What’s the News: Looking at images of odd undulations in the rings of Saturn and Jupiter, astronomers have discovered that comets are to blame. The finding means that a planet’s rings act as a historical record of passing comets, possibly leading to a better understanding of comet populations. “We now know that collisions into the rings are very common—a few times per decade for Jupiter and a few times per century for Saturn,” Mark Showalter, from the SETI Institute in Mountain View, California, told the Daily Mail. “Now scientists know that the rings record these impacts like grooves in a vinyl record, and we can play back their history later.”

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Six years ago, NASA visited the comet Tempel 1 with a fury: Its Deep Impact mission launched a projectile into the comet that kicked up dust and ice for the spacecraft to capture as a sample. Tonight, NASA is taking another pass by the comet—but a little more gently this time.

The hardy Stardust explorer, which has passed by other comets and brought samples back to Earth, will pass by Tempel 1 tonight to try to get a good look at what NASA’s blast did to the comet. Last time around, Deep Impact’s projectile was actually too effective. It blasted so much debris off the comet that it blinded itself.

Now, Stardust will be able to obtain images of that crater up close for the first time. Moreover, in the nearly six years since that initial encounter, the comet has completed an orbit around the solar system, passing close to the sun. “For the first time, we’ll go back to see what happens to a comet” after it passes close to the sun, said Pete Schultz of Brown University, a scientist for the new mission, which has been dubbed Stardust-NExT. [Los Angeles Times]

Stardust’s pass presents an opportunity not just to see what’s changed in the last five to six years of the comet’s life, but also to peel back more of Tempel 1‘s ancient history.

“Here’s a chance where we can see what has changed, how much has changed,” said Joseph Veverka, a professor of astronomy at Cornell and the mission’s principal investigator, “so we’ll start unraveling the history of a comet’s surface.” For example, photographs taken by Deep Impact in 2005 showed areas that looked old and others that seemed much younger. But the snapshots did not tell the ages of any of them. “We have no idea whether we’re talking about things that have been there for a hundred years, a thousand years, a million years,” Dr. Veverka said. [The New York Times]

The flyby is scheduled to begin at about 11:30 p.m. Eastern tonight (Monday). Stardust should pass within about 125 miles of the comet, taking snapshots of the cosmic traveler and also catching some of the particles flying off it. The approach will mark the first time two spacecraft have studied the same comet up close, and probably mark the last hurrah for Stardust: After returning with samples from the comet Wild 2 in 2004, the craft was still running fine and got the OK from NASA to go on this second comet chase. But after a dozen years in space logging billions of millions, Stardust will soon be, simply, out of fuel.

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Image: NASA

This NASA probe had its Valentine’s Day plans set well in advance. On February 14 at 11:37 p.m. eastern time, the Stardust-NExT spacecraft will swoop past the comet Tempel 1 to snap photos and collect data about this solar system wanderer. The probe will pass just 124 miles from Tempel 1.

Tempel 1 is of particular interest because another probe has dallied with it in the past: In 2005, NASA’s Deep Impact probe approached and fired an 800-pound impactor at the comet’s surface to study its composition. While that mission was a success, the dust kicked up from the crash prevented the Deep Impact probe from getting a good look at the crater created. By sending Stardust-NExT along now, researchers can finally get a good look at the crater.

And that’s not all they’ll be looking at.

Since the 2005 impact, the comet has passed closer to the sun and then headed out to the orbit of Jupiter, before heading back for another visit to the inner solar system. The team hopes to see not only the size of the crater plowed by the 2005 impact, but also to see how a close pass by the sun resurfaces a comet. “We’re going to find out a lot about how comets evolve,” says Stardust-NExT co-investigator Steve Chesley. [USA Today]

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Phil Plait has the report on NASA’s latest pictures from the flyby of Hartley 2, which reveal that the comet is surrounded by a blizzard of snow and ice:

Wow! Most of those dots are not stars: they are actual snowballs, frozen matter that has been ejected by the comet itself! They range in size from a few centimeters to a few dozen across, so they really are about the size of snowballs you’d use in a snowball fight… or to make a snowman. But I wouldn’t recommend it: a lot of that material is not frozen water, it’s actually frozen carbon dioxide, or dry ice.

Find out what else we’ve learned about Hartley 2 and see more photos at Bad Astronomy.

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Image: NASA / JPL-Caltech / UMD

This morning, NASA’s EPOXI mission whizzed by the comet Hartley 2, coming as near as 450 miles to the comet at 8 a.m. and snapping pictures all the while. The icy comet is less than a mile in diameter and has an irregular shape that one NASA researcher recently described as “a cross between a bowling pin and a pickle.”

The images are already streaming in: Head to Bad Astronomy for more pictures and a discussion of what these snapshots tell us about Hartley 2.

This is the second cometary encounter for this spacecraft–under a previous mission called Deep Impact the spacecraft sent an impactor hurtling into the comet Tempel 1, allowing the craft to photograph the excavated debris and the impact crater.

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Image:NASA

On November 4, NASA’s mission EPOXI will make a flyby less than 450 miles from the comet Hartley 2. Here’s what to know about this dirty snowball.

1. It’s a frequent visitor.

Malcolm Hartley discovered this namesake comet 24 years ago, and it’s returned to swing around the sun a few times since.

Like the famous Halley’s Comet, Comet Hartley 2 is a periodic comet that follows a years-long loop around the sun. It takes 6.46 years to complete one circuit, compared with Halley’s 75.3 years. [Christian Science Monitor]

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When it comes to explaining why the woolly mammoths died out, “death from above” could be down for the count.

Nearly 13,000 years ago, North American megafauna like the mammoths and giant sloths—and even human groups like the people of the Clovis culture—disappeared as the climate entered a cold snap. As DISCOVER has noted before, there’s been a controversial hypothesis bubbling up saying that a comet impact caused it all, but other scientists have been shooting holes in that idea of the last couple years. In a study in this week’s Proceedings of the National Academy of Sciences, a team led by Tyrone Daulton pooh-poohs what may be the last major evidence that supports the impact idea.

That evidence takes the shape of nano-diamonds in ancient sediment layers, a material said to form during impacts only.

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This week brings the annual return of the Perseids, one of the most stunning meteor showers of the year, visible from just about anywhere.

WHAT: The height of the Perseid shower comes every August, because that’s the time our planet passes through a certain debris path.

The Perseids are created by the tiny remnants left behind by comet Swift-Tuttle. The Earth passes through this material once a year, creating a spectacular show as the cometary particles burn up in the atmosphere [Discovery News].

WHERE: Like the Orionid meteors, which come around in October, the Perseids are so named because of the constellation from which they appear to originate.

If you trace the Perseid meteor trails backward, they meet within the constellation Perseus the Hero; this is how the shower got its name [Astronomy].

WHEN: Tonight (Wednesday) through Friday night we’ll see the height of Perseid visibility once the sky reaches full darkness, from 11 p.m. to midnight wherever you might be until the first light of dawn. On Friday night the crescent moon will set before twilight ends, giving stargazers a dark sky to gaze at.

Swift-Tuttle’s debris zone is so wide, Earth spends weeks inside it. Indeed, we are in the outskirts now, and sky watchers are already reporting a trickle of late-night Perseids. The trickle could turn into a torrent between August 11th and 13th when Earth passes through the heart of the debris trail [NASA Science News].

Indeed, the opening shot of the Perseids appeared as a bright fireball over Alabama on August 3.

WHAT YOU NEED: Your two eyes, and a place away from the city lights. For more cool Perseid details, check out Astronomy’s coverage.

Follow DISCOVER on Twitter

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Image: flickr / aresauburn

In the 1980s, fossil record research showed a curious cycle: Every 27 million years, Earth hosted a mass extinction. Some scientists suggested that a dim star dubbed Nemesis was in a deadly dance with our sun, periodically kicking comets out of the distant Oort Cloud to shower our planet with destruction. Morbidly fascinating as it may be, the authors of a new study argue that this “death star” theory doesn’t hold up.

The cyclical extinctions do make a solid pattern, say Adrian Melott of the University of Kansas and Richard Bambach of Smithsonian Institution Museum of Natural History, whose paper is available through arXiv.org. The two have gone back in the record to 500 million years ago, further than any other researchers, and have confirmed the 27 million year cycle at a 99 percent confidence.

According to Bambach, there’s no doubt at all that every 27 million years-odd, huge numbers of species suddenly become extinct. He says this is confirmed by “two modern, greatly improved paleontological datasets of fossil biodiversity” and that “an excess of extinction events are associated with this periodicity at 99% confidence”. This regular mass slaughter has apparently taken place around 18 times, back into the remote past of half a billion years ago. [The Register]

The problem, Nemesis fans, is that the cycle is too precise, the researchers say. If these extinctions result from a dance between our sun and Nemesis, the researchers note, the period of these mass extinctions would change as other stars buffeted the pair and changed the courses of Nemesis’s orbit around the sun.

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On Saturday, the European Space Agency’s Rosetta probe took the world’s closest pictures of the 80- by 50-mile-wide asteroid known as 21 Lutetia. Though the Lutetia visit is just a stop on the way to Rosetta’s real destination–a 2014 visit to the comet 67P/Churyumov-Gerasimenko–Saturday’s pictures document the closest visit to this big asteroid, the largest we’ve ever visited with a spacecraft.

We’ve known about Lutetia for quite a while: since 1852, according to Sky and Telescope. In November of that year, Hermann Goldschmidt spotted the space rock from his Paris balcony. The asteroid is now around 280 million miles from the Sun. From only 2,000 miles away, Rosetta got a much closer look at Lutetia, whipping around it at about 10 miles per second (30,000 miles per hour) as its OSIRIS camera snapped pictures recording details down to a few dozen meters.

“The fly-by has been a spectacular success with Rosetta performing fautlessly,” ESA said in a statement. “Just 24 hours ago, Lutetia was a distant stranger. Now, thanks to Rosetta, it has become a close friend.” [AFP]

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It makes for a good movie: 12,900 years ago, a comet slams into Earth, igniting forest fires across North America and sending the planet into a thousand cold years, killing off mammoths, giant sloths, and a bunch of other big mammals. But scientists have fiercely debated whether such a movie, about the cause of the planet-wide cooling period called the Younger Dryas, should be documentary or science fiction. According to a paper recently published in the Geophysical Research Letters, new evidence–or refuted, old evidence–points to science fiction.

Those that think a comet hit the planet cite “carbonaceous spherules” and nanodiamonds found in sediment from the period of the suspected impact. They argue that these particles formed from the intense heat of the collision.

Lead author of this new study, Andrew Scott of the University of London in Egham suspects those spherules are not from a comet collision, but are bug poop, fungal spores, or charcoal pellets.

From a test that measures how much light the spherules reflect, Scott’s team has determined that the spherules were slow-roasted in a low-intensity heat (perhaps from natural wildfires) instead of in intense, comet impact heat. As shown in the figure, the researchers compare the charred spherules to fungal sclerotia, emergency cell balls created by stressed fungi that can germinate after a bad growing period is over, and saw a striking similarity.

Some of the more elongate particles are “certainly fecal pellets, probably from termites,” says Scott…. “There’s certainly no evidence [that any of these particles are] related to intense fire from a comet impact,” says Scott. Part of the problem, he says, is that “there was nobody [among impact proponents] who ever worked on charcoal deposits, modern or ancient. If you’re not familiar with the material, you can make mistakes.” [Science Now]

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When you saw the Hale-Bopp comet, you may have seen material from a distant star passing by. In a new study, a team of astronomers argues that most of the comets that streak through our solar system were actually born in other solar systems.

Given their eccentric orbits and infrequent visits, comets seem like worthy candidates for an exotic origin. But the prevailing thinking said no, they are rather ordinary. Researchers thought most of the comets that pay us a visit initially formed from the sun’s protoplanetary disk—the same swirling mass that formed our own planet—and came to reside in the weird Oort cloud region at the periphery of our solar system. From there, the gravitational bullying of larger bodies can dislodge a few like Halley’s Comet or Hale-Bopp, which swerve into an orbit that sees them visit the inner solar system now and then.

In a study in Science this week, researchers led by Harold Levison posit a different idea: Many of the comets hanging around our solar system are stolen. It goes like this:

Like most stars, the sun may very well have been created in a tightly nestled birth cluster, a stellar nursery with tens, hundreds or possibly even thousands of stars. During millions of years of intimate infancy, the newborn stars could have exchanged vast numbers of comets from the fringes of their disks, each of them winding up with an ensemble of hand-me-downs from their stellar siblings [Scientific American].

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Its doom was sealed six years ago.

In 2004, UC Berkeley researchers say, this comet was tugged by Jupiter’s gravity into a path bound for destruction in the cauldron of the sun. And when its end finally came this March, astronomers captured the comet plunging deep into the sun on video (see below), watching it go farther into the light than any suicide comet seen before.

Seeing comets and other small objects approach the sun is difficult because the objects are overwhelmed by the sun’s brightness. Scientists were able to track this one closer to the sun than ever, before it it burned up in the sun’s lower atmosphere [Wired.com].

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Since NASA’s Stardust mission returned in 2006 from its trip of billions of miles collecting the dust of a comet called Wild2 and dropped it samples down to Earth in the Utah desert, the samples have raised all sorts of questions about how comets formed and what the early solar system was like. In a study this week in Science, there’s a new surprise. Scientists say that the comet sample contains chemicals that must have formed in our home turf, the inner solar system.

Lead researcher Jennifer Matzel studies a tiny particle taken from Stardust’s sample, a piece just five micrometers across. In it her team found the mark of materials that would have formed under high temperatures. Matzel, who specializes in using the decay rates of radioactive chemical elements to assess ancient dates, determined that the Stardust particle must have crystallized just 1.7 million years after the oldest solid rocks in the solar system were forming [San Francisco Chronicle]. After that, the researchers says, the particle must have been flung out to the Kuiper Belt, the region of icy comets revolving around the sun at a distance far past Neptune.

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While Jupiter’s two largest moons, Ganymede and Callisto, are nearly the same size, they’re far from identical twins. Now, in a Nature Geoscience study, Amy Barr and her team might have figured out this tale of two similar moons with very different histories.

Voyager and Galileo mission images showed Ganymede, seen here on the right, to be a geologically active place, with a surface that scientists think changes through tectonic processes like those that we have here on the Earth. Callisto, seen on the left, looks totally different: Its rock and ice have not mixed in the same way, and it doesn’t seem to have such active geology, despite being approximately the same size as Ganymede. For 30 years, researchers have wondered what process could have got enough heat into Ganymede to drive its geological evolution without setting off Callisto as well [ScienceNOW Daily News].

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