Comet Encounter

Like a potato emerging from the shadows, the icy core of comet Tempel 1 looms into view in a new picture from NASA’s Stardust spacecraft released Tuesday.

The image was snapped at 11:39 p.m. ET during a close flyby that brought the office desk-size craft within 110 miles (178 kilometers) of the 3.7-mile-wide (6-kilometer-wide) comet. Stardust took 72 high-resolution pictures during the encounter, 60 of which had been successfully beamed back to Earth as of Tuesday afternoon ET.

The comet and spacecraft are both seasoned pros when it comes to NASA missions. In 2005 Tempel 1 was visited by a NASA probe called Deep Impact, which smashed an 800-pound (363-kilogram) metal slug into the comet’s core, or nucleus (see pictures of the comet impact). And in 2004 Stardust collected samples from the comet Wild 2 (pronounced “Vilt 2″).

The Stardust-NExT mission involved retargeting Stardust for a Valentine’s Day rendezvous with Tempel 1 after the comet had made a complete trip around the sun.

“This is the first time we’ve ever had the opportunity to visit a comet twice,” Stardust-NExT project manager Tim Larson said today during a press briefing.

Tempel 1, he added, “comes as close to the sun as the orbit of Mars, and that’s about where we met it last night with the spacecraft.”

Tempel 1′s Close-up

Proposed in 2006, the Stardust-NExT mission had three main science objectives that involved taking pictures, according to Joe Veverka, the Stardust-NExT principal investigator, based at Cornell University in Ithaca, New York.

“First of all, we wanted to look again at areas on Tempel 1 that we had seen before with Deep Impact and, for the first time, see what changes that occur on a comet when it gets close to the sun,” Veverka said during the briefing.

“We also wanted the opportunity to look at the Deep Impact impact site, and we wanted to take the opportunity to extend our exploration and see areas on Tempel 1 not seen before.”

Based on the images that scientists have seen so far—such the one above taken at 11:39 p.m. ET Monday—the mission hit the mark: “Was this mission 100 percent successful?” Veverka asked. “I’d have to say no, it was 1,000 percent successful!”

Valentine’s Visit

Two small, circular craters—visible in a picture taken by the Stardust craft Monday night—flank the region of comet Tempel 1 that was hit during the 2005 Deep Impact mission. The cosmic smashup had sent up a bright spray of ice and dirt, obscuring the crater site in pictures taken at the time.

“That created a lot of mystery, but it also helped create this mission,” said Pete Schultz, a Stardust-NExT co-investigator at Brown University in Providence, Rhode Island.

The path and timing of the Stardust-NExT flyby was carefully calculated so that scientists could get a look at the impact site. The new pictures prove that Deep Impact made a crater, about 492 feet (150 meters) wide—too small to be seen in this picture. The pit is not very sharply defined, though, with a small mound in its center.

“The bottom-line message is that the surface of the comet at the impact site is very weak, so the crater partly healed itself,” Schultz said.

Target Sighted

From thousands of miles away, Tempel 1 looks like no more than a mote of dust in a picture taken by the Stardust spacecraft as it approached the comet Monday night. Not visible in the new pictures is the larger halo of ice grains and dust that surrounds the comet’s nucleus.

When a comet gets close to the sun—as Tempel 1 was during the flyby—ices on its surface sublimate, or turn directly from solid to gas. As this happens, “comets send out clouds of dust and ice and rock that come apart,” said Stardust-NExT co-investigator Don Brownlee of the University of Washington in Seattle.

“It’s a very dramatic environment.” (Also see “Comet Is Cosmic Snow Globe, NASA Flyby Shows.”)

In fact, Stardust recorded thousands of impacts with dust and ice grains as it swooped close to Tempel 1. Even so, the spacecraft emerged healthy, and mission managers plan to collect more data and pictures of the comet over the next few weeks as Stardust bids Tempel 1 adieu.

Surreal Sunrise

Clouds flow like a river over the lights of Swiss villages as the rising sun crowns the Alps with morning gold in a January 30 picture. To the right, a crescent moon and the bright dot of Venus decorate the paling sky.

Venus is closer to the sun than Earth, so—like a car going around a racetrack—Venus periodically overtakes Earth as it orbits. This means Venus changes from the evening star, visible after sunset, to the morning star, visible before sunrise, every 584 days.

Star-Struck Aurora

Stars wheeling across the sky seem to cut through a fiery aurora in a recently released long-exposure picture taken in western Sweden.

Auroras can appear in different colors depending on the types of gases in the atmosphere and where these gases are. Auroras happen when energized particles form the sun interact with air molecules and give them extra charge. These “excited” molecules then emit light. Oxygen, for example, can create auroras in yellow-green to red, while nitrogen emits light in blues and purples.

Aft View of Earth

An astronaut aboard the International Space Station snapped this recently released shot of the station’s rear end with Earth in the distance from aboard the ISS Progress 40 supply vehicle.

The unpiloted Progress 40 craft has since undocked from the station carrying waste items. The craft will be used for scientific experiments until it is burned up in Earth’s atmosphere.

Double Trouble

An arch of plasma called a solar filament erupts from the sun on January 28 in a video still from NASA’s Solar Dynamics Observatory spacecraft.

The craft caught the sun producing two events at once: At the same time the filament erupted, a coronal mass ejection on the opposite side of the solar disk (not pictured) blasted a huge spray of particles into space.

Have a Nice Day?

People with coulrophobia might want to avoid the south pole of Mars: Seasonal carbon dioxide frost has given rise to a pit that bears an eerie resemblance to a deranged clown face.

Scientists compared this newly released picture from the Mars Reconnaissance Orbiter with another taken in 2007 to see how the pit has changed over time. The team saw signs of growth inside the “happy face,” which they think is caused by frost that sublimates—turns directly from solid to gas—from the pit walls and then recondenses on its surfaces.

Snowy Berlin

Bright white snow and dark vegetation create a crystalline patchwork over the urban landscape of Berlin in a recently released satellite image of the German capital. Home to 3.4 million people, Berlin has the second largest population, within city limits, of any city in the European Union after London.

The picture was taken by the Japanese ALOS satellite and processed by theEuropean Space Agency. ALOS was designed to chart land cover in visible and near-infrared light.

WISE Comets

After a year of mapping the sky in infrared, NASA’s Wide-field Infrared Survey Explorer, or WISE, space telescope discovered 20 comets, seen above in a mosaic of false-color pictures. The backgrounds appear fuzzy because WISE also captured the faint heat signatures of dust in our solar system.

In addition to comets, WISE discovered more than 33,000 asteroids in the main belt, between the orbits of Mars and Jupiter, and 134 near-Earth objects—asteroids and comets that come within 28 million miles (45 million kilometers) of Earth’s orbit around the sun.

New research challenges the controversial theory that an ancient comet impact devastated the Clovis people, one of the earliest known cultures to inhabit North America.

Writing in the October issue of Current Anthropology, archaeologists Vance Holliday (University of Arizona) and David Meltzer (Southern Methodist University) argue that there is nothing in the archaeological record to suggest an abrupt collapse of Clovis populations. "Whether or not the proposed extraterrestrial impact occurred is a matter for empirical testing in the geological record," the researchers write. "Insofar as concerns the archaeological record, an extraterrestrial impact is an unnecessary solution for an archaeological problem that does not exist."

The comet theory first emerged in 2007 when a team of scientists announced evidence of a large extraterrestrial impact that occurred about 12,900 years ago. The impact was said to have caused a sudden cooling of the North American climate, killing off mammoths and other megafauna. It could also explain the apparent disappearance of the Clovis people, whose characteristic spear points vanish from the archaeological record shortly after the supposed impact.

As evidence for the rapid Clovis depopulation, comet theorists point out that very few Clovis archaeological sites show evidence of human occupation after the Clovis. At the few sites that do, Clovis and post-Clovis artifacts are separated by archaeologically sterile layers of sediments, indicating a time gap between the civilizations. In fact, comet theorists argue, there seems to be a dead zone in the human archaeological record in North America beginning with the comet impact and lasting about 500 years.

But Holliday and Meltzer dispute those claims. They argue that a lack of later human occupation at Clovis sites is no reason to assume a population collapse. "Single-occupation Paleoindian sites — Clovis or post-Clovis — are the norm," Holliday said. That’s because many Paleoindian sites are hunting kill sites, and it would be highly unlikely for kills to be made repeatedly in the exact same spot.

"So there is nothing surprising about a Clovis occupation with no other Paleoindian zone above it, and it is no reason to infer a disaster," Holliday said.

In addition, Holliday and Meltzer compiled radiocarbon dates of 44 archaeological sites from across the U.S. and found no evidence of a post-comet gap. "Chronological gaps appear in the sequence only if one ignores standard deviations (a statistically inappropriate procedure), and doing so creates gaps not just around [12,900 years ago] but also at many later points in time," they write.

Sterile layers separating occupation zones at some sites are easily explained by shifting settlement patterns and local geological processes, the researchers say. The separation should not be taken as evidence of an actual time gap between Clovis and post-Clovis cultures.

Holliday and Meltzer believe that the disappearance of Clovis spear points is more likely the result of a cultural choice rather than a population collapse. "There is no compelling data to indicate that North American Paleoindians had to cope with or were affected by a catastrophe, extraterrestrial or otherwise, in the terminal Pleistocene," they conclude.

History of Clovis people:

The Clovis culture is a prehistoric Native American culture that first appears in the archaeological record of North America around 13,500 years ago, at the end of the last ice age.

The culture is named for artifacts found near Clovis, New Mexico, where the first evidence of this tool complex was excavated in 1932. Earlier evidence included a mammoth skeleton with a spear-point in its ribs, found by a cowboy in 1926 near Folsom, New Mexico. Clovis sites have since been identified throughout all of the contiguous United States, as well as Mexico and Central America.

The Clovis people, also known as Paleo-Indians, are generally regarded as the the first human inhabitants of the New World, and ancestors of all the indigenous cultures of North and South America. However, this view has been recently contested by various archaeological finds which are claimed to be much older.

There are a number of controversial sites vying for the position of the earliest site in the region. The best evidence, however, suggests that a society of hunters and gatherers known as Clovis People were the first to settle in the Southwest, probably sometime before 9,500 B.C. The Clovis People were so named after the New Mexico town, site of the first discovery in 1932, near Clovis, N.M.

Since the mid 20th century, the standard theory among archaeologists has been that the Clovis people were the first inhabitants of the Americas. The primary support of the theory was that no solid evidence of pre-Clovis human inhabitation has been found. According to the standard accepted theory, the Clovis people crossed the Beringia land bridge over the Bering Strait from Siberia to Alaska during the period of lowered sea levels during the ice age, then made their way southward through an ice-free corridor east of the Rocky Mountains in present-day western Canada as the glaciers retreated.

The culture lasted for about a half a millennium, from about 11,200 to 10,900 years ago. People of the Clovis culture were successful, efficient big-game hunters and foragers. Judging from sites on the North American Great Plains, the Clovis people were skilled hunters of huge animals, especially Ice Age mammoths and mastodons.

It is generally accepted that Clovis people hunted mammoth: sites abound where Clovis points are found mixed in with mammoth remains. Whether they drove the mammoth to extinction via overhunting them – the so-called Pleistocene overkill hypothesis – is still an open, and controversial, question, keeping in mind that Archaeology is purely a theoretical endeavor.