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Asteroids: Close and Closer, but Not Too Close for Comfort


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Herschel space telescope spots asteroid Apophis

New Herschel telescope images of near-Earth asteroid Apophis. Credit: ESA/Herschel/PACS/MACH-11/MPE/B.Altieri (ESAC) and C. Kiss (Konkoly Observatory)

Early this morning, while most of the U.S. slept, a once-menacing asteroid drew close to Earth on its usual rounds through the inner solar system. The 300-meter asteroid, known as Apophis, kept a comfortable distance, flying well beyond the orbit of the moon. But Apophis has not drawn so near to Earth since 2004, when it was first discovered—and when it was briefly feared to be on a possible collision course with Earth. Further observations of the asteroid have cooled those worries, but Apophis will come much closer in 2029, passing within about 35,000 kilometers of Earth, and still holds a minuscule chance—about 1 in 230,000—of an impact when it comes back around in 2036.

Such is often the case with near-Earth asteroids (NEAs) and the troublemakers among them, potentially hazardous asteroids (PHAs). Uncertainties in a PHA’s orbit allow the possibility that the object will strike Earth until better observations reduce those uncertainties and all but eliminate the risk.

Old orbital estimate for asteroid 2011 AG5

Projection of asteroid 2011 AG5's orbit in 2040, based on old data that allowed the possibility of a collision with Earth. Credit: NASA

That is just what happened last month with a PHA known as 2011 AG5, which carried a small risk of impact in 2040 [right]. Discovered two years ago, the 140-meter rock had been one of only two known asteroids to score a 1 on the Torino Impact Hazard Scale, which ranks comet and asteroid hazards from 0 to 10. A 1 on the Torino scale means a collision is “extremely unlikely with no cause for public attention or public concern,” but 2011 AG5 was still tied for the most threatening known PHA. (Put another way, all the known asteroids are pretty tame—the only remaining PHA to rate above a zero on the Torino scale is 2007 VK184, which holds a 1-in-1,820 chance of impact in 2048.)

Asteroid 2011 AG5 orbital projection (new)

Orbital prediction for asteroid 2011 AG5, incorporating new data. Note that the asteroid's projected path now misses Earth in 2040. Credit: NASA

Now the 2011 AG5 threat has been retired, thanks to new data released in December. Astronomer David Tholen of the University of Hawaii and his colleagues observed the asteroid several times throughout the month of October and pinned down its orbit with sufficient precision to significantly limit the possible paths it might follow in the future. The new data [left] show that 2011 AG5 will cruise past Earth at a comfortable distance of roughly 900,000 kilometers. “The bottom line is that there is no impact risk for 2040,” Tholen says.

For researchers who study asteroid properties and orbits, close approaches of Earth are not cause for hand-wringing—rather these flybys are rare opportunities to get a good look at objects that usually hide from view. European researchers, for instance, used the Herschel space telescope to observe Apophis as it drew near over the weekend. Preliminary data from Herschel indicate that the asteroid may be somewhat larger, and significantly more massive, than had been assumed.

Orbit of asteroid 2012 DA14

Orbital path for asteroid 2012 DA14, which is expected to buzz Earth in February. Credit: NASA

Next month astronomers will have a much closer target to examine. A near-Earth asteroid called 2012 DA14, just 45 meters or so in diameter, will approach Earth on February 15, passing inside the realm of some orbiting satellites. No reason to worry, though—the asteroid rates as a solid zero on the Torino scale, with a negligible impact risk until 2026 and only a 1-in-3,000 shot after that. Nevertheless, during its February flyby 2012 DA14 is expected to miss Earth by just 30,000 kilometers or so—the closest approach of a known asteroid in the foreseeable future.

About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. bkgutt31 2:10 pm 01/10/2013

    We can debate all we want about whether or not we need to worry about comets and NEO’s, but what does “The Man”, Stephen Hawking think? Here it is…

    http://snip.ps/HawkingBelievesThatCometCollisionIsBiggestThreattoHumansinGalaxy

    Link to this
  2. 2. Crasher 4:38 pm 01/10/2013

    I don’t think there is any debate about if we need to ‘worry’ about NEO’s. The debate is about who will fund the search. The reality is, as Hawking points out, that we will be hit. Its just when and if we can do anything about it. The sooner we are aware the more likely we can take action.

    Link to this
  3. 3. Postman1 7:17 pm 01/10/2013

    This, friends, is exactly why we must build self-sufficient colonies on the Moon and Mars, ASAP. Unless one wants to follow the dinosaurs, there is no other viable choice. Yes, I know we hope to find methods to steer them away, but what about the one we don’t spot until it is too late? Actually, building colonies will help us in developing interception and steering technologies and locating and tracking. What will it take to light a fire under people to demand we get off planet? I don’t know, but I hope it happens in time.

    Link to this
  4. 4. vulvox 12:44 am 01/11/2013

    we should keep the asteroids on the radar scape. some big surprises have slipped through.

    Link to this
  5. 5. Quinn the Eskimo 2:09 am 01/11/2013

    Of course, if a planet-crusher asteroid does hit Earth spot-on, and think this over carefully, there would still be no reason to worry now, would there?

    And no taxes due next year. None!

    Link to this
  6. 6. bucketofsquid 3:31 pm 01/11/2013

    This is why the heating up of the space race is a good thing even if it seems like a race between snails.

    Link to this

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