Combinations of space- and ground-based telescopes may be the most economically palpable defenses NASA can mount against asteroids and comets heading toward Earth, but there are more advanced defenses involving spacecraft and nuclear explosions that might be plausible in the future.
Those were just some of the conclusions included in a report, “Defending Planet Earth: Near-Earth Object Surveys and Hazard Mitigation Strategies,” issued today from scientists at the National Research Council on what options NASA has to detect more near-Earth objects (NEOs) -- asteroids and comets that could pose a hazard to Earth.
The same council issued a preliminary report in August saying imminent impacts (such as those with very short warning times of hours or weeks) require better current discovery capabilities. Existing surveys are not designed for this purpose; they are designed to discover more-distant NEOs and to provide years of advance notice for possible impacts. In the past, objects with short warning times have been discovered serendipitously as part of surveys having different objectives. Search strategies for discovering imminent impacts need to be considered, and current surveys may need to be changed.
No matter what though, the report says the $US4 million the US currently spends annually to search for comets and asteroids is insufficient to meet a congressionally mandated requirement on NASA to detect NEOs that could threaten Earth.
The report states that while impacts by large comets or asteroids are rare, “a single impact could inflict extreme damage, raising the classic problem of how to confront a possibility that is both very rare and very important. Far more likely are those impacts that cause only moderate damage and few fatalities.”
An asteroid or comet about 10 kilometers in diameter struck the Yucatan peninsula 65 million years ago and caused global devastation, probably wiping out large numbers of plant and animal species including the dinosaurs, the report states.
Objects as large as that strike Earth only about once every 100 million years on average, the report notes. NASA has been highly successful at detecting and tracking objects 1 kilometer in diameter or larger, and continues to search for these large objects. The report notes that NASA has managed to accomplish some of the killer asteroids mandate with existing telescopes but with over 6,000 known objects and countless others the task is relentless.
Objects down to sizes of about 140 meters in diameter -- which NASA has been mandated to survey for -- would cause regional damage; such impacts happen on average every 30,000 years, the report says.
The report recommends that NASA monitor for smaller objects -- those down to 30 to 50 meters in diameter -- which the report says recent research suggests can be highly destructive.
The report states that detailed studies of ways to mitigate collisions are best viewed as a form of insurance. How much to spend on these insurance premiums is a decision that must be made by the nation’s policymakers.
The report goes on to say that with sufficient warning four types of mitigation could meet the threat from all NEOs, except what it called the most energetic ones:
• Civil defense (evacuation, sheltering in place, providing emergency infrastructure) is a cost-effective mitigation measure for saving lives from the smallest comet or asteroid hit and is a necessary part of mitigation for larger events.
• “Slow push” or “slow pull” methods use a spacecraft to exert force on the target object to gradually change its orbit to avoid collision with the Earth. This technique is practical only for small NEOs (tens of meters to roughly 100 meters in diameter) or possibly for medium-sized objects (hundreds of meters), but would likely require decades of warning. Of the slow push/pull techniques, the gravity tractor appears to be by far the closest to technological readiness.
• Kinetic methods, which fly a spacecraft into the NEO to change its orbit, could defend against moderately sized objects (many hundreds of meters to 1 kilometer in diameter), but also may require decades of warning time.
• Nuclear explosions are the only current, practical means for dealing with large objects (comets or asteroids with diameters greater than 1 kilometer) or as a backup for smaller ones if other methods were to fail.
“Although all of these methods are conceptually valid, none is now ready to implement on short notice, the report says. Civil defense and kinetic impactors are probably the closest to readiness, but even these require additional study prior to reliance on them,” the report stated.
NASA has been increasing its ability to track dangerous comets and asteroids. For example, part of the space agency’s recently launched Wide-field Infrared Survey Explorer spacecraft to uncover objects never seen before, including the coolest stars, the universe's most luminous galaxies and some of the darkest near-Earth asteroids and comets.
In addition, NASA's Jet Propulsion Laboratory recently launched the Asteroid Watch Web site to act as a centralized source for information on objects hurtling at Earth.