Space is dark and full of nightmares: asteroids, bolides, and comets, oh my. Millions of invisible objects pass through Earth's orbit every year. And at least one is whizzing by today.

Exhale and have a seat. 2013 TX68 is probably not going to hit the Earth. Scientists' best estimates have it passing more than three million miles away. And though they sound pretty certain about that figure, it's a pretty recent revision. Less than a month ago, NASA moved the estimated flyby date back three days, and extended the asteroid's predicted intercept point. That's ... unsettling. If they were wrong before, how can Earthlings trust them to be so certain now?

Answering that question is a matter of understanding how astronomers catalog any object that comes close to Earth. The fundamentals date back to the earliest skywatchers, who noticed that some stars moved across the sky relative to the others. (Planet is derived from a Greek word that means "wanderer.") Except unlike the planets, these objects are so small that they are often barely perceptible to even the highest powered telescopes. "They take a series of pictures of the same part of the sky at 15 to 30 minutes apart," says Gareth Williams, associate director of the International Astronomical Association's Minor Planet Center, based in Cambridge, MA.

If an astronomer finds a blip in the stellar backdrop, he or she passes the note along to Williams and his colleagues at the MPC. They'll calculate the object's trajectory based on its position relative to the fixed stars, and make note of those moving fastest. "Typically anything moving more than a degree a day is going to be interesting, because it is probably going to be close to Earth," says Williams.

When the the Hawai'i-based Pan-STARRS telescope first noticed TX68 in October 2013, the object was moving fast enough to warrant closer inspection. So Williams and his colleagues at MPC catalogued the object and added to their confirmation page. Then they passed it along to NASA’s Center for Near-Earth Object Studies at the Jet Propulsion Laboratory in Pasadena, which looks for impact threats far into the future.

NASA, with its comfortingly worst-case mindset, added TX68 to the Sentry Risk Table, a clearinghouse for objects with high (or in TX68's case, uncertain) probability of hitting Earth in the next 100 years. "When a new near-Earth object gets detected close to Earth there is a low probability that it could impact at some point in the future, and as we get more observations generally that probability shrinks," says Williams. When JPL made its first estimates, it was running off just three days of observations from October 2013. The tentative flyby date: March 5, 2016. Closest approach: anywhere between 11,000 and 9 million miles away.

NASA/JPL-Caltech

After those first three observations, TX68 passed into Earth's daytime side and the astronomers lost their ability to track it—no stars to measure movement against. Luckily, astronomers weren't left worrying about what happened to their missing asteroid: They found pictures of TX68 in night sky photographs taken a month before its discovery.

These so-called "precovery" pictures are actually pretty common. Neptune itself is the most famous case (Galileo thought it was a fixed star when he first observed the slow moving planet in 1612). TX68's precovery pictures put the total number of observations at ten days, and JPL scientists adjusted their estimates. Now the flyby would come on March 7, and the asteroid would probably pass around 3 million miles away.

So what if the scientists are still wrong—if TX68 beats all the odds and hits Earth? Based on its brightness, Williams says the asteroid is less than 90 feet in diameter. That's big enough to squash a school bus, if you don't account for all the material the atmosphere would strip away. At the very least, TX68 would create a fireball bigger than the one caused by the 2013 Chelyabinsk meteor—which injured more than 1,000 people with broken glass and falling objects in rattling homes. And if the meteor did pass through the atmosphere and collided with Earth, the crater would be small. An extinction level event? Only if you happen to be riding your bike in the impact zone.

Williams assures this is an impossible scenario for TX68. But less so for the hundred million or so unknown objects that periodically cross Earth's orbit. Tracking data is so incomplete that the Chelyabinsk meteor was the second object to pass close to Earth that day in 2013. Of course, only a couple thousand are big enough to make a dent. Which definitely soothes the nerves, right?