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When a spaceship is orbiting around the Earth, it is usually powered by the solar panel expanded. Can the spaceship be powered by nuclear energy instead of solar energy? Also, there is gravitation force from the Earth to make it move around the Earth, then what is the use of solar energy? Please reply.

- Louis Leung (13.1.2001)

A spacecraft can be powered by chemical, solar or nuclear energy. In general, installation related to power source could take up to about 15 - 25% of the total weight of a spacecraft.
In the outer space, the level of solar radiation is about 1360 W/ m2, or 1.3 - 1.7 times more than that on the Earth. In addition, the use of solar energy is free and can reduce the total weight of spacecraft. That is why it is the most popular power source for satellites or spacecraft working in the inner solar system.
Although a satellite can keep going around the Earth by inertia, equipment onboard (e.g. navigation system, communication system or other specific equipment) still need power to function properly.

CHAN Ki-hung
Asst. Curator, Hong Kong Space Museum
22.1.2001