Impact! Tracking Near-Earth Asteroids

In Hot Pursuit of Asteroids

All Automated, All The Time

Such asteroid analysis, which was hand-worked a century ago, is now fully computerized. “These days, astronomical observation is almost never done by a person looking through a telescope,” says astrophysicist Grant Stokes. Still, it wasn’t until the late 80’s—the 19 80’s—when photography’s successor came about: the CCD, or charge-coupled device.

Stokes is the head of the NASA-funded Lincoln Near Earth Asteroid Research (LINEAR) program. LINEAR’s two telescopes, located near Socorro, New Mexico, hunt asteroids in fulfillment of a mandate Congress issued to NASA in 1998 to find 90 percent of near-Earth objects a kilometer in diameter and larger by 2008. “A kilometer is near the threshold for global environmental damage if the object indeed hits Earth.” says Stokes.

Every clear night, automated digital cameras on LINEAR’s telescopes image large sections of the sky accessible from the New Mexico site. Five frames of each section are collected over a two-hour period. A 6 cm x 5 cm CCD chip in each camera acts as an electronic photographic plate, but is a hundred times more sensitive, so long exposures are no longer needed. When photons of light hit a grid of sensors on the CCD, the asteroid’s brightness can be captured, quantified, and analyzed much more precisely than with a photograph. Also, because the CCD output is digital, LINEAR’s computers can find asteroids in it immediately.

LINEAR takes upwards of 6,000 frames per night and can cover all the available sky in a month. Objects moving with unusual speeds or directions are flagged as “interesting”: potential near-Earth objects. The nightly lot of positional data is then handed over to the MinorPlanetCenter. So far, LINEAR and the several other large sky surveys have found 700 km-and-up asteroids—an estimated 60 percent of the objects in the Congressional mandate.

Orbit Hunting

Each day, the Minor Planet Center computes orbits for every single object reported from observatories worldwide. “It takes probably 6 to 10 images over several nights to determine an orbit,” says Spahr. “And to make a prediction of how close that could come to Earth in the future, we need more observations—over several weeks or months.”

Minor Planet Center posts the initial orbit solutions of a few dozen of the new “interesting” objects online per month. That way, astronomers at universities and amateur observatories—often one person sitting in their den, checking their backyard telescope via computer—can follow up with more observations. If LINEAR is the bulldozer in a gold mine, these smaller outfits are the pan-sifters, pointing their smaller-field-of-view telescopes at the right patches of sky to track intriguing objects.

The longer the baseline of observations, the more accurate the orbit calculations get. “We can, of course, observe a new object in the future,” says Marsden. But for near-Earth objects, “astronomers are often impatient to find out whether there really is a danger to Earth. So it helps a lot if we can find images of them on old photographic plates.”

“Over geologic time there's a hundred percent chance of a very large asteroid impact on Earth,” says Stokes. “But will it happen in the next hundred years, in the next couple of hundred years?  It's highly unlikely.” Still, the risk is clearly worth the government funding projects like LINEAR and the Minor Planet Center. If an object is found to be Earth-bound for certain, the save-the-world idea is that we’d somehow nudge it off course before it has a chance to arrive.

“Suppose that $3 million does ensure us against a really large object that would put an end to the human race,” suggests Marsden. “Well, the world’s gross product is about $30 trillion, so here we're spending $3 million to save $30 trillion. That's a pretty good insurance rate.”

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