at 10:23 UT, and ATLAS Mauna Loa (T08) recorded 439°/hr around 11:46 UT.

• smokin! But where does 761 arcsec in 24 seconds come from on that page? – uhoh Oct 18 '19 at 7:15
• @uhoh, Those seem to be differences derived between the last two observations in the table taken on 2019-07-25.... – amateurAstro Oct 18 '19 at 14:16
• @amateurAstro got it, thanks! – uhoh Oct 18 '19 at 15:42
• 2020 QG is probably a new record! minorplanetcenter.net/neo/… – uhoh Aug 19 at 7:18
• @uhoh Absolute magnitude based on 1 AU from both Earth and Sun, and opposition phase. – Mike G Aug 19 at 10:35