NASA is planning to put astronauts on Mars one day and since the Red Planet is about as off the grid as you can get, the space agency is developing a new generation of modular nuclear reactors to power manned outposts. Under funding from the Space Technology Mission Directorate (STMD), the Kilopower project is a multi-year effort to build simple, inexpensive reactors that can be used for a wide variety of planetary and deep space missions.
One of the primary problems with almost any space mission is how to provide it with power. Depending on the goal of the mission and its duration, there are any number of options. The very first satellites used batteries that supplied them with electricity for a few days. Soon, solar panels were added that extended the mission life to years. Fuel cells provided manned missions with not only power, but drinking water as well as hydrogen mingled with oxygen to create electricity and a potable waste product.
Unfortunately, all of these options turned out to be very limited in application. The most successful of them, solar power, only works when sunlight of sufficient brightness shines on the panels. This means that it's a system largely confined to the inner Solar System with Jupiter as the extreme limit, doesn't provide much in the way or power density, is bulky, and is useless at night or on planetary surfaces that may be obscured by dust and clouds.