A small comet named 46P/Wirtanen is currently approaching Earth and is expected to reach peak proximity on December 16, when it will swing by at a distance of 11,586,350 kilometers (7,199,424 miles) – about one-thirteenth the distance between our planet and the Sun.
According to skywatching expert Joe Rao, this inter-orbital encounter will be the tenth closest since 1950. As one of more than 400 “Jupiter-family” comets, 46P/Wirtanen has a long elliptical orbit with an aphelia – the point farthest from the Sun – near Jupiter. As it treks toward its perihelion – yep, you guessed it, the closest point to the Sun – on its 5.4-year orbit, the comet also comes closer to Earth. And every so often, our planet’s orbit happens to sync up in a way that puts us particularly cozy to its perihelion path (see video below). Luckily, 2018 is one of those years.
Here’s what you need to know if you want to witness this event. In the beginning of December, 46P will be present in the sky at a declination near -20°, between the constellations Cetus and Eridanus. By its drive-by on the 16th, the comet will be midway between the Hyades and Pleiades star clusters.
“By New Year's Eve, it will have rocketed on a north-northeast trajectory to a declination of +56 degrees into the constellation Lynx,” Rao wrote for Space.com. “For most midnorthern latitude locations, it will become circumpolar on the day after Christmas; in other words, just as the Big Dipper or Cassiopeia appear for most northern locations, it will neither rise nor set, but rather be visible in the northern sky all night long.”
In terms of how easy it will be to see 46P, the answer is less clear-cut.
Some astronomers have predicted that the icy object may appear with an apparent magnitude – measure of brightness that uses a logarithmic scale where negative numbers are brighter – of +8. Others believe it will be brighter, with a magnitude of +4 in December, peaking to +3 on the 16th. For reference, a positive 3 to 4 on the scale corresponds to the faintest stars visible to the naked eye when viewed from an urban location, so hold your breath for a brilliant streaking comet experience à la Hail-Bopp in 1997.
Adding another hurdle, 46P will not even look like a dimmer version of a comet, or at least, what the general public thinks a comet looks like. The object, with an estimated diameter of 1.2 kilometers (0.7 miles), is not large enough to generate a long tail of light-reflecting dust particles. Instead, its paltry tail is mostly composed of gas. All in all, 46P/Wirtanen will appear in our sky as a “fuzzy and diffuse ball of light,” about the size of the moon, “which may be quite difficult to see against the backdrop of a light-polluted sky,” said Rao.
For the best chance of seeing it, he recommends traveling to the darkest area accessible and looking at the correct portion of the sky with your naked eyes, using averted vision – astronomy speak for look right near the object in question instead of right at it. This allows your eyeball’s low-light sensitive rod cells, which are clustered just off the center of your retina, to pick up as much detail as possible.