On November 6, we had a close encounter with a near-Earth object, 2009 VA (a NEO is a near-Earth object, including both asteroids and dormant comets). The space boulder, a 7-meter-diameter asteroid, streaked by at a distance of only 14,000 km, well inside the orbits of our geosynchronous satellites. NASA’s Jet Propulsion Lab estimates that we have two such encounters each year, on average, with objects of this size. About every five years, Earth is struck by such a body, but objects this small burn up in the atmosphere, resulting in a fireball and the release of several kilotons of energy (TNT equivalent).
The close pass of 2009 VA surprised some news outlets, which speculated on why the small asteroid had not been detected sooner by astronomers (The University of Arizona’s Catalina Sky Survey picked up 2009 VA about 15 hours before closest approach). The answer is that these small cosmic rocks are so numerous, and so difficult to observe, that we only discover them at random. NASA runs a search program, Spaceguard, to detect larger objects, 1 km and up, that may pose a civilization-ending threat to Earth. So far about 85% of those objects have been found; none pose an immediate threat to Earth, but may in future decades.
Impacts of small objects like 2009 VA create only sky-high fireworks, no harm to us here on the ground. But the Tunguska impact in Siberia a century ago devastated 2,000 square km of Siberian forest. That airburst of about 5 megatons (Mt) of TNT equivalent was caused by an object 30-40 m in diameter; large enough to level a city center. Such an object strikes us every few hundred years. The last one was a century ago; the next one to come along may hit us tomorrow. With current telescopes, we have only a small chance of seeing such an object before it strikes Earth.
Congress has asked NASA to look into what it would cost to search systematically for NEOs down to 140 m in diameter; if we found most of those objects, we would have greater confidence that no “city-buster” NEO is headed for an imminent collision with a populated area on Earth. A report to NASA on the prospects of detecting and even deflecting such potentially hazardous NEOs is due out by year’s end from the National Research Council.
Impact, or cosmic bombardment, is a process that has been altering the faces of the planets since the dawn of the solar system 4.6 billion years ago. Impacts by giant comets and asteroids have changed the course of life on Earth, ending the reign of the dinosaurs 65 million years ago, and possibly causing other mass extinctions through Earth’s long history. We now have the technology to both detect damaging NEOs heading for Earth, and with proper warning, to nudge them out of the way. What we lack is the international will to take action should a hazardous NEO be found on a collision course with Earth. The Association of Space Explorers is working with the United Nations to draft such a NEO decision-making agreement.
At the Houston Museum of Natural Science on Tuesday, Nov. 17, I will be speaking about impact and the other processes that shape the worlds of the solar system, in a talk called Planetology. My talk will discuss these processes — tectonics, volcanism, erosion, for example — and our search for life and “other Earths” across the galaxy. Please join me for the lecture that evening at 6:30 pm, or turn the pages of Planetology, written by me and noted planetary geologist Ellen Stofan. After the talk, I’ll be answering questions and signing copies of the book.
See reviews and more info on Planetology at:
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