Whenever you see interstellar ships in fiction, they’re almost always immense, something close to the size of an aircraft carrier. There are a lot of good reasons for that. Traveling between the stars requires immense amounts of energy, so you’ll need reactors to generate it all or a huge set of solar sails to keep going, shielding from reactors and cosmic debris, and because you’re not going to be able to easily diagnose and fix problems light years away from mission control, you’re going to need a crew which needs living quarters, supplies, and means to generate and renew air, food, and water. Accelerating all that mass to relativistic velocities is going to be very difficult with anything short of fusion reactors and antimatter, and even then you’re going to be dealing with drag from dust and microscopic debris littered across the universe. Since trying to bend space and time is still only a vaguely theoretical endeavor at best, we’ve come to see the prospect of interstellar travel as something probably a) best done by machines, b) require long periods of planning and waiting, and c) very unlikely to happen in our lifetimes anyway.
Enter billionaire investor Yuri Milner with a $100 million plan to create a proof of concept for an amazing mission to Alpha Centauri that will take only 20 years and be powered by a laser that sounds like something Bond would be assigned to destroy before a genius villain bent on world conquest finishes its construction. In order to make it happen, he’s going to take a hatchet to a conventional view of an interstellar mission and slash anything that can slow it down. Fuel and power generation? Gone. Crews? Gone. Dust shields? Gone. The only things left are batteries, one solar sail, and a camera that you couldn’t find even on the cheapest phones you could buy today, with a resolution of just two megapixels. In other words, he’s going to create what would be the fastest Razr flip phone and shoot it into space with a multi-megawatt laser. On paper, it seems like a pretty sound plan. Such a huge jolt to a solar sail on a spaceship weighing a mere few hundred grams would accelerate it very, very effectively, and since it’s such a simple, small device, not much on it can really go wrong so you don’t need elaborate rescue scenarios or an adventurous crew of experts on board should something go terribly wrong along the way.
Unfortunately, the devil is in the details, his preferred hiding spot. One of the biggest problems any interstellar probe would face is collisions with high energy particles and dust that makes up the interplanetary and interstellar mediums. While in interstellar space, this dust and debris will not be a problem until you get up to half the speed of light, and even then most particles aren’t going to even register until you’re going 0.95c which is far beyond anything Milner expects from his device. However, that assumes a fairly hefty ship rather than a cell phone sized little box we hurled into deep space. Going by the generally accepted calculations, the dust will erode a very painful 20 kg of shielding material, if we use the metric system to run the numbers and account for the law of inverse squares when it comes to the energy of the impacts as we accelerate. While the math works for accelerating less than a kilogram of spaceship to a significant percentage of the speed of light, it also says that this probe will be shredded into grain sized particles before it leaves the solar system as we know it, since interplanetary medium it would have to traverse as it gains velocity is much denser. To borrow a phrase, Milner’s gonna need a bigger ship.
But all that said, if we set our sights on interplanetary travel with larger, crewed ships and build lasers capable of powering their solar sails to navigate to the outer solar system and back, this project could really pay off over the long term. Imagine launching inflatable space stations with massive sails that surf our lasers to their destinations, then ride it for a slingshot around nearby worlds and make their way back to Earth. The only problem one could see in this scenario is a political fight over a laser that would put today’s best military technology to shame and have the capability of vaporizing satellites innocently orbiting in its path, but that’s a completely different sort of problem than we’re trying to solve here. When it comes to interstellar travel, however, a powerful laser and solar sails just aren’t going to be enough even though intuitively it seems to be a no-brainer that the smaller the craft, the faster and farther it can go while in reality, you’re pretty much doomed without enough heft to counter the rigors of relativistic flight. At least until we invent force fields and can really test them out using Milner’s ultra-lightweight probe…