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updated 12/23/2011 6:08:37 PM ET 2011-12-23T23:08:37

Earth has two moons, a group of scientists argues. One is that waxing and waning nightlight we all know and love. The other is a tiny asteroid, no bigger than a Smart Car, making huge doughnuts around Earth for a while before it zips off into the distance.

That's the scenario posited by the scientists in a paper published Dec. 20 in the planetary science journal Icarus. The researchers say there is a space rock at least 1 meter (3.3 feet) wide orbiting Earth at any given time. They're not always the same rock, but rather an ever-changing cast of "temporary moons."

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In the scientists' theoretical model, our planet's gravity captures these asteroids as they pass near us on their way around the sun. When one is drawn in, it typically makes three irregularly shaped swings around Earth — sticking with us for about nine months — before hurtling on its way.

According to the researchers, surprisingly little attention has been paid to Earth's natural satellites other than the moon, despite the fact that they're sure to exist. "There are lots of asteroids in the solar system, so chances for the Earth to capture one at any time is, in a sense, not surprising," said co-author Jeremie Vauballion, an astronomer at the Paris Observatory in France.

The group claims that its paper is the first effort to theoretically model the orbits and sizes of Earth's temporary second moons. The researchers' results are consistent with observations of one such "temporarily-captured asteroid" that is believed to have orbited Earth for about a year starting in June 2006. The object, labeled 2006 RH120, was discovered by the Catalina Sky Survey in Arizona; estimated to be between 10 and 20 feet (3 and 6 meters) wide, it appeared to be orbiting Earth from two moon-distances away. [ What Would Earth Be Like with Two Suns? ]

Mikael Gravnik, a physicist at the University of Helsinki and lead author of the new paper, says 2006 RH120 was probably discovered because it was slightly larger than most of the other "temporary moons" that come traipsing through our planetary system. Most of the hobo moons are only about 1 meter wide.

"Objects of this size are too faint to be detected when being at a distance of, say, a few lunar distances from the Earth," Gravnik told Life's Little Mysteries."When coming closer in during their orbit, they are moving too fast to be detected, because the limited amount of photons is spread over too many pixels."

These limitations mean we don't currently have a way of finding our second moons. But an observatory called the Large Synoptic Survey Telescope (LSST), planned to open in Chile in 2015, could change that.

"We hope that LSST will do something about this, but dedicated programs will without doubt be even better," Vauballion said. "Statistic study is still needed to see where and how to look for them."

NASA's Spaceguard Survey tracks the paths of all near-Earth objects (NEOs) in Earth's neighborhood that are larger than 0.6 miles (1 kilometer) in diameter, but the scientists are less concerned with bodies that are too small to pose a threat to Earth — as is the case when they're just 1 meter wide.

But if our distant, noncommittal moons don't threaten Earth, and are much too dim to act as nightlights, does it matter that they're there at all?

According to astronomers, it does. Some researchers say it might be possible to go and get one of these temporary moons and bring it back to Earth for analysis.

"When found, such an asteroid will immediately raise the question whether or not we should go, and I'm ready to bet that many astronomers will argue that we definitely have to go!" Vaubaillon said in an email. "The reason is simple: What astronomers would not want to have a full and intact (unaltered by any physical process) piece of space rock? Meteorites are all altered because they go through our atmosphere. The only piece of asteroid we have comes from the Japanese Hayabusa mission (a few grams at the very most). The comet grains the Stardust mission got back from comet Wild 2 were all altered."

Clark Chapman, senior scientist at the Southwest Research Institute in Boulder, Colo., said a lot could be learned from the retrieval of a temporary satellite. "No doubt it is true that temporarily captured NEOs would be comparatively easy to get to and get back from ― it wouldn't take an especially powerful rocket, and round-trip times would be short," said Chapman, who is an expert on asteroid impact hazards.

Gravnik said, "We certainly hope that a space mission to a natural Earth satellite would someday materialize, and have actually already started a collaboration with experts in spacecraft orbital mechanics to find out how a mission from the Earth to a temporary satellite could be accomplished."

Follow Natalie Wolchover on Twitter @ nattyover. Follow Life's Little Mysteries on Twitter @ llmysteries, then join us on Facebook.

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Photos: Year in Space 2011

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  1. Ultimate space shot

    2011 was a year of farewells in space: an end to the space shuttle program ... NASA's official abandonment of the Spirit rover on Mars ... and the leavetaking of NASA's next Mars rover. This unprecedented image shows a different kind of leavetaking. Italian astronaut Paolo Nespoli snapped the picture of Endeavour docked to the International Space Station on May 23 as he was leaving in a Soyuz spacecraft. This was the only opportunity to photograph the space station and shuttle together from an orbital vantage point. (Paolo Nespoli / NASA / ESA) Back to slideshow navigation
  2. Tribute to Gabby

    During a post-landing ride on a Russian helicopter, NASA astronaut Scott Kelly wears a blue "Gabby" wristband in honor of his sister-in-law, wounded U.S. Rep. Gabrielle Giffords. Kelly and his fellow crew members from the International Space Station returned to Earth on March 16. Kelly's twin brother, Mark Kelly, is Giffords' husband. The two Kellys were the only twins to serve together in NASA's astronaut corps. Mark Kelly retired from NASA in October. (Bill Ingalls / AFP - Getty Images) Back to slideshow navigation
  3. Up from the clouds

    Stefanie Gordon captured this remarkable picture of the shuttle Endeavour's ascent on May 16 while she was on a commercial airline flight from New York to Palm Beach, Fla. (Stefanie Gordon / for msnbc.com) Back to slideshow navigation
  4. Hanging on

    NASA astronaut Greg Chamitoff holds a handrail during the fourth spacewalk conducted by the shuttle Endeavour's crew at the International Space Station. During the seven-hour, 24-minute spacewalk on May 27, Chamitoff and astronaut Michael Fincke (visible in the reflections of Chamitoff's helmet visor) moved a 50-foot-long inspection boom to the station, officially completing U.S. station assembly. (NASA via EPA) Back to slideshow navigation
  5. First Family on the final frontier

    Astronaut Janet Kavandi leads President Barack Obama, First Lady Michelle Obama and their daughters, Sasha and Malia, beneath the shuttle Atlantis' underbelly during an April 29 tour of an Orbiter Processing Facility at NASA's Kennedy Space Center in Florida. The Obamas visited the space center in hopes of seeing the shuttle Endeavour's final launch, but liftoff was delayed due to a technical glitch. The Obamas couldn't return to the cape for the Endeavour launch on May 16. Atlantis' launch in July closed out the 30-year space shuttle program. (Saul Loeb / AFP - Getty Images) Back to slideshow navigation
  6. Waiting for the last launch

    Spur King, from Armarillo, Texas, sleeps on the roof of a van in Titusville, Fla., as he waits to watch the liftoff of space shuttle Atlantis from NASA's Kennedy Space Center on July 8. Atlantis' mission marked the end of the 30-year space shuttle era. (Joe Raedle / Getty Images) Back to slideshow navigation
  7. Last liftoff

    NASA managers watch from Firing Room Four of the Launch Control Center at Kennedy Space Center as the space shuttle Atlantis lifts off from Launch Pad 39A on July 8. (NASA) Back to slideshow navigation
  8. Look! Up in the sky!

    Spectators watch the shuttle Atlantis ascend for the last time from NASA's Kennedy Space Center in Florida on July 8. (Shawn Thew / EPA) Back to slideshow navigation
  9. Back to Earth

    The space shuttle Atlantis blazes a trail back home through the atmosphere in this photograph, captured by the crew aboard the International Space Station on July 21. Airglow over Earth can be seen on the horizon. (NASA via EPA) Back to slideshow navigation
  10. Night landing

    The space shuttle Atlantis glides down from a moonlit sky to the runway at Kennedy Space Center in Florida on July 21. Atlantis' touchdown marked the end of a 30-year odyssey for NASA's shuttle fleet. (Pierre Ducharme / Reuters) Back to slideshow navigation
  11. On the beam

    A glowing laser shines forth from the European Southern Observatory’s Very Large Telescope in Chile, in a picture captured by ESO Photo Ambassador Gerhard Hüdepohl. The beam energizes sodium atoms high in Earth’s mesosphere, causing them to glow and creating a bright dot that looks like a star to observers on the ground. That artificial star serves as a guide for the telescope's adaptive optics system. (ESO) Back to slideshow navigation
  12. Does 'Pacman' have teeth?

    In visible light, the star-forming cloud cataloged as NGC 281 in the constellation of Cassiopeia appears to be chomping through the cosmos. Astronomers nicknamed NGC 281 the "Pacman Nebula," after the famous Pac-Man video game of the 1980s. This infrared view from NASA's Wide-field Infrared Survey Explorer, released Oct. 26, reveals jagged rows of "teeth" that are actually pillars of interstellar dust. (NASA / JPL-Caltech / UCLA) Back to slideshow navigation
  13. Getting the rover ready

    NASA engineers stand by a conical shell that will help protect the Curiosity rover, a robot the size of a car, from the searing temperatures of atmospheric entry when it lands on Mars next year. This picture of the rover preparations was taken at NASA's Jet Propulsion Laboratory on April 4. Curiosity was launched from Cape Canaveral Air Force Station in November and is due to land on Mars in August 2012. (Damian Dovarganes / AP) Back to slideshow navigation
  14. Millipedes on Mars

    Martian sand dunes ripple across this false-color image from the high-resolution camera on NASA's Mars Reconnaissance Orbiter. What's fascinating about this image, released Oct. 17, are the ridges running the length of the dunes, creating the spectacular illusion that we're looking at millipedes. This is a good example of what's called "pareidolia," where our brain interprets a pattern as representing a familiar object - such as the Face on Mars. (NASA/JPL/University of Arizona) Back to slideshow navigation
  15. Walking on a mock Mars

    A mock mission to Mars "landed" on a simulated Red Planet on Feb. 14, and in the days afterward, volunteer crew members went on three make-believe Marswalks. The simulated surface was actually a giant sandpit, built inside a Moscow research institute. The exercise was the climax of a 520-day isolation experiment aimed at studying how a future real-life crew would handle the psychological stresses of a Mars mission. (Lightroom Photos / Zuma Press) Back to slideshow navigation
  16. Celestial snow angel

    The bipolar star-forming region called Sharpless 2-106, or S106 for short, looks like a soaring, celestial snow angel in this image from the Hubble Space Telescope, released Dec. 15. The outstretched "wings" of the nebula record the contrasting imprint of heat and motion against the backdrop of a colder medium. Twin lobes of super-hot gas, glowing blue in this image, stretch outward from the central star. A ring of dust and gas orbiting the star acts like a belt, cinching the expanding nebula into an hourglass shape. (NASA / ESA / STScI / AURA) Back to slideshow navigation
  17. Outer-space ornament

    The moon hangs over Earth's limb like a holiday ornament in a picture from the International Space Station.. Original tweet from Oct: 21, 2011: "#TGIF Here's a beautiful moon shot to start your weekend #NASA #ISS" http://twitpic.com/73povh (Ron Garan / NASA) Back to slideshow navigation
  18. Golden Gate ... to space?

    A new Virgin America A320 jet, aptly named "My Other Ride Is a Spaceship," flies in tandem with the SpaceShipTwo rocket plane and its mothership over the Golden Gate Bridge on April 6. The aircraft landed at San Francisco International Airport, becoming the first planes to arrive at the new $388 million, 640,000-square-foot Terminal 2. SpaceShipTwo is expected to begin rocket-powered suborbital test flights during 2012 - not from San Francisco, but from the Mojave Air and Space Port near Los Angeles. (Mark Greenberg / Virgin America) Back to slideshow navigation
  19. A little lunar base

    Hillary Livingston adds the finishing touches to a scale-model lunar base camp in the "Beyond Planet Earth" exhibit at the American Museum of Natural History in New York on Nov. 10. The exhibition looks forward to the next 50 to 100 years of spaceflight, with the intention of fueling dreams of colonizing the moon and Mars. (Piotr Redlinski / New York Times via Redux) Back to slideshow navigation
  20. After the landing

    An aerial view shows vehicles with their headlights on converging on a Russian Soyuz spacecraft in northern Kazakhstan after its landing on Nov. 22. The capsule brought NASA astronaut Michael Fossum, Russian cosmonaut Sergey Volkov and Japanese astronaut Satoshi Furukawa back to Earth from the International Space Station. (Shamil Zhumatov / Reuters) Back to slideshow navigation
  21. Galactic firestorm

    The fiery birth of stars is chronicled in this view of the galaxy Centaurus A, captured by the Hubble Space Telescope and released on June 16. (NASA/ESA via AFP - Getty Images) Back to slideshow navigation
  22. In the dish

    Engineers carry out maintenance on the focus box inside the 76-meter dish of the Lovell Telescope on June 21 in Holmes Chapel, England. (Christopher Furlong / Getty Images) Back to slideshow navigation
  23. Groovy view of Vesta

    This image obtained by the framing camera on NASA's Dawn spacecraft shows the south pole of the giant asteroid Vesta. The probe entered orbit around Vesta on July 16 for a year's worth of observations. Scientists are discussing whether the circular structure that covers most of this image originated by a collision with another asteroid, or by internal processes early in the asteroid's history. Images in higher resolution might help answer that question. (NASA) Back to slideshow navigation
  24. A falling star in autumn

    An Orionid meteor streaks through the skies above French Creek State Park in Pennsylvania early Oct. 22, with the reds, yellows and oranges of autumn reflected in the trees below. (Jeff Berkes) Back to slideshow navigation
  25. Colorful crash

    The Antennae are a pair of colliding galaxies about 70 million light-years away in the constellation Corvus. This color-coded image, released Oct. 3, combines views from the Hubble Space Telescope and the newly inaugurated ALMA radio telescope array in Chile. (ALMA (ESO/NAOJ/NRAO)) Back to slideshow navigation
  26. That's heavy, dude

    An unmanned Boeing Delta 4 Heavy rocket rises from its launch pad at Vandenberg Air Force Base in California on Jan. 27. The heavy-lift launch vehicle sent a spy satellite into orbit for the National Reconnaissance Office. This was the largest rocket ever launched from the West Coast. (Bryan Walton / AP) Back to slideshow navigation
  27. Monster blast from the sun

    When an M-3.6-class flare occurred near the edge of the sun, it blew out a gorgeous, waving mass of erupting plasma that swirled and twisted over a 90-minute period on Feb. 24. The event was captured in extreme ultraviolet light by NASA's Solar Dynamics Observatory. Some of the material blew out into space, and other portions fell back to the surface. (SDO Goddard Space Flight Center) Back to slideshow navigation
  28. Quartet of moons

    Four Saturnian moons, from tiny to huge, make an appearance amid the planet's rings in this composition from the Cassini orbiter, released Oct. 24. Bright Dione is in the foreground, with Titan in the background. The dot just to the right of Saturn's nearly edge-on rings is Pandora, and Pan is just a speck embedded within the rings, to the left of Titan and Dione. (NASA / JPL-Caltech / SSI) Back to slideshow navigation
  29. Lights, camera, action

    Norwegian photographer Tommy Eliason captured this amazing view of the northern lights, the Milky Way and a meteor streaking across the sky over Ifjord, Norway, on Sept. 25. The year was notable for producing frequent auroral displays. (Tommy Eliassen / Caters News Agency) Back to slideshow navigation
  30. Pool practice

    With the aid of scuba divers, spacesuit-clad astronaut trainees take part in drills in a pool at Russia's Star City cosmonaut training center outside Moscow on Feb. 18. Underwater training simulates conditions of weightlessness and is a part of space crew training. (Sergey Ponomarev / AP) Back to slideshow navigation
  31. Chinese ship seen from space

    This Dec. 8 satellite image provided by the DigitalGlobe Analysis Center shows the Chinese aircraft carrier Shi Lang (a.k.a. Varyag) sailing in the Yellow Sea, approximately 60 miles (100 kilometers) south-southeast of the port of Dalian, China. (Digitalglobe / AP) Back to slideshow navigation
  32. The glow below

    A picture taken from the International Space Station on Sept. 17 shows two docked Russian spacecraft with the southern lights below. The auroral display is caused by the interaction between solar particles and Earth's magnetic field. (NASA via AP) Back to slideshow navigation
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