The Christmas Comet!

To celebrate the season, here is a special Christmastime Sunday space post! Discovered in 1948 Comet 46P/Wirtanen orbits the sun every 5.4 Earth years.   The comet’s apoapsis (the point of its orbit farthest from the Sun) is out in the vicinity of Jupiter’s orbit, but the closest point in its orbit brings it to Earth’s orbit.  Unfortunately, because of the dance of the planets it only in relative proximity to Earth every 11 years, and even then, it is generally barely visible except to hardened astronomers.  The comet is also known as the Christmas comet because its periapsis (when it is closest to the sun—and thus, sometimes to Earth) is in December and because the comet has a distinct viridian tinge!

This year, 46P/Wirtanen’s periapsis is unusually close to Earth.  Tonight, the comet will be a mere 11.4 million kilometers (7.1 million miles) from Earth.  That sounds like a fairly large distance but it is quite close, astronomically speaking:  only 10 comets have come in such near proximity to our home planet in the past 70 years!  Filled with excitement, I glanced out my window only to see that it is raining in Brooklyn and the sky is filled with clouds.  But don’t worry, the comet will nearly as visible for another week.   If you have an internet connection (and if you don’t, how are you reading this?) you can go to this link and find the comet in the sky from your location (that link is an amazing resource, so maybe hold onto it).

 

So why is this comet such a delightful color?  Comet 46P/Wirtanen is mostly melted—it consists of a solid kernel approximately a kilometer in diameter trailing a cloud of gases hundreds of thousands of kilometers long.  The majority of these gases reflect light in green wavelengths. Additionally, the comet is hyperactive—which, in this case, does not mean that overpaid physicians will prescribe it unnecessary medications so it can learn rote facts. In an astronomical context, hyperactive bodies are emitting more water than expected.

Unless you are avidly examining the comet with a gas spectrograph, its color is likely to be a source of awe and reflection. Does the comet’s color reflect the seasonal green of Yuletide or is it an ironic reprimand for the envy and jealously which grip all of human society?  Is it the eye of a great sky panther or a kindly celestial sea turtle (hint: actually more of a ball of gas with an icy nucleus).  Whatever your conclusions, I hope you enjoy this close-up view of “the Christmas Comet” before it zips back towards Jupiter’s orbit. Season’s greetings to all of my readers.  I will try to find some special posts for this solstice week, before we all take a much-needed Christmas break.

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