The Gliese 710 Close Encounter

Over the course of a single human lifetime the night sky does not change much at all. Except for the motions of the planets and the occasional comet or rare guest star such as the coming Boom Star, the night sky will look the same from the day you were born to the day you die. But in the sense of geologic time, the galaxy is a roiling ever-changing place full of stars moving to and fro and sometimes passing by and disrupting each other's solar systems.

We are not immune to this. About 7.3 million years ago the Algol star system passed by our sun at a distance of just under ten light years. That's not that close, Proxima Centauri is currently less than half that distance away. But unlike Proxima, which is a tiny red dwarf star, Algol is a multiple star system that contains a lot of mass. That mass  may have been enough to disturb the Oort cloud and may have slightly increased the number of comets entering the inner solar system at the time. While the danger of that specific event was negligible, comets have smacked into planet earth in the past and will do so in the future unless we do something about it.

But the most pronounced effect of that close encounter would have been on the appearance of the night sky. Algol would have been the brightest proper star in the heavens, significantly beating out the current brightest star which is Sirius, the dog star. While not as bright as Venus or the Moon, Algol would have otherwise dominated the late night sky rivaling Jupiter and Mars in apparent magnitude.

But that encounter is in the past and no harm was done that we know of. But what of the future? About 1.35 million years from now we will have a far, far closer encounter with another star. It's called Gliese 710 and not only will it perturb our Oort cloud, it has a high likelihood of directly passing through it.

While there's still a lot of uncertainty as to just how close this stellar intruder will pass, the current best prediction is that its closest approach will be about 13,000 astronomical units out. For comparison, an astronomical unit is 149.6 million kilometers which is essentially the distance between the earth and the sun. That may sound like a lot until you convert that distance into the more familiar light-year system that we use in astronomy.

A light-year is the distance a wave of light travels in a year at 186,000 miles per second, so a star system like alpha centauri is just under 4.5 light years away at 40 trillion kilometers or roughly around 300,000 astronomical units. Space, even the local area, is unbelievably huge .

As a result 13,000 astronomical units is not that much at all, so much so that the light-year is not really the best distance scale you could apply to Gliese 710. It will pass about 77 light-days from us.

That's a very close pass indeed. Now, Gliese 710 is in a different class than Algol and won't light up the sky as a second sun. It only possesses about 60 percent of the mass of our own sun and will be 13000 times further away. But it will rival Mars and Jupiter in the night sky as Algol once did.

But unlike Algol, the proximity of Gliese 710 could have disastrous results for earth due to a prediction that there is an 86 percent chance that it will enter our Oort cloud and a near certainty that it will gravitationally disturb it.

The star currently poses no danger at all as it's 63.8 light-years away still. But the star's small proper motion suggests that it's heading more or less straight for us. When it does finally approach us it could hypothetically unleash a hail of comets entering the inner solar system and endangering earth with an impact event. Now, comets are nothing new of course, visible ones pass through the inner solar system every few years or so. But the estimates for the Gliese 710 encounter could push those numbers up to as many as 10 visible comets per year.

That would be a spectacular sight indeed, and not necessarily a hugely dangerous one, one model for example predicts an increased cratering rate of no more than 5 percent. But we do live in a cosmic shooting gallery on a planet that has been hit many times before so anything could happen. But let's not get excited, we're talking about an event 1.35 million years in the future. By that time, if we're still here, we would likely be a highly advanced civilization that could easily deflect comets, in fact we're not far from that ability with our current technology, and there may be nothing to deflect at all since by that time we may have scoured the Oort cloud and Kuiper belt for raw materials leaving nothing left behind and allowing Gliese 710 to pass by harmlessly.

By why waste such a great opportunity? Gliese 710 is, after all, a huge ball of free energy heading our way. Why not colonize it? We could found a second human stellar colony by building a dyson sphere or swarm around gliese 710. Sounds good. But, over time this colony would grow increasingly distant unless we figured out a way to eventually, over geologic time scales, sync up the two star systems. While moving stars is not impossible, you can build megastructures that redirect stellar radiation to become thrust and accomplish it. But it takes enormous amounts of time so we would instead likely wave a long, slow goodbye to our new colony. And while that might be sad, it could also be a blessing if the two cultures end up not getting along.