NASA’s near Earth object program reported this week that the only known near-Earth object (NEO) on its current Sentry asteroid impact hazard list thought to pose any risk to Earth over the next 100 years is now crossed off the list.*
2007 VK184, an asteroid about 130 meters in diameter, has been on JPL’s Sentry Impact Risk Page for several years, with an estimated 1 in 1800 chance of impacting Earth in June 2048. This risk prediction translates to a rating of 1 on the 10-point Torino Impact Hazard Scale. In recent months, 2007 VK184 was the only known NEO with a non-zero Torino Scale rating. (The Torino scale is a probabilistic risk assessment tool. While NEO observers find Torino ratings useful, they don’t mean much to non-experts. Some discussion is under way in the NEO community about developing new, non-statistical ways to characterize NEO impact risks. Stay tuned.)
2007 VK184 was discovered by the Catalina Sky Survey (CSS) at the University of Arizona in November 2007 and tracked by NEO observers for two months before moving out of view in January 2008. Last month, NEO observer David Tholen at the University of Hawaii looked for and found 2007 VK184. His observations of the asteroid led to reducing its impact risk from a Torino rating of 1 (“merits careful monitoring”) to zero – “no likely consequences.”
Ho hum, you might say. And right you are, for the removal of this object from the risk list was a routine process. What’s interesting about this routine event is that it shows how the global NEO monitoring system works efficiently. In the public discourse about NEO impact hazards, attention-grabbing statements about close calls, near-misses, and impending catastrophes seem more likely to draw attention than the more mundane business of 24/7 NEO observing and tracking. Predicting the orbital movements of asteroids is a largely mathematical enterprise….
Tholen sighted 2007 VK184 on March 27 and 27, using the 3.6-meter-diameter Canada-France-Hawaii Telescope at the Mauna Kea Observatories in Hawaii. Because the asteroid had not been sighted for almost six years, its predicted position was approximate. Nonetheless, Tholen was able to find it within the predicted search region. He forwarded his tracking data to the Minor Planet Center (MPC) in Cambridge, Massachusetts.
NASA’s automated Sentry asteroid monitoring system, run by the Jet Propulsion Laboratory, retrieved the new observations of 2007 VK184 from the MPC database, computed a new orbit for the object, and updated its impact hazard assessment for it. These new calculations showed that 2007 VK184 will pass no closer than 1.9 million kilometers from Earth in June 2048, with no other close encounters predicted. JPL removed 2007 VK184 from its Sentry Impact Risk Page after it sent its updated calculations to the MPC.
As the Risk Page explains, JPL’s Sentry system continually scans the most current catalog of known asteroids and predicts potential hazards of impacts with Earth over the next 100 years. NEOs will disappear from Sentry’s impact risk page when sufficient data become available to eliminate impact risks. Objects typically appear on the Sentry Impact Risk Page because a limited number of available observations indicate a potential hazard of impact with Earth but do not allow observers to precisely define their orbital movements. Whenever a newly discovered NEO is posted on the Sentry Impact Risk Page, the most likely outcome is that the object will eventually be removed as new observations become available, the object’s orbit is improved, and its future motion is more tightly constrained.
While some may find this story unexciting, perhaps others may find it comforting.
Reminder: I’m a consultant to NASA’s NEO program. No one at NASA asked me to write this post, nor did anyone at NASA review it.