Asteroid 2012 TC4 hasn't been seen since its last brush with our planet five years ago, but calculations of its trajectory told astronomers that it would return in October 2017. Right on cue, the building-sized rock has now emerged from the darkness of space as it hurtles towards the Earth. NASA and the ESA plan to use it as a test run for the international Planetary Defense network, and have now been able to calculate its trajectory.

With asteroids grazing past Earth with worrying regularity, it would definitely pay to have some forewarning if any were on a collision course. The Chelyabinsk meteor is a perfect example of a hazardous space rock sneaking up on us, causing widespread damage and injury when it exploded over Russia in 2013. To help prevent a disaster like that (or worse) from occurring again, NASA has established the Planetary Defense Coordination Office to detect and track near-Earth objects.

Knowing exactly when TC4 would return makes it a perfect candidate for testing the equipment and infrastructure of that system, and NASA, ESA and other institutions set up the 2012 TC4 Observing Campaign to determine its orbit more precisely. Previous calculations could only be sure that it would pass within a window of 4,200 to 170,000 miles (6,760 to 274,000 km) of Earth, and its size couldn't be precisely pinned down.

This time around, the asteroid was spotted on July 27 by ESA astronomers using the Very Large Telescope Observatory in Chile, before further observations on July 31 and August 5 confirmed its identity as TC4. Its brightness at this distance tells the team that its diameter is about 15 m (49 ft).

Based on these observations, the astronomers have determined that TC4 will pass within 50,100 km (31,130 miles) of Earth at 5:41 Universal Time on October 12, 2017. That's 13 percent of the distance between the Earth and the Moon.

Astronomers report that viewing conditions will remain clear over the next few months as TC4 approaches, allowing them to gather more data for this and other near-Earth objects. Then, like last time, it should vanish into the darkness again almost immediately after it passes.