Eye witness photograph of the 2013 meteor event near Chelyabinsk, Russia. [Nikita Plekhanov/Wikimedia Commons]
Scientists have now identified an estimated 90 percent of all larger asteroids – those at least six-tenths of a mile in diameter – that could come too close to Earth's orbit. Yet, our solar system is full of many smaller objects such as the previously undetected Chelyabinsk meteor, and current capabilities limit scientists' ability to spot such hazards early enough to do anything about them, speakers said at a 8 July event co-sponsored by AAAS.
Lindley Johnson, program executive for NASA's Near-Earth Object Observations (NEOO) Program , emphasized that people should not be concerned about an impending cosmic collision. "Currently, we know of no impact threats to the Earth," he said at an informational session organized by AAAS and the Secure World Foundation . "We know of a few objects that are in orbits that have a possibility of impacting the Earth sometime in the distant future but nothing that has a significant probability within the next 100 or so years."
The NEOO program, launched in 1998, this year reached its original goal to pinpoint 90 percent of all space objects that are at least six-tenths of a mile, or 1 kilometer in size and likely to come within about 30 million miles of Earth's orbit. Smaller space objects are more prevalent, however, and therefore more likely to strike the Earth, Johnson and other speakers said. He added that existing ground-based facilities are not well-suited for detecting the population of objects as small as 140 meters, or 460 feet in diameter, which is NASA's new NEOO program objective.
According to Johnson, finding these smaller near-Earth objects in less than a few decades should involve space-based facilities that allow scientists to observe large areas of space at night, even when the moon is bright.
Johnson joined two other experts – Irwin Shapiro, Timken University Professor with the Harvard-Smithsonian Center for Astrophysics, and Mark Boslough, a principal member of the technical staff at Sandia National Laboratories – in reviewing the research challenges associated with identifying, tracking and appropriately addressing space objects that might pose a threat to life on Earth.
Their presentations took place on Capitol Hill as U.S. policymakers were preparing to debate which of NASA's research activities should be authorized for support in 2014. President Obama's Fiscal Year 2014 R&D budget proposal, released  10 April at AAAS, includes a proposal for an asteroid retrieval mission, which has been opposed by a number of Members of the Science, Space and Technology Committee , including its Chairman Rep. Lamar Smith (R-TX) as being too costly and with limited scientific contribution. NASA Administrator Charles Bolden responded to such opposition in an op-ed article  published by The Hill: "By involving humans for the first time in an asteroid sample return mission," he wrote, "we will demonstrate our new deep-space technologies, move closer to our goal of sending humans to Mars and learn more about how to protect our planet and prevent natural disasters from space."
Johnson said that NASA's near-Earth object research plans encompass efforts to identify threatening space objects, characterize their physical properties, and to redirect them in space if ever needed. The agency is participating in a public-private partnership, the Space Act Agreement with B612 , which would build a spacecraft called Sentinel to complete an infrared survey of large areas of space from the vantage point of a Venus-like orbit around the sun. In addition, NASA on 18 June issued a Grand Challenge  to find all asteroids that could be hazardous to humans.
The meteor that streamed at a shallow angle toward Chelyabinsk, Russia, and then exploded in an event known as a "low-altitude airburst" on 15 February this year was 17 to 20 meters, or about 56 to 66 feet in size – "really a tiny thing," compared with many other asteroids and comets, speaker Shapiro said. Nonetheless, he emphasized, the Chelyabinsk impact "was a major event in the lives of people who lived in the nearby town."
In fact, Johnson said that the explosion injured 1,613 people and caused an estimated $30 million in structural damage, including a flattened factory and shattered windows that left residents exposed to the harsh Siberian winter. Boslough pointed out that the Chelyabinsk meteor would have been far more devastating if it had entered the Earth's atmosphere steeply. Still, it was moving at a speed of more than 40,000 miles per hour, with 40 times the energy, per pound, of the well-known explosive TNT. To put this into perspective, the energy released from the 2013 Chelyabinsk bolide exceeded 400 kilotons of TNT – a level of energy about twenty times greater than the first atomic bomb.
By comparison, the Chicxulub crater, located beneath Mexico's Yucatan Peninsula, is believed to have been caused by a massive asteroid that was about 6 miles in diameter, resulting in a globally catastrophic impact that has been associated with the extinction of dinosaurs and many other species. A much-smaller space object of 50 meters, or 55 yards in diameter carved out Arizona's Barringer Crater some 50,000 years ago, bombarding the Earth with destructive energy equivalent to 10 megatons of TNT and sending hurricane-force winds beyond Winslow, Johnson noted.
This year's Chelyabinsk airburst was a less-powerful version of the Tunguska event of 1908, when an exploding space object flattened trees and scorched the Earth near Krasnoyarsk Krai, Russia, Boslough explained. If the Tunguska event had happened over Washington, D.C., he said, the destruction would have extended past the Beltway. Scientists have speculated that another type of airburst, such as one that took place 30 million years ago in what is now the Libyan desert had enough destructive force to vaporize trees and melt rocks.
"The next destructive near-Earth object will probably be another low-altitude airburst," Boslough said. "We really lucked out that [the Chelyabinsk meteor] was kind of a grazing impact; somehow, miraculously, no one was killed."
Lindley Johnson (left to right), Mark Boslough and Irwin Shapiro answer questions after their presentations. [AAAS/Ginger Pinholster]
Boslough said that the "vast majority" of smaller near-Earth objects have not yet been discovered. "There's still quite a bit of uncertainty about how frequently a Chelyabinsk-sized object could hit the Earth," he added. "It might be once a century, it could be once every 30 years or so. We just don't know."
He added, however, that scientists' ability to model and better understand airbursts is continuing to improve. During his presentation, in fact, Boslough shared a simulation of a 5-megaton nuclear explosion compared with an asteroid blast of the same magnitude. A nuclear explosion causes surface damage as a result of the blast and the radiation, he said, but the mushroom cloud rises relatively quickly off the ground. In the case of an asteroid that explodes while making a steep, nearly vertical approach toward Earth, "The fireball is rammed downward into the ground, and you get this melting," he said. "So it's much more damaging than [an equivalent] nuclear explosion at the same altitude."
Certain comets known as "long-period comets" can also pose a unique risk, Shapiro said. "These comets come from beyond Jupiter's orbit, where they cannot be seen and could head at great speed directly for Earth, giving us very little time between discovery and impact," he explained. Shapiro expressed support for a key, ground-based observational facility, the Large Synoptic Survey Telescope , which he noted has not yet been fully funded, as well as other facilities reviewed by Johnson. "LSST could find 90 percent of 140-meter sized objects in well under two decades, mostly because of its large, 8.4 meter diameter, mirror and the large area of sky that it can view at once," Shapiro said after the event on Capitol Hill.
Existing detection capabilities include ground-based telescope facilities such as the Catalina Sky Survey  at the University of Arizona; the Panoramic Survey Telescope & Rapid Response System  at the University of Hawaii; and the Lincoln Near-Earth Asteroid Research facility  run by the Massachusetts Institute of Technology's Lincoln Laboratory, as well as radar-based observational facilities –the Arecibo Observatory  in Puerto Rico and the Goldstone tracking station  in California, which can reveal many of their physical properties.
A globally catastrophic collision with a near-Earth object is unlikely to take place anytime soon, said Victoria Samson, Washington Office director of Secure World Foundation , and yet the possibility raises many critical questions. Scientists must consider how best to discover such hazards before they reach the Earth, for example, and global cost-sharing, warning and disaster-recovery strategies must be developed. Planetary defense against near-Earth objects should be seen as a "shared international responsibility," Samson said.
The most frequent NEO visits come from small-diameter meteorites that disintegrate at high altitudes. [Courtesy of Lindley Johnson]
View the Capitol Hill presentations by: