Bad Astronomy
The entire universe in blog form

May 24 2015 7:30 AM

Wait. That’s the Same Comet?

The folks at the European Space Agency released a new picture of comet 67P/Churyumov-Gerasimenko, and I have to admit that it threw me.

Huh. That’s the comet? Where’s the second lobe?


For a second I thought we were just seeing it end-on, so that the bi-lobed rubber ducky shape wasn’t obvious. But then I realized the part we’re seeing is too thin; the big bottom lobe is much wider than seen here. I did a bunch of rotations and such in my head, and quickly concluded there’s simply no angle on the comet that would produce this view.

My brain really jammed at that point, and I had to concede: I didn’t understand the photo. I read the accompanying text to find out what was going on, and got a good chuckle. I forgot about the Sun.

Here’s the same photo, with the contrast/brightness wildly stretched:


Photo byESA/Rosetta/NavCam – CC BY-SA IGO 3.0

Aha! The smaller lobe is there, a barely darker black against the black sky. The reason it can’t be seen is that it’s in the bigger lobe’s shadow. And also, the bottom of the bigger lobe is flattened, shaped more like a river rock than a potato. At this angle it looks foreshortened, so that fooled me as well.

I love puzzles, and I love getting as far as I can before going to the answer key, but this still felt a little like cheating, since I couldn’t figure it out all out by myself. Drat!

But there’s one thing I did see I do understand. Look to the left, just below the tip of the lobe. See that luminous line dropping down? Care to guess what that is?

Hint: The plumes you see coming from the comet are actually jets of gas, caused by the Sun heating the ice in the comet, turning it directly into a gas*.

Got it now? That vertical line is the shadow of the solid part of the lobe on the gas surrounding the comet. Comets are so weird: They can cast shadows on themselves!

I’ve spent a lot of my life interpreting astronomical images, squeezing the science out of them by analyzing their shapes, contours, brightness, colors, and more. This picture is a good reminder not to take experience for granted, nor to invest too much confidence in it.

Any of us can be fooled at any time. That’s an uncomfortable but necessary piece of information to always keep in mind.

* Also making it useful for the annual Pacific Tech “Smart People on Ice” show. And yes, I did just watch “Real Genius” for the 300th time the other night. Why do you ask?

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May 23 2015 7:15 AM

How Do Clouds Form?

As someone who loves looking at clouds, and may have a somewhat scientifically directed brain, I’m fascinated by the shapes and structures of clouds. Where I live, at the foot of the Rocky Mountains, provides endless examples of them.

I write about them a lot, but I don’t think I’ve ever explained just how clouds form. After all, it’s a little weird: A typical storm cloud can have a mass of millions of tons, yet they float! I know this is because they’re less dense than air, but that still seems weird.


But, if you watch this great video on how clouds form by my friends at Minute Earth, you’ll understand exactly how this all works.

There were two parts in that video that I want to emphasize. One was just why clouds are buoyant; it’s for the same reason a helium balloon is. At a given temperature, a volume of a gas will have the same number of atoms or molecules in it no matter what those atoms or molecules are. Helium atoms have way less mass than oxygen and nitrogen, so it weighs less, so up it goes. And it’s the same for humid air!

The other part was why clouds have flat bottoms. The video makes that really obvious: As you go up in altitude, the temperature of the air drops (this is called the lapse rate, which for some reason is a term I think is really cool). Water vapor (water as a gas) is transparent, so you don’t see that big bubble of rising humid air until gets to the part of the atmosphere where it’s cool enough for the water to condense into droplets (and becomes visible). If the air over a wide area over the ground all reaches that temperature at the same altitude, it forms a plane parallel to the ground. As the humid air rises through that plane it condenses, forming a cloud that is round on top and flat on the bottom. It’s actually a bit of an illusion; the parcel of rising air is still balloon-shaped (very roughly), but you only see it where the water condenses.

I have to admit: I knew all the pieces of this, but hadn’t really put it all together in my head all at once. Seeing it drawn out this way in animated form made it very clear (so to speak, har har).

At the top of this post is a picture of clouds forming with flat bottoms, and you can see their bottoms are all at about the same altitude. I have always thought this would explain why we see the sky as a flattened dome over our heads; A cloud directly overhead is close to us, but one near the horizon is much farther away, giving the sky a flattened look. The perspective effect is strong; you can see how the clouds appear to bunch together when they’re farther away. That’s not real; they’re probably scattered just as much 50 km away as they are overhead, it’s just that when they’re farther away you see more of them in the same area of sky because they appear smaller with distance.

If all of this seems obvious to you, then yay! But I know that many times, when we live our lives in the natural world, there’s a lot of stuff we see, and even a lot of pieces of it we understand. But putting it all together, turning it from a lot of jigsaw puzzle pieces into a single glorious picture, well, sometimes you need someone to show you that part.

If you want more about clouds, then watch this other video by my pal Joe Hanson from It’s Okay To Be Smart. There’s a lot more there.

Tip o’ the brolly to the good folks (who are also friends of mine!) at Science Alert, too, for linking to both videos.

May 22 2015 2:42 PM

Dragon-Eye View of SpaceX Pad Abort Launch

On May 5, 2015, SpaceX tested its launch abort system: A set of powerful rockets on the Dragon space capsule that can pull the Dragon away from the Falcon rocket underneath in case of catastrophe.

SpaceX just released video taken from a camera on the Dragon capsule, and it’s pretty dang dramatic. Come along for the ride:


Whoa. SpaceX said the capsule went from 0 to 160 kph in 1.2 seconds, which is an acceleration of four times Earth’s gravity. It reached a top speed of 550 kph, arcing nearly 1200 meters into the air.

You can see the trunk jettisoned at 0:30 (in a real flight, this sits under the Dragon and contains unpressurized cargo and the capsule’s solar panels). At the time I wondered where the trunk landed after the test; from this it looks like it came down in the water; it looks to me the capsule was already past the shoreline when the trunk jettisoned.

Seconds later the drogue chutes deploy to stabilize the capsule, then the three main parachutes release. Weirdly, the video stops just before the capsule splashes down. Perhaps we’ll see more of that later.

This test was critically important: NASA requires any human-rated vehicle pass stringent tests, including the ability to get astronauts away from the rocket stack in case of emergency. If SpaceX had failed this test, it would have been a major setback to getting Americans back into space on an American rocket. As it happens, things look to have gone pretty well.

There's also video of the test taken from cameras on the ground, and you can see just how fast the capsule blasted away from the pad. 

The SuperDraco thrusters used for this test have double duty with Dragon; besides being there if needed in an emergency, they can be used on-orbit for maneuvering the capsule. SpaceX plans on being ready to put humans into space by 2017. They are also currently building the next generation Falcon Heavy rocket with plans for a test launch later this year.

May 22 2015 7:00 AM

Crash Course Astronomy: The Lord of the Rings

Who out there doesn’t need a little more Saturn in their life? I can deliver.

This was a cool one to record. Even though we’d done 17 episodes before, I like to play with the format a little bit. I was a little more relaxed when we shot this, leaning back in the chair more and just having more fun with it. I hope that shows.


I know I’ll get asked about this, so to head the question off: Yes, Saturn’s rings really are thinner to scale than paper, by a lot. I’ve done the math.

For clarity, I’ll note that there are places where Saturn’s rings are thicker than 10 meters; they range up to a kilometer thick in some regions. But bear in mind that’s still compared to their 300,000 km diameter! That’s a heckuva ratio.

And why not: Here’s an article I wrote about what would happen if Saturn made a close approach to Earth, inspired by a very cool video showing what it would look like. You’ll like it.

As for Saturn itself, now is a great time to go out and see it. By a funny coincidence, Saturn is at opposition tonight: That means it’s opposite the Sun in the sky, rising at sunset and setting at sunrise. It’s up all night, and that also means it’s as close to Earth as it will get for the year (about 1.34 billion km). If you want to see the planet for yourself — and oh my, yes you do — the next few weeks are the best time to do it.

Find a local observatory or nearby astronomy club; I imagine they’ll have viewings. I expect some people will be thinking of buying a telescope, too, so read this first!

Saturn through a telescope can be literally life-changing. It changed mine, and it’s done so for others. Go look.

May 21 2015 12:00 PM


I’ve been listening to electronica music for a long time (like, a really long time; as in I have Isao Tomita albums), and it’s interesting to me that variations of it are still popular. I can’t keep up with kids these days (STAY OFF MY ELECTRONS), so when a publicist sent me a note about Jamie XX, well, I’d never heard of him.

But she said the video for his new song “Gosh” has cool planetary visuals, so I figured what the heck. I clicked the link, and I have to admit: Yeah, the video has stunning visuals. It starts slow, so give it a chance.


I have to admit to chuckling when the rotating arm of the spaceship swings into view fully a minute into the video, after the very long approach sequence to Mars. That was well done. And the parts with spaceships and space stations set against the planet’s limb are really beautiful. Not to mention looking down on the dunes, craters, and other landscapes. The bits at the end with water-filled craters are really, really nice, jumping into the future after the planet is terraformed. That was pretty cool.

The music isn’t exactly my cup of tea, but after listening to it a couple of times it grew on me. I have pretty eclectic taste (I’ll listen to the Captain America 2 soundtrack then ABBA then Shostakovich all without a break), but this was still something of a stretch. But I do kinda like it.

And I like that the video is slow, languid, letting you linger over each scene. It’s nice to know that some people making visual art still appreciate simple beauty and give you a chance to soak it in.

And who knows? What’s art today may very well be fact in a century or two. That’s the point.

May 21 2015 7:00 AM

The Cliffs of Churyumov-Gerasimenko

The Rosetta spacecraft hasn’t been in the news much lately, but wow, is it the gift that keeps on giving. Check out this magnificent view of the comet 67P/Churyumov-Gerasimenko, looking past the Seth region on the big lobe to the towering Hathor cliffs on the small one:

Oh, my.


67P is a double-lobed beast, a 4-kilometer-long cosmic rubber ducky. It’s not clear why it (and so many other comets and asteroids) have this bowling pin or dumbbell shape; maybe it’s from two objects colliding slowly and sticking together, or one bigger object slowly eroding away as ice inside the comet gets heated to a gas by sunlight and vents into space.

Those cliffs are a clue, but what they’re telling us isn’t clear. They reach 900 meters high and are striated with linear features, looking very much like the comet has been cleaved right there.

That view is strengthened, perhaps unfairly, when you look at the comet in context. This mosaic shows 67P taken at a different time, but viewed from almost the exact same angle, and the lower left quadrant shows the same part of the comet as seen above:

When you see the whole thing, it looks like it was a snack for Galactus.

Photo by ESA/Rosetta/NAVCAM (CC BY-SA IGO 3.0)

It really looks like it has a bite taken out of it! It’s not hard to imagine this started off as a more elongated, somewhat cylindrical object, and that over the eons a vent in the side kept getting bigger and bigger, eating away into the body of the comet.

The same as the top image, but enhanced to show the gas venting away.

Photo by ESA/Rosetta/NAVCAM (CC BY-SA IGO 3.0)

Interestingly, you can see ice doing just that from Seth; if you brighten the top image a bit and drop the contrast, the streams of gas flowing away can be clearly seen.

But all this could be coincidence; the comet is a bizarre, alien landscape, and interpretation is difficult. Our sense of what can happen there may be biased by our more terrestrial experience, and we have to be careful to remember: This is in space, has little gravity, is composed of loosely conglomerated gravel and ice, and is sculpted by escaping gas and the rare impact rather than wind and sea. It’s easy to follow the garden path of reason to a completely wrong conclusion here.

But it’s fun to think about, isn’t it?

May 20 2015 12:01 PM

Get Thing Explainer

A great drawing person has made a book talking about cool stuff using only the ten hundred most used words. He is a friend of mine and very funny and brain-good, and his drawing stuff is a lot of fun to look at. Hundreds of hundreds of hundreds of people love what he does, and I do too. You should buy his new book by touching these red words.

Here is a picture of the cover of his new book:

Thing Explainer
Book cover by drawing-stuff guy.

Photo by Randal Munroe


And here is a picture of one of the drawing things he made about a car that looks at things on the Red World:

red world car
Red World car.

Drawing by Randall Munroe

Touch these words to make the picture bigger and easier to see.

If you don't understand what I am writing here and why I am doing it this way then you should touch these red words to find out more. My friend has more stuff about his book and when you read it you will understand.

You can order the book early and then get it when it comes out near the end of the year. I really really think you should.

May 20 2015 7:00 AM

Rainbow Tornado

Last year, I thought myself pretty lucky to catch a lightning bolt zapping across a rainbow while I was taking video of it.

But that was nothing compared with what some storm chasers saw near Eads, Colorado, on May 9, 2015: a tornado forming across a rainbow!



This was taken around 4:30 p.m. The Sun was about still halfway up the sky, putting the top edge of the rainbow just over the horizon. The raindrops forming the rainbow were between the storm chasers and the tornado, so technically the rainbow is in the foreground. Watching the video gives the illusion the rainbow is behind the tornado to me … but that is an illusion.

This tornado (and another nearby) reportedly didn’t do any damage or cause any injuries, but there were some in Texas spawned by the same massive system that did. I’ve never seen a tornado, nor do I feel the overwhelming need to. It’s rare to get them just where I live in Colorado, but towns nearby get their share. I’m glad we’re getting so much rain here this spring (especially given that Colorado supplies so much water to regions west of us that so desperately need it), but I also hope we don’t get the dangerous conditions that so often occur with it in this area … even if it sometimes gives us a glimpse of rare beauty.

Tip o' the storm cellar door to Geekologie.

May 19 2015 11:58 AM

Sen: Lunar Cartography

The United States Geological Survey just released two incredible and incredibly beautiful maps of the Moon, made using data from the Lunar Reconnaissance Orbiter. They are highly detailed and quite lovingly produced. 

I wrote about them for my twice-monthly column at (subscription required). The USGS site says they will be available as prints, but they're not online to order just yet. Still, the PDFs are very nice and, I'd think, very handy. If it ever stops raining here in Colorado, I'd like to compare them to the view through the eyepiece!


Correction, May 19, 2015, at 21:00 UTC: In the picture caption, I misidentified the mare as a basin.

May 19 2015 7:00 AM

Tripping the Light: Fantastic!

Update, May 20, 2015 at 15:15 UTC: The Atlas V rocket launch went well, taking to the skies just after 15:00 UTC. 

N.B. Scroll to the bottom for launch info.


On Wednesday, if all goes well, the Planetary Society will test a new technology that could open up a new way to explore the solar system. On board a United Launch Alliance Atlas V is a tiny cubesat, a parallelepiped just 10 x 10 x 30 centimeters in size—roughly the same size as a loaf of sandwich bread.

Packed into that tiny enclosure is a prototype called LightSail, a spacecraft that carries no fuel. Instead, it will use sunlight to propel it.

Light of any kind can exert pressure, called radiation pressure, when it interacts with matter. The amount is incredibly small, but it’s there. And if you’re out in space, where sunlight is eternal and there’s no atmosphere to counteract it, that pressure exerts a teeny force that can be used to accelerate a probe.

To be useful, you need a probe that has very little mass (to reduce overcoming its own inertia) but a lot of surface area, to catch as much sunlight as possible. This naturally leads to the idea of a light sail, a huge but very thin sail made of Mylar (4.5 microns thick; a human hair is about 20 times thicker). Attach this to a small satellite and you have yourself a space probe.

light sail
A full-size model of LightSail held by Planetary Society CEO Bill Nye.

Photo by the Planetary Society

To be sure, the acceleration is small, and it takes a while to build up speed, but that acceleration can be applied over long periods of time. Months. Interplanetary speeds are achievable this way!

This test is sponsored by the Planetary Society, whose purpose for existence is to “Empower the world's citizens to advance space science and exploration.” They do a lot of great work educating the public and advocating for space exploration, including funding actual projects.

LightSail is one of them. The launch Wednesday is to test a prototype of this technology and show that it can be deployed—it won’t be up high enough in orbit to overcome the very tiny but persistent drag caused by the very thin atmosphere. But if this goes as planned, the next step will be to build the full-fledged LightSail demonstration craft itself in 2016.

Here, let Bill Nye, CEO of the Planetary Society, explain:

This is amazing stuff, right out of science fiction … but it’s fact. By all accounts, this should work. The prototype should go a long way toward proving that.

As for the next step, the Planetary Society needs your help. It’s raised most of the funds needed to build the full-up LightSail for next year, but they’re looking for donations from the public to complete it. They’ve set up a Kickstarter for it that blew through the $200,000 initial goal pretty quickly, but the more the merrier. If you’ve got a few extra bucks lying around, you could do worse than help a group of really good folks try to create a new way to explore the solar system.

ULA will webcast the launch of the LightSail prototype live. The exact time of the launch window has not yet been announced, but the launch is scheduled between 14:45 to 18:45 UTC (10:45–2:45 p.m. Eastern time). You can keep up with the latest news and articles about the LightSail project at the Planetary Society, too.