An asteroid zooms past the Earth in this artist's impression -- but how many small space rocks are captured by Earth's gravity?
Asteroids and comets capture the human imagination unlike any other objects in space. They're speedy, craggy, not too far away -- and dangerous. Geologic investigations around the globe guarantee a sizeable space rock is bound to kiss our fragile blue marble within the next few centuries years. And if one may have wiped out the dinosaurs, the thinking goes, why not us? Browse through 10 of our favorite ominous objects that astronomers have plucked from the foreboding skies.
Asteroid 951, also known as Gaspra, is safely tucked away within the Asteroid Belt between Mars and Jupiter. For now. This image is the outcome of the very first robotic rendezvous with an asteroid. The Galileo spacecraft swung by Gaspra at about 1,000 miles away in November 1991 on its way to Jupiter. The intel Galileo brought back on the asteroid was breathtaking; Gaspra's irregular, tooth-like shape and lack of big craters suggest that it was born out of a recent (300 to 500 million-year-old) smashup in the Asteroid Belt.
What asteroid was the first known to have a mini-companion orbiting it? Asteroid 243, better known as Ida. The Galileo spacecraft also scouted out this 1,900-by-2,375 mile space rock during the long trip to Jupiter in August 1993. Ida spins around once about every 4 1/2 hours, and its little "moon" Dactyl is roughly 20 times smaller. Like asteroid Gaspra, Ida is oddly shaped -- yet unlike Gaspra, Ida has remained in one piece for much of the solar system's 4.5 billion year existence.
NASA's Galileo spacecraft was first to get up close and personal with an asteroid, but the Near-Earth Asteroid Rendezvous (NEAR) mission quickly stole all of the thunder. Designed specifically to study asteroids close to Earth, NEAR's main objective was to spy on 433 Eros. Second only to 1036 Ganymed, Eros is one of the largest asteroids drifting dangerously close to Earth. Measuring 21 by 7 by 7 miles, its size easily rivals the space rock thought to have wiped out the dinosaurs. This false-color image shows where the asteroid's surface is denser (red) and less dense (blue) compared to an average reading.
Itokawa (asteroid 25143) may look like a harmless, peanut-shaped pile of rubble -- but the writing is on the wall. In a few million years, it's probably going to wallop Earth. Hayabusa, Japan's asteroid-snooping spacecraft, was set to land on a smooth patch, scoop up a sample and return it to Earth. Although the plucky robot never did make contact, its payload -- set to arrive at Earth by June 2010 -- probably contains at least a few bits of dust and other material from Itokawa. If it returns safely, Hayabusa will be only the second spacecraft to bring back a sample of the solar system from beyond the moon.
No robot has come out to meet New Jersey-sized asteroid 216 Kleopatra. Rather, this series of images was put together by the expansive Arecibo Observatory in Puerto Rico in mid-2000. As for Kleopatra's dog bone shape? Likely the leftovers of a violent collision within the Asteroid Belt that stuck two similarly sized space rocks together.
Northwestern Univ./Johns Hopkins Univ.
When it comes to 253 Mathilde, it's a wonder there's any asteroid left at all. This pockmarked, porous wad of rock is denser on its surface than it is at its core -- a complete puzzle to scientists. The leading explanation? Constant pounding from impacts squashed Mathilde's surface into a compact layer of rock. Clockwise, from top left: Mathilde in true color; Mathilde's color enhanced; Mathilde in black and white, as seen by the NEAR spacecraft.
Tempel 1 is a comet, not an asteroid, but in fact the difference between the two types of wayward objects is still poorly understood. What appear to be plain ol' asteroids sometimes erupt into comets as they swing by the sun and heat up, sporting beautiful ionic tails. To better understand the difference, NASA smashed a robot called Deep Impact (scientific pun intended) into this wily comet. The probe created some July 4th fireworks in 2005 by deliberately slamming into the 3.1-by-4.3 mile core, directly between the two dark-rimmed craters at far right. Talk about a hole-in-one.
NASA/Lance Benner, JPL
In a few centuries, this ominous-looking asteroid known as 1999 JM8 may impact the Earth, putting it on the "watch out!" list for humans. Astronomers used deep-space radar to create these snapshots of the asteroid in 1999 as it tumbled through space. Clockwise, from top left: July 28; Aug. 1; Aug. 2; Aug. 5.
Also known as comet 81P, Wild 2 used to have a near-perfect circular orbit for billions of years -- that is, however, until Jupiter pulled it into a more eccentric loop around the sun. NASA's Stardust spacecraft sampled bits of the comet's tail and was the first-ever mission to return a sample of material from beyond the moon safely to Earth. Shown here is the comet's 3.1-mile-wide nucleus as seen by Stardust on Jan. 2, 2004.
Courtesy of Celestia Motherlode
Asteroid Apophis is last on our list, and certainly not the best. It is, after all, named after the Egyptian god of "uncreation" for a reason. Known in scientific circles as both 2004 MN4 and 99942 Apophis, this 885-foot-long space rock used to have a fairly good chance of impacting the Earth within a quarter century. After making a breathtakingly close pass on Friday the 13th in April 2029 -- less than 10 percent of the distance between Earth and the moon -- Apophis was thought to have a 1 in 45,000 chance of hitting Earth in 2036 when it swings back around. However, recent calculations have reduced this risk to a 1 in 250,000 chance. Phew! Despite Apophis' diminished threat to Earth, some scientists advocate sending a transmitter to the asteroid's surface to better track its movement. And now, Russia wants to send a deflection mission there, 'just in case' the calculations wrong.
Slide show presentation originally compiled by Dave Mosher.
Earth's gravity may not have the gravitas of Jupiter, but the planet regularly plucks small asteroids passing by and pins them into orbit. The mini-moons don't stay for long. Within a year or so they resume their looping, twisting paths like crazy straws around the sun. But others arrive to take their place.
Simulations show that two asteroids the size of dishwashers and a dozen half-meter (1.6 feet) in diameter are orbiting Earth at any given time. Every 50 years or so something the size of a dump truck arrives. So far, there's been just one confirmed sighting.
"We'd eventually like to see a mission to a mini-moon," astronomer Robert Jedicke, with the University of Hawaii, said this week at a workshop in Huntsville, Ala., to discuss proposals for two spare Hubble-class spy telescopes donated to NASA by the National Reconnaissance Office.
Jedicke would like to use one of the telescopes to hunt for near-Earth objects, including mini-moons, which could be captured whole and brought to the ground for study.
"It's the Rosetta stone of the solar system. You bring back a chunk of material that's never been processed through the atmosphere, that's not been sitting on the ground. It's going to be a tremendous wealth of information about how the solar system formed -- even more so if you can bring back more than one and get different types of material," Jedicke told Discovery News.
"The great thing is you don't have to go very far," he added. "These things are sitting there right in geocentric orbit and they're relatively easy to get to."
But hard to find.
A paper published last year showed that, in theory, a cloud of temporarily captured asteroids circles Earth at all times, but that the largest object is just about a meter (3 feet) in diameter.
"These are really difficult to detect with current technology," said astronomer Paul Chodas, with NASA's Near Earth Object program at the Jet Propulsion Laboratory in Pasadena, Calif.
So far, the only confirmed captured asteroid that orbited Earth was RH120, which most recently visited from September 2006 to June 2007. Initially, the object was suspected of being a spent upper-stage motor from an Apollo rocket, but follow-up observations by ground-based radars determined the object was not metallic.
"There is great interest in tracking these Temporarily Captured Objects (TCOs), because for a short time they are easily accessible for both scientific study and, possibly, eventually, resource utilization," Chodas wrote in an email to Discovery News.
In addition to being small, mini-moons are difficult to find because they only hang around for a relatively short time, between six and 18 months.
"We would want to discover one of these as soon after capture (by Earth’s gravity) as possible to have enough time to get a spacecraft out there, and then still have time to study the object before it escapes back into heliocentric orbit," Chodas said.
In addition to science, there is economic interest in asteroids. Two U.S. firms, Planetary Resources and Deep Space Industries, announced plans last year to mine asteroids for raw materials. Both companies are developing precursor missions to scout potential targets.
"Maybe you're not going to mine a half-meter diameter wide asteroid, but you can test out your techniques on it," Jedicke said.
Hunting mini-moons wouldn't be the telescope's only job.
"It would be best asteroid survey ever of the entire solar system," Jedicke said.
"That survey could be used to build up a very deep image of the sky in that region which could be used to look for supernova, gamma ray bursts, asteroid collisions -- basically anything that changes, this survey would find," he said.
A list of proposals for the donated telescopes is expected to be presented to NASA managers next week.