While everyone is paying attention to New Horizons' arrival at Kuiper Belt object Ultima Thule, Osiris Rex will be completing a very delicate thruster burn lasting a few seconds that put it into a gravitational orbit of tiny Bennu (only 500 meters across). It's going to be the smallest object that a spacecraft has ever orbited.
https://twitter.com/KinetXSNAFD/status/1...1236205568
While orbital velocity on Earth is about 18,000 mph, Osiris Rex will be orbiting Bennu at the blistering pace of 0.13 mph (6 cm/sec), relative to Bennu.
https://twitter.com/KinetXSNAFD/status/1...3765167105
New Horizons will be passing Ultima Thule at 32,212 mph, relative to Ultima Thule.
It's interesting that Ultima Thule is far enough away, about 6 light-hours, that relativistic effects become relevant. Timing for the spacecraft commands is 'SCET' time, 'Spacecraft Event Time', time relative to the New Horizons vehicle itself according to the vehicle's own on-board clock.
Edit: Osiris Rex has completed its thruster firing and has gone into in orbit around Bennu.
https://twitter.com/OSIRISREx/status/107...4278344704
Jonathan McDowell reports that the dimensions of Osiris Rex's orbit is a whopping 1.5 by 2.0 kilometers! So not only is this the smallest object ever orbited, it's the closest that an orbiter has ever been to object it's orbiting.
https://twitter.com/planet4589/status/10...0644944896
https://www.asteroidmission.org/?latest-...ing-record
It's kind of amazing that an object only 500 meters in diameter can still have a useful gravitational field that permits an object to orbit it.
So this New Years Eve is a big night for NASA. Two records. Both the smallest object ever to receive an orbiting satellite, and the farthest object ever to receive a reconnaissance fly-by.