Sciency Words: Antitail

Hello, friends!  Welcome to Sciency Words, a special series here on Planet Pailly where we talk about the definitions and etymologies of scientific terms.  In today’s Sciency Words post, we’re talking about the word:


Did you see the comet?  Pretty much everyone I know has been asking me that question lately.  Comet C/2022 E3 (ZTF) had a wild ride these last few weeks.  First, she started glowing a lot brighter and a lot greener than expected, leading to some people calling her “the green comet.”  Then, due to some intense solar activity, a gap formed in one of the green comet’s two tails.  Shortly thereafter, almost as if the comet were trying to compensate for the damage to one tail, an apparent third tail became visible to observers here on Earth.  This apparent third tail is what astronomers call an antitail.

Definition of antitail: Comets typically have two tails: a dust tail and an ion tail.  These tails are supposed to point away from the Sun.  They’re caused by the solar wind sweeping gas, dust, and other lightweight material away from the comet and off into space.  An antitail is an apparent third tail pointing toward the Sun.  At least antitails look like they’re pointing toward the Sun, but this is actually an optical illusion.

Etymology of antitail: The prefix “anti-” can mean several things.  In this context, it means “opposite,” because antitails point (or look like they point) in a direction opposite to the direction cometary tails are supposed to point.  Based on my research, I believe this term was first introduced in the late 1950’s, following the appearance of comet Arend-Roland.

Okay, I’m going out on a bit of a limb claiming that the term was introduced in the 1950’s.  I cannot find any sources explicitly stating that, but almost every source I looked at seems to agree that Comet Arend-Roland had the most famous and noteworthy antitail in the history of antitails.  In 1957, Arend-Roland developed a large and protruding “sunward spike.”  In photos (like this one or this one), the comet reminds me a little of a narwhal.

Arend-Roland cannot possibly be the first comet ever observed to have an antitail, but it does seem to be the most spectacular and most widely studied antitail in recorded history.  Crucially, I was unable to find any sources mentioning cometary antitails prior to 1957.  Ergo, I think I’m right that the term was first introduced around that time, in reference to that particular comet.  But I could be wrong, and if anyone knows more about this topic than I do, please do share in the comments below.

Regardless of how much of a first Arend-Roland’s antitail really was to the scientific community at the time, it was not much of a mystery.  Within a matter of months, scientists were able to offer explanations, like this explanation published in Nature:

No extraordinary physical theory appears necessary to account for the growth of the sunward tail […]  The sunward tail must almost certainly have resulted from the concentration of cometary debris over an area in the orbital plane.  Seen at moderate angels to the plane, the material possessed too low a surface brightness to be easily observed, but seen edge-on it presented a concentrated line of considerable intensity.

So several things have to happen in order for us Earth-based observers to see an antitail.  First, a comet needs to shed some debris that’s too big and heavy to be swept off by the solar wind.  This extra debris will accumulate along the comet’s orbital path, rather than billowing off in a direction pointing away from the Sun.  Second, Earth has to be in just the right place at just the right time to see this debris field “edge-on.”  Otherwise, the light reflecting off the debris will be too diffuse for us to see.  And third, this has to happen at a time when the comet’s tails don’t overlap with the debris field (i.e., the debris and the tails have to be pointing in opposite directions, as seen from Earth).  Otherwise, the glow of the tails will obscure the light reflecting off the debris.

Last week, I was lucky enough to see the comet, but I didn’t see her bright green color (she was a hazy grey in my telescope), and I certainly didn’t get a chance to see the antitail.  I’m pretty sure I was a few days too late for that, and besides, there’s too much light pollution where I live to see faint details like that.

Still, I consider it a great joy and privilege that I got to see as much of the comet as I did.  And for all the cool sciency stuff I couldn’t see for myself, I can always turn to my research if I want to learn more.


Here’s the 1957 report from Nature that I quoted above, explaining what “must almost certainly” have caused Arend-Roland’s “sunward tail.”

And here’s a more recent article about Arend-Roland, reviewing the comet’s discovery, observation history, and the appearance of his antitail.

Lastly, here’s an article from Live Science about the recent “green comet” and her antitail.

Sciency Words: The Torino Scale

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:


Are you worried about an asteroid or comet smashing into Earth and annihilating human civilization?  Well, you should be worried about that a little bit.  But only a little bit.  Let me tell you about the Torino Scale, and while that won’t put all your fears to rest, it may help put things in perspective.

In the late 1990’s, M.I.T. Professor Richard Binzel came up with a system which he initially called the Near Earth Object Hazard Index.  In 1999, Binzel presented his system to a conference on Near Earth Objects (N.E.O.s) in Torino, Italy.

People at that conference loved Binzel’s idea and voted that the system should be adopted by the scientific community at large. They also voted to rename Binzel’s system the Torino Scale.

The Torino Scale asks two questions about any given N.E.O.: how likely is it to hit us, and how much destructive energy would be released if it did?  Taking those two factors into consideration, the Torino Scale then produces a score between zero and ten.  Zero means we have nothing to worry about.  Ten means “WE’RE ALL GONNA DIE!!!  AAAHHHHHH!!!” as the experts would say.

According to Wikipedia, the comet that caused the Tunguska Event would have probably scored an eight, and the asteroid that caused the K-T Event (the event widely believed to have killed off the dinosaurs) would have scored a ten.  Wikipedia also tells me that the 2013 Chelyabinsk meteor would have scored a zero, because while that particular N.E.O. was definitely on a collision course with Earth, it’s destructive energy was relatively low (I wonder if the residents of Chelyabinsk, Russia, agree with that assessment).

As of this writing, there are no known N.E.O.s that score higher than zero on the Torino Scale, as least not according to this website from NASA’s Jet Propulsion Laboratory.  It is possible for an N.E.O.’s threat level to change as we learn more about it.  As explained in this article from NASA:

The change will result from improved measurements of the object’s orbit showing, most likely in all cases, that the object will indeed miss the Earth. Thus, the most likely outcome for a newly discovered object is that it will ultimately be re-assigned to category zero.

Sooner or later, another eight, nine, or ten on the Torino Scale will come along.  Fives, sixes, and sevens could also be bad news for us.  But for now, at least within the next one hundred years, it sounds like we probably don’t have too much to worry about.


Sciency Words: Centaur (An A to Z Challenge Post)

Today’s post is a special A to Z Challenge edition of Sciency Words, an ongoing series here on Planet Pailly where we take a look at some interesting science or science related term so we can all expand our scientific vocabularies together. In today’s post, C is for:


As I mentioned in my first Sciency Words: A to Z Challenge post, some scientific terms are kind of dumb. This isn’t one of them. I actually think this one’s pretty clever. There’s a class of large objects in the Solar System that astronomers have decided to call centaurs.

Eh… no. These objects have nothing to do with horses, but they are sort of half one thing and half another! When they were first discovered, astronomers were confused because centaurs appeared to have the characteristics of both asteroids and comets.

I first learned about centaurs in this article from Discovery News. It’s now believed that centaurs originally came from the Kuiper belt—a sort of second asteroid belt that lies beyond the orbit of Neptune. Basically, they came from Pluto’s neighborhood.

Due to gravitational interactions with the gas giants, these objects were pulled inward. The now have highly unstable orbits crossing between the orbits of Neptune and Jupiter. Eventually, further gravitational interactions may hurl a centaur into the inner Solar System, putting it within melting distance of the Sun and transforming it into a full-fledged comet.

Originally, the International Astronomy Union wanted to name all the centaurs after actual centaurs from Greek mythology. But they quickly ran out of names. Now the official naming theme includes all mythical hybrids and/or shape-shifters. Examples include Typhon (half man, half dragon), Ceto (half woman, half sea monster) and Narcissus (a man who transformed into a flower).

Next time on Sciency Words: A to Z Challenge, we’ll find out why dimetrodon is not a dinosaur.

Sciency Words: The Tunguska Event

Sciency Words PHYS copy

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us all expand our scientific vocabularies together. Today’s term is:


June 30, 1908, was a bad day to be a tree. At least, it was a bad day for a heck of a lot of trees in the middle of nowhere, Russia.

My03 Tunguska Event

Something—there’s debate over what exactly is was—fell from space that day. It didn’t make it to the ground. Instead, it exploded midair above the Tunguska River.

According to human eyewitnesses, who were many miles away, the sky appeared to be split in two by fire. They heard a series of loud booms. The ground shook, and there was a sudden and intensely hot burst of wind so strong it apparently knocked people off their feet.

According to tree eyewitnesses… actually, we can’t ask the trees what they saw. They died. About 2,000 square kilometers (almost 800 square miles) of forest were scorched and flattened.

Scientific debate continues over what caused the Tunguska Event. It could have been an asteroid; however, asteroid debris can usually be identified by the presence of certain rare metals, such as iridium and osmium. These metals have not been found in the Tunguska region, at least not in quantities that would be atypical for Earth.

Another possibility (which makes the most sense to me) is that is was a comet. Comets are composed of lighter, more volatile chemicals like water. So when the Tunguska comet exploded, it would have been completely vaporized, leaving no debris.

The absurdly huge asteroid that killed the dinosaurs gets a lot of attention in popular culture, but asteroids (and comets) don’t have to be mass-extinction-sized to cause considerable damage. If the Tunguska Event had been the New York Event, there would be no more New York. And if something that big fell over the ocean, the resulting tsunamis could obliterate hundreds of miles of nearby coastlines.

Much like the Carrington Event of 1859, the Tunguska Event serves as a warning. Space is dangerous. Space is deadly. Earth can’t protect us from everything.