Earlier this week, on March 8, asteroid 2013 TX68 came within about 3.1 million miles of Earth, astronomers estimate. Original predictions suggested its closest approach might be within the orbits of geostationary satellites around the Earth, or it could be as far away as 9.5 million miles. Additional data changed the distance range to between 3 million and 15,000 miles. At about 100 feet in diameter, the object was too small to be seen at the 3.1 million-mile distance, but it clearly did not hit Earth.

We keep an eye on such space debris. NASA’s Center for Near-Earth Object Studies at the Jet Propulsion Laboratory in California currently tracks 13,947 near-Earth objects, defined as those coming within 130 million miles of our planet’s orbit. The center considers about 12 percent of those potentially hazardous, according to Paul Chodas, manager of the center. That means they come within 5 million miles and pose about a one in 1 billion chance of hitting Earth in the next 100 years. (TX68 isn’t one of them.)

Asteroids travel on elliptical orbits around the Sun, explains Judit Györgyey Ries, an asteroid observer and researcher at the University of Texas at Austin’s McDonald Observatory. An asteroid’s path changes slightly from the effect of gravity when it passes close to a planet or from the energy of it absorbing and emitting sunlight.

The orbit of asteroid TX68. Image credit: NASA/JPL-Caltech

The more data scientists collect on a specific asteroid, the more accurate their predictions of its path and probability of colliding with Earth. TX68 is a perfect example. It was first observed by the Catalina Sky Survey in October 2013, while approaching Earth at night. Three days later, the asteroid passed into the daytime sky and could no longer be observed. Based on those three days of data, TX68 appeared to have a four in 1 billion chance of hitting Earth.

That may sound like a long shot, but the odds were nevertheless four times higher than the threshold NASA has set for potentially hazardous objects. “That caught our attention,” Chodas says. Then Italian astronomer Marco Micheli, with the European Space Agency, noticed faint traces of the asteroid in archived telescope images, which directed a search for more archived images. Based on that additional data, TX68’s potential for impact dropped back to the more acceptable one-in-a-billion chance.

All calculations come with uncertainty, of course, and with asteroids, that uncertainty grows the farther into the future the orbit projection. At the scale of the Earth, this uncertainty equals large distances, on the order of millions of miles. (For perspective, the average distance from Earth to the Moon is about 239,000 miles.) That makes it important for scientists to continue to monitor known objects.

Now scientists know where to look for TX68 when it returns to our part of the solar system. If it turns up where expected, that will decrease uncertainty about its future orbit. If not, says Györgyey Ries, the uncertainty will grow.

Three years ago, a meteor about 60 feet wide broke up in the atmosphere over Chelyabinsk, Russia. Observers didn’t see it coming because of its small size and approach from the direction of the Sun, but the dashcam and smartphone recordings of its fiery descent and glass-shattering sound wave were subsequently seen worldwide.

Any object between about 100 and 165 feet should burn up and disintegrate in the atmosphere, Chodas says, with some small meteorites reaching the ground, as they did in Chelyabinsk. NASA mostly worries about roughly 1000 known objects measuring at least one kilometer, or about six-tenths of a mile.

NASA-funded surveys began scanning the night sky in 1998 for near-Earth objects, and about 1500 NEOs are now detected each year. The strategy, according to Chodas, is to find as many of these objects measuring 330 feet and larger as possible, to provide as much time as possible for attempts to deflect a potential impact. For example, preparations for diverting a large asteroid of 650 to 1000 feet might involve building and launching a rocket, which would take years.

“You would just have to nudge it,” Chodas says. “Presumably, we could launch as heavy a rocket as we possibly could to run into the asteroid and change its velocity slightly. A change of one meter per second would likely be enough to divert it from impact.” NASA has plans for two missions to test deflection methods.

In January, NASA announced that its NEO detection and tracking project, now called the Planetary Defense Coordination Office, will supervise all NASA-funded projects working to find and characterize asteroids and comets passing near Earth's orbit and also coordinate response to potential impact threats.

For Chodas, TX68’s fly by presented an opportunity. “We know this particular asteroid can’t impact Earth in the next 100 years,” he says. “It is more of an opportunity to remind people we are working on the problem so that, if an asteroid should be headed for Earth, we would have enough warning time, possibly decades, to do something about it.”

But as Györgyey Ries notes, “I only worry about the ones we don’t know of.”