Planetary Profile: Abnormal Asteroids

It’s been a while since I made an entry in my weird solar system series (here are the entries for weird Mercury, Venus, Earth, and Mars) but we’re onto the asteroids. What’s that you say, the asteroids are not planets? Well yeah, but they’re undeniably part of our solar system, and there are at least a few that are fully deserving of appreciation for their weirdness.

I know we’re generally used to thinking of the asteroids as living in the main belt between Mars and Jupiter (which is why I’m writing this entry next after the weird Mars post), and a lot of them do, but we’ll be looking at a few that wander far from the belt. No reason not to revel in solar system weirdness just because it happens a little closer to or farther from the Sun.

The Bizarrest

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Obviously everyone is entitled to have their own opinions about what asteroid qualifies as the most bizarre, but for me the title goes to Psyche. This thing is a big honkin’ chunk of metal. It’s the 17^th largest asteroid in terms of size but leapfrogs to 10^th in mass. It makes up 1% of the mass of all known asteroids. This thing is chonky.

We’re not sure how a chunk of metal got to be this big. One possibility is that it’s the exposed core of a failed planet, which is an incredibly awesome sentence to get to write. It’s possible that Planet Psyche didn’t happen because huge impacts stripped away the outer layers of the forming planet.

It’s also possible that Psyche is a metallic core that has a silicate mantle on top of it, which opens up the possibility that this object undergoes ferrovolcanism. This is when you have a volcano, only instead of erupting molten rock it erupts molten metal. Theoretically anyway. We’ve never actually seen ferrovolcanism in action. If it happens on Psyche it might be unique, and if that doesn’t qualify this hunk of iron as weird, I don’t know what else will. (We’ll get to know this asteroid much better when the Psyche spacecraft—yes, the same name, ugh—arrives in 2029.)

Scaling New Heights

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Perhaps you’ve heard of the mountain Olympus Mons on Mars. This landscape monstrosity is roughly three times the size of Mt. Everest. You may have heard it called the tallest mountain in the solar system. Other sources will refer to it simply as the tallest volcano. That’s because there’s another contender for the title of tallest mountain, depending on your exact way of defining things. And it’s not on a planet, it’s on an asteroid. 

The asteroid Vesta is the second largest known asteroid, over 350 miles (563 km) across at its widest point. It also happens to be the most reflective asteroid, making it the brightest, occasionally even bright enough to be seen in the sky with the naked eye. It reflects about 40% of the sunlight that hits its surface. Even Earth, with its bright clouds, icy poles, and giant oceans, only reflects about 30%. So it’s the shiniest asteroid which, as someone who has always been a human magpie, attracted to shiny thing, I fully appreciate. 

But for me the weirdest thing about Vesta is Rheasilvia. This is an impact basin so huge that it makes up most of the asteroid. Like many big craters it has a central peak, also known as Rheasilvia. And, at around 14 miles (22.5 km) tall, it rivals and might just surpass Olympus Mons in altitude. I spent an entire afternoon scratching my head over that one when I first learned it years ago. Now I just accept that Vesta and its strangely giant mountain are just weird.

Birthing the Geminids

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Meteor showers are periodic wonders of nature occurring in our sky. They are caused when Earth’s orbital motion carries it into the debris field left behind by a comet. It’s just a fact, meteor showers come from comets. Except...well, there’s one that doesn’t. The Geminids, which are capable of putting on pretty decent sky shows in mid-December, come from an asteroid, Phaethon. And Phaethon is so very weird for an asteroid.

Phaethon isn’t icy, the way comets are, but it behaves like a comet in many ways. It brightens as it approaches the Sun, and it grows a tail. Not a big, spectacular tail like comets do, but it’s there, tenuous yet undeniable. It seems to mostly be made of sodium gas, which means it’s not the tail that’s generating the Geminids (gas tails don’t make meteors, dust tails do).

In fact, we have no idea how Phaethon is making the Geminids. It has the same orbit as the debris stream from whence the meteor shower comes, so it seems that Phaethon must be the shower’s parent body. Yet the rock seems to be far too small to be generating the amount of debris needed to supply the Geminids. So it remains a mystery how this asteroid-that-wishes-it-could-be-a-comet is creating one of Earth’s more prolific showers.

Did I mention that this rock appears to be blue? Which is a highly unusual color for an asteroid? There’s a whole heckuva lot going on with Phaethon that definitely qualifies as weird. Hopefully we’ll learn more when Japan’s DESTINY+ spacecraft, scheduled for a 2028 launch, performs a Phaethon flyby in 2030.

Junk piles

For this section I’m not going to call out an individual rock but an entire class of objects that only qualify to be called rocks in the loosest sense, because a loose bunch of pebbles is exactly what they are. Colloquially known as rubble pile asteroids, their existence has always been, for me, a particularly odd consequence of gravity.

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Rubble pile asteroids are pretty much what they sound like, large conglomerations of rocky debris held together by gravity and moving through space like a single solid object. This works to make them the opposite of the hyper-dense Psyche—rubble piles are very light for their sizes. And there’s a lot of them! It’s possible that the majority of smaller asteroids, by which I mean less than, say, 6 miles (10 km) might be rubble piles.

Not being a rigidly solid object, of course, means that there are a number of things that can affect a rubble pile asteroid’s shape. It blows my mind that Itokawa, which looks like a worn-away model of a sea otter, and Ryugu, which looks like a D8 die, are both rubble piles. The asteroid Bennu, which has a similar shape to Ryugu, proved to be such a loose pile that when the OSIRIS-REx spacecraft descended to the surface to take a sample the asteroid almost swallowed the spacecraft.  Just proves that it’s the little ones you’ve got to watch out for.

The Big One

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In any group of objects as large as all the things that fall under the category of “asteroid”, you’re going to find extremes. But probably the most well-known extreme is size, and among asteroids the undisputed winner is Ceres. At 590 miles (950 km) across, Ceres is about a quarter the size of the Moon and about 150 miles (240 km) wider than Vesta.

Remember earlier when it was impressive that Psyche made up 1% of the mass of all known asteroids? Well Ceres makes up about 40% of it. That’s enough mass for Ceres to be pulled into a spherical shape by gravity. It’s the only asteroid that is officially categorized as a dwarf planet.

It’s also a highly complex world. It’s suspected that Ceres may have differentiated insides, like Earth does, with a crust on top of a mantle on top of a core. It has some extremely bright spots on its surface that might be sites of past cryovolcanism.

Ceres has enough ice in its makeup that it’s thought that it might have been possible for it have had pockets of liquid water under its surface in the past. Some studies suggest that it actually formed much farther out, between Saturn and Jupiter, and got flung inward during the chaotic kerfuffle of movement and gravity that was the inner solar system. It’s a crazy, bizarre world, this asteroid!

Weird Rocks

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Obviously with so many rocks I’ve only just scratched the surface of the peculiarities that can be found amongst the solar system’s asteroids. But if I tried to be thorough about it, this blog post would have to become a textbook. The universe seems to adore variety, after all, and physics has found all sorts of fun ways to stick rocky bits together.

And that’s just the stuff we’ve already found out about. We have no idea how many asteroids are in the solar system! We’ve detected about 1.4 million already, and it’s hoped that the new Vera Rubin Observatory will be able to detect over 5 million more in the next decade or so. And you can bet that at least some of those will be weird. After all, the universe really can’t seem to help itself!