One of the tenets of science fiction is that of inter-planetary travel. The dream of boarding a spacecraft and launching out to the far reaches of the universe is an exciting idea, but the reality of this happening anytime soon is slim to none. Despite NASA’s Kepler mission identifying almost 3,000 exoplanets in its mere three years of research, there are limitations to reaching any of these newly-discovered planets. Through some research I’ve done for my science fiction novel, Frozen Planet, four boundaries are keeping us from traveling to the thousands of planets in the known universe:

  1. Habitable Zone
  2. Distance
  3. Gravity Slings
  4. Escape Velocity

The first problem with many of these planets is that they aren’t the right distance from their respective star for us. Too close and we’d burn up, too far and we’d freeze to death. The zone right in the middle is known as the “Goldilocks Zone” for being just right to sustain life. Fortunately, because the research that discovered these planets also provides an idea of their distance from their stars, this is an easy enough problem to solve. Unfortunately, of the 3,000+ exoplanets we know about, only about a dozen of them are potentially habitable.

Exoplanet graph
We’ve discovered many exoplanets in just the last few years! (Source: Wikipedia)

So, of these twelve exoplanets, only one of them is less than ten light years away from us. This fact highlights the second boundary: distance. Even if an exoplanet has ideal conditions, if it’s over 1,000 light years away, we’re probably never going to get there in any of our lifetimes. Our current technology doesn’t allow us to go that fast. Sure, we can go fast enough to get satellite probes out to the planets in our solar system, but most of them are terribly inefficient. If we were able to implement nuclear options, or even “out of the box” ideas like mass drivers on the moon and other solar system locations, we might be able to get closer to the speeds we’d need to travel these long distances.

As it just so happens, we do have a technique for increasing our speed in space: gravity slings. Launched in the 1970’s, the Voyager spacecraft took advantage of the “Grand Tour,” which was an ideal alignment of Jupiter, Saturn, Uranus, and Neptune that used their enormous masses to accelerate the spacecraft to much higher velocities. Unfortunately, this is also a limitation, as this alignment only occurs once every 175 years. Our next “Grand Tour” possibility will be in the 2150’s. That being said, even with three of the four planets used for a gravity sling, we could potentially use the alignment by 2035. By then, we might even have an idea of where “Planet 9” is hiding and be able to use it to boost any spacecraft to even faster velocities.

Conditions need to be just right to leave our solar system.

While gravity slings are ideal for essentially getting “free momentum,” there is still another problem with them (aside from the aforementioned alignment timing). It’s easy to use gravity slings when you’re traveling in the same plane since any failed or sub-optimal maneuvers can be readjusted to hit the next planet. If you try to accomplish this outside of the plane of the planets, the accuracy for each maneuver needs to be incredibly high. These out-of-plane maneuvers have much lower odds of an optimal alignment, but they can be done if needed. Consequently, our best shot of leaving our solar system via gravity slings would be on the ecliptic plane.

If there are any exoplanets on the ecliptic plane which are close enough to travel to and are in the habitable zone of their star, we still have one enormous boundary to reach them: escape velocity. We’ve done pretty well at escaping the Earth’s gravity, but that’s because we “only” need to accelerate up to just over 11 km/s. The Voyager probes are traveling at 17 km/s, partly because they were assisted by the gravity slings mentioned above. This speed isn’t even close enough to escape our solar system! Despite these probes traveling faster than almost anything man has ever made, they are not going fast enough to escape the gravity of our sun. To escape our solar system, we need to be going over 42 km/s! Furthermore, once we get outside our solar system, we’d need to be going over 500 km/s to escape the Milky Way! Gravity is a harsh mistress, and right now she’s not only limiting interstellar travel to our solar system, but to our galaxy as well.

Even the Voyager spacecraft will never leave the sun’s gravity!

These last two boundaries have mainly constrained our reachable exoplanet to the intersection of the ecliptic plane and the Milky Way. If we were to find a close-by exoplanet, our best bet of reaching it would be if it were in the constellation of Sagittarius. Now, if we somehow manage to invent relativistic spaceships that can travel close to the speed of light, a few of these boundaries can be broken. Unfortunately, the distance boundary would mean we could travel the 1,000+ light years to get somewhere, but only with the understanding that we’d never be able to communicate with Earth ever again (after all, transmissions can only go at light speed as well).

I don’t mean to make people depressed by pointing out these limitations, but I do want people to know that the discovery of an exoplanet, especially a potentially habitable one, does not mean that we can visit it. While it’s nice to see that we can discover these distant worlds, we need to understand that physics limits us to what we can explore. That being said, I’d love to be proven wrong!

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