Get ready to look up, lucky humans: Not one but two comets will fling themselves past Earth over the next two days, one of which will be the third closest comet flyby in recorded history. As reported by NASA’s Jet Propulsion Laboratory (JPL), these two frozen celestial spheres may be twins, in a manner of speaking.
Comet 252P/LINEAR was discovered by the Massachusetts Institute of Technology’s LINEAR survey at the turn of the new millennium. The second, P/2016 BA14, was discovered just this year by the University of Hawaii’s PanSTARRS telescope on the island of Maui; initially thought to be an asteroid, follow-up observations with both the University of Maryland and Lowell Observatory team with the Discovery Channel Telescope uncovered its true identity as a tail-wielding comet.
252P, about 230 meters (750 feet) in length, is barreling past our planet as you read this at a distance of 5.2 million kilometers (3.3 million miles). At half the size, BA14 will careen past at a far closer distance of 3.5 million kilometers (2.2 million miles) tomorrow, March 22, with the time of closest approach at 2:30 p.m. GMT (10:30 a.m. EDT). This is the closest approach of a comet to Earth since Lexell's Comet approached at 2.2 million kilometers (1.4 million miles) in 1770, according to Sky & Telescope.
The orbital paths of these two currently passing comets are remarkably similar, but is this all there is to it? Not quite. “We know comets are relatively fragile things,” said Paul Chodas, manager of NASA’s Center for Near-Earth Object Studies at JPL, in a statement. “Perhaps during a previous pass through the inner Solar System, or during a distant flyby of Jupiter, a chunk that we now know of as BA14 might have broken off 252P.”
(It should be noted that NASA incorrectly claims that comet IRAS-Araki-Alcock in 1983 came closer than BA14 in its statement.)
The green glow of Comet 252P (left) due to the ionization of its diatomic carbon gas. M. Mattiazzo (CC BY-NC-ND)
So it could be that these two were once together. The break-up of comets isn’t without precedent: Shoemaker-Levy 9, which smashed into Jupiter in 1994, was already in 21 individual pieces by the time it began colliding with the gas giant – to give just one example.
Astronomers estimate that it had been captured by Jupiter’s gravity up to 30 years earlier, and that on one approach it moved perilously close to the planet. At this hazardously close distance, the gravitational forces of the world were strong enough to rip it apart into separate fragments, which slammed into the surface over the course of five days.
It isn’t certain, however, that a huge planet’s gravitational field was the culprit behind the break-up of the original 252P. Another comet by the name of 73P may hold a clue, in that it is currently disintegrating merely by being heated by the Sun. It’s now 66 individual icy pieces, and looks set to continue breaking down. Perhaps our local star, then, is to blame for 252P’s schism.
The break-up of component B of comet 73P/Schwassmann-Wachmann 3 into even more pieces. NASA/ESA
To see these two broken comet fragments you’ll probably need a relatively good telescope. Even at the astronomically close distance of BA14’s flyby, that’s still about nine times the distance from Earth to the Moon.
On the other hand, despite having a more distant flyby, 252P is rapidly brightening as it approaches Earth, and some think it'll be 100 times brighter than expected. There's a chance it'll be visible to the naked eye.
Either way, if you miss the pair this time, you’ll have to wait 150 years for their return.