NASA Scientists: A Lot More Dangerous Comets Exist Than We Thought


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Andrew Follett Energy and Science Reporter
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NASA scientists say a new study proves there are seven times as many big and potentially dangerous comets flying through deep space than previously thought.

The study analyzed data from the Wide-field Infrared Survey Explorer (WISE) spacecraft that tracks “long-period” comets (LPCs) that take at least 200 years to orbit around the sun. WISE data showed far more LPCs that potentially pose a serious impact risk to Earth.

“Previously the sense was that for every 100 asteroid impacts (of all sizes) we get 1 comet impact,” Dr. Joseph A. Nuth, a senior asteroid scientist at NASA’s Goddard Space Flight Center, told The Daily Caller News Foundation. “Now that we know there are more LPCs for every 100 asteroid impacts we might get 5 comet impacts.”

Astronomers previously gave broad estimates of the number of LPCs in the solar system, but researchers didn’t have a good way to detect them since they were obscured by clouds of gas and dust.

Gravitational forces can move LPCs back into the inner solar system, potentially putting them on a collision course with Earth. An LPC impact would be devastating.

“A comet impact is, on average, much more devastating than an asteroid impact, Nuth said. “Think BBs and Bullets vs. Howitzers and Nukes.”

Comets are rarer than asteroids, but can carry more than 100 times the energy of a typical asteroid. An LPC would likely impact Earth at a much higher velocity than other space objects.

“[A] higher velocity impact would release more energy and therefore would be worse,” Paul Chodas, manager of the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory, told TheDCNF. “This is offset somewhat by the fact that comets are less dense than asteroids: the energy release of a comet impact would be less than that of a similarly sized asteroid impact if the velocities were the same.”

The study also determined that LPCs are more dangerous than more conventional asteroids or comets because they are twice the size on average.

“Yes, a body twice the size has eight times the mass of a smaller body and therefore delivers eight times the kinetic energy if all other factors are equal,” Nuth said. “The impact energy is proportional to the velocity squared.  So a comet moving three times the average asteroid velocity would impact with nine times the energy.”

“The potential impact of a long-period comet would be more hazardous than that of a Jupiter family comet both because the long-period comets are more likely to be larger, and because they follow orbits that would impact at higher velocities,” Chodas said.

The final reason researchers think LPCs are dangerous is their relative stealthiness compared to other space rocks. Detecting an LPC on a collusion course with Earth would be more difficult that spotting a more conventional near-Earth asteroid.

“The larger distance of comets, and the long orbital periods affect the warning time more than higher velocities: the generally larger distance of comets make the tracking observations less effective (since they are angular measurements), and the longer orbital periods mean that we don’t have multiple opportunities to see these objects at closer ranges,” Chodas said. “The distance at which a comet is discovered depends largely on the activity level of the comet.”

The difficulties inherent in detecting LPCs mean that NASA would likely have far less warning that one was on a collision course with Earth. The comet Siding Spring was detected on a trajectory that would strike the planet Mars in October of 2014, just 22 months after its discovery.

“Since Comet Siding Spring was quite active, it was discovered at a distance of 7 AU, about two years before it passed very close to Mars,” Chodas said. “A less active long-period comet might be discovered only a year before entering the inner Solar System. Establishing a precise trajectory for the comet would be challenging and likely take several months, so the warning time would likely be less than a year.”

Twenty-two months warning is far less than NASA scientists think they’d need to intercept an incoming comet. The best way to stop an asteroid or comet from hitting the Earth on such short notice may be to send a spacecraft up to intercept it. But even then, NASA researchers think they would need at least five years to construct a reliable spacecraft and man it.

“The case of Siding Spring is a reasonable example: much less than 5 years is a good guess,” Nuth said.  “Siding Spring came in from the celestial pole and was not spotted until it began to exhibit a coma. A larger body would show a coma at similar distances.”

Earth is just as vulnerable to LPCs and other space rocks as Mars nearly was to the Siding Spring Comet. Earths’ closer proximity to the Sun wouldn’t make it easier to detect a long-period comet on a collision course, according to NASA researchers.

“The discovery time is dictated mostly by the comet’s distance from the Sun and how active the comet is,” Chodas said. “If Comet Siding Spring were headed for an encounter with Earth, its discovery time would be the same.”

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