|Fig. 1: Photograph of the International Space Station, post-undocking. (Source: Wikimedia Commons)|
As humanity continues to develop its interest and awe with space, we are discovering more and more problems and eventually solutions as we go. In this paper I will address the four main methods of powering face vehicles today: batteries, fuel cells, solar, and nuclear.
The very first vehicles to enter space were powered only by batteries. Theses were effective for a little bit, but lasted a very short amount of time. And are therefore only efficient for very short trips. They still are, however, used as a supplement quite often.  The lander Philae that I will discuss in further detail in the "What is there's no sun?" section of this paper had a couple of batteries in addition to its solar power. Curiosity and the Apollo missions made use of batteries for descent and re-entry respectively. 
Fuel cells combine two chemical components at a controlled rate to produce heat, electricity, and some chemical waste. When hydrogen-oxygen fuel cells are used, the chemical waster product is water. This is ideal because the water can then be used by the crew. 
An advantage of fuel cells is that they don't depend on exposure to the sun in order to produce energy. A disadvantage, however, is that they don't last for very long, in the order a magnitude of a few days.
Additional disadvantages of fuel cells include that they require a lot of support equipment (and therefore a lot of extra weight) and that they use pipes and valves to deliver the fuel, introducing the risk of mechanical failure. 
Most of the satellites in Earth's orbit are powered by solar panels. They have some great advantages as well as disadvantages. A significant disadvantage is that as you get farther from the sun, it gets harder to produce energy. I will address this issue more in the next section. An advantage that solar panels have over nuclear power is that the materials that go into making and implementing them are extremely safe, whereas using nuclear power involves handling dangerous fuels.  You can see an example of solar panels on a spacecraft in the image of the International Space Station on the right.
Sometimes, you run into the problem of the vehicle in question not being exposed to sun. This could happen, for example, if it is orbiting a planet. With every rotation around the planet, the vehicle will be blocked from the sun for a chunk of its rotation.  Some have simply made bigger solar panels to make up for less sun, but this isn't ideal because it means the craft becomes bigger and heavier and therefore harder to maneuver. 
In November of 2014, NASA landed a robotic lander, called Philae, onto a comet more than 300 million miles away from Earth. As exciting as this is, there is worry in the air too. This worry is a result of the fact that the harpoons on the lander, intended to keep the lander in place at its contact point, didn't go out. As a result, the lander bounced a couple of times before settling in the wrong place. The place where it settled is in the shadow of a cliff, where it doesn't get as much sunlight as its solar panels need. As a result, the scientists at NASA found themselves unsure that it would be able to continue on with its intended purpose, due to a lack of energy. 
The last type of energy that is used for space exploration right now is nuclear energy. There are many tasks that we may want a space craft to accomplish that have to survive durations with a lack of sun that the previously mentioned sources of energy simply cannot accommodate. When a space craft needs to be in the outer reaches of the solar system where there is only faint sunlight, or it needs to power through lunar nights that last 14 days, or through long periods of the dark and the cold that are found in the higher latitudes of Mars, space technology often looks to Radioisotope Power Systems. Specifically, plutonium-238 is the only practical isotope to fulfill this need. 
None of these solutions to fuel in outer space are perfect, but together they have allowed us to make amazing advancements in space travel.
© Rachel Marincola. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 B. Brumfield, and C. J. Carter, "On a Comet 10 Years Away, Philae Conks Out, Maybe For Good," CNN, 18 Nov 14.
 B. Johnson, "Power Sources for Space Exploration," Physics 240, Stanford University, Fall 2012.
 "Powering the Future," NASA Glenn Research Center, PS-00537-0811, July 2011.
 Radioisotope Power Systems (National Academies Press, 2009).