An asteroid passing within 26,000 mi (42,000 km) of Earth, which is one tenth the distance to the Moon and not far above the orbital altitude of communications satellites, might seem like cause for concern. But when the 10 to 20 m wide asteroid 2012 TC4 did just that in the early hours of today, it provided the perfect opportunity to test a global asteroid-impact early-warning system.

TC4 was discovered in 2012 by the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) in Hawaii as part of the NASA Planetary Defense Coordination Office's (PDCO) NEO Observations Program. Though it passed out of observational range before a detailed study could be made, it was still possible to plot its orbit well enough to predict its return this year.

According to NASA, TC4 posed no threat of hitting the Earth and confirmed that no asteroid is predicted to impact our planet in the next century. However, the space agency recognized the close pass of TC4 was an opportunity to test the ability of international observer teams to coordinate and optically and radar track TC4 as part of a volunteer global asteroid-impact early-warning system conceived, organized and supported by PDCO.

"Asteroid trackers are using this flyby to test the worldwide asteroid detection and tracking network, assessing our capability to work together in response to finding a potential real asteroid-impact threat," says Michael Kelley, program scientist and NASA PDCO lead for the TC4 observation campaign.

TC4 was reacquired in July by the European Space Agency and the European Southern Observatory using its eight-meter aperture telescopes. As the asteroid drew closer to Earth, other teams joined the hunt and have tracked the asteroid in wavelengths from visible to near-infrared to radar, sending their data to the Minor Planet Center. NASA says that most of the work is being done by the larger professional telescopes because TC4 only has a visual magnitude of 17 and is moving very fast. Magnitude 17 objects are so dim that they push the limits of telescopes smaller than 0.5 m.

"This campaign is a team effort that involves more than a dozen observatories, universities and labs around the globe so we can collectively learn the strengths and limitations of our near-Earth object observation capabilities," says Vishnu Reddy, principal investigator for a NASA-funded near-Earth asteroid characterization project. "This effort will exercise the entire system, to include the initial and follow-up observations, precise orbit determination, and international communications."

However, in a post-flyby question and answer blog post, ESA's Detlef Koschny revealed that not everything went as planned. "Not all telescopes worked as planned due to technical issues, which can happen," he said. "One large, important asset is the big radar system in Arecibo, Puerto Rico – which was not functioning due to damage from the recent hurricane there. Luckily another US-based radar system could be used a couple of nights ago. This is exactly why we do this exercise – to not be surprised by these things."

Despite these minor problems, the exercise was considered a success. Initial size estimates put the asteroid at between 10 and 30 m, but radar observations during the flyby reduced that figure to 10 - 20 m, probably 10 - 12 m. As such, had it not zoomed past and actually entered the Earth's atmosphere, it would have exploded and been destroyed.

"But in that case we would need to inform the public to, for example, stay away from windows – it was the shock wave from the Chelyabinsk object in 2013 that damaged many windows, causing injuries due to flying glass," said Koschny.

Work is now underway to analyze the data to determine TC4's composition and refine positional information to help make future predictions more accurate.

The first video below by Kiso Observatory, the University of Tokyo, shows asteroid 2012 TC4 using the observatory's one-meter Kiso Schmidt telescope. The second video was recorded by astronomers Peter Schlatter and Dominik Bodenmann working at the ZIMLAT telescope at the Swiss Optical Ground Station and Geodynamics Observatory.

Sources: NASA, ESA