Finding the right fuels for space travel
If you were somehow able to drive a car to the Moon, assuming your vehicle had sufficient life support and a surface to gain traction on, you’d still be stuck once you ran out of gas. Of cars currently on the market, your best option might be something like a Volksagen Passat, which can go around 814 miles before needing to refuel. To get the Moon 238,900 miles away, you’d still need to fill up over 293 times to finish the trip, which demonstrates one of the many reasons why power sources for space travel can’t just be borrowed from how we get around on Earth.
One problem with any chemical source of fuel, such as gasoline that is to be burned to produce power, is the weight involved. For our hypothetical roadtrip to the Moon without gas stations along the way, our midsize sedan would need to be able to carry around 5,430 gallons of gas with it, adding an additional 48,323 pounds of payload to the vehicle. Getting that much extra weight away from a planet’s gravitational pull requires even more power, which then needs more fuel. This is part of the reason many rocket systems use some kind of booster system that pushes your capsule, satellite or rover off the surface of the planet and then drops away, lightening the load once the biggest fuel cost has been dealt with.
This isn’t to say that a journey through space is then free of power concerns. Aside from questions of propulsion, many other electrical components need to be powered for the spacecraft to be useful, including radio transmitters, transponders, and other equipment for measuring details of the cosmos and sending home great photos. Aside from keeping critical systems powered on, they need to be somehow protected from extreme temperatures and radiation exposure.
Sunny side of the solar system
As the last 60 years of space exploration prove, none of the above is insurmountable. For trips in the inner-solar system, solar panels can collect a fair amount of energy from the Sun. The arrays of panels on the International Space Station, satellites and other probes are usually built to higher efficiency standards than those on buildings, and even then they need to occasionally be angled to avoid too much exposure to the sun to avoid being overpowered.
If you can’t count on access to the Sun, which is a given if your craft will be passing behind a planet from time to time, you’ll need batteries of some kind. Currently, that usually means some nickel-hydrogen batteries, which can handle over 50,000 recharges over 15 years. For future missions to colder locations, NASA is developing batteries that can operate at -148° Fahrenheit, as well as lithium-ion batteries with higher energy density.
For the really long trips though, when you’re planning on operating for more than 10 years, or too far from the Sun for much solar power, you can use Radioisotope Thermoelectric Generators (RTG). They generate heat from decaying radioactive materials like plutonium, and as such pack a fair amount of power into a relatively small space. The lack of moving parts helps ensure reliable power for as much as 30 years, or in the case of the Voyager 1 and 2 spacecraft, at least 38. Many of our headlining space missions, including the Cassini spacecraft and Curiosity rover use RTGs, but they may not be the only option for long-term missions much longer. Stirling Radioisotope Generators operate similarly, but are more efficient and smaller than RTGs, but at the cost of complexity.
Eventually, our space-faring sedan may have options like fusion reactors (if the passengers can be safely shielded from them), hydrogen fuel cells (with drinkable waste-water!) and ion thrusters (hopefully cutting down costs and trip times). For now, we can just be thankful that our hypothetical, 133-day drive should at least be equipped with cup holders and air conditioning.
Source: What will power tomorrow's spacecraft? by Peter Ray Allison, BBC Future