A space rock bigger than a skyscraper will sail past Earth on Halloween, zipping by just beyond the moon. The asteroid won't collide with our planet, but as a cosmic trick-or-treat it offers a rare chance to see a near miss.
Astronomers from NASA's Near Earth Object Program first spotted the incoming asteroid on October 10, just three weeks before its closest approach. It was too small and faint to detect until it came within the range of large survey telescopes.
Since then, scientists having been scrambling every night to keep an eye on the interloper, tracking its orbit as well as its shape and size.
Nicknamed Spooky , the asteroid (officially called 2015 TB145) is estimated to be about 950 to 2,100 feet wide (290 to 650 meters). Scientists won't be sure of its exact size until they can do radar measurements—and the most accurate will be on Halloween, when it passes the closest. But even at the middle of that range, or 1,300 feet, Spooky will be about 32 times larger than the asteroid that burned up in spectacular fashion above Chelyabinsk, Siberia, in 2013.
NASA has calculated the asteroid's trajectory and says it will safely zip by us, traveling at 78,000 miles per hour (35 kilometers per second), some 29 times faster than a speeding bullet. Earthlings can breathe a sigh of relief as the rock flies just past the moon’s orbit on Saturday, October 31, at 1:05 pm ET (17:05 UT).
“At the point of closest approach, it will be no closer than about 300,000 miles [480,000 kilometers]," said Paul Chodas of the Center for Near Earth Object Studies at NASA’s Jet Propulsion Laboratory in a statement.
Spooky is not the largest nor the closest of Earth's near misses in recent years. Just this past January, BL86 came to within a similar distance to Earth.
The next time we'll get a chance to see a known object this big pass so close to Earth will be in August 2027, when 1999 AN10 will fly by at the distance of the moon. But expectations are that there will be other such encounters, even sooner, as we continue to stumble across new NEOs regularly. So NASA hopes to keep an eye on this and other asteroids so as to predict its path and help determine any potential hazard it may pose in the future.
Scientists say that because of its proximity and large size, Halloween's near miss will be a great chance to snag some high-resolution images. NASA plans to use the 110-foot (34 meter) Goldstone antenna to bounce radio waves off the asteroid's surface and catch the radar echoes with radio telescopes in Virginia and Puerto Rico. This will be the first time the space agency can collect images of an asteroid swinging by Earth showing details down to two meters in size.
In total astronomers have so far cataloged some 13,000 near-Earth asteroids. Yet it's estimated that only 30 percent of those in the same class size as Spooky have been detected, and fewer than one percent of those 30 meters and greater—ones that could damage a city.
How to Watch
“Even though that is relatively close by celestial standards, it is expected to be fairly faint, so night-sky Earth observers would need at least a small telescope to view it,” Chodas said.
In the few hours when Spooky will be at its closest and brightest on Halloween, skywatchers will need a large backyard telescope to hunt it down in the skies above. Its brightness is expected to reach magnitude 10, which would make it visible through an eight-inch mirrored telescope.
The best time for Spooky-hunting will be the night before its closest approach, on Friday, October 30. As it slowly moves across the field of stars within the bright constellation Orion, the hunter, the asteroid will pass a relatively brighter 8th magnitude star HIP 23301 (visible through telescopes only) at 3:20 a.m. ET, Saturday morning. Since these are faint objects, a computerized telescope with the ability to select an object to slew to, would come in quite handy.
But Slooh, an Internet-based space tracking service, will be keeping an eye on the asteroid with its robotic telescopes on the Canary Islands and Europe. You can watch Slooh's broadcast of the flyby online starting at 1 p.m. ET/ 17:00 GMT on October 31.