15,000 and counting, baby. That’s the number of known near-Earth asteroids (NEAs), with an average of 30 new discoveries added each week. Astronomers define “near Earth” as objects that come within 121 million miles of the sun at their closest, a distance is equal to 1.3 astronomical units (a.u.). An a.u. equals the distance between the Earth and sun or 93 million miles. This distance brings the asteroid within about 30 million miles of Earth’s orbit, close by solar system standards.
This milestone marks a 50% increase in the number of known NEAs since 2013, when discoveries reached 10,000 in August of that year. In addition to the asteroids, there are also more than 100 comets that are considered Earth-approachers. Collectively, all these small bodies are called NEOs or Near-Earth Objects. Surveys funded by NASA’s Near Earth Object (NEO) Observations Program account for more than 95% of discoveries so far.
2016 TB57, discovered on Oct. 13, by observers at the Mount Lemmon Survey in Tucson, Arizona, became the 15,000th. It’s just a little guy, about 50 to 115 feet (16-36 meters) across, and will whizz safely by Earth on Halloween at just beyond five times the distance of the moon. Sky surveys, which are now largely robotic, have already discovered more than 90% of the estimated population of the big one — those larger than 0.6 miles (1 km) — that pretty much would destroy civilization as we know it should Earth and asteroid cross paths.
“The rising rate of discovery is due to dedicated NEO surveys and upgraded telescopes coming online in recent years,” said NASA’s NEO Observations Program Manager Kelly Fast. “But while we’re making great progress, we still have a long way to go.” Astronomers estimate that only about 27% of the NEAs that are 460 feet (140 meters) and larger have been found to date. That means there’s at least another 45,000 asteroids and comets out there waiting to be discovered. Congress directed NASA to find over 90% of objects this size and larger by the end of 2020.
Of course, there are far more smaller objects, numbering probably in the hundreds of thousands or even millions, orbiting in the neighborhood. Some of them become the 10-or-so-a-year witnessed meteorite falls that fall as eyegasmic fireballs over the planet. The NEO Observations Program is a primary element of NASA’s Planetary Defense Coordination Office, which is responsible for finding, tracking and characterizing potentially hazardous NEOs, issuing warnings about possible impacts, and coordinating U.S. government planning for response to an actual impact threat. It’s nice to know our country takes the potential threat seriously.
The main drivers of discovery right now are the Catalina Sky Survey and the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) in Hawaii which account for about 90% of new NEO discoveries. A recent upgrade to one of the Catalina Sky Survey’s telescopes resulted in a tripling of its average monthly NEO discovery rate. When the Pan-STARRS system increased the observing time it devoted to NEO searching to 90%, it increased its rate of discovery by a factor of three. Pan-STARRS also will add a second telescope to the hunt this fall. As more and more scopes are devoted to the effort, the survey effort will be able to As more capable telescopes are deployed, the number of objects as small as and smaller than 140 meters (460 feet) should steadily climb.
No known NEO currently poses a risk of impact with Earth over the next 100 years, and we’ve found most of the larger asteroids, but that’s no reason to think we can take a breather. We still need another set of eyes to be on the lookout for asteroids closely approaching our planet from the direction of the sun in the daytime sky. Since darkness is required to see and discover asteroids, what’s needed is a satellite orbiting interior to Earth’s orbit that could spot space rocks against the dark backdrop of space, when it’s not possible to see them from Earth.
Currently, the private B612 Foundation, established by NASA astronauts Ed Lu and Rusty Schweickart, is at work building such this asteroid-scouring satellite called the Sentinel Mission. It would be put into an orbit trailing behind Venus. Originally slated to launch this year, the project has suffered from a lack of money, so the group began a crowdfunding campaign, which along with private donors, might raise the needed $450 million to realize its goal.
Want to stay in touch with the latest asteroid discoveries? Subscribe free to the Daily Minor Planet. Each day, you’ll get news of new discoveries and links to interesting stories about asteroids. Today’s edition brought news of 2016 UR36, which will miss Earth by just 1.3 times the distance to the moon on Halloween.