NASA

The formation of Saturn's rings is but one of the planet's many mysteries. They look solid, but they're made of particles, mostly dirty ice, from small grains to big boulders

By Joe Rao
Space.com

On occasion I'll get an e-mail from someone who says they've just received a telescope as gift and while they've enjoyed looking at the moon and the planet Jupiter or Venus with it, they really would love to get a view of Saturn. 

The problem for most people is that unlike Venus and Jupiter, which because their great brilliance readily stand out against background stars, or Mars because of its distinctive fiery orange color, there is nothing overly distinctive above Saturn when viewed with the naked eye. In terms of color, it appears white with a hint of yellow.  In terms of brightness, it certainly is quite bright, though it lacks the kind of "here I am!" brightness that Venus and Jupiter can boast. 

But this spring and summer I can promise you this: You'll have no problem in identifying Saturn even if you can't make heads or tails out of a simple star chart. And on Wednesday night, the task will be made all the easier by the proximity of a waxing gibbous moon to the great ringed beauty. [Skywatching Events for May 2013]

Using the moon as your guide
Just wait until it gets sufficiently dark — say about one hour after sunset. Look about one-third of the way up from the horizon to the point directly overhead (called the zenith) and you'll see the moon. Most people will think it's a "full" moon, but that won't happen until late Friday night. The moon actually will be 94-percent illuminated.

Then look off to the left of the moon and a bit above it for a bright, non-twinkling star shining with a whitish-yellow glow. That will be Saturn.

Of course, from our Earthly vantage point, we know that the moon is much closer to us than Saturn. In fact, it's 3,673 times closer to be precise. On Wednesday, the moon will be 225,400 miles (363,000 km) away compared to 828 million miles (1.33 billion km) to Saturn. As such, the moon appears to move much more quickly against the background stars compared to Saturn. During the course of one hour, the moon moves to the east by roughly its own apparent diameter (about one-half degree).

JPL

This sky map from NASA's Jet Propulsion Laboratory shows the locations of the constellations Leo and Virgo in relation to the moon and Saturn looking southeast on Wednesday (May 22).

That's why during the overnight hours of Wednesday night to Thursday morning, the moon's position relative to Saturn will noticeably change. By 1:30 a.m. local daylight time — if you're still awake — look toward the southwest sky for the moon and Saturn. The direction you'll need to face by then will have changed thanks to the rotation of the Earth.

But notice also that the moon's position relative to Saturn has changed; now Saturn is not to the moon's left, but soaring almost directly above it. That's because the moon will have moved about 2 degrees — four moon widths — to the east of Saturn over that four-hour time frame.

Find the isosceles triangle
Of course on other nights you won't be able to use the moon to find Saturn, since it will have moved to another part of the sky. I mentioned, however, that you wouldn't have any difficulty in identifying Saturn and that's true too, because this spring and summer Saturn will be part of a bright and conspicuous triangle in the sky. 

Not to be confused with the famous Summer Triangle (now emerging into view in the east-northeast), we can see an almost perfect isosceles triangle covering the southeast part of the sky. 

This triangle is formed by the bright stars Arcturus, Spica and Saturn. The triangle appears to point almost straight up, with the brilliant yellow-orange star Arcturus at the vertex. The bluish star Spica and Saturn form the bottom of the Triangle; Spica is on the right and Saturn is on the left. Even from a brightly lit city you should be able to make out this triangle, and once you see it you should have no trouble recognizing it on other nights.

And now you know where to point your telescope to see Saturn.

And of course, through a telescope magnifying by at least 30-power, Saturn is quite a spectacle! Saturn's amazing rings are currently tilted about 17.5 degrees, with its north face exposed. Try also to see Titan, Saturn's brightest moon. It will appear as a tiny star, and should be readily visible at its greatest eastern elongation (four ring lengths from the rings' edge) on the night of May 27.   

Editor's note: If you snap an amazing picture of the three planets or any other night sky view that you'd like to share for a possible story or image gallery, send photos, comments and your name and location to Managing Editor Tariq Malik at spacephotos@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium, and is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow Space.com on Twitter @Spacedotcom, Facebook and Google+. Original article on Space.com.

• Amazing Night Sky Photos by Stargazers (May 2013)
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• 'Ring of Fire' Solar Eclipse of May 2013: Photos and Maps

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China Photos / Getty Images file

A partial eclipse creeps over the moon's disk in 2007, as seen from China's Chongqing Municipality. Thursday's partial lunar eclipse will be similarly shallow.

Looking for a darkening moon? Thursday's partial lunar eclipse will be particularly subtle, and it won't be visible at all from North America — but you can still catch the show, such as it is, on the Web.

Lunar eclipses occur when Earth's shadow blots out part of the full moon's disk. When the shadow covers the whole disk, the moon takes on an eerie reddish glow. The effect is much less pronounced during a partial eclipse. And NASA's eclipse expert, Fred Espenak, says Thursday's eclipse will be "barely partial": Earth's umbral shadow will reach less than 1.5 percent across the moon at the most.

That means the partial phase will last just 27 minutes, from 3:54 to 4:21 p.m. ET. That's the shortest duration for a partial lunar eclipse since 1958. But there's more to the event than those 27 minutes: Before and after the partial phase, the moon passes through a semi-shaded region of space during what's known as the eclipse's penumbral phase. When you add that in, the darkening of the moon lasts more than four hours.

Unfortunately for North Americans who want to watch the subtle spectacle with their own eyes, it's an inconvenient four hours — lasting from 2:03 to 6:11 p.m. ET, when the sun is in the sky and the moon isn't. Europeans and Africans, Asians and Australians are in a much better position.

This map shows how much of the eclipse is visible from where:

NASA

North America is the only continent that is totally out of the picture for Thursday's partial lunar eclipse. P1 marks the beginning of the penumbral phase, U1 is the start of the partial phase, U4 is the partial phase's end, and P4 is the penumbral phase's end.

Thursday's event is the only partial lunar eclipse of 2013. Two other moon-darkenings, on May 25 and Oct. 18, only get as far as the penumbral phase. There'll be solar eclipses in May and November of this year — but if you're partial to lunar eclipses, this is as good as it gets until next April.

If you're outside the eclipse zone, or if the skies are cloudy, you can turn to the Web:

• Slooh Space Camera is planning to air free live video from an array of cameras starting at 3 p.m. ET. You can watch the Slooh webcast, or you can download an iPad app and touch the broadcasting icon to watch it on a tablet. Lucie Green, a frequent BBC contributor and solar researcher based at the Mullard Space Science Laboratory, heads up Slooh's team of commentators. "The broadcast is scheduled for one and a half hours," Slooh's president, Patrick Paolucci, told NBC News in an email. "We will have feeds from South Africa, Dubai, India and maybe Cyprus — although some of these may have to drop out due to weather." Find out more from Slooh's news release.
• Virtual Telescope Project 2.0 will begin its webcast coverage from Italy at 3:30 p.m. ET and keep the signal up until 4:50 p.m. ET. "This will not be a spectacular event, as the moon will enter only marginally the Earth's shadow, but it will be well worth a look," says Gianluca Masi, who manages the Virtual Telescope Project as well as the Bellatrix Astronomical Observatory in Ceccano.
• Indian television may offer other options: For Hindus, a lunar eclipse is a religious occasion known as Chandra Grahan. "Chandra Grahan in India will be most probably live telecast by news channels like NDTV, CNN-IBN, Aaj Tak, Sun News, Times Now, ABP Star News, Zee News, India TV, etc.," K. Kandaswamy says on his Live Trend blog.

Even if you miss out on the live feeds, it's a good bet that SpaceWeather.com and Space.com will have pictures of the eclipse afterward. If you snap a nice photo of the darkening moon, please share it with us via NBC News' FirstPerson photo upload website.

More about lunar eclipses:

• Flash interactive: What causes a lunar eclipse?
• Think pink during April's full moon
• Eclipse dims the moon's glow

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Starry Night Software

This is for Wednesday, April 24, 8 p.m. local time. As seen from Central America, the Caribbean, and southern Africa, the moon will actually pass in front of Spica.

By Joe Rao
Space.com

The moon will rendezvous with two bright objects in our current spring evening sky on consecutive nights this week, giving skywatchers a treat.

The first meeting comes on Wednesday, when Earth's nearest neighbor passes close to the bright star Spica in the constellation Virgo. Then, on Thursday night, the moon will pass the ringed wonder of the solar system, the planet Saturn.

The Wednesday pairing will be interesting, because it will look a bit different depending on where you’re located. [Amazing Photos of the April Night Sky for 2013]

For those in the eastern part of North America, about an hour after sunset when it will be reasonably dark, you’ll see the nearly full moon rising in the east-southeast, with bluish Spica twinkling just over one degree above it. (Your clenched fist held at arm's length measures about 10 degrees across.)

The moon and Spica actually appeared even closer together — less than 0.5 degrees, which is the apparent width of the moon in our sky — only a couple of hours earlier. Unfortunately, it was still daylight and neither the moon nor Spica was above the horizon at that time.

So when you first see the pair on Wednesday evening, they will be in the process of gradually drawing farther apart. This will be especially evident later on in the night as they get higher in the sky. By around 1 a.m. local daylight time, the moon will have noticeably moved well off to the left of Spica. (As it moves in its orbit around the Earth, the moon shifts to the east at roughly its own width per hour). 

And that’s also why those living in the western part of North America will see a much wider gap between Spica and the moon as darkness falls compared to those living farther east – it is, after all, three hours later.     

Although Spica is one of the 21 brightest stars in the sky, it will appear a bit overwhelmed by the brilliant light of the moon, which is just one day from full phase. Yet as the moon slowly moves away to the east during the night, Spica may appear to become a bit more prominent. 

Even more interesting is the sight that you’ll have if you live in the Caribbean, the northern part of South America, southern Africa or Madagascar.  For observers in those regions, the moon will pass directly in front of Spica — an occultation, as astronomers call it.  To see a map depicting the zone of visibility of this event, along with predictions for nearly 400 locations, click here.

Thursday, it’s Saturn’s turn
On Thursday evening, as the full moon rises in the east-southeast sky, it will be accompanied by a very bright yellowish-white “star” shining about 4 degrees to its upper left. 

That will be the planet Saturn, just a few days from opposition and shining at a brilliant magnitude of -0.1.  That makes Saturn nearly three times brighter than Spica and equal to the star Arcturus in brightness. 

There is a distinct difference between Saturn and Arcturus, though: Although both are of the same brightness, Arcturus twinkles and glimmers with an orange hue compared to Saturn, which shines with a steady and sedate yellowish-white color.  

So despite its relative close proximity to the moon, Saturn will stand out better than Spica did on the previous night.

Saturn reaches opposition (180 degrees from the sun) on Sunday and will then be visible from dusk until dawn. A telescope will show the rings tilted 18 degrees with their northern face in view. The rings are continuing to open toward a maximum of almost 27 degrees in 2017.

Mark your calendars
A final note: Some hours before the moon rises for North America on Thursday, a very slight partial lunar eclipse will take place, visible to those in Europe, Africa and much of Asia. 

But less than a year from now, on the night of April 14-15, 2014, the moon will undergo a total eclipse, and North Americans will have a ringside seat for that event, as it will occur high in the sky in the middle of the night. 

And what’s more, the totally eclipsed moon will be very close to the star Spica, making an eye-catching celestial scene even more spectacular.

Editor's note: If you have an amazing picture of the moon or any other night sky view that you'd like to share for a possible story or image gallery, send photos, comments and your name and location to Managing Editor Tariq Malik at spacephotos@space.com.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y. Follow us @Spacedotcom, Facebook and Google+. Original article on Space.com.

• 10 Surprising Moon Facts
• Gas Giants, Leo Triplet and More - April 2013 Skywatching Guide | Video
• Moon Master: An Easy Quiz for Lunatics

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NASA

Apollo 11 astronaut Buzz Aldrin walks on the moon in July 1969 in this photo snapped by Neil Armstrong.

By Leonard David
Space.com

When it comes to preserving history, a group of archaeologists and historians are hoping to boldly go where no archaeologist has gone before.

Researchers are increasingly urging humanity to protect off-Earth cultural resources. That may well mean preserving NASA's Apollo landing sites on the moon as national historic landmarks, regarding far-flung spacecraft as mobile artifacts and even working to preserve some pieces of space junk.

"The cultural landscape of space includes both sites and objects on and off Earth," said Beth O'Leary, an associate professor of anthropology at the University of New Mexico in Las Cruces. "It is necessary to evaluate the significance of the latter and treat them as important objects and places worthy of legitimate archaeological inquiry." [Historic Apollo Moon Landers Found! (Photos)]

O'Leary spearheaded a NASA-funded effort to make the 1969 Apollo 11 lunar landing site a national historic landmark. She and other experts in the emerging field of space archaeology gathered at the annual meeting of the Society for American Archaeology (SAA), held April 3-7 in Honolulu.

Legitimate archaeological inquiry
O'Leary and Lisa Westwood of California State University, Chico co-chaired the SAA session on space archaeology. The field seeks to scrutinize the routes for preservation of space objects and places.

Westwood said that in 1972 — near the end of the Apollo program — the General Conference of the United Nations Educational, Scientific, and Cultural Organization (UNESCO) adopted the World Heritage Convention in a pioneering effort to protect universally important monuments, buildings, archaeological sites,and natural and cultural landscapes from being depleted.

"At that time and within that context, cultural heritage was defined by its location relative to then-current political boundaries on Earth. We now can broaden that view to encompass many other historic properties on Earth, on the moon and beyond," Westwood said.

In applying a cultural landscape approach to early space exploration heritage, she asked: Is it possible to designate a World Heritage List district of sites and properties that spans not only multiple countries, but planetary bodies as well?

NASA / GSFC / Arizona State University

The Lunar Reconnaissance Orbiter Camera snapped its best look yet of the Apollo 11 landing site on the moon. The image, which was released on March 7, 2012, even shows the remnants of Neil Armstrong and Buzz Aldrin's historic first steps on the surface around the Lunar Module.

Historic preservation
"I am a preservationist trying to protect a human archaeological site 233,000 miles away," said Joe Reynolds of Clemson University in South Carolina. He detailed his analysis of international space law and how it affects historic preservation.

From 1969 to 1972, NASA's Apollo astronauts completed six separate lunar landing missions, "creating historically significant sites that now sit frozen in the lunar desert," Reynolds said.

Protection of lunar sites is complicated by the Outer Space Treaty of 1967, which prohibits countries from exercising territorial sovereignty over the moon or other celestial bodies.

Reynolds reviewed international treaties, such as those governing the ocean floor, Antarctica and the heavens. He also examined the language of the World Heritage Convention, the Geneva Conventions, the National Historic Preservation Act of 1966, The Antiquities Act of 1906 and various state preservation laws.

"The Apollo 11 Lunar Landing Site can be legally protected," Reynolds said. "What my colleagues and I are trying to accomplish is to legally protect a site of unprecedented human achievement on land that cannot be owned by anyone."

Conserve and protect base camps
According to Reynolds, legal protection of historic or culturally significant sites on land not claimed by any nation is not unprecedented. "There are areas on Earth that share the designation of Res Communis with the moon, such as international waters and the Antarctic continent, and there are a few examples of preservation in those areas," he said.

One example Reynolds cited is the New Zealand Antarctic Heritage Trust. Created in 1987, NZAHT is focused on conserving and protecting the base camps for the four major Antarctic explorations of the early 20th century.

"The Apollo 11 Lunar Landing Site is similar to the camps protected by the NZAHT because at the most basic level, the objects left on the moon are more or less just another base camp, for another historic scientific expedition, on land that cannot be owned by anyone," Reynolds said.

The objects left on the moon by Apollo 11 moonwalkers Neil Armstrong and Buzz Aldrin are U.S. government property. Ownership of those objects has never been relinquished.

"This makes the legal protection of these objects a very simple classroom exercise," Reynolds said. "However, getting Congress to agree to it — or anything these days — is another story. One of the reasons for a lack of action to protect this site on the moon may be because it could be construed as a claim of sovereignty on the lunar surface."

Pinnacle of American bravado
Reynolds thinks the Apollo 11 Lunar Landing Site could become a national monument with the stroke of a U.S. president's pen. The Antiquities Act of 1906, he said, gives the president the power to create national monuments via executive order.

Congress could also allow for the protection of the site by passing the Tranquility Base National Historic Landmark Act, written by Reynolds' colleagues Westwood and O'Leary.

The Apollo moon landing sites should be included as national historic landmarks, Reynolds said, because they may represent "the pinnacle of American bravado ... (the) physical manifestation of that innovation, hope and discovery. That is why the U.S. should preserve these sites," he concluded.

Robot avatars
Nearer to planet Earth, Alice Gorman of Flinders University in Australia sees cultural value in orbiting space junk.

There are thousands of defunct satellites, rocket bodies and other pieces of junk currently in Earth orbit. Gormancalled this cloud "a robotic colonial frontier" that reflects the nature of our social and political interactions with space and adaptations to a new environment. [Worst Space Debris Events of All Time]

But unlike terrestrial artifacts, orbiting objects are barely visible to us and are not designed to interact with human bodies (with a few notable exceptions, such as the International Space Station).

"They may represent the beginnings of a technological trajectory that will transform how human cultures relate to time and space," Gorman said.

Representatives of Homo sapiens
In a session at the SAA conference, Peter Capelotti of Penn State University reviewed dead or soon-to-be dead interplanetary spacecraft.

Capelotti noted that space probes navigating the boundaries between our solar system and interstellar space seem to represent "whole new categories of archaeological methodology ... if we are to consider the possibilities of heritage, preservation and, eventually, fieldwork."

For example, NASA's far-flung Pioneer 10, Pioneer 11, Voyager 1, Voyager 2 and New Horizons probes will "eventually enter interstellar space and become the archaeological representatives of Homo sapiens to the rest of the galaxy," Capelotti said.

Once a spacecraft no longer responds to signals from Earth, it ceases to be used for the original mission for which it was designed, and becomes instead a discarded, and hence, archaeological, object, Capelotti said.

Leonard David has been reporting on the space industry for more than five decades. He is former director of research for the National Commission on Space and is co-author of Buzz Aldrin’s new book “Mission to Mars – My Vision for Space Exploration” out in May from National Geographic. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.

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NASA/JPL-Caltech/Space Science Institute

The recently formed south polar vortex stands out in the color-swaddled atmosphere of Saturn's largest moon, Titan, in this natural color view from NASA's Cassini spacecraft.

By SPACE.com

New photos from NASA's Cassini spacecraft show that an icy cloud is growing over the south pole of Saturn's largest moon, Titan, hinting that a seven-year fall has taken hold at the cloudy moon's southern realm.

Scientists are not sure what the budding cloud is made of, but the same icy haze has been clearing over Titan's north pole, where it is currently springtime.

"We associate this particular kind of ice cloud with winter weather on Titan, and this is the first time we have detected it anywhere but the north pole," Donald E. Jennings, of NASA's Goddard Space Flight Center in Greenbelt, Md., said in a statement. The research by Jennings and his colleagues is based on observations with the composite infrared spectrometer (CIRS) on the Cassini probe, which has been studying Saturn for nearly a decade. [Amazing Photos of Saturn's Moon Titan]

Titan is the second-largest moon in the solar system and the only one with clouds and a dense, planet-like atmosphere. Earlier observations by Cassini showed that warm air from Titan's southern hemisphere was rising high in its atmosphere and then being dumped over the moon's north pole, where it cooled and descended, forming an icy cloud. (The pattern is similar to the Hadley cell on Earth, which transports heat from the tropics to the subtropics.)

The new Cassini observations suggest this large-scale pattern of air flow over Titan has reversed direction, and winter is coming for the moon's southern hemisphere.

Titan's north pole officially began its transition from winter to spring in August 2009, and the researchers now believe that the circulation shift occurred that year. But the southern ice cloud wasn't spotted until July 2012, and scientists only saw the first hints of the change at Titan's south pole in early 2012, when Cassini detected a high-altitude "haze hood," a swirling polar vortex and other features linked to cold weather.

"This lag makes sense because first the new circulation pattern has to bring loads and loads of gases to the south pole," Carrie Anderson, a CIRS team member and Cassini participating scientist at Goddard, said in a statement from NASA. "Then, the air has to sink. The ices have to condense. And the pole has to be under enough shadow to protect the vapors that condense to form those ices."

 As for the ice clouds' composition, scientists say they have ruled out some chemicals, including methane, ethane and hydrogen cyanide. Whatever the makeup, the clouds could play a role in the complex chemistry of Titan's atmosphere.

"What's happening at Titan's poles has some analogy to Earth and to our ozone holes," Goddard's F. Michael Flasar, CIRS principal investigator, said. "And on Earth, the ices in the high polar clouds aren't just window dressing: They play a role in releasing the chlorine that destroys ozone. How this affects Titan chemistry is still unknown. So it's important to learn as much as we can about this phenomenon, wherever we find it."

Follow SPACE.com on Twitter @Spacedotcom. We're also on Facebook and Google+. Original article on SPACE.com.

• Latest Saturn Photos From NASA's Cassini Orbiter
• Titan, Saturn's Largest Moon, Explained (Infographic)
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NASA / Caltech

Vesta was round—until it got hit by something nearly one-tenth its size. The resulting impact basin, named Rheasilvia, fills most of this image, taken from some 5,200 kilometers away. Vesta's south pole lies in the mountains at lower right, while the parallel grooves at left (also created by the impact) run roughly along the equator.

By Elizabeth Howell
Space.com

The same population of space rocks that battered Earth's moon during the early days of the solar system also slammed the huge asteroid Vesta, scientists say.

While the cosmic bombardment – which occurred when Jupiter and Saturn shifted orbits – has been known for a while, this is the first time scientists found evidence of it on Vesta, one of the biggest asteroids in the solar system.

NASA Apollo astronauts collected evidence of the bombardment on the moon during the lunar landing missions of the 1960s and 1970s. On Earth, erosion washed away most of the evidence of the violent chapter during the solar system's formation, researchers said.

"We wanted to study the evolution of the solar system. That was the main topic. So we tried to tackle that with a different scenario approach," said Simone Marchi, who is with the NASA Lunar Science Institute in Boulder, Colo., told Space.com. [Photos of Asteroid Vesta by NASA's Dawn Probe]

But it was a surprise to find that the moon and Vesta share the same bombardment history, NASA officials said in a statement. The discovery found that the same population of rocks that etched craters on the moon also affected the asteroid belt's history.

The research, led by Marchi, appeared in the March 24 issue of the journal Nature Geoscience.

Courtesy of Space.com

Meteorites from the giant asteroid Vesta have revealed clues into an event known as the "lunar cataclysm," when a repositioning of the gas giant planets destabilized a portion of the asteroid belt and triggered a solar-system-wide bombardment.

Heavy cosmic artillery
At 319 miles (523 kilometers), Vesta is big enough for an amateur using  binoculars to see. It is so large that it is considered by some scientists as a "protoplanet," or large body that is similar in size to the genesis of the planets in the solar system today.When the solar system was still forming, some planets experienced a sort of dynamic instability as they orbited around the young sun. It was at this period of time that Jupiter and Saturn began moving in their orbits, according to the Nice model of planet formation.

The planets' movements — which took place in only about a million years or so — spurred what is now known as the Late Heavy Bombardment. This coincides with the time that life began to arise on Earth roughly 3.9 billion years ago. Icy and rocky bodies careened into the inner solar system, pummelling the moon, the Earth and other large objects.

Asteroids ejected into high-speed planetary-crossing paths, by their nature, should only have a lifetime of a few tens of millions of years before crashing.

Scientists said it was unlikely that they all were ejected at once. Rather, they were moved in periods stretching over hundreds of millions of years as the planets moved.

The planets' movements carried some asteroids into the inner solar system. The planets also altered the orbits of other asteroids that, after their orbits coincided with other bodies, eventually were kicked out into new orbits veering toward the sun.

Melting rock
Simulations showed that the greatest bombardment on Vesta happened between 4.1 billion and 4.55 billion years ago, as the mass of the young asteroid belt was at its highest. However, only 0.2 percent of impacts was high enough to melt the underlying rock.

That proportion jumps to about 11 percent in the next epoch of Vesta's history, about 3.5 billion to 4.1 billion years ago. This occurred when asteroids began "resonating" with each other and the planets in their orbits, sending some objects careening into the solar system and crashing into Vesta. While these encounters were more rare, they took place at a much higher speed.

A typical asteroid collision on Vesta today occurs at just 3 miles (5 km) a second, which is not fast enough to produce rock melting. On the moon, by contrast, a collision is nearly four times as fast: 11 miles (18 km) a second — that's about 39,600 mph (63,730 km/h). This is because Vesta is orbiting in a swarm of rocks moving at similar speeds, while the moon is on its own and closer to the sun's gravity, researchers said.

A new interpretation of radiometric dating of Vesta's ancient asteroids, however, revealed small bodies smashing into the surface twice as quickly — at velocities exceeding 6 miles (10 km) a second. Craters from these smaller meteorites on Vesta's surface vanished long ago due to gradual erosion from newer impacts.

Because argon is lost during impacts if the "target is heated for a long enough time beyond a threshold temperature," the paper stated, there's enough argon loss on ancient Vesta meteorites to show that they were moving much faster 4 billion years ago than previously believed.

Even later in asteroid's development, about 1 billion to 2 billion years ago, two nearly cataclysmic collisions changed the nature of the Vesta's interior. Scientists, who performed that research separately from Marchi and his colleagues, said this could explain why the asteroid has a thicker crust than could be explained previously.

A suite of NASA scientists were involved in the new research, including some from the Marshall Space Flight Center and the Jet Propulsion Laboratory. The agency-funded Lunar and Planetary Institute also participated, along with institutions in California, Tennessee, Arizona, Italy and Germany.

Follow Elizabeth Howell @howellspace, or Space.com @Spacedotcom. We're also on Facebook and Google+. Original article on Space.com.

• Photos: Asteroids in Deep Space
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NASA / GSFC / Arizona State University

LROC NAC stereo-derived topographic map of the GRAIL Impact area -- the map is 8,400 meters wide, north is up. The image was released on Tuesday.

By Mike Wall
Space.com

An eagle-eyed NASA spacecraft has spotted the tiny craters two moon probes created when they crashed intentionally into the lunar surface last year.

NASA's Lunar Reconnaissance Orbiter (LRO) snapped a series of photographs of the two 16.5-foot-wide (5 meters) craters, which mark where the space agency's twin Grail probes ended their gravity-mapping mission, and their operational lives, on Dec. 17.

"It was really fun to find the craters," Mark Robinson of Arizona State University, principal investigator for the Lunar Reconnaissance Orbiter Camera (LROC), said Tuesday during a press conference at the 44th Lunar and Planetary Science Conference in The Woodlands, Texas.

It's a bit of a surprise that the LROC team was able to find the craters at all, Robinson added. LRO orbits the moon at an altitude of about 100 miles (160 kilometers), and the craters are small, nondescript features on a body riddled with impact scars. [Grail Probes' Final Moments (Video)]

The two Grail spacecraft — known as Ebb and Flow — slammed into a mountain near the lunar north pole at 3,771 mph (6,070 km/h), striking the surface about 20 seconds apart. They were running out of fuel and were bound to crash at some point, so the Grail team brought them down in a controlled fashion away from areas of historical importance such as the Apollo landing sites.

NASA / GSFC / Arizona State University

The twin GRAIL spacecraft impacted the moon on Dec. 17, 2012. LROC imaged the impact craters this Feb. 28, showing them both to be about 5 meters in diameter. Upper panels show the area before the impact; lower panels after impact. Arrows point to crater locations.

The Grail craters first showed up in LROC photos from January, but images taken on Feb. 28 show them in much greater detail. Robinson and his team used these later photos to produce a topographic map of the impact zone, which was named after the late NASA astronaut Sally Ride, who had led Grail's educational MoonKAM project before her death last July.

This map revealed that the two craters are separated by about 7,250 feet (2,210 m) in straight-line distance and 985 feet (300 m) in altitude, researchers said. Surprisingly, the crashes ejected material that appears darker than the surrounding lunar dirt.

"Fresh impact craters on the moon are typically bright, but these may be dark due to spacecraft material being mixed with the ejecta," Robinson said in a statement. This material may be residual fuel left in the probes' lines, or bits of their carbon-fiber bodies, he added.

LRO also managed to observe the immediate aftermath of the Dec. 17 Grail impacts after performing some precision maneuvering, team members announced today.

LRO didn't get any images of the actual crashes, which occurred in the dark. But its ultraviolet imaging spectrograph did see emissions from mercury and atomic hydrogen in the ejected plumes when they rose high enough to reach sunlight.

"This gives insight into how volatile material is transported around the moon," LRO chief scientist John Keller, of NASA's Goddard Space Flight Center in Greenbelt, Md., said in a statement. "It gives us a data point that helps constrain models of volatile transport, especially for models that describe how volatile material can get transported from warm to cold areas on the moon."

The analysis of the Grail impact plumes is ongoing, researchers added.

The $496 million Grail mission — short for Gravity Recovery And Interior Laboratory — launched in September 2011. Ebb and Flow arrived at the moon about three months later, then raced around Earth's satellite in tandem, mapping out its gravity field in unprecedented detail.

The probes' measurements have allowed scientists to create the best-ever gravity map of any celestial body, Grail scientists say. And that map is getting better all the time, as researchers continue to analyze the data Ebb and Flow gathered in their last weeks and months.

The twin probes, which were each about the size of a washing machine, zipped around the moon at an average altitude of just 7 miles (11 km) in their final days.

"They dedicated their existence to science," Grail principal investigator Maria Zuber of MIT said during Tuesday's news conference, which also revealed an updated lunar gravity map. "Their demise allowed us to map the moon at a very low altitude that enabled the high-resolution maps that you see today."

Follow Mike Wall on Twitter @michaeldwall. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.

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