The entire universe in blog form

July 23 2014 7:30 AM

The Longest Year

I don’t usually write about newly discovered record-breaking objects found by astronomers, because in general it’s not long before that record falls. But in this case, I’ll make an exception for Kepler-421b. It has the longest year—that is, it has the longest orbital period around its star—for any exoplanet yet seen.

That by itself is enough to make this an interesting object, but even cooler (literally) is where that puts this planet: Far enough from its star that it may have formed in a different way from the other planets we’ve detected around other stars. It may very well be an ice giant, like Uranus or Neptune, and not a gas giant or rocky planet.

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First, let’s go through the basics: The host star is Kepler-421, a star much like the Sun but a bit smaller and cooler. It’s located about 1,000 light years away, which is a fair ways (the Milky Way galaxy is 100,000 light years across). From Earth, that makes the star pretty faint.

The planet, Kepler-421b, was discovered by the Kepler observatory, a space-based telescope that has found so many of the recently-discovered exoplanets. It uses the transit method to find planets; if we see the planet’s orbit around its star edge-on then every time the planet passes between us and the star it blocks a bit of the star’s light. It’s tricky; for example Kepler-421b only blocks about 0.3 percent of the star’s light. But with modern detectors, that sort of dip in light is detectable.

Kepler-421b light curve
The "light curve" for the star Kepler-421, a plot of brightness over time. The blue points are from the first transit seen, and the red ones from the second transit 704 days later. The bottom points are the residuals after subtracting a model fit to the transit curve. The takeaway: It's a really good fit.

Plot by D. Kipping et al.

Generally speaking, you need three transits to be sure you’ve got something. If you see just one it could be a starspot, or some other nonplanetary object interfering with your observations. A second transit tells you the orbital period (the year) of the planet, but it could still be a coincidental starspot. If you get a third transit at the right time interval after the second, then you can be more confident.

transit diagram
If a planet's orbit is aligned with our line of sight, we see a transit (bottom), otherwise, we don't (top).

Adapted from a diagram by Greg Loughlin

For Kepler-421b, the astronomers only saw two transits, which made me suspicious, but after reading their paper I’m more inclined to think they got it. The shape of the “light curve” and the incredible match between the two transits make it very likely they did find a planet. For the rest of this article I’ll just assume it exists, but remember that it has yet to be confirmed independently.

Kepler-421b is about four times the diameter of Earth (judging from how much of the starlight it blocked), and has a year that’s 704 Earth days long. That’s amazing; most exoplanets found have much shorter periods, like days or weeks. That orbit puts it about 180 million kilometers (110 million miles) out from the star. Since the star is cooler than the Sun, the planet actually receives about one-fourth the light from its star as Earth does from the Sun. That’s even less than Mars gets, so the planet is pretty chilly.

And that brings us to the second cool thing about this planet. Planets form from broad disks of material orbiting the star when it’s young. Close in it's hot (duh) so you don’t get much gas or ice. The material in the disk is mostly metal and rock. Farther out there’s still metal and rock, but water is in the form of ice (this distance is called the “snow line,” a term I like), and there’s lots of it. Giant planets that form at least that far out have a lot more ice than ones farther in, and we call them ice giants. To be clear, these aren’t giant ice balls; they look a lot like gas giants but have more ice in them as opposed to rock and denser stuff.

In our solar system, Uranus and Neptune are ice giants. Given Kepler-421b’s location, it should be one as well. If we assume it’s about as dense as Uranus, it has 16 times the mass of the Earth. That will likely give it a thick atmosphere (and it’s very cold, remember) so it’s not Earth-like at all.

But it’s the first ice giant seen orbiting another star. We’ve seen other planets with similar masses and sizes, but they orbit closer in, and are likely gas giants. Ice giants may very well be pretty common among exoplanets, but they’re pretty hard to detect. For one, the long period means you have to wait a long time to confirm them. Also, the bigger the orbit is, the less likely it is we’ll get a transit — a planet close in to its star can be seen to transit from a wide range of viewing angles, but a more distant planet needs a more tightly constrained viewing geometry (the orbit has to be more precisely edge-on) for us to spot it.

Finding Kepler-421b means that astronomers may be able to start finding more. Seeing one planet might be an anomaly, but if you find 20 more like it you can start categorizing them. This means they can use physics and models to understand better how planets form, especially that far from their parent star. We’re still figuring that out for our own solar system, so having other examples with which to compare and contrast is very helpful.

And so that’s why I’m willing to write about a record-breaker, even if that record is soon broken. As usual in astronomy, I hope it is! That turns this planet from a weirdo into the first member of its class, and that means we get to learn stuff. And astronomers love learning stuff.

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July 22 2014 11:29 AM

Another Mind-Crushing Illusion: Straight or Curved Motion?

From the twisted mind of brusspup comes another brain-hurting illusion. This one is really, really convincing, so tell me: When you look at this video, you’re seeing a circle of eight dots rotating as it spins around inside a bigger circle, right?

No, you’re not. As brusspup shows, each individual white dot is moving in a straight line! The trick here is two-fold: One is that the dots aren’t moving at constant velocity (you can see that in the video at the 0:44 mark), and that combined their motion mimics what we’d see if a smaller circle is rolling around inside a big one.

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Try as I may, when I look at this video I can’t make my brain see the dots moving linearly; it looks like a circle rolling. If I focus on one of the dots I can see it moving back and forth along a line, but the others still look like the rim of a circle rolling around. For most illusions there’s a moment when your brain can see what’s going on and the illusion shatters, but not with this one. It’s maddening.

When I was a kid, Spirograph was a very popular “game.” It wasn’t really a game, but a set of clear plastic disks with gear teeth around them (or rings with teeth on the inside). They had holes in them; you’d pin a ring down on a piece of paper, then take another disk, place it inside the ring, put your pencil tip in a hole, and roll the inner disk around inside the outer ring. The results were really lovely and graceful interlocking and overlapping curves. If you’re a lot younger than me and missed this craze, here’s a video that’ll help you picture it:

Man, I miss Spirograph. It was so much fun*! And this dots illusion is related. In Spirograph, when you’d use your pencil to roll around the inner disk, the motion you made was very similar to what you’re seeing in the illusion; it was more of a back-and-forth motion than an around-and-around one. It’s difficult to explain without math, which I find funny; I have a visceral feeling for it because of all those hours I spent playing with a Spirograph when I was little.

If you want the math, then here you go: The shapes made this way (tracing the motion of a point on a circle as it rolls) are called cycloids, and there are a lot of varieties: epicycloids, hypocycloids, and others, depending on how the inner circle is rolled. I once modeled the shell of gas around a supernova as an epicycloidic shell (like a peanut shell), and it reproduced what we saw with Hubble pretty well (even though it formed in a very different way than a cosmic Spirograph!).

cycloid motion
Cycloid motion: As a circle rolls on a line, a fixed point on its rim traces out a shape called a cycloid.

Photo from Wolfram Alpha

I’ll note that when a circle rolls along a straight line, if you watch a single point on it you can break the motion up into two dimensions: Horizontal and vertical motion. These motions aren’t constant, but depend on the sine and cosine of the time elapsed. They start off motionless, accelerate to a maximum speed in the middle, then slow back down to zero … and in each direction, the point moves linearly! It’s only when you combine them that you get the cycloid.

That’s how this illusion works. By mimicking this trigonometric motion, your eyes and brain are fooled into thinking the dots are acting together, portraying the rim of a circle. But they aren’t; their motions are related but independent of one another (what’s called “parametric” in mathematics).

A-flippin’-mazing. But also MATH! And SCIENCE!

And another in a long, long series of illusions that shows very well that seeing is not necessarily believing. Our brains are very easily fooled, and that’s very important to remember in life.

[Tip o' the Necker cube to a lot of folks, including Oren Geva, Mitchell Moffit, and Gizmodo.]

Related Posts: More illusions by brusspup and others

* Holy cow! Even in today’s app-based world, a good old manual analog Spirograph set is still available to buy! I may buy a bunch just to give to friends who have kids. Or even if they don’t.

July 22 2014 8:24 AM

Plunging Deep Into Colliding Galaxies

Roughly 50 million light years from Earth is the most spectacular example of galactic collision in the sky: the famed Antenna Galaxies, two huge spiral galaxies in the middle of a cosmic train wreck. Playing out over hundreds of millions of years, the gravity of the two galaxies has distorted their shapes, flung out streamers of stars a million light years long, and triggered a burst of star formation so intense that billions of new stars are being born in the galaxy’s hearts.

As mighty as this event is, 50 million light years is a long, long way. The details of this collision used to be discernible only using giant observatories like the Very Large Telescope or Hubble.

But not anymore! Behold Rolf Wahl Olsen’s newly-released picture of the Antennae and prepare to scrape your jaw off the floor:

July 21 2014 11:03 AM

Go Forth and Science

Thank you.

In April 2014, Slate announced its new paid membership program called Slate Plus. The Hive Overmind at Slate asked us writers to promote it and even had a contest: Whoever got the most people to sign up would get a $500 bonus.

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Seeing that I have a pretty big audience here and that I was asking that audience to pay for something, it didn’t feel right to keep that money if I won. So I announced that if I did win I’d give it all to Donors Choose, a non-profit group that organizes donations to classroom teachers around the U.S. (think of it as a Kickstarter for learning).

Well, guess what? A lot of you folks signed up for Slate Plus. Enough so that I won the contest.

Yay!

donorschoose_completed
Seeing this felt really, really nice.

Photo by Donors Choose

So I just had a delightful time going through the various science teachers’ pages at Donors Choose, looking for projects that needed funding. I found quite a lot, so I narrowed my criteria: Projects that were near full funding but just shy of the goal, coupled with good science, coupled with classrooms that needed the money, coupled with teachers who seemed to have that special quality, that spark, that got me so excited about science when I was a kid.

I found four that (literally) fit the bill: Learning in Motion (Mr. Estrada), Bringing Space to Our Room! (Mrs. Gibson), Mad Scientists Explore (Ms. Sunnucks), and Bring Vital Learning Technology to My Classroom (Ms. Carr). All four of these projects are now fully funded, and these educators can go forth and teach their kids science.

To all of you who helped out, thank you. You got more than just a subscription to Slate Plus; you helped hundreds of children across the country get a chance to explore the Universe around them.

You should feel really good about that. I know I do.

And if you like, there are plenty more worthy projects still needing funding at Donors Choose. Go.

July 21 2014 7:30 AM

Robert F. Kennedy Jr.: Still Fighting the Wrong Fight

Let’s get this out of the way right at the start: I’m not a big fan of Robert F. Kennedy Jr. This is almost entirely due to his grossly erroneous belief that a preservative in some flu vaccines causes autism. This preservative, thimerosal, has been tested thoroughly by many different groups and has never been found to have any connection with autism.

Got it? Vaccines don’t cause autism. It’s really that simple.

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Despite this, RFK Jr. continues to beat this drum. In a recent Washington Post article, journalist Keith Kloor wrote about RFK Jr.’s attempts to talk to two senators about his crusade against thimerosal and about a new book he’s published about this topic. In the past, Kloor has been pretty matter-of-fact about RFK Jr.’s bizarre claims, so I expected this would be a pretty tough profile.

It wasn’t. Now, I don’t mean that Kloor treats RFK Jr. with kid gloves; the article actually shows his claims to be dead wrong and portrays him as an outcast from the mainstream. That’s all fine. I just don’t think Kloor really showed RFK Jr.’s true nature; something we here at Slate have seen for ourselves.

In 2013 I wrote an article giving great details on just how over-the-top anti-vax RFK Jr. is, including his giving a talk at a rabidly anti-vax conference. After it posted, Slate got a call from RFK Jr. himself, requesting a chance to rebut over the phone. I declined; having read his writings and listened to his radio shows, I knew better than to let him rail away at me.

I was right. My editor, Laura Helmuth, decided to accept the call, and was subjected to RFK Jr.’s, um, opinions, for nearly an hour. She wrote an excellent article about it, describing his conspiracy theories and how he misrepresented what scientists told him. It’s an eye-opener.

And now, with this WaPo profile, Helmuth decided it was time to reiterate this point, so she has written another takedown of RFK Jr. I highly recommend reading it; it starts with this:

Most paranoid, grandiose, relentless conspiracy theorists can’t call a meeting with a U.S. senator. Then there’s Robert F. Kennedy Jr. 

… and it keeps going from there.

RFK Jr. has a lot of clout, quite a bit of which comes from his family name. But to me he is in the same heap as people like Jenny McCarthy—those who make baseless, unwarranted claims about vaccines, sowing doubt and fear about one of the greatest medical triumphs in human history. In fact, the similarity between the two is striking, since RFK Jr. claims—despite his own actions—that he is not “anti-vax”, a claim McCarthy recently made as well.

As the entire U.S. sees a huge spike in measles outbreaks, largely caused by unvaccinated people, and we’re also seeing a resurgence of other preventable diseases like whooping cough and polio (polio, for Pete’s sake!), making these outrageous claims about vaccines as RFK Jr. and McCarthy do is more than just irresponsible. It’s dangerous.*

I’ve been vaccinated my whole life, and I make sure to get my boosters as needed, too. I walk the talk. Don’t listen to people just because they have famous names. Talk to a board-certified doctor and get the facts.

* To be fair, RFK Jr.’s claims relate to vaccines that contain thimerosal, which are a tiny minority. But to be completely fair a) he’s still wrong, and b) it’s all grist for the mill for the overall anti-vax movement. Wrong is wrong, and RFK Jr.’s claims are wrong.

July 20 2014 7:00 AM

From the Earth to the Moon to the Earth

Forty-five years ago today—and for the first time in human history—human beings set foot upon another world.

It was one of the proudest moments in America’s history, arguably the proudest. Despite being initially motivated by small-minded territoriality, it ironically brought our planet together, with people all over the world watching breathlessly as Neil Armstrong placed his boot on the desolate surface of the Moon.

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Every year on this anniversary I reflect a bit on what happened then, how it affected me, and what it means today.

Some people fret over whether it was all worth it, taking this one small step. I have no such doubts; we are better as a species for having ventured into space. The evidence for this is overwhelming, from learning about our planet (and the dangers to it), to the very nature of humanity’s need to explore that is so fundamental to our psychology.

Venturing into space is not just something we can do. It’s something we must do.

And yet here we are. It’s been 45 years since we put men on the Moon and 42 years since the last men left it. We’ve not gone back since, at least, not with humans. Sure, we’ve made a lot of progress about living and working in space: We’ve launched several space stations, put more than 500 people into space, and built countless satellites and space probes. I’m fully aware of the awe-inspiring achievements we’ve made, and how much they mean.

But still, there is a hole in that picture. All of those people we’ve launched into orbit haven’t gone more than a few hundred kilometers above the Earth’s surface. The yawning chasm between the Earth and Moon hasn’t seen a human in it for over four decades.

Now, there's a lot to be said for low Earth orbit. It is a fantastic resource for science, and I strongly think we should be exploiting it even more. But it's not the goal. It's like walking halfway up a staircase, standing on your tiptoes, and admiring the view of the top landing.

When I look back over the time that’s elapsed since 1969, I wonder what we’re doing. I remember the dreams of NASA, and they were too the dreams of a nation: Huge space stations, mighty rockets plying the solar system, bases and colonies on the Moon, Mars, and asteroids. Those weren’t just the fantasies of science fiction. We could’ve done them. Right now, today, those dreams could have been reality.                                                                 

Instead, we let those small-minded human traits flourish. We’ve let politics, greed, bureaucracy, and short-sightedness rule our actions, and we’ve let them trap us here on the surface of our planet.

It needn’t have been this way, and it still needn’t be this way. There are those who still dream, who understand the call to space, and who are devoting their lives to make it reality. We’ve faced adversity before, and have not let it stop us.

I think we can overcome our own petty blindness. Sometimes we humans look up, not down, and see not just the Universe stretching out before us, but also our place in it.

We’ve done it before and we can—we must, and we will—do it again.

Per somnia et ardua ad astra.

July 19 2014 7:30 AM

Cruisin’ Over the Bahamas

I just got back from travel, and now I'm deep into planning my panels for San Diego Comic-Con next week, so at the moment I'm enjoying a slow, broiling panic.

But I couldn't pass up the chance to post this breathtaking picture of the ocean around the Bahamas taken by an astronaut on the International Space Station as it sailed over on July 15, 2014:*

Bahamas
The Bahamas and environs, froooooom spaaaaaaace. Click to archipelagenate.

Photo by NASA

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Yegads. You very much want to embiggen that.

The bright lights to the upper left outline Florida (the long glow is from Miami), and you can trace cities up the East Coast of the U.S. Cuba dominates the lower left (cut off a bit by an ISS solar panel), but the teal and turquoise waters are what draw the eye. The islands right in the middle are the Bahamas, and the bright glow smack dab in the middle of the picture, is (I believe) Nassau—remind me not to go stargazing there! The lights must wash out the sky. But that's probably not why people go to Nassau in the first place.

Speaking of the sky, note the green arc of light over the Earth's limb. This is called airglow, and it due to the slow release of energy from sunlight the upper atmosphere stores during the day. It's actually a fascinating physical process that I've described before. In that link I also talk about the brownish-yellow glow beneath it: That's from glowing sodium in the air, and the source of that sodium may be meteors that have previously burned up in our atmosphere!

Amazing. There's no such thing as just a pretty picture taken from space—there is always a lot more going on than you might think. And just like any artwork, knowing the story behind the beauty makes it that much more wonderful.

*Correction, July 19, 2014: I had originally identified the area photographed as the Caribbean Sea (taking my cue from the original NASA page about the photo), but the Caribbean is actually south of Cuba; what's pictured is the Atlantic Ocean.

July 18 2014 5:30 AM

Cosmic Duckie, You’re the One ...

(With apologies to Ernie.)

Right now, in deep space 400 million kilometers from Earth, the European Space Agency probe Rosetta is easing its way toward the comet called 67P/Churyumov-Gerasimenko (which, for obvious reasons, I’ll just call ChuGer). On July 14 it was a mere 12,000 km (7400 miles) from the comet—less than the diameter of the Earth! It took a series of images of the frozen ice ball, and it’s becoming very clear that this comet is really, really weird:

Rosetta comet
... you make space exploration so much fun. The original image is on the left, and a smoothed version on the right.

Photo by ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

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It’s not an ice ball … it’s an ice rubber duckie! And a big one, about 4 kilometers across.

As you can see, it appears to have two distinct components; a large, flattish piece and a smaller, rounder one attached off-center. You can see this a lot more clearly in an animation composed of 36 images taken about 20 minutes apart:

What the what? Note that the comet is only a couple of dozen pixels across as seen by Rosetta; the images in the animation (and on the right in the still image above) have been smoothed to give you an idea of what it looks like. Don’t take small details too literally, but the overall shape is apparently real.

It’s not clear why it has this shape, but there are several possible explanations of how it may have been molded this way.

Comets are big lumps of frozen water with dust and rocks mixed in. In fact, we sometimes call them “dirty snowballs.” It’s possible that two comets suffered a very low speed collision and stuck together, forming the weird shape.

However, that strikes me as very unlikely. Why? Space is big. That’s why we call it “space.” The odds of two comets coming that close together at just the right speed and angle to do this seem very low to me.

It’s more likely, in my mind, that ChuGer got whacked by a much smaller object, say a chunk of asteroid. Some comets aren’t solid, like rocks, but more like loosely bound rubble piles held together by the ice. Over the eons, small impacts would shatter it, creating deep cracks inside the comet. A slightly bigger collision could actually dislodge large chunks. If the impact speed were right, the big chunks might separate a bit and then slowly re-accrete over time due to their feeble gravity. What you’d get is a weirdly shaped object, like if you took a small snowball and stuck it on the side of a bigger one.

Which is just what we see with ChuGer. I’m totally guessing here, but I have a sneaky suspicion that’s what we’re seeing.

Another way it could have gotten this shape is that it used to be rounder and smoother, but over time eroded away. Comets have lots of ice, and as they approach the Sun that ice turns directly into a gas (which is called sublimation) and streams away. Anything less volatile (like rocks inside) will remain. If the comet has big lumps of rock inside it, or just big pockets of ice distributed through it, it could erode asymmetrically, leaving huge lumps like ChuGer’s.

Again, I’m guessing. We’ve only visited a handful of comets by spacecraft, and to be honest they’re all weird. Halley is a lumpy potato. Hartley 2 looks like a bowling pin or a dumbbell, with lobes made of carbon dioxide and a waist of frozen water. Other comets have been found to be similarly lobed, lumpy, and basically asymmetric.

The whole point here is that we’re exploring—we don’t know what we’ll find. If we did, it wouldn’t be exploring.

And we’ll find more, much more. Rosetta is still approaching ChuGer, and in early August will be a mere 100 km (60 miles) from the comet. It will enter orbit, examining the nucleus (the solid part of the comet) in excruciating detail. Then, in November, it will send out a lander named Philae to physically touch down on the comet’s surface! It will examine the comet and send the data back to Earth via Rosetta. Its planned mission will last about a week, and should be a huge bounty for astronomers and planetary scientists.

Over the next weeks I’ll be writing more about this amazing mission and the strange comet it’s studying. ESA doesn’t have the same policy of freely releasing images that NASA does, but hopefully when something new and spectacular is available, you’ll be able to read all about it here.

In the meantime, you can take a look at some of the amazing highlights from the Rosetta mission so far, including some jaw-dropping images of Earth, Mars, an asteroid named Lutetia (with a fantastic gallery), another lumpy diamond-shaped asteroid named Steins, and a lovely shot of the Moon rising over the limb of the Earth. These images show the promise of what we’ll see very, very soon once Rosetta is in orbit around 67P/Churyumov-Gerasimenko.

July 17 2014 7:30 AM

The Chaotic Glory of Starbirth

Robert Gendler is the gift that keeps on giving. He’s an accomplished astrophotographer (why, I would never make such a claim without ample evidence) who takes observations from professional observatories and adds to them images he’s taken himself and with other people. The result is substantial beauty … and here’s another bit of proof for you in the form of the ridiculously chaotic star-forming nebula NGC 1333:

NGC 1333
Turbulence, eruptive stars, gassy discharge, and sheer beauty: NGC 1333 has it all.

Photo by NAOJ/DSS/NOAO/Robert Gendler & Roberto Colombari

This cloud is a sprawling stellar nursery, a dense cloud that’s still collapsing, fragmenting, and creating stars hither and yon. One feature of many young stars is that they rotate rapidly, and are still surrounded by the thick disk of gas and dust from which they formed. Their still-intense magnetic fields get wound up like a corkscrew, and this can launch twin beams of matter out from the poles of the star.

HH object
Detail from the image above; a Herbig-Haro object in NGC 1333.
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These are called Herbig-Haro stars, and NGC 1333 is littered with them. I found several perusing Gendler’s image, and inset one here. The cloud is choked with thick, opaque dust, which hides a lot of the details. Infrared light can pierce that veil, though, so images from observatories like the Spitzer Space Telescope can reveal far more details. In fact, in that linked article I have an in-depth discussion about both of this nebula and the HH objects in it (and a close-up of one that actually has curved beams, which are lovely). I suggest giving it a read to truly grasp the awe-inspiring nature of this object.

Gendler’s image is comprised of observations from the 8.2 meter Subaru telescope, the Digitized Sky Survey, telescopes with the NOAO, and his own data in collaboration with Roberto Colombari. These are magnificent telescopes, one of the reasons the final image is so spectacular. Another is that NGC 1333 is only about 1,000 light years away, relatively nearby on a galactic scale (the Milky Way is 100,000 light years across). This makes it one of the closer massive star-forming regions in the galaxy, so we get a little more detail than we otherwise might.

It always amazes me that so much science can be found in objects so beautiful. I wonder what this tells us about our minds, our perceptions, and how we evolved to appreciate beauty ... and how useful an ability like that can be?

July 16 2014 7:30 AM

Jovian Armageddon +20

Twenty years ago on this day—July 16, 1994—Jupiter got slammed, hard, by a comet.

It was the first time we had ever witnessed such a collision on a planetary body other than the Earth. I’ll note I’m very glad it wasn’t Earth; we wouldn’t be here talking about it if the comet had had us in its crosshairs instead of Jupiter.

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The comet was discovered on March 24, 1993, by the team of Eugene and Carolyn Shoemaker and David Levy, who were taking pictures of the sky to look for comets. Right away they knew they had something odd—although comets are generally fuzzy and elongated, that’s usually due to their long, sweeping tails. This one appeared to have a long, fuzzy head. The head is the central part of the comet, the solid nucleus (generally made of ice and rock) surrounded by gas. Heads are usually pretty compact, but this comet had one that was stretched way out, like a thick line (I have a fun post describing all the parts of a comet and how they work).

SL9
The comet as it was seen a few days after discovery. Note the elongated head, and the tail which fans out above and below.

Photo by Spacewatch

Being the ninth comet the team had found, it was dubbed Shoemaker-Levy 9, or, more technically, D/1993 F2. Those in the know just called it SL9.

It was noticed that the comet was near Jupiter in the sky. The team backtracked the orbit in time, and found it had recently passed very close to Jupiter … and in fact, it was in orbit around the planet! At this point, a more complete picture emerged.

The comet had orbited the Sun for billions of years. But in the 1960s or 1970s it made a close pass at Jupiter, and the giant planet captured it. This is a relatively rare scenario and takes a fairly specific set of circumstances to occur. However given enough time and billions of comets, it’s inevitable.

The comet orbited Jupiter on a highly elongated path. On July 7, 1992, it passed so close to Jupiter that the planet’s mighty tides tore the comet apart. The pieces separated along a line, and when it was discovered a year or so later these distributed pieces are what gave the comet its squashed look.

And then astronomers got the big surprise: Calculating the orbit of the comet into the future, they saw it would physically collide with the planet in July of 1994!

SL9
The "string of pearls": The fragments of SL9 all in a row. Each piece? Doomed.

Photo by H. Weaver (JHU), T. Smith (STScI),NASA

I was in graduate school at the time, and boy, howdy, do I remember the excitement. No comet or asteroid had ever been seen hitting a planet besides Earth before. And SL9 was made up of dozens of individual pieces, each on a doomsday path to Jupiter! This was going to be big.

Unfortunately, the pieces were due to hit on the far side of Jupiter, which means we wouldn’t directly see the impacts. But it turns out we still got quite a show.

When the day came, practically every telescope on and above the Earth was watching. The first piece impacted at 20:13 UTC (2:13 p.m. ET) on July 16 … and within minutes pictures started pouring in from telescopes. Although the impact was on the other side of Jupiter, the huge plume from the impact rose so high it could be seen over the limb of the planet. Within a few minutes the impact site rotated into view, and we were all stunned to see photos showing a vast scar from the collision. The first piece (Fragment A) was about 2 kilometers in diameter and moving at 60 kilometers per second when it hit Jupiter—the energy equivalent of over a million one-megaton nuclear bombs exploding at the same time.

So yeah. We saw the impact scar. Here’s an infrared animation of the Fragment A impact made from observations using the 3.5 meter telescope on Calar Alto in Spain:

The bright spot to the upper right is Jupiter’s moon Io, and you can see the Red Spot, too. Then the impact blossoms as Jupiter’s rotation swung it into view. Yikes.

Within minutes, Hubble Space Telescope images were sent down, revealing the impact site: a vast splotch with a crescent-shaped fan of material blown ahead in the direction of the comet’s motion. Fragment A’s collision left a scar on the top of Jupiter’s clouds larger than Earth itself. It was stunning.

SL9 impact site
Fragment A impact site (using a filter that accentuates methane in Jupiter's upper atmosphere).

Photo by Heidi Hammel/MIT/NASA

Over the course of the next week the remaining pieces hit, one after another. Fragment G was the biggest, but several were over a kilometer across. Jupiter’s powerful gravity accelerated them to their doom, and each gave up their individual ghost with a titanic explosion.

In the end, this was one of the most studied events in the history of astronomy. And it was full of surprises. We expected to see a lot of water released by the impacts, since it was thought there was a layer of water vapor in Jupiter’s atmosphere. Some was seen, but not nearly as much as predicted. The impacts generated tremendous waves in the atmosphere of Jupiter that were seen traveling away from the collision for hours. The chemical composition of the atmosphere was temporarily altered as well.

plume
The plume from the explosion of Fragment A as it rose and collapsed over the limb of Jupiter.

Photo by NASA/ESA

A few things remain stuck in my memory from those days. One was how dang slow the Internet was; we’d wait forever to download a 100kb image of the impact from the SL9 sites where they were being uploaded. It didn’t help that a million other people were trying at the same time. Back then, the ‘Net was pretty new, and most servers couldn’t handle so many people trying to access them.

Another fun memory was a live press conference the Shoemaker-Levy team was giving. They were discussing the comet on air when my friend and planetary scientist Heidi Hammel ran in (live on TV, remember) with printouts of the Hubble observations showing the impact event. All three of the team members’ faces lit up, the fruition of their work in front of them. Not long after, news shows ran a clip of several joyous astronomers cheering as they stood around a computer monitor watching the images come in (one of them was another friend, Melissa McGrath, with whom I went to grad school, and I always got a kick out of seeing her jumping up and down with excitement). All of this was shown on a National Geographic show, and a clip is on YouTube:

The other vivid memory was from a few days later. I was at the University of Virginia, and we had a huge 24-inch telescope there, built in 1885. I went out to observe Jupiter, but the air was unsteady that night, and all I saw was a blur.

So I went around the observatory to “The Doghouse,” a smaller brick enclosure that housed a vintage 6-inch brass ‘scope. Pointing it at Jupiter, I was stunned to my core to clearly see several black scars punctuating the planet’s cloud tops. The bigger telescope accentuated the blurry air, but the smaller one didn’t have as much magnification, and the unsteady atmosphere of Earth didn’t affect the viewing as much. I remember standing there, slack-jawed, knowing I was witnessing history.

Since those days two decades ago we’ve seen Jupiter hit several times; in July 2009, in June and again in August 2010, and a fourth time in September 2012. We’ve seen Saturn’s rings get hit by debris (actually multiple times), the Moon hit by small asteroids, asteroids themselves collide, and we just missed seeing an asteroid impact on Mars … but a comet will pass so close to Mars later this year that it could rain debris down on the planet. We’ll see.

The lessons of SL9 are many. Impacts still happen across the whole solar system, even after 4.56 billion years. We need to keep watching the planets and look for such rare events. And, of course, the Earth is no exception as a target.

But looking back to those days, what I really remember is the excitement, the wonder, the anticipation, and how the public was just as fired up as we were. Every time I deal with some anti-science nonsense, some attack on what we know to be true, I remember this:

This stuff is real. And the public, when it comes right down to it, loves it as well. On this anniversary of one of the biggest bangs in the solar system, I think we all need to be reminded of that.

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