The entire universe in blog form

Aug. 1 2014 12:54 PM

Mesocyclone or Mothership?

National Geographic Traveler just announced its winners for its 2014 photo contest. The shots are all very cool, but I can’t argue with the No. 1 pick: a mesocyclone over Colorado, taken by photographer Marko Korosec:

I expect to see Thor riding down the middle of that thing. Click to supercellenate.

Photo by Marko Korosec, used by permission

Holy wow! I love this shot. Korosec was storm-chasing on May 28, 2013, when this vortex appeared near Julesburg, Colorado, in the extreme northeastern corner of the state, near Nebraska. Despite what you might think, eastern Colorado is extremely flat, and prone to, um, interesting summer storms.


I’ve written about mesocyclones before (see Related Posts below for more jaw-dropping pictures and time-lapse animations). I can’t describe them much better than I did before:

A supercell is a rotating thundercloud; the spinning vortex in the middle is called a mesocyclone. Conditions need to be just so to create one. First you need a wind shear, where wind blows faster in one spot than another, so a blanket of air is flowing over another one. This sets up a rolling vortex, a horizontally rotating mass of air like the way a wave breaks when it gets to a beach. An updraft then lifts that vortex, which then spins vertically.
The warmer air in the vortex rises; this is called convection. If there’s a boundary layer of air above it, called a capping layer, it acts like a lid, preventing the vortex air from rising. It builds up power and can suddenly and explosively grow to a huge size. Wikipedia has a good description and diagrams of how this works.

Impressive. And what amazes me is that the photo above isn’t even what I think is the best one Korosec took of a mesocyclone. This one is:


Photo by Marko Korosec, used by permission

WOW. It looks like a spaceship touching down, its thrusters disturbing the ground underneath it.

And just because I want to make sure you see it, here is a time-lapse of a supercell growing in Kansas, taken by Stephen Locke, because holy yikes it’s magnificent.

The next time someone tells me they’re feeling “under the weather,” they can be sure to get a lot of sympathy from me.

You can find more of Korosec’s photography—and it's all amazing, he’s really seriously gifted—at 500px, and on his website

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Aug. 1 2014 7:30 AM

The 2012 Solar Disaster That Almost Was

In July 2012 the Earth dodged a bullet. Or more accurately, the bullet was misaimed. But had it hit, we’d have been in big trouble.

The bullet in this case was a solar storm, an eruption of a billion tons of plasma exploding outward from the Sun. This kind of event—called a coronal mass ejection, or CME—is actually relatively common. But this particular CME was a monster … in fact, it may have been the most powerful one ever seen.


People sometimes ask me if anything in astronomy actually worries me. Something like this is near the top of the list Why? Because a storm this size is big enough to cause widespread blackouts that can last for months, and can damage critical satellites we rely on for our modern existence.

Although this one missed us, it did hit a solar orbiting satellite called STEREO A, which observed the whole thing. Here’s a video showing the event as seen from different spacecraft (the different segments are explained on the SDO YouTube page):

The part at 0:20 gave me chills. All that “snow” you see is actually the result of countless subatomic particles slamming into STEREO A at about 1 percent the speed of light. That’s fast.

So what’s going on here? And was this event really all that dangerous?

The quick version of this is that the Sun has a very complicated magnetic field. Inside the Sun there are enormous packets of hot plasma (gas with its electrons stripped off) that rise from deep within. These blobs have their own internal magnetic field, and as they rise to the surface the huge loops of magnetic field lines (similar to what you see in bar magnet diagrams) pierce the surface. These loops carry a vast amount of energy with them, and normally carry that energy up the loop and back down into the Sun.

But if a bunch of these loops get tangled, they can interact and connect with each other, releasing their energy all at once. The resulting explosion dwarfs our planet’s entire nuclear arsenal and is what we call a solar flare. Sometimes, this can trigger an even larger release of energy in the Sun’s outer atmosphere (the corona). That’s a coronal mass ejection. The expanding bubble of subatomic particles and energy sweeps out into the solar system, and if it hits the Earth, it can connect with our own magnetic field, creating all kinds of havoc.

Damage done to a transformer during the 1989 solar storm. Click for more info.

Photo from NASA

The fierce blast of subatomic particles can fry satellites and can induce large currents of electricity deep within the Earth. This can in turn create a surge of energy that can cause blackouts; just such an event in 1989 blew out transformers in the U.S. and caused a blackout in Quebec.

The largest such solar storm ever seen was also the first one ever seen: the 1859 Carrington Event. This storm was so powerful it blew out telegraphs across the U.S. and caused aurorae all over the planet.

Daniel Baker, a solar astronomer at the University of Colorado, estimates that the July 2012 storm was at least as powerful as the Carrington Event. Had it hit us, the results would have been catastrophic.

I want to be careful here. I always try to be very careful not to either overplay or understate the risks from astronomical events—some people panic over things that are very low risk, for example, and I certainly don’t want to exaggerate dangers. But in this case, there’s no other way to say this: If this 2012 CME had hit us, it would’ve been a global disaster.

Many satellites would have been fried, their electronics shorted out. That alone would mean we’d lose billions of dollars in space assets, not to mention the loss of international communications, weather prediction, and all the other critical systems that depend on satellites.

On the ground things wouldn’t have been so hot either. There would have been widespread power outages. Large transformers would’ve been destroyed by the enormous current flowing through them induced by the storm. These transformers can take months to build and deploy; imagine a large portion of the United States without electricity for several months—especially in the staggeringly hot months of July and August, when the load on the power grid is already very high.

So no kidding, an event like this would have been very, very bad. I’m glad it missed!

The Sun blasted out a series of high-energy flares over the course of a day in 2013, seen here in the far ultraviolet using the Solar Dynamics Observatory. Solar flares like these can sometimes trigger CMEs, like the one that blew out of the Sun in 2012.

Photo by NASA / SDO

But mind you, that was entirely due to chance. It could easily have hit us. Looking over storms from the past 50 years, Baker estimates the odds of the Earth getting hit by a similar storm in the next 10 years as 12 percent. That's a bit higher than makes me comfortable, to put it delicately.

There’s literally nothing we can do to prevent such storms from occurring on the Sun. However, there are ways we can mitigate the damage they can cause. Satellites can be made to resist the effects of such a storm, for one. Another is that the power grid in the US was designed and built in the days when the load was much lower than it is today; it could be upgraded to carry or dissipate the extra current generated by a big CME.

Of course, these techniques cost money. A lot of it, certainly billions. But estimates of the damage the 2012 event could have caused top $2 trillion. And that doesn’t include the resulting human suffering.

Perhaps our best course of action is early warning. Ashley Dale, an engineer at the University of Bristol, and his colleagues on the SolarMAX initiative, say that deploying small satellites around the Sun can measure its magnetic field and the environment into which the solar storms flow. This can help us predict when and where big storms might occur, and whether they are aimed at Earth. They estimate this could give us at several days warning; critical time needed to divert power on the grid, shut down vulnerable power lines, reorient satellites, and more. All of these actions could prevent much of the damage a storm could inflict.

I’ve been beating this drum for some time; I wrote about it in my book Death From the Skies!. I take this threat seriously, and given what we’ve learned about this event, I think everyone needs to.

July 31 2014 12:53 PM

50 Years Ago Today, Ranger 7 Took a Lunar Death-Dive

On July 31, 1964—50 years ago today—the Ranger 7 probe snapped the very first picture of the Moon ever taken by a U.S. spacecraft. Here's what it saw:

Ranger 7 pic of the Moon
Not long before the end, Ranger 7 had this view of the lunar surface.

Photo by NASA

Ranger 7 was the first of many to follow. It was designed to do two things: impact the Moon and take pictures along the way. It worked beautifully. That picture above was taken at 13:08:45 UTC, when it was just over 2,000 kilometers from the surface. It took thousands of pictures before impacting minutes later (I've always loved the final image it took).


I have to say, thinking about this gave me chills. We've been exploring other worlds via spacecraft for (just barely) longer than I've been alive. Since that time, more than 50 additional spacecraft have achieved orbit around another celestial body (not including those orbiting the Sun). Many more have performed flybys; every big planet in the solar system has been visited, and we have a probe flying past Pluto next year. A dozen humans have walked on another world, and more than 500 have been into space.

I'm tempted to say we live in a time of miracles of wonders, but they're not miracles. They're the achievements of human beings using math, physics, and engineering. Using science. And using their imagination and determination because they knew, no matter what, we will and we must explore the Universe.

July 31 2014 10:48 AM

2,000 Kilometers to Go

The Rosetta spacecraft is now less than 2,000 kilometers from the comet 67P/Churyumov-Gerasimenko (which I’ll just call ChuGer, for obvious reasons). In less than a week the European probe will enter orbit around the 4-kilometer-wide chunk of ice and rock, but even from this distance it’s starting to return pretty great images of it. On July 29, 2014, it took this shot with its main OSIRIS camera:

comet 67P/Churyumov-Gerasimenko
The two mini-worlds of the comet 67P/Churyumov-Gerasimenko.


The comet’s solid nucleus is definitely a freaky place. Not only does it have those two big chunks, there appears to be a brighter collar or ring of material in the neck region. It’s too early to know what’s going on there, though it may provide a clue as to why the comet has that overall shape. It may have resulted from two different-sized comets sticking together after a low speed impact, or it could be it was a single comet and has eroded away to leave that shape (a lot of the solid material in a comet is what we would think of as frozen gases—carbon dioxide, carbon monoxide, ammonia, and so on—and as it approaches the Sun they sublimate away).


My own best guess is that it was a single comet that got hit by a smaller rock and shattered from the impact. The pieces spread out, but not very far, and their combined gravity, feeble as it was, pulled it all back together in a jumble with those two big pieces dominating.

The bright collar is puzzling. It’s not like a snowball a few kilometers across has a lot of gravity—a good, solid jump would launch you into space if you were standing on it—but there’s some. The neck region would have the weakest gravity, so pebbles, rocks, dust, and so on would tend to flow downhill, to the two lobes. That might leave the brighter ice behind to create that ring. I’m completely speculating here, but hopefully we’ll know more soon.

At around the same time that higher-resolution photo was taken, Rosetta also took one with its lower resolution NavCam, and got this:

comet 67P/Churyumov-Gerasimenko
The cosmic rubber duckie has eyes!

Photo by ESA/Rosetta/NAVCAM

Interesting! While fuzzier, you can see a handful of craters, including the two big ones, one on each lobe. This is definitely a promise of things to come.

Rosetta will enter orbit around ChuGer in early August, and the imagine quality will continue to improve. Exciting times are ahead! My friend Emily Lakdawalla at The Planetary Society has lots of details on what’s going on and what will happen then, too.

July 31 2014 7:30 AM

A Weird Angle on a Weird Crater

The Lunar Reconnaissance Orbiter has been circling the Moon since 2009 and has taken approximately eleventy kajillion pictures of the lunar surface. They’re all pretty amazing, and it’s seen some incredible things. But of all of the images LRO has taken, I think my favorites are the ones where an object is seen at an oblique angle. The main camera is designed to look straight down, so that circular craters always look round, but sometime the angle is changed so that craters are seen more from the side.

When that happens, you get some pretty cool shots, like this one of an unnamed crater about 11.5 kilometers (7 miles) across, located in the Moon’s southern hemisphere:

double ring crater on the Moon
Looks like your soufflé collapsed. Click to impactenate.

Photo by NASA/GSFC/Arizona State University


That picture was taken from an angle 57° away from vertical, so the crater is foreshortened. Still, it looks weird, doesn’t it? The crater rim looks normal enough, pitted with smaller impact craters and being slowly eroded by micrometeorites, the solar wind, and the day/night thermal cycle pulverizing rocks over the eons.

But the floor is odd. Instead of being flat, like some craters, or dropping down into a bowl shape, there’s that peculiar humped ridge running along the inside of it (it looks like part of the wall of the crater has slumped, too, causing those pile-ups around the slope). It almost looks like there’s another ring inside of that one, too. What causes this?

An image of the crater taken from LRO on a different pass, when it was looking straight down. The peculiar polygonal shape is likely due to collapsed material sliding down the crater wall. Click to embiggen.

Photo by NASA/GSFC/Arizona State University

Actually, the physics of creating double or multiple ringed craters isn’t well understood. However, it seems like a good bet that they’re due to an impact from a comet or asteroid into terrain that’s layered. Instead of a single layer of rock, there may be a layer underneath the surface made of some other material. If the layers have different structural strengths, they’ll each make a ring; the surface layer will make the outer rim, and the inner layer will make the inner ring.

On Mars, this is seen when rock overlays ice. The Moon doesn’t have a thick ice layer under the rock, though. Interestingly, this lunar crater sits in the Apollo Basin, a huge, flat area about 500 km (300 miles) across. Basins on the Moon are caused by enormous impacts from asteroids or comets themselves dozens of kilometers across (way bigger than the one that wiped out the dinosaurs). The impact can punch a hole through the Moon’s crust, and lava can partially fill the impact crater. That gives the impact site a flat interior, and you can see that’s true for Apollo … as well as it being double-ringed as well! That sometimes happens in very large impacts, and the physics of why that happens is even less well-understood than for smaller ones.

Apollo Basin
An elevation map of Apollo Basin taken using an altimeter on LRO (blue is low, red high), with the location of the unnamed crater marked. The inner ring of the basin is obvious.

Photo by NASA/Goddard

The unnamed crater sits about halfway between the Apollo Basin’s inner ring and outer rim. It’s possible that the surface there is layered, but interestingly other craters in the area don’t show that double ring structure. Maybe the layering under the unnamed crater is local. Maybe something else is at play here. It’s hard to say.

And one final point. The crater we’re looking at here is 11.5 km across, 10 times wider than the famous Barringer Meteor Crater in Arizona. The diameter of a crater increases roughly as the cube root of the energy of the explosion, so the rock that smacked into the Moon to make this crater did so with 1,000 times the energy of whatever made Barringer … which was itself equivalent to roughly a 15 megaton bomb!

Whatever carved out this crater on the Moon, it did so with a single explosion far larger than every nuclear weapon on Earth combined. And yet, on the Moon there are so many craters that size (or far bigger) that no one has even bothered to give this one a name.

The history of the solar system is violent. And yet it has spawned incredible beauty.

July 30 2014 1:01 PM

James Inhofe Slammed on Global Warming

Sen. James Inhofe (R-Okla.) is more than just a global warming denier. He’s a conspiracy theorist (calling warming a “hoax”) and, shockingly, hugely funded by the oil industry. Reading things he’s said about global warming is like perusing a denier’s playbook of nonsense.

So this story comes as no surprise to me, even as it elicits a long sigh: Sen. Amy Klobuchar (D-Minn.) submitted a resolution simply acknowledging that global warming exists and poses a threat to the interests of the United States—which it does, on both counts.


Of course Inhofe blocked it.

Inhofe’s action was so egregious, so ridiculous, that Sen. Sheldon Whitehouse (D-R.I.) took to the floor and magnificently schooled him on reality. Watch:

It should be said that Whitehouse is a new hero of mine. Inhofe’s stance is stupidly dangerous: Global warming is real, the climate is changing faster than it has in thousands of years, and the fault lies in ourselves.

But denial is in the blood of too many of our representatives. It’s become a Republican mantra to say, “I’m not a scientist, but …” as if that excuses them to say any nonsensical thing they want. The proper response to a statement like that is what Charlie Crist, gubernatorial candidate for Florida, said: “I'm not a scientist either, but I can use my brain and I can talk to one.”

If only all politicians did that … and actually listened to them, instead of to the pipeline of dollars flowing to them to which they’re apparently beholden.

Tip o’ the thermometer to Amy Klobuchar and Michael Mann.

July 30 2014 7:30 AM

Geysers on Enceladus Run Deep and Are Powered by Saturn Itself

In the outer solar system, a moon of Saturn is erupting. Enceladus, a ball of ice 500 kilometers across, has more than 100 geysers erupting near its south pole, and they’re blasting nearly a thousand tons of water into the sky every hour. After six years of observations using the Cassini spacecraft, astronomers are finally starting to get a handle on what’s causing this massive outbreak. They’ve also found clues that liquid water is making it from deep inside the moon all the way to the surface.

A little background: The geysers were discovered when Cassini took backlit images of Enceladus; that is, when the Sun was behind the moon. The surface of the moon was dark, so longer exposures were taken. A faint fan of light was seen coming from the moon’s south pole, and it was quickly discovered that it was in fact due to gigantic plumes from water geysers reaching hundreds of kilometers high.


After they were discovered, Cassini’s orbit was altered so it would pass the little moon many times. At one point the spacecraft actually flew right through the plumes, and it was able to sample the constituents. It found they were mostly made of water ice, but there was also the presence of salt and organic compounds. Not life, of course, but an intriguing list of ingredients that contained some of the needed elements for biology.

The questions remained: What is causing these geysers? Where is the water coming from? And there’s more: Hot spots (well, warmer than the surrounding ice, but still well below the freezing point of water) were seen using thermal imaging, and they lined up with the geysers. Were they causing the geysers, or were the geysers heating up the ice around them?

Enceladus hot spots
Hot spots (orange and red) follow deep cracks in the surface of Enceladus and are associated with the geysers.

Photo by NASA/JPL/Space Science Institute

Two new papers have just been released that are the culmination of all those years of work: The geysers—101 in total—are powered by the mighty gravity of Saturn, they are the source of the heat (and not vice versa), and the water does appear to be coming from deep within the moon. Here’s how that all works.

The strength of gravity drops with distance. That means the side of Enceladus facing Saturn is tugged a bit harder than the far side. This results in a stretching force, called the tidal force. Moreover, Enceladus doesn’t orbit Saturn in a perfect circle. Sometimes it’s a bit closer, sometimes a bit farther. The orbit is also tilted a wee bit, so different parts of the moon experience different tides from Saturn.

This constantly changing stress has created deep cracks near the south pole of Enceladus. Called sulci, there are four of them, roughly parallel, stretching across the terrain. (They’ve been nicknamed “tiger stripes.”) The tides from Saturn’s gravity opens and closes these cracks as the moon orbits around the planet. The geysers are erupting from these cracks, as you can see in this nifty 3-D video created using Cassini data:

Cassini observations have shown definitively that the plumes wax and wane in strength at different parts of the orbit, so they are certainly caused by tides. However, oddly, there’s a 5.7 hour delay in the maximum brightness in the geyser plumes versus what you’d expect from where the moon is in its orbit. While tides are driving this phenomenon, the delay means there’s something else going on as well. It’s not clear what it is, which means there’s more yet to understand here.

Cassini has a high-resolution thermal camera on board, too. It found hot spots corresponding to the geyser locations all along the tiger stripes. It was proposed that these may have been due to friction; literally the edges of the cracks rubbing together as the moon flexed over its orbit. If true, that meant the hot spots may have been causing the geysers.

However, that turns out not to be the case. The sharp images of the hot spots show them to be very small, just a few tens of meters across, roughly the size of a tennis court. That’s too small to be explained by frictional heating. So what’s causing them?

Diagram of geysers
What lies beneath. Click to oldfaithfulenate.

Diagram by NASA/JPL/Space Science Institute

It turns out to be the geysers themselves! The diagram above shows what’s going on. Deep under the surface of Enceladus is a vast repository of water (possibly a global ocean, or it may just be local lakes under the pole). Tidal flexing keeps the water liquid, and at the times in the orbit when the cracks open, the water can travel up the cracks. Under pressure, it erupts at the surface, blasting out the plume. As it does, it cools, releasing its internal heat. That warms the surface, creating the hot spots.

This is actually pretty amazing. It means that the geysers are occurring near the surface, but begin their journey much farther down. It also means that water from deep underneath the surface is mixing with the ice on top, and temporarily creating liquid water at the surface—the only other place in the solar system where that’s known to happen.

As Cassini imaging team leader Carolyn Porco points out, there are about 1,000 geysers on Earth, which means that about 10 percent of all the known geysers in the solar system are on Enceladus. That’s a nifty factoid for your next cocktail party.

As I said, there’s still many unknowns here, and the data are still coming in. Unfortunately, time is running out for Cassini: The mission is scheduled to end in 2017. I’m of two minds about that; I know that funds are limited, and the mission began in earnest when Cassini entered orbit around Saturn in 2004. That’s a long time for a planetary mission to go!  On the other hand—and this is a much bigger hand—Cassini is doing phenomenal work. Much of what it’s discovered has been because it’s been in orbit so long. It’s had a long baseline to observe and has seen amazing events that a shorter mission would have missed entirely.

Extending Cassini’s life would be a great idea, obviously, if NASA had the funding. That’s a rant I’ve made many times before, but Cassini is grand evidence supporting it. The longer we stay at Saturn, the more we learn about this ridiculously fascinating world and its fleet of moons. And that’s exactly what we should be doing.

July 29 2014 9:15 AM

Noctilucent Clouds … Frooooom Spaaaaaace!

I recently wrote an article talking about noctilucent clouds—relatively rare high-altitude clouds usually seen just after sunset and before sunrise. They have a milky, silvery appearance, and are usually pretty hard to capture on photos.

It can be even harder from space, where lighting conditions are harsher and getting the right exposure balance is difficult. But astronaut Reid Wiseman got it just right recently, snagging a photo of the odd clouds from the International Space Station:

noctilucent clouds
An eerily glowing noctilucent cloud above the curved edge of the Earth, as seen from the space station. Click to embiggen.

Photo by NASA


Conditions to create noctilucent (literally, “night shining”) clouds are touchy, which is why they’re rare. But there have been a lot seen recently—check out this astonishing photo taken over an alpine lake in Germany—and that has many folks wondering what’s going on. There could very well be a link with them and global warming, which is intriguing but doesn’t have a lot of evidence to support it yet.

But if we keep seeing more of these clouds, we may yet get a better understanding of them, and whether or not they are a canary in a coal mine of global warming.

July 29 2014 7:30 AM

The Littlest Victims of Anti-Science Rhetoric

After all these years advocating for science, and hammering away at those who deny it, I’m surprised I can still be surprised at how bad anti-science can get.

Yet here we are. Babies across the U.S. are suffering from horrific injuries—including hemorrhages, brain damage, and even strokes (yes, strokes, in babies)—because of parents refusing a vitamin K shot. This vitamin is needed to coagulate blood, and without it internal bleeding can result.


Vitamin K deficiency is rare in adults, but it doesn’t cross the placental barrier except in limited amounts, so newborn babies are generally low in it. That’s why it’s been a routine injection for infants for more than 50 years—while vitamin K deficiency is not as big a risk as other problems, the shot is essentially 100 percent effective, and is quite safe.

Mind you, this is not a vaccine, which contains minuscule doses of killed or severely weakened microbes to prime the immune system. It’s a shot of a critical vitamin.

Nevertheless, as my friend Chris Mooney writes in Mother Jones, there is an overlap with the anti-vax and “natural health” community. As an example, as reported by the Centers for Disease Control and Prevention, in the Nashville, Tennessee, area, more than 3 percent of parents who gave birth in hospitals refused the injection overall, but in “natural birth” centers that rate shot up to 28 percent. My Slate colleague Amanda Marcotte points out that vitamin K levels in breast milk are very low as well, and that’s the preferred technique for baby feeding among those who are also hostile to vaccines. In those cases, getting the shot is even more critical.

But the anti-vax rhetoric has apparently crossed over into simple injections. Chris has examples in his Mother Jones article, and there’s this in an article in the St, Louis Post-Dispatch:

The CDC learned that parents refused the injection for several reasons, including an impression it was unnecessary if they had healthy pregnancies, and a desire to minimize exposure to “toxins.” A 1992 study associated vitamin K and childhood leukemia, but the findings have been debunked by subsequent research.
“We sort of came to the realization that parents were relying on a lot of sources out there that were providing misleading and inaccurate information,” said Dr. Lauren Marcewicz, a pediatrician with the CDC’s Division of Blood Disorders. 

By “sources,” they mean various anti-science websites and alt-med anti-vaxxers like Joe Mercola (who has decidedly odd things to say about the vitamin K shot, which you can read about at Science-Based Medicine). Despite the lack of evidence of harm, some parents are still buying into the nonsense, and it’s babies who are suffering the ghastly consequences.

These include infants with brain damage, children with severe developmental disabilities, and more, because of parents refusing a simple shot for their infants. The irony here is extreme: These are precisely the sorts of things the anti-vaxxers claim they are trying to prevent.

The Centers for Disease Control and Prevention has a great Web page about Vitamin K: what it is, why we need it, and why babies need it even more so. It will answer any questions you have about this necessary vitamin.

If you’re about to have a baby or have had one recently: Congratulations! It’s one of the most amazing things we can do as humans, and I will always remember watching and participating in my daughter’s birth. I would have done anything to make her ready for the world, and for me—for every parent—that includes getting the real facts about health.

July 28 2014 11:00 AM

Phobos’ Googly-Eyed Transit

Mars has two moons: Phobos and Deimos. Both are lumpy, rather irregular potatoes, and quite small. Deimos is only 15 kilometers along its long axis, and Phobos is about 27.

Phobos is weird for more reasons, too. It orbits the planet very low, only 6,000 km or so above the surface of Mars. It moves so quickly around the planet that it actually goes around faster than Mars rotates, so it rises in the west and sets in the east—twice each Martian day.


It just so happens that the rover Curiosity is in a location on Mars where things line up just right such that every so often Phobos passes directly in front of the Sun. I’ve written about these transits before, but I somehow missed this one from Aug. 20, 2013. Happily, Robert Krulwich on his NPR blog wrote about it, to my delight.*

The photo above shows two images of the transit, which I couldn’t resist because it looks a bit googly-eyed. Someone tell Anne Wheaton and Bonnie Burton!

But those are just two images; Curiosity took quite a few … and the folks at NASA’s JPL put them together into this amazing video showing the moon moving right across the face of the Sun:

I love this video because it shows the whole transit, because the transit isn’t partial (with the moon cutting a shallow chord across the Sun, say), and because it’s in real time! The frame rate was set so that what you see here is pretty much what Curiosity saw: a 37-second long event.

On Earth, the Moon and Sun are about the same apparent size in the sky, because in a cosmic coincidence the Sun is 400 times bigger than the Moon but also 400 times farther away. Phobos is smaller than the Moon, but much closer to Mars, so it appears about half the size of our Moon. But Mars is also farther from the Sun, so the Sun looks smaller there too, and Phobos does a decent job covering it up.

It’s really odd to see something like this, knowing that what you’re seeing isn’t an airplane or a cloud, but an actual rocky moon orbiting far overhead.

… and of course, that’s nothing compared with knowing that this sequence was taken by a machine sitting on the surface of another world.

*Correction, July 28, 2014: This post originally misspelled the name of NPR.