Not one, but two, possibly related comets will make exceptionally close flybys of Earth on March 21–22. Here's what we know and a guide on how to see them.
You'd have to go back to 246 years to find a comet that passed closer to Earth than Comet PanSTARRS (P/2016 BA14) will on March 22nd. Predicted to come within about nine lunar distances (2.1 million miles or 3.4 million kilometers) of Earth around 16:00 UT on that day, P/2016 BA14 will soon claim the distinction of third closest comet ever recorded.
Lexell's Comet has them all beat. It missed Earth by 1.4 million miles (2.2 million kilometers) on July 1, 1770. Although discovered in June that year by the comet ferret himself, Charles Messier, it became popularly known as Lexell's Comet after astronomer and mathematician Anders Johan Lexell computed its orbit. As the comet sprinted across the sky in late June and early July, Messier described the coma as more than four times the size of the full Moon and as bright as magnitude +2. Wow!
We'll see plenty of action from P/2016 BA14 as it speeds from Canis Major to Ursa Major in the span of just 7 nights, but you'll need a telescope to spot it. When first discovered on January 21st of this year by the PanSTARRS 1 telescope, it was given an asteroid designation, 2016 BA14. Soon after, Russian astronomer Denis Denisenko noticed that it's orbit was remarkably similar to comet 252P/LINEAR, discovered in 2000 and on course for a similar close approach to Earth this month.
Could they be related? Astronomers Michael Kelley and Matthew Knight wanted to find out, so they observed 2016 BA14 with Lowell Observatory's Discovery Channel Telescope in February. Sure enough, their photos revealed a tail. Two comets on nearly identical orbits with nearly the same period of 5.32 (252P) and 5.25 years (BA14) imply a common origin. The most likely scenario? A chunk of 252P/LINEAR spalled off to become P/2016 BA14. The "P" designation stands for periodic and indicates that both comets make repeat orbits around the sun.
Surging rapidly towards naked-eye visibility, 252P/LINEAR has riveted the attention of the comet community but for now is only visible in the Southern Hemisphere. Hold onto your hats. It moves rapidly northward and will pop up in Scorpius' tail at 6th magnitude in about a week. In the meantime, its sibling is likewise headed north at rocket speed. Currently tiny and a pitifully faint 15th magnitude, P/2016 BA14 is expected to brighten rapidly and reach magnitude +12 during closest approach on March 22–23. I suspect it will also show a large coma, especially in long time exposure images.
Although the full Moon will make a mess of things at that time, we get a break by the 24th when the Moon rises at twilight's end, allowing for a brief window of darkness. At that time, the comet will be beautifully placed, flying across Ursa Major at the rate of 13° per day or more than 1/2° per hour. Between March 21st and 22nd, it covers an incredible 20° in 24 hours, fast enough to detect motion in less than a minute through a telescope. Provided it brightens as hoped, that is.
Although I've included a detailed map for March 25-26, I encourage you to visit the Small-Body Database Browser via the JPL HORIZONS Web-Interface, grab the comet's orbital elements, and create your own chart using a planetarium-style program such as Starry Night, Stellarium (free) or TheSkyX.
We'll soon know much more about these two unequal popsicle halves. Michael Kelley has secured time on the Hubble Space Telescope for a closer look at 252P. Spectra should help astronomers determine if the comets' compositions are similar enough to prove they're related. Radar observations are planned for BA14 using the Green Bank and NASA's Deep Space Network's DSS-13 dishes to determine the shape and size of its nucleus and other characteristics. Unfortunately, it appears that the 1000-foot (305-meter) Arecibo dish, which might have been used to image 252P, will most likely be down for maintenance during the flybys.
Their rapid flight brings to mind another similar comet, the previous third-place holder (after Lexell's and 55P/Tempel-Tuttle in 1366) at 2.9 million miles (4.7 million kilometers), IRAS-Araki-Alcock, discovered in 1983. I recall the sight of a 3rd-magnitude, tailless blob of haze leap-frogging across the sky at the rate some 30° per night. Through the telescope, fascinating structures lit up the inner coma as the comet moved against the stars in real time. When I look back, I'm grateful for those three clear nights in a row during during closest approach ... and hoping for a repeat!
Whether you have a telescope or not, you'll still have an opportunity to see Comet P/2016 BA14. Astronomer Gianluca Masi, who maintains the Virtual Telescope website, will broadcast a live telescopic image of the comet's historic encounter on March 21st and 22nd starting at 21 UT (4 p.m. CDT). Sky & Telescope will also keep you updated with news, photos, and maps of both comets in the coming weeks.