Careful analysis of the gravity fields of our solar system's "ice giants", collected by Voyager 2 decades ago, suggests that weather patterns on these worlds are confined to a layer less than 600 miles (1,000 km) deep.

Two of the greatest achievements in planetary exploration occurred when Voyager 2 made flybys of Uranus (1986) and Neptune (1989). Officially the Voyager mission called for reconnaissance of Jupiter and Saturn only, but getting to the solar system's two ice worlds completed a "grand tour" of the giant planets that had been in the cards all along.

You would think that, by now, planetary scientists would have milked those data for every tidbit of scientific value. But research published this week offers surprising new insights using Voyager 2's quarter-century-old measurements.

In some ways, Uranus and Neptune are near twins. For example, their outer atmospheres are both very deep mixtures of hydrogen, helium, and methane, and their diameters are within 500 miles (800 km) of matching. The planet's winds create a pattern of east- and west-moving jets that mirror one another closely.

Yet the spin axis of Uranus is tipped over by 98°, which means its poles and midsection experience huge swings in sunshine throughout its 84-year orbit. And though Neptune is twice as far from the Sun, it displays much more vigorous cloud activity — one jet stream was clocked at 1,500 miles (2,500 km) per hour, the fastest winds in the solar system — and its interior emits 1.6 times more energy than it receives from the Sun.

So does the energy to drive the storminess on these two planets come from their deep interiors, or is the weather layer skin deep, powered only by the weak light of a faraway Sun? Five researchers led by Yohai Kaspi (Weizmann Institute of Science, Israel) have taken a fresh look this question and conclude, in the May 16th issue of Nature, that the weather layers on Uranus and Neptune are at most 600 miles (1,000 km) deep.

They base this claim in part on the wind patterns seen by Voyager 2 and, in the decades since, by the Hubble Space Telescope. More critically, they used theories of global circulation developed by Kaspi and co-author Adam Showman (University of Arizona) to predict what the gravitational fields of Neptune and Uranus should look like. Then they compared those models to Voyager 2 gravity data. During each flyby, precise tracking revealed how the gravitational pull of Uranus and Neptune altered the spacecraft's trajectory. This in turn provided a rough estimate of the distribution of "rock," "ice" compounds like water and ammonia (virtually all in liquid form deep down), and gas inside each globe.

The researchers included a fudge factor for the mass of each planet's upper atmosphere, which can vary because — just as on Earth — regions of high and low pressure, and thus density, are in constant motion. Kaspi and his colleagues got the best fit when they restricted the dynamic "weather" layer to just the outermost 0.15% of Uranus's total mass and 0.20% of Neptune's. "Our analyses show that the dynamics are confined to a thin weather layer no more than about 680 miles deep," says team member William Hubbard (University of Arizona).

The result is at odds with a longstanding notion, first proposed by theorist Fritz Busse in 1976, that the fluid interiors of these planets could be thought of as a set of nested cylinders rotating at different rates.

So why wasn't this paper written soon after the Voyager 2 encounters, I wondered. Hubbard took a stab at it in 1989, but the results were inconclusive. "Today of course we have much better methods than two decades ago," he comments in a University of Arizona press release, "so we can put a more accurate constraint on these phenomena than I was able to at the time.”

The loss of a flywheel-like device to ensure precise pointing means that NASA's planet-hunting telescope has perhaps made its last observations.

After rocketing into deep space and passing its initial tests, the Kepler telescope began its search for planets around other stars on May 12, 2009.

Engineers designed the spacecraft to last four years, and apparently the warranty has expired. Last Sunday, four years to the day after science observations began, a component critical to pointing the spacecraft failed. Kepler placed itself in safe mode, a kind of electronic hibernation, and chances are not good that it can resume its work.

As deputy project manager Charles Sobeck explained during a NASA briefing today, ground controllers learned yesterday that one of the craft's four reaction-control wheels (RCWs) had failed — the same one that showed worrying symptoms of excess friction back in January. At that time engineers shut it down for 10 days in the hope that "rest" would extends its life.

But the problem continued, and even though the spacecraft resumed science operations the mission personnel knew they were on borrowed time. "We've been anticipating this for a while, unfortunately," notes John Grunsfeld, the ex-astronaut who now heads NASA's Science Mission Directorate.

On paper, NASA's Kepler spacecraft has done exactly what it was supposed to do. The task is conceptually simple: use a big telescope (95 cm in aperture) to stare at roughly 150,000 solar-type stars all at once. If a star's brightness displayed periodic dims, chances are good that the brief dips are caused by a planet crossing in front of the star's disk.

RCWs are essentially precision flywheels that maintain the telescope's rock-solid pointing on the target field, a generous 115 square degrees near the Cygnus-Lyra border. Kepler needs three wheels to maintain attitude, but its only spare was lost last July. Now, with a second failure, only two RCWs are working, and observations have ground to a halt. Unless engineers come up with some mechanical magic — and it might take months to exhaust all the possible options — Kepler's planet-hunting days are over.

In the meantime, the spacecraft has been placed in a fuel-saving mode that relies on brief occasional thruster firings to keep its solar-cell arrays pointed toward the Sun and its antenna toward Earth. The spacecraft travels around the Sun in an Earth-trailing orbit and is currently some 40 million miles (65 million km) away. "Kepler's not in a place where I can go rescue it," laments Grunsfeld, a member of two Space Shuttle servicing missions to the Hubble Space Telescope.

"I am just devastated. My hands are trembling, and my heart is aching," exoplanet hunter Geoffrey Marcy (University of California, Berkeley) after hearing the news. But sifting through the existing observations will continue, he adds. "We will be able to detect Earth-size planets just inward of the habitable zone, and also planets a bit larger than Earth within the habitable zone. We will be working seven days a week, day, evenings, and weekends to extract the Earths from the existing data. Still I'm so sad."

One person who clearly hasn't written off Kepler is William Borucki, who championed the mission concept for decades before NASA finally gave it a green light and tapped him to be its principal investigator. "There's a reasonable possibility that we can mitigate the problem," he insists. "I don't want to be a pessimist here."

Even if the spacecraft stops sending observations back to Earth, the team still has plenty to chew on. Borucki estimates that it will take two years to comb through all the archived data, and buried in those are almost certainly transits from exoplanets with years-long orbits around their host stars. These are precisely the kinds of worlds, situated in "habitable zones" with moderate temperatures, that would be most conducive to life.

Just last month astronomers announced Kepler's discovery of a five-planet system that includes a "hot Mars" and four "super-Earths", two of which have temperatures that might be right for liquid water.

"We're really pretty positive that we'll find earthlike planets in habitable zones around stars like the Sun," Borucki said today. "The most interesting and exciting discoveries are coming in the next two years — the mission is not over."

As I noted in mid-2011, Kepler ran into an unexpected complication because its target stars proved "noisier" (less stable in brightness) than predicted. This meant that the spacecraft would have to amass brightness data for more than more than 3½ years — the $600 million mission's planned duration — to coax out the very slight dimmings (less than 0.01%) produced by transits of Earth-size worlds. A longer timeline became crucial to achieving the mission's goal of finding habitable, Earth-size worlds, and in 2012 NASA managers obliged by approving a four-year mission extension.

So far, Kepler has identified more than 2,700 candidate planets using this transit method. Follow-up observations show that 130 of these really are planets (as opposed to false alarms of some kind), and statistically most all of the remaining candidates will likewise be confirmed. Another major achievement, though less well known, is the revolution that Kepler has triggered in the nature of the stars themselves.

Harvard College Observatory is digitizing its famed collection of more than 500,000 glass sky-survey plates and has just released the first data set.

I walk past row after narrow row of dark green filing cabinets. Each one, I know before I peer inside, is filled with hundreds of astronomical glass plates in paper sleeves: old-fashioned photographic negatives of the night sky. A typical plate holds 50,000 stars in its delicate emulsion coating. Most are nearly as big as an ordinary sheet of paper — and thin enough to be pretty fragile.

The collection of more than 500,000 plates fills an old brick building on Harvard’s Observatory Hill in Cambridge, Massachusetts. Here is preserved roughly a century of information about faint happenings across most of the celestial sphere. For more than a lifetime, astronomers have come from around the world to examine these plates — one by one on light tables under magnifiers — for the histories of the innumerable objects they contain.

For a long time, breaking one of these glass plates meant destroying data — there was no backup. Some of the sleeves contain two or more pieces of broken glass. Nor was there a way to access this data without traveling to Harvard and spending a lot of time fetching, handling, and refiling.

But that’s changing even as I step down the tight spiral staircase to yet another floor of the Harvard College Observatory Plate Stacks. I’m following Alison Doane, the current Curator of Astronomical Photographs, on a tour of three floors of irreplaceable sky history. She opens a cabinet to show me rows and rows of plates in sleeves. On each sleeve, cursive handwriting in fountain pen records the plate’s position, date, and other information. She carefully pulls out the plate she’s looking for and walks back to a lightbox.

This is how astronomers of old examined the plates, she explains, handing me a magnifying loupe that I would normally associate with jewelers. The stars she shows me are breathtakingly tiny: black specks on a light-gray sky. On this particular plate, small notations in ink are written on the plate itself next to some stars. These are magnitudes and identification numbers written by Harvard’s famed women astronomers who examined these plates in detail starting in the late 19th century, after Edward C. Pickering began Harvard’s project of imaging big swaths of sky every clear night. The plate collection runs from roughly 1890 to 1990.

Examining the plates the old-fashioned way is fun at first but slow, and for a large project it becomes massively time-consuming and tedious. We expect better in the digital age. So Doane leads me yet another level deeper, to the basement, for the tour’s grand finale. In what looks like a darkroom, Assistant Curator David Sliski carefully places a plate on a digital scanner that's slowly working its way through all 500,000. Built under the guidance of Bob Simcoe of the Amateur Telescope Makers of Boston (ATMoB), the instrument is specially designed for high-precision scanning. It measures the position on the plate of each tiny star speck to half a micron, and measures its brightness with an average uncertainty of 0.1 magnitude. The finished project will amount to a petabyte (1 million gigabytes) of data.

In an ironic reversal of the work done a century ago, when the plates were taken at night and processed during the day by women “computers,” the plates are now put in the scanning machine by hand during the day (nominally 400 every day). At night, modern computers take over. They use the measured stars to identify and fit a custom coordinate grid across each plate to account for all distortions, and then determine each star’s sky position and its brightness — while also taking into account the different photographic emulsions used, the different telescopes, exposure times, and observing conditions. The project has relied on ATMoB volunteers, as well as Alison Doane, professional astronomers, technicians, and funding from the National Science Foundation and private donors.

The project is called the Digital Access to a Sky Century at Harvard (DASCH). In early May it released its first dataset. After years of development, followed by scans of more than 45,000 plates (most of them during the last two years of “production scanning”), anyone can now access a 100-year light curve of any bright object within 15° of the north galactic pole. This data release also includes test fields elsewhere: around the quasar 3C 273, the Beehive open cluster (M44), Baade’s Window near the galactic center, the field of the Kepler planet-hunting mission, and the Large Magellanic Cloud.

A typical star of blue magnitude 12 or 13 offers a light curve of about 1,500 points. Astronomers have already discovered a new type of stellar variability, long-term dimming of a certain type of giant star, and much more. The century of data allows researchers to detect slow variations over decades, something otherwise impossible with today’s digital data — putting data online, after all, is only a recent innovation.

Even with about 90% of the scanning yet to go, Jonathan Grindlay, DASCH’s project leader, hopes to finish by 2016. But that goal depends on funding. The National Science Foundation gave DASCH two grants to enable development and the first data release, and DASCH is now depending on NSF funding to keep going. Grindlay expects to find out in June whether the next dataset (an additional 15° away from the north galactic pole) will be released in October as planned.

Astronomers have been waiting for our galaxy’s slumbering supermassive black hole to stir for a snack. Instead, the universe handed them a different treat.

A mysterious object is hurtling toward the supermassive black hole lurking in our galaxy’s center. Known as G2, the object looks like a tiny bit of fuzz in images taken by some of the most powerful infrared telescopes. In fact, it could be anything from a gas cloud with the mass of three Earths to an enshrouded star or even an evaporating protoplanetary disk.

Whatever it might be, G2 will whizz past our galaxy's central black hole (often called Sgr A*) in mid-September. It'll pass just 180 times the distance between Earth and the Sun away from the black hole, an event that affords astronomers an unprecedented opportunity to watch the beast devour a snack. What exactly will happen is anyone's guess, but astronomers are at the ready, regularly monitoring the galaxy’s central black hole.

On April 24th, the Swift telescope witnessed an X-ray flare coming from the galactic center, tantalizing lengthy compared to Sgr A*'s typical flares. And one day later, Swift’s Burst Alert Telescope captured a fleeting, 32-millisecond-long burst of higher-energy X-rays

Needless to say, the galactic center had astronomers’ attention.

But did the flare signal G2's imminent demise? The ultra-short flare emitted on April 25th looked more reminiscent of the type of outburst emitted by magnetars, spinning stellar corpses with extreme magnetic fields.

Unlike a pulsar, whose whirling rotation powers the lighthouse-like jet that we see as radio-wave and X-ray pulses, a magnetar's emission is powered by its decaying magnetic fields, which can be 100 million times more powerful than the strongest man-made magnet. To put it in another perspective, a magnetar half the Moon’s distance away from Earth would wipe the magnetic strips of every credit card on the planet.

Swift didn’t see any pulsations from the X-ray source, but it wasn't designed to — its detector can only register incoming X-ray photons every 2.5 seconds. NuSTAR, on the other hand, has a time resolution of 2 milliseconds. And on April 26, two days after the initial flare, the space telescope gathered X-rays from the galactic center for a full 26 hours, spotting a complex, three-peaked pulse emitted every 3.76 seconds. A follow-up observation 9 days later confirmed that the source was a magnetar.

Still, it wasn't clear where the source was; NuSTAR's vision is even less sharp than Swift's. To locate the source, astronomers needed the eagle-eyed Chandra X-ray Observatory, which placed the pulsing X-ray source just 3 arcseconds away from Sgr A*. And Sgr A* wasn’t flaring, confirming that all recent X-ray activity came from the magnetar.

“It appears Nature was playing a little game with us,” Mark Reynolds (University of Michigan) says a tad ruefully. Reynolds is part of the Swift team monitoring Sgr A*.

But instead of witnessing galactic fireworks, astronomers had been handed an unexpected treat.

“I have been working on pulsars and magnetars for years,” says Kaya Mori (Columbia University), “and I have to say, this source is an extraordinary object found at the most extraordinary place in our galaxy.”

Though it’s still possible that the source now named SGR J1745-2900 lies elsewhere in the galaxy, and just happens to be superimposed 0.3 light-years away from Sgr A* (100 times further away than G2’s closest approach), X-ray measurements and later radio observations show that the source probably lies in the galactic center. Whether the magnetar is close enough to orbit the black hole is a question for follow-up observations.

Reynolds acknowledges, “magnetars are strange beasts,” and since only two-dozen magnetars are currently known, this newest discovery makes a valuable addition to the menagerie. (For comparison, over 1,500 pulsars have been discovered.) Some magnetars are extremely active, but others can remain quiet for a decade or more. SGR J1745 appears to be one of the transients — archival observations from Chandra show that the X-ray source didn’t exist at detectable levels before the recent flare.

Mori, who released a paper on the astronomy arXiv yesterday evening, says the recently emitted X-rays and radio waves come from electrons and positrons swirling in twisted magnetic field lines bundled at the magnetar’s poles. Swift continues to monitor the galactic center, and its follow-up observations will help test Mori’s theory.

Meanwhile, those hoping to catch G2’s fireworks will have to wait a little longer for their treat.

The proportion of deuterium ("heavy hydrogen") in water trapped inside lunar rocks suggests it came from the same primitve-asteroid feedstock that supplied most of Earth's water.

The title of this post sounds like the beginning of a joke you might hear on The Big Bang Theory, but instead it's a tipoff that the geochemical evidence linking Earth and Moon is more robust than ever.

The new insight comes from studies of primitive volcanic rocks returned from the Moon by the crews of Apollos 15 and 17. These contain tiny bits of glass trapped within olivine crystals, and those glassy bits (called melt inclusions) have a higher concentration of water — up to 1,200 parts per million (0.12%) — than any known lunar sample.

Alberto Saal (Brown University) and colleagues first found water in lunar material several years ago, and Saal again led the team whose analyses and conclusions appear in this week's Science Express.

They found the proportion of deuterium ("heavy" hydrogen that includes a neutron) in water trapped inside lunar rocks is close match to the deuterium-to-hydrogen (D:H) ratio in primitive meteorites called carbonaceous chondrites. Almost all of Earth's water must likewise have come from carbonaceous chondrites and other water-rich asteroids.

Decades ago, astronomers thought comets were the source of Earth's water, but in most cases their D:H ratio is too high, so comets contributed at most just a small fraction of what fills our oceans. While certain "Jupiter-family" (short-period) comets have Earth-like (and, now, Moon-like) D:H ratios, they can't have been major players because their nitrogen-isotope ratios are quite different.

The new finding means that lunar water likewise came from carbonaceous chondrites rather than comets. And since the Moon apparently formed from superheated matter splashed out during an enormous impact on Earth, the thinking now is that whatever water ended up inside the Moon must have come from Earth. In other words, our young planet was already wet (or at least very damp) when it got clobbered.

"The simplest explanation for what we found is that there was water on the proto-Earth at the time of the giant impact," Saal explains in a Brown press release. "Some of that water survived the impact, and that’s what we see in the Moon."

These weren't easy measurements to make, notes co-author Erik Hauri (Carnegie Institution of Washington). That's because, once the lava flows reached the lunar surface, it started losing water molecules due to slow leaks from the rock itself and from molecular-scale damage caused by cosmic rays. These effects depleted hydrogen more readily than deuterium, artificially driving up the D:H ratio. So the researchers concentrated on water trapped in the melt inclusions, because it was protected from loss.

In candid moments, planet modelers will tell you that they need to figure out how the Moon managed to hang onto any water at all. But, that said, the realization that lunar water once resided on Earth strengthens the genetic link between the two worlds. In fact, evidence like this is forcing wholly new thinking about the type, mass, and velocity of the object that hit the young Earth. Specifically, it's becoming ever clearer that that the Moon assembled itself mostly at Earth's expense, not the impactor's.

Seven clouds of hydrogen dotting the space between two iconic galaxies might be crumbs from a past encounter or evidence for the elusive cosmic web theorized to fuel galaxy growth.

Despite appearances, galaxies don’t float in total voids. There’s a lot of diffuse gas milling around the cosmos — in fact, computer simulations suggest that two-thirds of the universe’s regular matter exists not in galaxies but in the stuff between the galaxies. This gas is hard to track down, although previous studies have revealed hints.

Following up on radio observations of the Andromeda (M31) and Triangulum (M33) Galaxies that detected neutral hydrogen gas between the two spiral galaxies, astronomers working with the 100-meter Green Bank Telescope in West Virginia now think they’ve detected seven clumps in that gas. The clouds are each a few thousand light-years wide, the size of dwarf galaxies, the team reports in the May 9th Nature.

Spencer Wolfe (West Virginia University) and his colleagues are cautious in their interpretation, but they think the best explanation for these clouds might be that they’re condensations in the elusive cosmic filaments proposed to funnel gas into galaxies. While galaxies do grow by cannibalizing one another, astronomers suspect that the universe’s vast star cities also grow by feeding on gas siphoned into them by a giant cosmic web. This gas would be the fuel galaxies need to form stars and beef themselves up.

The detection of cosmic filaments is an attractive conclusion, but other astronomers hesitate to back that interpretation. The two galaxies are known to be an interacting pair — the Triangulum travels around its big brother in a wide orbit, and it’s possible that the interaction stripped out material to form a string of clouds between the two galaxies, says Gurtina Besla (Columbia University). The clouds move at similar speeds to the galaxies, which would make sense if the gas is a part of the pair’s system.

Mary Putman (also at Columbia) favors the tidal leftovers scenario, too. The denser parts of filaments would typically be closer to the galaxies than these clouds are, she says. “Still, getting a few clumps out there like this is definitely possible,” she adds. The cosmic web could indeed be the source.

While his team doesn’t rule out the interaction scenario, Wolfe thinks it’s less likely than the filament one. “If the clouds were due to an interaction between M31 and M33, then it would have occurred a few billion years ago,” he says. “It's not obvious that these clouds would persist for that long a time.” Furthermore, the clumps don’t have any stars, which would likely form in gas kneaded by the tidal stripping process.

Either way, the results might help astronomers understand gas flows in the Local Group, the bunch of galaxies of which Andromeda, Triangulum, and the Milky Way are members. “The Local Group is the only place these kinds of clouds can be detected and mapped in detail,” Putman says. “And how gas accretes onto galaxies to fuel star formation depends heavily on what happens with the gas between galaxies, as this is where the majority of the baryons lie.”

Further observations of the hydrogen distribution around other parts of the M31-M33 system will help distinguish between the scenarios. One question that needs answering is whether the pressure of the surrounding environment is high enough that interaction leftovers would last this long, Wolfe says.


Below, you'll find an animation showing where the clouds lie between the galaxies and how improved resolution revealed what previously looked like diffuse gas.



Reference: S.A. Wolfe et al. "Discrete clouds of neutral gas between the galaxies M31 and M33." Nature, 9 May 2013.

Sky & Telescope introduces our new eBook library, where you can download digital books about your favorite topics free with registration.

To celebrate Saturn’s recent pass through opposition, its closest approach to Earth, the editors of Sky & Telescope compiled an eBook drawn from the pages of S&T that will tell you everything you ever wanted to know about the planet and its lunar family. Saturn's Bounty is free with registration.

"May’s Amazing Planet Trio" by Fred Schaaf tours May’s sky. Saturn rises a little later every evening in the East, but there’s also a treat in store later in the month as Venus, Jupiter, and Mercury converge in a tight knot in the west, visible just after sunset.

"Ringworld Revelations" by Matthew S. Tiscareno focuses on Saturn’s rings. Unwrapped, they would stretch from Earth to the Moon, yet they're not much thicker than a troupe of acrobats standing on each other's shoulders. Find out how something so fragile can stay stable over time.

In addition to its vast rings, Saturn hosts 62 moons, of which Titan is the largest and most secretive. In "Titan: Earth in Deep Freeze" author Jason Barnes discusses the moon’s eerily Earth-like landscape, sculpted into mountains, lakes and dunes. Despite the similarities, it's a vastly different place, with "ghost rain," polar seas, and smog that falls out of the sky. Only recently have robotic probes returned fascinating detail about this curious world, and tantalizing secrets remain for future space missions.

Titan often gets the attention, but every one of Saturn’s moons is a unique geological wonder. "Ice Worlds of the Ringed Planet" by Emily Lakdawalla shows what NASA’s Cassini spacecraft has uncovered over the past decade, including ice-spewing geysers on Enceladus's surface, secrets from two-faced Iapetus, and tantalizing hints of another ring system around Rhea, Saturn's second-largest moon.

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The gamma-ray burst GRB 130427A erupted on April 27th with record-setting power. That made it an easy target for two of NASA's orbiting observatories, major ground-based telescopes, and even one lucky backyard observer. It reached visual magnitude 7.4.

Over the past four decades, orbiting observatories have recorded thousands of gamma-ray bursts (GRBs) coming from the depths of space. The lion's share of those have been snared by NASA's Swift, launched in 2004; and Fermi Gamma-ray Space Telescope, launched in 2008.

But one that erupted at 7:47 Universal Time on April 27th turned out to be a record-setting blast. "We have waited a long time for a gamma-ray burst this shockingly, eye-wateringly bright," notes Julie McEnery, Fermi's project scientist, in a NASA press release. One of the blast's gamma rays had an energy at least 35 billion times that of a visible-light photon.

Moreover, this GRB, designated 130427A, lasted for hours — easily long enough for numerous ground-based telescopes to swing around to watch at visible-light and infrared wavelengths. Its location, in northeastern Leo, was right ascension 11^h 32^m 33^s, declination +27° 41′ 56″.

Not many GRBs become bright enough in visible light to be within range of amateur observers. But this one was, and it caught the attention of Patrick Wiggins, who just happened to be awake — and imaging the night sky with his 14-inch telescope in Tooele, Utah. Wiggins was in the middle of snack break when notice arrived about Swift's detection. "I figured I was too late to catch anything, but I was currently working a spot on the sky just a few degrees from the predicted location," Wiggins told me via email, "so I slewed over and made a quick 60-second exposure."

There, clearly evident in the middle of his image, was a 13th-magnitude dot — too bright to be a GRB, Wiggins thought. So he slewed over a bit and took another image — and there it was again. He kept recording throughout the night, finally shutting down as dawn approached. At right are one of his images (made hazy by strong moonlight) and the light curve he derived. "It was my first GRB detection," exults Wiggins. "That it happened on my birthday made it even more special to me."

And that was just the afterglow. Three RAPTOR all-sky monitors recorded an optical counterpart at magnitude 7.4, 50 seconds before the Swift satellite trigger. Within a minute the optical glow was fainter than magnitude 10. Several other robotic telescopes were pointing to the spot within minutes; they caught the afterglow at about 11th magnitude. This compares to the visible-light record holder GRB 080319B, which reached magnitude 5.3 in 2008.

Gamma-ray bursts are typically short or long. Astronomers think that the latter type, which usually last no longer than a minute or so, herald the death of a supermassive star. The collapse of the star's core triggers jets of relativistic matter so powerful that they bore outward through the star and into the surrounding space. Interactions with shells of gas previously shed by the dying star creates dazzling outbursts of radiation — the most luminous explosions known.

GRB 130427A appeared so bright because it was relatively nearby, "just" 3.6 billion light-years away. This proximity ranks among the 5% closest GRBs recorded to date, and it gives observers hope that they'll be able to spot the star's shattered remains in the days and weeks ahead.

Ironically, gamma-ray scientists from around the world had just wrapped up a weeklong meeting to discuss their latest findings when the brilliant blast appeared.

Using two instruments aboard the Herschel space telescope, observers have mapped the abundance of water in the stratosphere of Jupiter — 95% of which was delivered by Comet Shoemaker-Levy 9 in July 1994.

How time flies! It's been more than 20 years since the discovery of Comet Shoemaker-Levy 9 on film images (yes, film) taken the night of March 24, 1993. Then, after much anticipation, a score of its fragments crashed into Jupiter 16 months later, with spectacular consequences. Comets Halley or Hale-Bopp might be more famous, but at no time ever have more of the world's great telescopes been trained on the same object as they were during S-L 9's demise.

We'll never know the true size of the comet's intact nucleus. Estimates varied from 1½ to 10 km across. Yet even at the small end of this size range, a typical fragment delivered 10²⁷ ergs of energy — the explosive equivalent of nearly 25,000 megatons of TNT. These crashes occurred high in Jupiter's stratosphere, creating a series of dark, ominous "powder burns" that faded away in a few weeks.

At the time telescopic observers recorded spectroscopic evidence for water (and lots of other compounds) in those fireballs, and researchers have assumed that any water found in the stratosphere of Jupiter in the years since must have come from the comet. But direct proof was lacking — until now.

In the April 23rd issue of Astronomy & Astrophysics, a team led by Thibault Cavalié (Laboratory of Astrophysics, Bordeaux, France) shows not only that water vapor exists in the stratosphere of Jupiter but also that almost all of it is in the planet's southern hemisphere, where the comet hit.

These observations, made in 2009 and 2010, utilized two instruments on the European Space Agency's Herschel telescope, which ceased operation just a few days ago when it ran out of cryogenic coolant. Herschel's big 3.5-m mirror provided both enough sensitivity at 66 and 180 microns (the far-infrared wavelengths of strong water emission lines) and enough spatial resolution (about 13 arcseconds) to map the water abundance in a 5-by-5-pixel grid across the planet's disk.

The Herschel observations, together with heat maps provided by NASA's Infrared Telescope Facility on Mauna Kea, showed the researchers that the Jovian stratosphere was 20° to 30°F (10° to 15°C) warmer than it would be if completely dry. One question is whether the stratospheric warming results from the gentle, continuous infall of interplanetary dust particles, which would be warmed by sunlight as they linger high up. Cavalié and his colleagues believe IDPs create some of the infrared emission but cannot explain it all.

Further, a continuously supplied source would migrate to lower depths, yet most of the emission is too high up, at pressures less than 2 millibars. And while the amount of water is roughly constant across the southern hemisphere, the emission gradually weakens northward until it's less than half as strong. It's not simply that Jupiter's bottom half is hotter — there's just more water down there. As the researchers note, "At least 95% of the observed water comes from the SL9 comet and subsequent (photo)-chemistry in Jupiter’s stratosphere according to our models, as of today.

Taken together, they conclude, these observations offer "clear evidence that a recent comet … is the principal source of water in Jupiter. What we observe today is a remnant of the oxygen delivery by the comet at 44°S in July 1994." You'll find more background about these results in this ESA press release.

Interestingly, water isn't the only "smoking gun" left by the comet's crash. Observers led by Mark Gurwell (Harvard-Smithsonian Center for Astrophysics) mapped the abundance of hydrogen cyanide (HCN) using an array of microwave dishes atop Mauna Kea. They too found an excess in the southern hemisphere and a lack of HCN close to the poles — another good match to where Comet Shoemaker-Levy 9 would have injected itself.

The ringed planet has been making news for a hurricane at its north pole, digestive troubles in its interior, and "rain" of water ice from its rings.

Right now our solar system's signature ringed planet is putting on a show. Saturn reached opposition just two days ago, so it rises at sunset and takes all night to arc across the sky.

But I'm not referring to that kind of show, as beautiful and visually satisfying as telescopic views of Saturn might be. Instead, I mean Saturn's performance for planetary scientists: it's been in the news three times recently.

Two days ago, the Cassini imaging team released dramatic views of the planet's north pole. Seasonal change happens slowly on Saturn, which takes 29½ years to circle the Sun. So ever since equinox came and went in August 2009, sunlight has been gradually getting stronger around the north polar region.

The waxing northern spring has allowed Cassini scientists to resume their tracking of the "hexagon," a broad, wave feature centered at 76° north that has six straightish segments. Roughly two Earths across, the hexagon has been around at least since its discovery in 1981 by Voyager 2 and appears to be a fast-moving jet, though the exact cause remains a mystery.

The researchers had hoped to gain some insight by seeing what lies in the hexagon's center, at the north pole, and the surprising answer is … a hurricane! Its ring-shaped "eye" spans some 1,250 miles (2,000 km), 20 times larger than its counterparts on Earth. Winds moving around the outer edge are racing at 330 miles (530 km) per hour.

It's unclear what's driving this vortex of motion or how long it's existed. On Earth, hurricanes are powered by heat drawn from tropical ocean water. The one at Saturn's north pole "is somehow getting by on the small amounts of water vapor in Saturn's hydrogen atmosphere," notes Andrew Ingersoll (Caltech) in a NASA press release.

Cassini took these revealing images when it glided over the north polar region last November 27th at a distance of 260,000 miles (418,000 km). Especially dramatic is the "red rose" close-up created from images taken at three near-infrared wavelengths. Seen at right, the blue channel corresponds to light at 890 nm, green at 728 nm (sensitive to high-altitude clouds), and red at 752 nm (sensitive to low clouds).



Meanwhile, closer to Saturn's equator, another investigation has turned up evidence that water derived from the rings is raining into the planet's upper atmosphere. This process had been suspected for decades — especially after Cassini found evidence when it arrived in 2004 that the rings are immersed in a tenuous water "atmosphere" but confirmation took a while.

In the April 11th issue of Nature, James O’Donoghue and seven others describe infrared observations of ionized trihydrogen (H₃⁺) in Saturn's ionosphere made two years ago with the Keck I telescope. The observers expected to see an even glow across the planet, induced by a steady bombardment charged particles from Saturn's magnetosphere. Instead, they found a series of bright and dark bands at the middle latitudes of both hemispheres.

Apparently, water molecules from the rings are being ionized by sunlight and then travel along magnetic field lines into Saturn's upper atmosphere, where they "quench" the expected infrared emission by reacting with H₃⁺. The ionospheric glow is strongest at the locations that map back to two big "holes" in the rings — the Cassini Division and Colombo Gap — where icy ring particles are sparse.

As the team explains in a Keck Observatory press release, this banding effect is not seen in the ionosphere of Jupiter. That's likely because the Jovian rings are far more tenuous and contain little water.

Observers have long realized that Saturn's entire globe has a healthy infrared glow, thanks to roughly 10¹⁷ watts escaping from its interior. All the solar system's planets have radiated heat as they've gradually cooled over 4½ billion years, but Saturn's glow is brighter than it should be for a planet of its age. For decades, the most reasonable explanation seemed to be that helium was separating from hydrogen in the planet's deep interior, "raining out" toward the core as droplets that liberate energy as they descend.

But maybe that's not the right explanation. Jérémy Leconte (Laboratory of Dynamic Meteorology, Paris) and Gilles Chabrier (University of Exeter, England) think Saturn's youthful glow comes about because deep-seated layers of dense gas prevent the internal heat from making its way outward efficiently. Instead of traveling via giant "conveyor belts" of buoyant gas, the heat must diffuse outward — and that takes longer.

"Our calculations show that Saturn appears young because it can’t cool down," Chabrier noes in an Exeter press release. "These separate layers effectively insulate the planet and prevent heat from radiating out efficiently. This keeps Saturn warm and bright.”

Helium rainout is probably still happening, Leconte and Chabrier note in the April 21st issue of Nature Geoscience, but it probably can't account for all of Saturn's infrared excess. Jupiter and Neptune also release a tremendous amount of internal heat. But Uranus, a curious planet in many ways, does not.

The largest infrared space telescope ever launched has run out of cryogenic coolant, permanently ending its science operations.

The European Space Agency announced today that its infrared Herschel Space Observatory has finally kicked the light bucket.

Herschel launched May 14, 2009 with the Planck satellite and was the first space observatory to cover far-infrared to submillimeter wavelengths, its observations spanning 55 to 672 microns. Both spacecraft went to L2, the second Lagrange point in the Sun-Earth gravitational system where a small mass can basically “hover” without being pulled this way or that.

Herschel’s 3.5-meter primary mirror worked with two cameras (both with spectroscopic abilities) and a high-res spectrometer, all of which were cryogenically cooled by superfluid liquid helium to a couple of degrees above absolute zero, -271°C (-456°F) — as least until Herschel ran out of coolant.

Mission planners estimated that Herschel would exhaust its helium supply in late March. It lasted one month beyond that projection. But without coolant its science operations are now impossible.

Herschel’s main objectives were to study the formation of stars and early galaxies, the universe’s molecular chemistry, and the chemical composition of solar system atmospheres and surfaces. Observing in infrared is key for this work: infrared wavelengths reveal stuff a few tens of degrees above absolute zero (such as nebulae) and also pass through the dusty clouds enshrouding young stars. In addition, cosmic expansion has shifted visible light from the early universe to infrared wavelengths, meaning studies of that era must be conducted in infrared. (Incidentally, that’s why the infamous James Webb will focus on infrared, too — the infrared range is possibly the most information-heavy of the entire electromagnetic spectrum.)

For far-infrared observers, Herschel was a major upgrade from NASA’s Spitzer Space Telescope, which has a primary mirror only 0.85 meters wide and observed from 3 to 180 microns while it had coolant. (Spitzer is still working in “warm mode” and observes at 3.6 and 4.5 microns — i.e. it only sees the toastier stuff now.)

Herschel allowed astronomers to zoom in on the formation of massive stars, those that will someday cataclysmically die to form neutron stars and black holes. Its observations revealed that these stars grow on the margins of rapidly expanding bubbles in the interstellar medium, where the expanding bubbles squeeze surrounding gas. In the distant universe, Herschel detected evidence that galaxies' star formation spiked within the first few billion years, and that it's the availability of gas, not the fireworks of galaxies crashing into each other, that fuels such bursts.

It also revealed that the water ice of Comet 103P/Hartley 2 has the same hydrogen-deuterium ratio as Earth’s water — a much-sought data point for those who argue Earth’s water comes from comets. However, Hartley 2 is the only comet with the right ratio, and other data suggest that the comet is actually not such a great match to Earth’s composition after all. Increasingly, astronomers are arguing that Earth formed with its water and didn’t need alien delivery.

Another water discovery came in late April from Herschel: the detection of water in Jupiter's upper atmosphere, left there by Comet Shoemaker-Levy 9 in 1994. Even though it's been nearly 20 years since the comet smashed into Jupiter, Herschel's observations show more water in the southern hemisphere (where the comet hit) than in the north and suggest that 95% of the planet's current water comes from that comet.

Using Herschel, astronomers found out that what they’d long thought was a dark cloud hiding star formation next to the reflection nebula NGC 1999 is actually a hole in space. While black in visible light, the purported cloud should have shown up in infrared — and it wasn’t there. Instead, nearby stars somehow blew out that section of gas.

I could make a long list of Herschel’s work, but hey, the ESA has already done that for me: check out the full list of Herschel press releases, which go back even before its launch. You can also read the ESA obit for Herschel, which provides a nice wrap-up.

With its usefulness ended, Herschel will have to leave L2. The mission team will send the spacecraft into a “no-return heliocentric orbit,” which means it will take at least 300 years to return to the Earth-Moon system’s gravitational playground. Maybe by then, space retrieval systems will be so good that astronauts will grab it and bring it back for 24th-century children to gawk at in the Smithsonian.

Where's the planet that should be to blame for this star's carved-out disk?

How do planets form? This deceptively simple question has sparked a new field in astronomy over the last two decades. Astronomers have used telescopes all over the world (and in orbit!) to study all stages of planet formation, from disks of material around infant stars to the scattered debris and planetesimals in systems such as Fomalhaut’s. The goal of all these studies is the development of a coherent scenario that explains the behavior and evolution of planet-forming disks around young stars and links them to the exoplanets observed around older stars (such as those glimpsed by Kepler).

New results on a particularly exciting system, V1247 Orionis, may shed light on an important, seldom-observed stage of planet formation. Stefan Kraus (Harvard-Smithsonian Center for Astrophysics, University of Michigan, and University of Exeter, UK) and his colleagues observed in visible to far-infrared wavelengths (about 400 nm to 100 microns) the star V1247 Orionis, a young star hiding in Orion’s Belt that is part of the extensive star-forming association there. V1247 Ori has a mass about twice that of the Sun but is very young, with an age from 5 to 10 million years. The star was known to harbor a disk, but Kraus and his team used a slew of ground-based instruments in the Northern and Southern Hemispheres to better understand the disk’s properties.

Disks form around young stars with the material left over from star formation. The most widely accepted planet-formation scenario predicts that it takes a few million years to build planets from these disks, with dust grains gradually clumping together to form larger objects. Once these planetesimals gain enough mass, they begin to gravitationally interact and disrupt the disk they formed from, creating gaps in an otherwise smooth pancake.

In the last few years, observations of very young stars with disks have shown tantalizing evidence of just this scenario. Most notably, observations of the star LkCa15 revealed a planetary companion carving out a gap in the star's disk.

Kraus and his team deduced the presence of a gap spanning from about 0.2 to 46 astronomical units in the disk of V1247 Ori, slightly larger than the distance between the Sun and the Kuiper Belt’s inner edge and about the same size as the gap around LkCa 15. Yet when they used the Keck 10-meter telescope to look in the gap, they couldn’t find any protoplanets. Instead, they detected a complex, perhaps clumpy distribution of diffuse dust. Because the astronomers observed at a range of infrared wavelengths (the gap is most readily visible in the 1.5 to 13 micron range), they were able to probe a larger variety of dust temperatures than usual, which might explain the novel results.

V1247 Ori is the first star of the team’s observing campaign, and it’s possible that the star is experiencing a normal, early stage of disk clearing. But astronomers expect to find a planet if there’s a gap, and that gap "should" be clear. Perhaps the dusty haze is a short-lived transition period between gap and planetesimal formation.

An unexpected result from this analysis concerns the diffuse material’s composition. Previous observations of other holey disks have pointed to a silicate-based dust around these stars, like pulverized rock. However, in the case of V1247 Ori, the dust is probably carbon-based. Carbon is the building block of organic material, and is also thought to be prevalent in some exoplanets (such as 55 Cancri e). One other system, Beta Pictoris, shows signs of carbon-rich debris.

As astronomers gather more information on the planet-formation process, new questions and details continue to emerge. The type of investigation led by Kraus and his collaborators paves the way for studies with new telescopes, such as JWST and ALMA. These telescopes will be capable of directly observing the locations of planet formation, and the future looks bright for unraveling the mysteries of this process.

Reference: S. Kraus et al. "Resolving the gap and AU-scale asymmetries in the pre-transitional disk of V1247 Orionis." Accepted to the Astrophysical Journal, posted to arXiv.org 9 April 2013

A massive neutron star and its lightweight companion provide a unique space laboratory to test general relativity.

You have to be really dense to prove Einstein right. And the pulsar J0348+0432 is really dense. Twice as massive as the Sun and roughly 20 kilometers wide, at its center this celestial lighthouse fits more than one billion tons into a sugar-cube-size volume. That gives it a surface gravity more than 300 billion times stronger than Earth’s.

So basically, forget breathing deeply.

J0348 also has a wimpy sidekick, a white dwarf less than one-tenth its mass. The white dwarf and neutron star whirl around each other in a fairly circular orbit every 2.46 hours, reaching velocities close to the speed of light.

These aspects of J0348 excite scientists who want to test Einstein’s theory of gravity in extreme environments. The binary isn’t the best all-around test of the famed physicist’s general theory of relativity, but it does probe a strong-gravity environment that hasn’t been accessible to observers before.

J0348 is one of only two neutron stars discovered with a mass of two solar masses. Before the discovery of the other object (J1614–2230) in 2010, astronomers had thought neutron stars didn’t grow above 1.5 solar masses, explains Thibault Damour (Institut des Hautes Etudes Scientifiques, France). “Now they find another one [at two solar masses], which is a big discovery in itself.”

General relativity’s no slouch when it comes to acing tests, but it’s possible that gravity might not follow those rules around massive, dense objects. Astronomers study such exotic objects to find out. They know that general relativity (the physics of big things) and quantum mechanics (the physics of the über-small) don’t meld well, so they suspect there’s a crack somewhere.

J0348 gives researchers a unique opportunity to test for those cracks. Other binary systems have similarly short periods or equally egregious disparities between the two members’ masses, but the combination of both allows observers to look for unique effects that shouldn’t be there if general relativity reigns.

The key is that the two dead stars don’t just orbit ad infinitum. They’re slowly spiraling in toward each other, radiating away energy in the form of spacetime ripples called gravitational waves. General relativity precisely predicts how much the orbit should decay with time. If extra effects are at work, other, unpredicted ripples could also appear.

With such a difference in the pulsar and white dwarf masses, this violation of Einstein’s gravity should be pretty obvious in J0348's orbit. But after careful study using radio observations to time the pulsar’s signal, John Antoniadis (Max Planck Institute for Radioastronomy, Germany) and his colleagues found no sign of extra physics. The orbital period shrinks by 8.6 microseconds each year (give or take 1.4), which is consistent with the predicted value.

J0348 complements another gravity-testing system, the double-pulsar pair J0737–3039, says study coauthor Michael Kramer (Max Planck Institute for Radioastronomy and University of Manchester, UK). J0737 has an orbital period only a couple of minutes different than J0348 — 2 hours 25 minutes versus 2 hours 28 minutes — but with two pulsars blinking steadily and a more elliptical orbit, J0737 reveals relativistic effects that the pulsar-dwarf pair doesn’t.

One really cool example is the Shapiro effect. This effect is a delayed arrival time for a signal coming from a massive object and is created because the beacon’s photons have to climb out of the ditch the object creates in the fabric of spacetime. (Even photons can’t leap over hills in a single bound.) Because J0737’s two pulsars follow elliptical orbits, they’re not always the same distance apart. That variation in turn changes the shape and depth of the gravitational well, which means that the delay also changes throughout the orbit.

J0737 will always be better in terms of the number and measurement precision of relativistic effects, Kramer says. But because its two pulsars have similar masses, effects that would only show up when the masses are different are difficult to test. That’s where J0348 comes in.

The new pulsar-white dwarf pair does rule out some non-Einstein effects (the extra spacetime ripples). But whether gravitational hiccups arise when two strongly interacting bodies are much closer together isn’t clear. Experiments dedicated to hunting for gravitational waves, such as Advanced LIGO, which is scheduled to begin observations by 2014, might reveal more. The growing Event Horizon Telescope network will also peer into our galaxy’s innermost sanctum to determine whether gravity behaves around the supermassive black hole as general relativity predicts.


Below, you can watch a video of a pulsar-white dwarf pair. The relative sizes of the two stars aren't to scale (the neutron star is a whole lot smaller in reality), but the animation does give you a sense of gravitational waves.



Reference: J. Antoniadis et al. “A Massive Pulsar in a Compact Relativistic Binary.” Science, 26 April 2013.

Astronomers are finding out how iconic disk galaxies form.

Sombrero-shaped galaxies like our Milky Way pepper the cosmic scene. If you picked a random star in the universe, it would most likely be in a disk-shaped galaxy with a mass similar to our home swirl of stars. But how did these galactic sombreros form? Astronomers thought they had a pretty good idea, but a growing body of evidence suggests that, when it comes to Milky-Way-mass galaxies, the standard picture is wrong.

Disk galaxies such as the Milky Way have a fat yellow bulge of stars in their centers surrounded by a thin gaseous wheel. Generally, astronomers thought that galaxies’ bulges formed first, coming together over a couple of billion years via mergers. Then, these growing orbs would slowly skirt themselves with disks.

If this picture were true, astronomers should see “naked bulges” in the early universe. So Pieter van Dokkum (Yale University) and his colleagues used archival Hubble and Sloan Digital Sky Survey images to study 401 Milky-Way-mass galaxies, dispersed across the last 11 billion years of cosmic time. (In astronomical parlance, they looked back to a redshift of about 2.5). Regardless of age, all the bulges were fully clothed.

If only all toddlers would follow these young galaxies’ example.

By comparing galaxies at different times the team found that, between 11 and 7.5 billion years ago, Milky-Way-mass galaxies beefed up their bulges and disks in tandem and at about the same rate. Fed by cool gas funneled by cosmic filaments into the growing galaxies, star formation was high during this growth phase, enough to create 10 to 15 Suns each year — compare that with the Milky Way’s current 1 Sun/year rate. This influx of gas and the burgeoning bevy of stars it nourished can explain the mass growth without resorting to mergers, the astronomers say. The bulges (and the supermassive black holes hiding inside them) therefore would have grown via more mundane processes, such as clumps migrating from the disk inward to the galaxies’ centers.

After this era of buildup, bulge growth simmered down and the galaxies continued to grow more lethargically at their edges. That’s in keeping with recent studies of gas distribution at the 7.5-billion-year mark and with spirals today.

Although a growing number of observations and computer simulations have been pointing to this picture, it’s in stark contrast to what happens in more massive galaxies, which do use mergers to build up their central regions. Marie Martig (Swinburne University of Technology, Australia) explains that massive galaxies generally occupy denser regions of the universe, where mergers are more common. They also have halos of hot gas that can ultimately stifle the flow of cold material into the galaxies and prevent star formation, which could explain why massive galaxies have grown more slowly than their smaller counterparts in the last 10 billion years or so: the team calculated that from 11 billion years ago to now, Milky-Way-mass galaxies increased their masses by a factor of 10, while more massive galaxies only grew by a factor of 3.

Takashi Okamoto (Hokkaido University, Japan), who simulates galaxy evolution, says that something must delay bulge formation in smaller galaxies similar to ours, something that doesn’t operate in bigger ones. The authors suggest that observations mapping gas’s distribution in and around galaxies in the last several billion years might help disentangle the formation picture.


Reference: P.G. van Dokkum et al. "The Assembly of Milky Way-like Galaxies Since z~2.5." Posted to arXiv.org 8 April 2013

Asteroid 99942 Apophis isn't a huge threat to Earth — at least not for the next century. Telescopic observations made in early 2013 show that it is both elongated and tumbling, characteristics that help predict its future location.

Asteroid 99942 Apophis has been worrisome ever since its discovery in 2004. Estimated to be a bit more than 1,000 feet (300 m) across, it would deliver the kinetic-energy equivalent of 500 megatons of TNT were it to strike Earth — which it might well do some day. At first, dynamicists predicted a dangerously close pass in 2029 and a 3% chance of collision. More observations eased those fears — but triggered worry instead about a threatening flyby in March 2036.

As I reported three months ago, radar observations made last January have seemingly ruled out any chance of an impact in 2036. Barely resolved views made with NASA's big radar dish at Goldstone, California, even show Apophis to be quite elongated (as many near-Earth asteroids are). Mechanical problems at the Arecibo radio telescope prevented radar astronomers there from obtaining even better views.

Yet some dynamicists weren't ready to dismiss a 2036 collision completely because of a computational wild card known as the Yarkovsky effect. This gentle but persistent nudging arises when sunlight is absorbed by a rotating object and then reradiated as heat in some other direction. In particular, if Apophis were spinning retrograde (opposite the way Earth does), then over time its orbit would change in a way that increases the chance of impact in 2036.

But now we can rest easy, because Apophis appears to be tumbling as it orbits the Sun. That's the conclusion reached by a team of telescopic observers who monitored the asteroid's light curve as it passed near Earth in January. Apophis is spinning around two axes at the same time, implying that any Sun-warmed surfaces are radiating heat in all directions, not just one in particular.

"This greatly reduces Yarkovsky drift as a dynamical consideration for Apophis," concludes Jon Giorgini, one of the NASA dynamicists who've been focused on this body's orbit for a decade. As the team reports on its Apophis page, this new development "eliminat[es] any impact chance in 2036."

It also simplifies the task of computing collisional probabilities down the road. One date of interest is a close flyby of Apophis on April 14, 2068, which currently has a 1-in-430,000 chance of impact.