asteroid heading earth's way ... tonight!, page-25

Speed an size are impossible to guess due to unknown distance. It traversed the three stars of Orions belt almost horizontally in around 10-15 seconds. Slow enough for me to give my sister the reference location and for her to find it. I forget the exact orientation of the house and garden but suspect it was moving towards the SE in the sky. That means it might have been going faster than it looked due to the earths surface rotating towards the east for sunrise anyway.

Size is impossible to estimate, but as the atmosphere is around 100km thick and this was clearly not being affected by the atmosphere and was big enough to distinguish as a large boulder, I am guessing at the very least, if skimming the atmosphere, at least 250m in diameter.
Hard to know because I have never looked at a boulder 250m wide at a distance of 100km to see how large it appears.

Obviously if it was say 1000km away, it was probably +2km in diameter, if 10000km maybe +20km in diameter and if one quarter of the distance to the moon of around 100000km, maybe 200km diameter. Hard to know, but at the time, (some time ago now) I formed an impression: due to it's smooth if irregular surface features and lack of large pieces of surface detritus, it appeared to be a single piece of rock, maybe with a powder dusting) rather than a agglomeration of pebbles and smaller boulders as you would find in larger asteroids, so I think it was *probably* between 250m and 500m wide and quite close to earth: less than 1000km.

I was a teenager at the time so I didn't really grasp the significance. We just thought it was cool and our parents probably thought we were lying when we bounded inside with the news.