A joint mission between NASA and the European Space Agency plans to study the binary asteroid system 65803 Didymos. ESA satellites, depicted in this artist's illustration, would monitor the asteroid system from orbit while the NASA-built Double Asteroid Redirection Test (DART) impacted the smaller asteroid.

Why incoming asteroids shouldn't keep you up at night

A close shave with a previously undetected asteroid last month serves as a pointed reminder to humanity that planet Earth and other celestial bodies can sometimes cross paths. Fortunately, space agencies are on the case.

Every once in awhile, as we Earthlings strive to explore the cosmos, we’re reminded that bits of the cosmos occasionally visit Earth, too.

One such reminder came in the form of a blazing green fireball streaking across the predawn New Jersey sky earlier this month. Police dashcam footage shot at 3:09 a.m. on Dec. 2 in Hamilton, N.J., shows a meteor plunging into the Earth's atmosphere and exploding in a brilliant flash. No injuries were reported.  

A slightly larger visitor hurtled past our planet on Nov. 9, one that astronomers didn’t detect until the following day. An asteroid designated 2017 VL2 came within 75,000 miles of Earth – less than a third of the distance to the moon. Despite news reports that the asteroid, which measures about 22 yards wide, carried enough energy to obliterate New York City, the asteroid – the 48th known one to pass within the moon's orbit this year so far – would have actually burned up in the atmosphere, causing little, if any, damage.

“The most important message to get across is that asteroid impacts are extremely unlikely,” Paul Chodas, manager for the Center for Near-Earth Object Studies at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “This one passed at a comfortable distance.”

The chance of a destructive meteor impact is small, but the consequences couldn’t be greater. Hence Spaceguard, an international project to track potentially hazardous asteroids and comets. So far, astronomers have spotted more than 15,000 objects, with an average of 30 new discoveries added each week.

“We can reassure the public that our observatories scan the night skies every dark night,” says Dr. Chodas. “Our current capabilities have been very successful in finding asteroids.”

Blind spots

In 1998, Congress mandated that NASA find 90 percent of asteroids more than 1 kilometer wide (0.62 miles) – about a tenth of the size of the asteroid believed to have abruptly ended the Age of Dinosaurs. NASA met this goal in 2011, but in the meantime, Congress expanded its mission to include include 90 percent of asteroids 450 feet or larger, whose impact could cause regional devastation. Scientists say they have detected about a third of these so far.

The bigger the asteroid, the lower the chance of impact: The odds of an asteroid 1 kilometer wide hitting Earth in any given year are 1 in about 500,000, and even an object 450 feet wide has just a 1-in-30,000 chance of impact.

Asteroid 2017 VL2, for its part, came from the direction of the sun, and it was lost in the glare. “We know less about the population of asteroids that spend time between Earth and sun since we find [asteroids] in dark sky, and those are not in dark sky much or possibly ever,” says Martin Connors, an astronomer at Athabasca University in Alberta, Canada. “But chances of impact from any source are very small. We do know theoretically and statistically that there is not a huge group of such hidden asteroids.”

NASA aims to close the gap on undetected asteroids with its Near-Earth Object Camera (NEOCam), a space-based infrared telescope scheduled for launch in 2021. NEOCam will peer between the Earth and the sun to spot potentially hazardous space rocks by their heat signatures.

“When it comes online,” says Chodas of the space telescope, “it will have a dramatically improved capability of detecting asteroids much farther out.”

The Large Synoptic Survey Telescope (LSST), a wide-field reflecting telescope built atop Chile’s Cerro Pachón mountain, could be operational by 2019. The LSST, which is funded by the National Science Foundation, is designed to detect a variety of cosmic phenomena, including near-Earth asteroids, dark matter traces, and supernovae. Outfitted with the world’s largest digital camera, the telescope will snap full panoramas of the night sky for 10 years; each image, at full resolution, would be the size of about 40 full moons.

Redirecting an asteroid?

If astronomers spotted a potentially devastating asteroid on a collision course with Earth, could it be deflected?

A joint project between NASA and the European Space Agency seeks to find out. The Asteroid Impact & Deflection Assessment (AIDA) mission proposes to crash an unmanned spacecraft into the smaller body of the binary asteroid system 65803 Didymos.

The mission would consist of two spacecraft: the ESA-built Asteroid Impact Mission (AIM), which would monitor the asteroid system from orbit, and the NASA-built Double Asteroid Redirection Test (DART) that would impact the smaller asteroid, known as “Didymoon,” at about 14,000 miles per hour.

The key in deflecting an asteroid is to hit it with just the right amount of force. “To mitigate an asteroid threat, we wish to cause a sufficient deflection to make the asteroid miss the Earth, but not more than necessary and not so much as to cause its disruption,” says Andrew Cheng, chief scientist of the space department at Johns Hopkins University’s Applied Physics Laboratory, in Laurel, Md., and co-leader of the DART team.

If approved, AIM will launch in October 2020, and DART in July 2021. The two spacecraft would begin orbiting 65803 Didymos in October 2022.

But before that, astronomers on Earth will get an up-close look at one of largest known near-Earth objects. 3200 Phaethon, a 3-mile wide object thought to be the source of the annual Geminid meteor shower, will approach within 6.4 million miles of Earth on Dec. 16.

Often called a “rock comet,” 3200 Phaethon is made of the same rocky stuff as other asteroids, but moves along an elongated orbit usually traveled by icy comets. Astronomers have dubbed the object “potentially hazardous” not so much because of its proximity – in 2017, it will be about 26 lunar distances away at its closest point – but because of its size. Though an impact would cause significant damage in populated areas, 3200 Phaethon’s predictable orbit won’t intersect our own in the foreseeable future. It will come close enough for ground-based telescopes to get detailed snapshots. [Editor's note: An earlier version mischaracterized the composition of 3200 Phaethon.]

“Expect to see great images,” says Chodas.

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