Sciency Words: Frost Line (An A to Z Challenge Post)

April 7, 2017

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, F is for:


They say it’s cold in space. That’s not quite true. First off, how do you define what temperature means in a vacuum? That’s a much harder question that you might think.

But secondly—and more importantly for today’s post—a lot depends on where you are in space, because if you happen to be anywhere near a star, I guarantee you will feel the heat.

If you read enough scientific literature about space, you’ll eventually encounter the term “frost line,” and you’ll probably be able to guess from context what it means. Objects on one side of the line are close enough to the Sun for ice to melt (or more likely, sublimate), while objects on the other side are far enough away that ice remains frozen.

In our Solar System, the frost line is usually placed somewhere in the middle of the asteroid belt.

But there’s a lot of disagreement about where specifically the frost line is, in large part because there’s a lot of disagreement about how, specifically, the term should be defined.

Some astrophysicists define the frost line based on temperature conditions in the Solar System today. Others define it based on conditions from back when the Solar System was still forming. Also, there can be different frost lines for different chemicals, because the freezing point of water is different than that of methane or nitrogen or carbon dioxide.

This is a case of how some scientific terms are more clearly and precisely defined than others. And yet despite all the ambiguity about the frost line (or lines), it is still an incredibly useful term to help describe the layout of the Solar System. Which is why, if you read enough scientific literature about space, you are bound to come across this term eventually.

Next time on Sciency Words: A to Z Challenge, where did the word gravity come from?

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

April 4, 2017

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: Frost Line

December 23, 2016

Welcome to a very special holiday edition of Sciency Words! Today’s science or science-related term is:


When a new star is forming, it’s typically surrounded by a swirling cloud of dust and gas called an accretion disk. Heat radiating from the baby star plus heat trapped in the disk itself vaporizes water and other volatile chemicals, which are then swept off into space by the solar wind.

But as you move farther away from the star, the temperature of the accretion disk tends to drop. Eventually, you reach a point where it’s cold enough for water to remain in its solid ice form. This is known as the frost line (or snow line, or ice line, or frost boundary).

Of course not all volatiles freeze or vaporize at the same temperature. When necessary, science writers will specify which frost line (or lines) they’re talking about. For example, a distinction might be made between the water frost line versus the nitrogen frost line versus the methane frost line, etc. But in general, if you see the term frost line by itself without any specifiers, I think you can safely assume it’s the water frost line.

Even though our Sun’s accretion disk is long gone, the frost line still loosely marks the boundary between the warmth of the inner Solar System and the coldness of the outer Solar System. The line is smack-dab in the middle of the asteroid belt, and it’s been observed that main belt asteroids tend to be rockier or icier depending on which side of the line they’re on.

It was easier for giant planets like Jupiter and Saturn to form beyond the frost line, since they had so much more solid matter to work with. And icy objects like Europa, Titan, and Pluto—places so cold that water is basically a kind of rock—only exist as they do because they formed beyond the frost line. This has led to the old saying:


Okay, maybe that’s not an old saying, but I really wanted this to be a holiday-themed post.

Sciency Words: Apollos and Atens

November 25, 2016

Sciency Words BIO 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 expand our scientific vocabularies together. Today we’ve got two terms:


Asteroid are classified into different “groups” based on their orbital properties. The Apollo asteroids and Aten asteroids are two such groups, and these groups are of particular interest to anyone who doesn’t want a repeat of the K-T Event (which wiped out the dinosaurs) or the Tunguska Event (which flattened a forest and could have done the same to a whole city).

Technical Definitions

  • Apollo asteroids have a semimajor axis greater than 1.0 AU and a perihelion less than Earth’s aphelion of 1.017 AU. The first known Apollo was 1862 Apollo, for which the group is named.
  • Aten asteroids have a semimajor axis less than 1.0 AU and an aphelion greater than Earth’s perihelion of 0.983 AU. The first known Aten was 2062 Aten, for which the group is named.

Less Technical Definition

  • Apollo asteroids spend most of their time beyond Earth’s orbit, but cross inside at some point.
  • Aten asteroids spend most of their time inside Earth’s orbit, but cross outside at some point.


The important thing to know is that both Apollos and Atens cross Earth’s orbit at some point. Keep in mind that space is three-dimensional, so their paths don’t necessarily intersect with Earth’s. They might pass “above” or “below” Earth, so to speak.

But the orbits of enough Apollos and Atens do intersect with Earth’s orbital path that they might one day hit us. Atens are particularly worrisome. They spend so much time inside Earth’s orbit, in relatively close proximity to the Sun, that it’s hard for astronomers to find them.

So if a giant asteroid ever does sneak up on us and wipe out human civilization, my guess is it’ll be an asteroid from the Aten group. Those are the asteroids that frighten me the most.


Don’t Panic: It’s Just Another Asteroid

November 23, 2016

People ask me all the time: “Hey, did you hear about that asteroid?” These people then tell me about some asteroid that’s supposed to “just barely miss us” is the next day or so. Sometimes, they also ask, “Aren’t you worried?”

There are certain kinds of space news that I simply can’t get excited about anymore. This is one of them. Why?


There’s actually a newsletter about asteroid flybys. It’s called Daily Minor Planet, and I have a subscription (it’s free). Every day in my inbox, I’m notified of the latest asteroid or other object skimming past Earth. Every day. Sometimes there are more than one per day.

Occasionally, one of these objects will pass within the radius of the Moon’s orbit. That’s not an everyday thing, but still… it happens more often than you might think.

So when people ask if I’ve heard about the latest asteroid flying past Earth, the only thing I can really say is, “Which one?” And if someone asks me if I’m worried, my answer is no. The asteroids that make headlines on the news and the asteroids that appear in Daily Minor Planet… those are asteroids we know about. It’s all the asteroids we don’t know about that scare the bejesus out of me.

Space Harpoons: They’re a Real Thing

June 22, 2016

If the future of space exploration requires an economic incentive, look no further than asteroid mining. All the rare and valuable minerals and metals contained in a single asteroid (except those lousy S-type asteroids) could be worth billions.

But catching an asteroid and landing on it for mining purposes… that’s much easier said than done. You see, no two asteroids are exactly alike, and they each present a host of challenges for asteroid hunters of the future.

Jn22 Crazy Asteroids

There are several ideas for how to catch an asteroid. You could throw a net around it, assuming the asteroid isn’t too big. Or you could latch on with magnets, assuming the asteroid has a high enough metal content.

But the most common idea that I’ve seen is the shoot the asteroid with a harpoon. It makes the whole endeavor feel oddly reminiscent of old timey whaling. You know, like in Moby Dick. Or Star Trek IV.

As I understand it, the harpoon has a cable attached, so once you’ve harpooned yourself an asteroid you can reel your spacecraft in to a secure landing. Or in the case of those wildly spinning asteroids, the asteroid will reel you in by wrapping the cable around itself (what could go wrong?).

So the next time you’re in space trying to grab billions of dollars worth of asteroid, remember to bring a harpoon. And a really strong cable.

P.S.: Also, if an asteroid somehow manages to bite off your leg, maybe it’s best to let it go. As Mission Commander Ahab will tell you, vendettas against whales and asteroids never lead to happy endings.

Sciency Words: Pixie

May 27, 2016

Sciency Words BIO 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:


The Future

It is the year 2217. The mining vessel Belvedere approaches a large, rocky body in the outer Solar System. The object is dark in color, barely visible against the inky blackness of space. It has every appearance of being a carbonaceous asteroid, no doubt rich in volatiles. It may even contain that most precious of substances in space: water.

The crew of the Belvedere stand to make a substantial profit, but the science officer reports that she’s getting a lot of strange readings. The asteroid may not be what it seems. “I recommend keeping our distance,” she says.

The Present

“Pixies” are part of the whole commercialization of space thing that’s going on right now. They’re made by a company called Asteroid Initiatives LLC, and they’re intended for use in asteroid prospecting and, ultimately, asteroid mining.

Basically, pixies are a new kind of space probe. As you might guess from the name, they’re really small. They’re sometimes referred to as femto-spacecraft, though they’re not actually femto-scale (that would make them smaller than atoms).

In terms of size, they’re often compared to credit cards, cell phones, or TV remote controls. In other words, a pixie spacecraft could fit in the palm of your hand.

Pixies may be going on their first mission soon. They’re under consideration to be part of the AIDA mission to the asteroid Didymos and its moon, Didymoon. If approved, a swarm of forty pixies will either surround Didymoon or land on its surface.

Granted, pixies are too small to carry much sensor equipment, but there will be forty of them. That’s forty extra data feeds, forty extra points of view, forty extra perspectives on how Didymoon responds during AIDA’s impact experiments. That’s a lot of additional information without putting any expensive hardware at risk.

The Future

The captain of the Belvedere steeples his fingers. He can’t pass up the opportunity to mine such a large carbonaceous asteroid, but if his science officer is right… if there’s any danger….

On the view-screen, the large, rocky object drifts through space. The captain comes to a decision. There’s a way to get more data without putting the ship at risk.

The captain issues the order: “Release the pixies!”