Not long ago I wrote about a pair of dim red dwarfs in a binary system called WISE J072003.20-084651.2 (or, more colloquially, Scholz's Star) that passed fairly close to the Sun about 70,000 years ago. The exact distance isn't easy to pin down, but it was likely just under a light-year. Close as stars go, though still too far to be visible to any humans banging rocks together back then. It's possible it did affect comets in the outer solar system, though.
But that's all in the past. What about the future?
In 1999, using data from the Hipparcos satellite (which observed the positions and motions of over 100,000 stars), astronomers found that the star Gliese 710 (or just Gl 710 for (barely more) short) will give us a pretty close shave in about 1.4 million years. Since that first prediction came out the distance estimate has been refined, and the most recent gives a pass at about 0.2 light-years. Given that it's currently 62 light-years away, this means it's headed nearly directly at us!
The Hipparcos data are good, but the new satellite Gaia is far more precise. Using this better dataset, a different team of astronomers looked at Gliese 710 and confirmed the older results. While exact data is hard to get — Gliese 710's trajectory being so aligned toward us means that its sideways motion is small and hard to measure — their best fit to the data indicates it will pass us at a mere 0.163 light-years in about 1.28 million years.
That's pretty dang close. Right now, the closest star to the Sun is Proxima Centauri at about 4.2 light-years (and it's a red dwarf so dim you need a telescope to see it). As close as this pass will be, it's not anywhere near close enough to affect the inner planets much — it'll be over 10,000 times farther from Earth than the Sun is — but that does put it in the middle of the Oort Cloud, the vast repository of icy bodies out past Neptune. It's entirely possible this will cause a great disturbance in the gravitational force out there, and send lots of these iceballs toward the Sun. As they get close they will start to warm, the ice will turn into a gas, and they will become comets.
Not to alarm you, but there could be planetary impacts from these comets… but I have to assume that by this time over a million years in the future, we’ll either a) be able to easily push them out of the way or 2) if we can't then we're in no position to complain about it.
I find this fascinating. Gl 710 is an orange dwarf, a star with about 0.6 times the mass of the Sun, and only about 4% as bright (the amount of energy a star emits depends very strongly on its mass). Still, at that distance it'll shine in the sky pretty brightly, reaching a magnitude of about -2.5. Dimmer than Venus, but still about as bright as Jupiter gets, and the color of it should be stunning. It'll be the brightest object in the sky outside the solar system by quite a bit.
I rather envy those future humans, seeing such a thing in their sky. I hope that if we are still around, and still recognizably human, someone in that distant future will wonder when we first knew that Gl 710 would become our nearest neighbor. Perhaps too they'll find that we discovered it back in the dim dark ages of our species when humans had barely left our world for the first time, but when we did, we looked to the heavens and mapped its motion.
That's a fine legacy.