I’m going to do a livestream discussion with EarthSky’s Deborah Byrd (!) today at 17:15 UTC (1:15 pm Eastern US time). We’ll be talking about one of my favorite astronomical space missions, Gaia, and what astronomers have learned form its observations. It’ll be streamed on YouTube, and I’ve embedded the stream here as well for your convenience:

I’m amused at the promo graphic they made for me:

If you don’t follow me on social media, that’s a drawing my friend Zach Weinersmith made of me years ago, which I use pretty much universally for my avatar because it cracks me up. Yes, it’s me as an adult baby in a onesie because are you not familiar with SMBC comic? That’s the least weird thing he could’ve done.

A couple of pretty cool sky events are going on this week!

First, the wonderful Perseid meteor shower peaks TONIGHT! It’s actually good for several days before and after the peak, but at its best it produces 60+ meteors per hour. I’ve written a guide on observing this meteor shower at Scientific American. I also have an older one at The Old Blog™, if you like, which is still relevant. 

Second, Mars and Jupiter are slowly approaching one another in the sky. On August 14 they pass a mere 0.3° apart (depending on where you are; folks in Asia and Australia can see them a bit closer together, but for the US that happens during the day), less than the width of the full Moon! This is called a conjunction, and it’s pretty rare to get one this tight. They’re still close together all week though, so if you’re an early riser (they’re best to see between 03:00 – 04:00) take a look. I have a guide for this at Scientific American as well. Bonus: you might catch some Perseids, too!

There is a whole population of small objects in the solar system that get near Earth and don’t fit neatly into the category of either rocky asteroids or icy comets, but are somewhere in between. Astronomers call them dark comets, and by studying some of them have found some interesting things: they tend to rotate quickly, they may have started off bigger and flung themselves apart as they spun too rapidly, they still have a little bit of ice left that affects their behavior, and that they imply the inner part of the main asteroid belt has objects with ice still in them [link to journal paper].

There isn’t really a sharp line between asteroids and comets. Both are relative small bodies (say tens of meters to a hundred or more kilometers wide) that orbit the Sun. Asteroids tend to be made more of metal or rock with some ice, while comets have rock and lots more ice in them. When a comet gets near the Sun its ice turns to gas, which then blows away from the surface to create what you think of when you hear the word “comet”.

But if they have shorter orbits, like a few to a few dozen years, every time they pass the Sun they lose more ice before heading back out to the colder depths of space. So, over time, eventually they’ll run out of ice. What’s left is a small rocky body that looks a lot like an asteroid — a dark comet.

However, even far out from the Sun they can still lose a little gas. Not enough to create the big fuzzy head and long tail, so we don’t see the activity directly, but enough that the gas escaping acts like a very low-thrust rocket. That changes the orbit of the object over time, which can be seen in long-term observations. Quite a few are known.

The astronomers looked more carefully at these dark comets. They’re small and irregular in shape. As they spin, their brightness changes because sometimes we see the long side (so they appear brighter) and sometimes the short side (and they appear dimmer). Using this change to measure the rotation rates, astronomer found they tend to be rapid rotators, spinning once in a matter of minutes or hours. That’s telling! They spin so quickly they can barely stay together; their tensile strength is incredibly low, like frothy cotton candy, and if they spun much more quickly they’d fly apart.

The astronomers think that these objects rotate more rapidly over time as gas escapes, spinning them up like a pinwheel. At some point they fly apart, and the smaller pieces themselves will spin faster, too. This is called a rotational fragmentation cascade, and if you think I decided to write this whole article just because of that extremely cool phrase, then you know me pretty well. But this does explain their small size and rapid rotation.

These objects likely started off orbiting the Sun toward the inner edge of the main asteroid belt between Mars and Jupiter. Gravitational influence from the planets can move them closer to the Sun, where they warm up, spin faster, and fly apart. But this means that a lot of asteroids in the inner belt must have a repository of ice under their surface! This has been suspected for a while — the biggest rock in the belt, the protoplanet Ceres, is loaded with ice — but this study indicates it’s true.

Why is that important? These rocks can get into orbits that bring them close to Earth, and even impact us. A really big mystery is how Earth got all its water, since it’s so close to the Sun that the material it formed from may not have been water-rich. It’s possible the water came from comets that formed way out in the solar system, but it’s also possible it came from asteroids closer in. This research indicates that may very well be the case.

The only problem is they didn’t study very many of these dark comets (not a lot are known), so it’s hard to get statistics — though it’s possible up to 60% of all near-Earth objects are actually part of this group. But of course we’re getting better at seeing smaller and fainter objects, with bigger telescopes coming online. As the known population of these objects grows, we’ll get a better handle on how they behave, and how big a role that had in making Earth a wet planet. 

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