Boo! A Decent-Size Asteroid Will Safely Pass Earth on Halloween.

Around 17:00 UTC (13:00 Eastern US time) on Halloween, an asteroid called 2015 TB145 will pass relatively close—but still well away—from the Earth.

It’ll slip past our planet at a distance of about 470,000 kilometers, which is a bit farther away than the distance to the Moon. In fact, it’ll get closer to the Moon than it will to us, passing it at around 280,000 kilometers distant.

This is an interesting asteroid. It’s very roughly 500 meters across, so it’s pretty big as near-Earth asteroids get. It’s also on a very elongated orbit that takes it closer in to the Sun than Mercury, and about half the distance out to Jupiter. The orbit is also highly tilted; it’ll come at us from the south, cross Earth’s orbit, move up to the north for a while, and then back down again.

This unusual orbit is why it was only discovered on Oct. 10! Most asteroids stick near the plane of Earth’s orbit. Worse, it’s coming from the south, and there are fewer observatories in the Southern Hemisphere scanning the skies to look for passing rocks. It was discovered in observations made by the Pan-STARRS telescope in Hawaii.

For NASA, this asteroid is more treat than trick: It gets close enough that it can be tracked by the Goldstone radio telescope, which can send pulses of radio waves at the rock to accurately measure its size, shape, and trajectory. Observations are already being planned. Here’s an example of such an observation made of the near-Earth asteroid 1999 JD6, which passed about 7 million km away in July 2015:

If you want to watch live, the Virtual Telescope Project will be observing the pass.

Again, I’ll note we’re in no danger from 2015 TB145. It misses us cleanly, and extrapolating its orbit forward into time, it won’t be back this way for at least a century. I won’t get too specific there because astronomers have only been tracking the asteroid for a little more than a week, so trying to project the orbit forward in time more than a few decades gets hazy pretty quickly. But it’s clear that while this is a close pass, this rock won’t be a danger to us for a long time, if ever (though I can imagine there will be, as usual, breathless articles exaggerating the danger from less-than-reputable venues).

But it does highlight that we’re still trying to find threatening asteroids. It’s pretty unusual for one this size to get this close without us knowing well in advance, but here we are. It can happen, if very rarely. If circumstances had been slightly different, TB145 could have been on an impact trajectory, and we still wouldn’t have seen it any earlier.

This is why I support more telescopes scanning the skies. The odds of an impact at any given time are very low—we get hit by something this large once every million years or so. But it doesn’t take a rock this big to do serious damage. And the data recorded by such surveys are incredibly valuable for other reasons; they uncover more distant objects that will help us make a better census of our solar system, and they reveal other objects that change brightness like supernovae, variable stars, galaxies, and other transient phenomena. All of this helps us understand the Universe better.

And they might just save the world. I find that to be a pretty good bargain.