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Space Access '15

Meteor Crater
Meteor Crater is a reminder that the Earth is struck regularly by asteroids and comets. What can we do to prevent another major impact? (credit: J. Foust)

Planetary defense: deflection and disruption

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The threat of an asteroid or comet impact has often fallen squarely on the boundary of science fact and science fiction. Because virtually no one in human history has experienced a catastrophic impact first-hand (save for the handful of eyewitnesses to the Tunguska event in 1908, who had no comprehension at the time of what was taking place), the idea of a celestial object destroying a city or a civilization has instead become a staple of science fiction; occasionally good, frequently bad. Yet, as the geological record has made abundantly clear, the Earth has been struck by objects large enough to cause regional and even global death and destruction.

The explosion of a meteor above the Russian city of Chelyabinsk in mid-February may change that assessment, though. While the explosion killed no one and spared the city from major damage, the event served as a stark reminder that objects do pose a risk to the Earth. The Chelyabinsk object was too small to be seen before its impact (see “Piecing together the Chelyabinsk event”, The Space Review, April 15, 2013), but larger objects that have the potential to cause greater devastation can be detected in advance. What could we do if we found one heading this way?

The case of 2011 AG5

As it turns out, NASA recently studied this question, when faced with an object that, at the time, posed a small risk of impacting the Earth. The study showed that, with sufficient warning time, and for small enough objects, a relatively modest mission could eliminate the impact risk.

“If we have a near miss in 2040, it could hit later,” Chodas said. “We want to move an asteroid far enough away that it will not have a serious chance of hitting in a future year; any time soon, anyway.”

That study was prompted by the discovery of a near Earth asteroid designated 2011 AG5 in early 2011. The asteroid, approximately 140 meters in diameter, was observed for half an orbit before it disappeared from view. Calculations of that orbit going forward showed that it had a 0.2 percent chance of impacting the Earth on February 5, 2040. “AG5 was for a while at the top of our risk page, one of our most hazardous objects,” said Paul Chodas of the Near Earth Object Program Office at JPL in a presentation at the Planetary Defense Conference 2013 earlier this month in Flagstaff, Arizona. He was referring to a page on the office’s website that tracks potentially hazardous objects. “If it hit, it would be 100 megatons.”

Chodas said JPL was asked in early 2012 to examine what it would take to prevent that potential impact. “The question is, how would we carry out a deflection mission on this real problem?” he said. Would there be enough time to develop and carry out some kind of deflection mission before that impact?

A key factor for dealing with 2011 AG5 was that, if the asteroid was to collide with the Earth in 2040, it would have to pass though a “keyhole” about 365 kilometers wide during a close approach to the Earth in 2023. A mission mounted before 2023 would only have to move the asteroid a few hundred kilometers so it missed that keyhole. After that, Chodas said, deflecting the asteroid to avoid an Earth impact would be 50 times harder.

The brief study examined what kind of mission could be developed to deflect the asteroid using a kinetic impactor: a spacecraft designed to crash into the asteroid, shifting its orbit enough to miss the Earth. But by how much? “If we have a near miss in 2040, it could hit later,” Chodas said. “We want to move an asteroid far enough away that it will not have a serious chance of hitting in a future year; any time soon, anyway.” In the study, they focused on solutions that caused the asteroid to miss the earth by at least 10 Earth radii, moving it into “safe harbor” regions that avoid future potential impacts through at least 2100.

The JPL study examined missions capable of providing that kind of deflection prior to the 2023 keyhole, involving first a spacecraft that rendezvoused with the asteroid, followed by the impactor spacecraft. It found that a sufficiently large deflection was possible without requiring the use of particularly large spacecraft or launch vehicles, or advanced technologies. In one scenario, an Atlas V 401—the most basic version of that vehicle—launches a rendezvous spacecraft in April 2018, arriving at 2011 AG5 in August 2019. The following January, another Atlas V 401 launches the impactor mission, a 3,060-kilogram spacecraft that would strike the asteroid in May 2021. The resulting change in velocity of the asteroid, 19 mm/sec, would be enough to cause the asteroid to miss by 11 Earth radii in 2040.

That mission involved only conventional chemical propulsion, but the JPL study also examined the use of solar electric propulsion. Those allowed delaying the launch of the rendezvous and impactor missions to mid-2020, allowing for more time for additional observations to potentially rule out an impact and thus cancel the mission.

If a deflection mission wasn’t possible before the asteroid’s 2023 keyhole passage, a deflection mission would be more difficult but still feasible. “You would have to use a larger launcher, a Delta IV Heavy,” Chodas said. “We could, in fact, move the asteroid a significant amount as late as launching in the 2030s,” including as late as 2036, although that scenario would move the asteroid just enough to avoid a 2040 impact, with no consideration for any future impact scenarios.

NASA has already demonstrated a kinetic impactor of sorts: the Deep Impact mission fired a projectile at the nucleus of comet Tempel 1 in July 2005, although that impact was designed for planetary science, not planetary defense. Deep Impact used software called AutoNav to autonomously guide the projectile to its target; that software has been used on several other missions as well, said Shyam Bhaskaran of JPL. He said they’ve run simulations using AutoNav on an asteroid impact mission with good success.

“Disruption, by which I mean breakup and dispersal of an object, is an option for smaller objects, particularly when there’s little warning time,” said Miller.

Still, some worried that keeping to the timetable identified in the JPL study would be difficult given the inherent delays in spacecraft development. “There is some flexibility in how long it takes to go through all of those phases [of spacecraft development], but it cannot be compressed too much,” Chodas said. With multiple mission opportunities, though, he said there was some flexibility in waiting to refine the asteroid’s orbit and determine if a deflection mission was needed.

The disruption option

The concept of avoiding an asteroid impact by plowing a spacecraft into it runs in sharp contrast to the preferred model of science fiction, of sending nuclear weapons (and, sometimes, astronauts as well) to an asteroid or comet to deflect or even destroy it. Is science fiction wrong?

Nuclear weapons, while a popular tool in the past to deflect asteroids, have fallen out of favor somewhat in recent years. Besides the complications inherent with nuclear weapons, there’s also the concern that a nuclear weapon could break apart so-called “rubble pile” asteroids, agglomerations of smaller rocks held together loosely by gravity. Many objects in that case would remain on an Earth impact trajectory.

Some at the Flagstaff conference, though, argued that nuclear weapons might still be useful even if they disrupted an asteroid. “Disruption, by which I mean breakup and dispersal of an object, is an option for smaller objects, particularly when there’s little warning time,” said Paul Miller of Lawrence Livermore National Laboratory. “We’re talking about dispersal of the material over huge distances,” so that most, if not all, of the disrupted material misses the Earth, and what’s left is not particularly dangerous.

Miller showed a simulation of a “megaton-scale” nuclear weapon striking a pure iron asteroid 50 meters across. “We found that, with this 50-meter object, the largest fragments were less than three meters in diameter,” he said.

Separate work at Iowa State University examined disruption using a combination of kinetic and nuclear impactors, with the former creating an impact crater for the latter to explode in, creating greater dispersion of objects. “A 100-meter target could be safely disrupted on all orbits if we’re given 15 days of lead time,” said Brian Kaplinger. Their criteria for safe disruption, he said, included leaving no more than .01 percent of the object’s initial mass on impact trajectories after disruption.

Fortunately, there’s currently no known asteroids that pose a threat that would require either deflection by a kinetic impactor or disruption by a nuclear weapon any time in the foreseeable future. So what happened to 2011 AG5?

“We were quite pleased that, for the first time, really analyzed a real deflection scenario and gone through the steps that we would have to go though to analyze a possible deflection mission,” said Chodas.

The final report of the JPL study was published last June, with the asteroid’s trajectory still uncertain. At the time it appeared that scientists would have to wait until a close approach in September to get new observations of the asteroid and refine its orbit. However, last October, University of Hawaii planetary astronomer David Tholen was able to observe the asteroid when it was very dim and required the use of large telescopes. Tholen captured images of the asteroid using the Gemini telescope and University of Hawaii 2.2-meter telescope on Mauna Kea.

Those observations, Chodas said, eliminated any risk of a 2040 impact. “But the study, nonetheless, was very worthwhile,” he added. “We were quite pleased that, for the first time, really analyzed a real deflection scenario and gone through the steps that we would have to go though to analyze a possible deflection mission.”



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