At least THREE very fun astro occurrences are coming up in December! An embarrassment of astronomical riches, really. Quite a refreshing change from the embarrassment that will soon be clearing out of the White House.
Let’s start off with the very quick summary:
December 6
The first-ever significant quantity of material retrieved from an asteroid returns to Earth aboard Japan’s Hayabusa2 spacecraft, which will swing by Earth and drop its payload into the Australian outback early in the morning (about 4:30 A.M.). For us non-Aussies, this will be December 5 at around 1 P.M. Eastern. It’s not yet clear when/if the Japanese space agency JAXA will try to livestream this.
December 13
The Geminid meteor shower peaks. This has, in recent years, taken over the distinction of Best Meteor Shower from August’s Perseids. This year’s viewing will be especially good because we’ll have a New Moon, and the show will last essentially all night (provided you don’t have clouds!)
December 21
Jupiter and Saturn meet in the sky! It’s the Great Conjunction of 2020, where these two planets will be so close together that you will be able to see both in the same field of view through a telescope. The last time they were this close together and clearly observable from Earth was back in 1228.
I’ve written about the December 6 and December 21 events before, so I won’t rehash those again, but I also really don’t want anyone to forget about them.
I haven’t written about the Geminids yet, but I need to, because this year’s meteor shower, which peaks the night of December 13-14, should be about as good as it gets. So let’s focus on that here.
First of all, we should have a nice, dark sky, because we get a “New Moon on Monday” at 11:17 A.M. on December 14. The peak of Geminid activity also occurs at nighttime in America, at around 2 A.M., but the viewing will be very good from 9 P.M. all the way until dawn, with as many as 150 shooting stars per hour.
Here is where to look starting … well, as soon as you can see Orion, who will be reasonably high up in the sky by 9 P.M.:
The “radiant” (the point from which the meteors seem to radiate) is within the constellation Gemini; hence the name “Geminids”. Not everyone can easily point out Gemini (myself included), but when it starts getting cold outside, we can all pick out Orion the Hunter:
When I’m walking outside in the winter and I’m getting uncomfortably cold, Orion is there as a sort of character builder, to remind me that I was born to endure this kind of weather. Sometimes I can almost hear the “Immigrant Song”…
Anyway, just look over to the left of Orion to find the radiant, but honestly you can look almost anywhere in the sky, because the trails of these meteors can be quite long.
The reason we have a radiant when there’s a meteor shower is that Earth is flying through a field of debris, much like driving a car through snow:
But where’s this debris from? Usually meteor showers are caused by debris from comets, but the Geminids are a little different. Astronomers are pretty sure that the Geminid debris is from the asteroid Phaethon.
Most asteroids orbit the Sun between Mars and Jupiter, but Phaethon is a weirdo that zooms in close to the Sun as part of its orbit:
I’m showing the position of Phaethon on June 20-21, 2009 in particular because a group of astute astronomers observed Phaethon on those days and found that not only did it brighten a bit, but it also had a mini-tail facing away from the Sun, sort of like a comet:
When Phaethon gets really close to the Sun, it can heat up as hot as 1300°F, which causes it to expand, crack, and release chunks of itself. These chunks probably fall off all through Phaethon’s orbit, leaving some at the place where it crosses our own orbit. So that’s the debris we Earthlings fly through every mid-December.
Two years ago, the Parker Solar Probe was able to spot this trail of debris and image it:
This debris field drifts through space, so every time the Earth goes through it, we experience a different cross-section of it. It’s thought that the thickest part of the debris field will cross Earth’s orbit around 2080, so the Geminids should continue to get better and better until then. The Geminids were not observed at all until 1862, so we are lucky to be entering the prime viewing years of what is now the best annual meteor shower.
The Geminid meteors are generally visually better than those caused by comets, because rocky material burns brighter in the atmosphere than mostly icy material. So you may catch a juicy fireball or two!
If you are fortunate enough to be mostly free of clouds the night of December 13 and you do go outside to view the Geminids, dress warmly and give your eyes at least 20 minutes to adjust to the darkness. Rest your head and neck by reclining in a comfortable chair. Don’t just go outside, look up for a couple minutes, and complain that you haven’t seen anything!
By the way, you don’t even have to wait until December 13. Geminids will be visible as early as December 4, and they’ll increase gradually until the 13th. They do diminish fairly quickly after the peak, so after December 16 or 17, that’ll be about all she wrote.
While we all wait for Joe Biden to raise his right hand in January, at least we can fill the time by appreciating the natural theater right above us.