A Tail Of Two Comets: Celestial Siblings Safely Pass Earth In Historic Double Fly By

Two comets — a big and little sister, if you will — will pass by Earth early this week in a historic dual fly-by.

One of those comets, described by National Geographic as green, passed by Earth on Monday and the second will follow it on Tuesday. The first, and largest, comet is 252P/LINEAR 12, and its trailing little sister is P/2016 BA14.

These dual comets are significant because they are among the closest to zip past Earth in recorded history. According to the Smithsonian, Monday’s flew by at about 3.3 million miles, and Tuesday’s will come even closer at 2.2 million miles.

They both pass us by with room to spare — about nine times our distance from the moon. Though that sounds pretty far, the littler of the two comets is the closest to pass by since 1770, and the second closest in Earth’s entire recorded history.

The comets are also unique because of their suspected relationship. According to Slate, 252P is the bigger of the two comets at 230 meters, or 750 feet, across. Its sister, BA14, is half that size; when it was first discovered in 2016, scientists thought it was an asteroid.

The larger of the two comets was discovered in 2000, and astronomers have known it’s been heading for a pass by Earth for years. The mysterious object trailing behind came as a surprise.


“What are the chances of such an unusual comet and a random asteroid having a similar orbit and Earth close approach?” an astronomer named Michael Kelley wrote on his blog. “Probably very small! A lot of suspicion was starting to be cast on this so-called asteroid.”

But then they looked closer and found its tail.

Both comets have similar, but not identical, orbits and it seems likely that they were once one object. Comets are actually pretty delicate, its rock, gravel, and dust glued together with ice. The nearer to sun it gets, its ice turns into gas and that loosely held material sloughs off. BA14 could’ve been ripped off 252P and then kept trailing along behind it.

“There are many more asteroids in near-Earth space than comets, which are significantly more rare,” Kelley added. “When a comet does come this close to Earth it is something to get excited about, and take advantage of to learn whatever we can.”


In recent weeks, 252P got pretty bright as it neared the sun, but was still pretty dim; the full moon, which arrives soon, will make it even harder to pick out of the night sky. As it made its pass by Earth, the Monday comet reportedly had a green hue — that’s its vaporizing nucleus, which is disgorging carbon-based gas that glows green in space. So far, the smaller of the two comets is even dimmer.

To explain how these two comets could be related, scientists are turning to Jupiter. In 1994, Comet Shoemaker-Levy 9 and 21 pieces of debris battered the planet. Astronomers think Jupiter’s gravitational force pulled it apart during a previous pass by, two years prior.

This could’ve happened to 252P during a previous pass through the inner solar system, perhaps even a fly-by of Jupiter. Smaller pieces may also be trailing behind.

After its pass by Earth on Monday, it will glide through the constellation Scorpius in the predawn skies on March 24 through 27. Its sister will soar through the handle of Big Dipper in the high northeastern sky, starting after dark on March 25. A couple days later, it’ll fly by the end of the constellation.

The last time the Earth was visited by an impressive comet was 1770. It was called Lexell and soared to within 1.4 million miles of Earth. Reports at the time described a head four times wider than the moon and as bright as the night sky’s brightest stars.

[Photo by kasha_malasha/Shutterstock]