Moscow. Solar energy supplies most of power in spacecraft nowadays.

Although the efficiency of solar cells has grown substantially recently, they have reached the limit of their development and can supply electricity only in near-Earth orbits and for satellite-borne equipment.

Large-scale projects as for the exploration of the Moon or a manned mission to Mars require nuclear power plants.

These plants are practically independent of sunlight. They can provide power not only for life support and equipment, but also drive electric or nuclear rocket engines.

Estimates made by researchers over recent years show that nuclear power, if used in long-distance space voyages, will save considerable funds and shorten interplanetary journeys.

In a Mars mission a nuclearpowered engine would cut flight time almost by two thirds, compared with a jet engine using ordinary chemical fuel.

The rim of the solar system could be reached within three, rather than 10, years. Nuclear plants can be used not only as sources of electric power, but also as sources of heat to support life and productive activities at bases beyond Earth.

Russia and the United States have laid a good groundwork for progress in this field. But Russia leads in such key factors as maximum hydrogen temperature and specific thrust impulse.

In fact, Russia is the only country in the world that has a hands-on technology for building space-based nuclear reactor plants.

The US tested a nuclear reactor like the Soviet Topaz unit only once. It was in 1965. The reactor lasted 43 days, although the satellite on which it was installed is still in orbit as part of space junk.

Russia has launched about 40 spacecraft with nuclear plants aboard.

Most of them were used for spying purposes and, once activated, stayed in low near-Earth orbits for several months on end.

The Topaz-II had a capacity of about 10 kW. This compares with the 120 watts that can be collected from one square meter of solar cells, which are the main source of power for space vehicles.
Moreover, the farther from the sun, the lower the efficiency of the battery.

Russian engineers have designed a series of conceptual nuclear plants with an initial capacity of 25 kW. A spacecraft incorporating such a plant and meant for Earth observations will mark a new stage in providing information for civilian and military users.

Nuclear power plants are more compact than solar ones, making it easier to direct and orient spacecraft especially when increased accuracy is required.

A nuclear power plant is noted for its resistance to environmental impacts and its lower weight-to capacity ratio. Whatever its capacity, a nuclear plant is always smaller than a solar one.

A plant with a nominal rating of 50 kW or a peak rating of 100 kW or more would help to build multi-purpose satellites of a new generation and radar spacecraft to monitor ground and air targets from geostationary and geosynchronous orbits.

In the past, research and development on space-based nuclear plants was halted both in Russia and in America for considerations of radiation safety. Today nuclear energy is more reliable and is having a rebirth.

Given proper funding, the humankind will not only send a manned mission to Mars and deep space soon, but also start using space for commercial purposes by establishing a habitable base on the Moon.

Nuclear energy provides the propelling answer.