Introduction to Beyond Rockets

Scientists with the National Aeronautics and Space Administration (NASA) realized a nearly 40-year-old dream in July 1999, when Deep Space 1 (DS-1), the first spacecraft to employ an ion engine as its main propulsion system, made a successful fly-by of an asteroid. Instead of a chemical reaction, the main engine of DS-1 created thrust by stripping electrons from the element xenon to create ions (electrically charged atoms) and ejecting them in a high-speed beam from the back of the vehicle. NASA built its first ion engine in 1960, but the expense of launching spacecraft into orbit made the technology too risky to use on an actual mission until DS-1. Although the ion engine aboard DS-1 produced only a tiny amount of thrust (about equal to the force of a single piece of paper resting on a table), the engine could run for very long periods using relatively little fuel, enabling it to eventually go much faster than a chemical rocket.

Although NASA focused much of its research and development effort in 2000 on reducing the cost of launching payloads into orbit around Earth, the most exciting area of space research was the progress being made in space propulsion technologies other than chemical rockets, which were still the workhorses of space. Although many of these systems are still experimental-and some are only on the drawing board-they may be the technologies that will power future missions to explore the universe.

The ion engine aboard the DS-1 is an example of a type of propulsion system known as Solar Electric Propulsion (SEP). This system uses solar panels to generate the electrical power needed to operate a highly efficient ion engine. However, because sunlight decreases in intensity as an SEP vehicle gets farther away from the sun, the technology was limited to robotic missions exploring only about to the distance of Jupiter. Scientists expected, however, that by 2005 they would be able to build SEP vehicles with much more efficient engines.

Harnessing Sunlight and Orbital Momentum

Another technology that scientists expected to fine-tune by 2005 is the solar-thermal rocket. This system would use large inflatable mirrors about 10 meters (33 feet) in diameter to collect sunlight and focus it into an engine to heat a propellant, such as liquid hydrogen. The heated propellant would expand and emerge at high speed through a narrow nozzle in the rear of the vehicle to provide thrust. Solar-thermal rockets would have about twice the exhaust velocity of the best chemical rockets. One possible application of solar-thermal rockets is to boost satellites from low Earth orbit to much higher orbits.

Space tethers are another experimental propulsion system that could be in operation by 2005. Unlike traditional propulsion systems, tethers require little or no propellant. Tethers exploit differences in the momentums of different orbits to change the velocities of orbiting objects, such as satellites. Objects in high Earth orbit move slower than ones in lower Earth orbit. A strong tether connecting objects in different orbits provides a potential to do work, because the object in low Earth orbit can be used to pull the object in higher orbit down, and the object in higher orbit can pull the lower object up. Using this principle, a series of spinning space tethers in orbit around the Earth could be used to fling payloads to the Moon, or even to Mars. Such a system would involve several spinning tethers, each in a higher orbit around the Earth than the last. The payload would be transferred between the tethers, picking up momentum before being released.

Between 2005 and 2010, scientists should be making progress in developing a “solar sail,” another propulsion system that would require little or no propellant. A solar sail would be a large reflective sheet propelled by the pressure of sunlight, similar to the way a sailboat's sails catch the wind. Because photons (particles of light) exert little pressure, the main challenge of building solar sails is developing ultra-lightweight films to enable even the slight pressure of sunlight to achieve an adequate rate of acceleration. Despite this challenge, solar sails may one day be the supertankers of the solar system, slowly transporting cargo between planets while consuming no propellant.

A concept similar to the solar sail, called a “magnetic sail,” could provide another way to use the sun to propel a spacecraft. A magnetic sail would harness the force of the solar wind, a steady stream of charged particles blowing outward from the outermost layer of the sun's atmosphere. A magnetic sail would create strong magnetic fields that the solar wind could push against, creating thrust to move the spacecraft forward.

Fission and Fusion

Between 2010 and 2020, scientists may be able to use a Nuclear Electric Propulsion (NEP) vehicle for unmanned missions beyond Jupiter, or to support manned missions to Mars and beyond. This system would use nuclear fission (the splitting of atomic nuclei) which provides much more power than chemical reactions. An example of a nuclear propulsion system is the NERVA (Nuclear Engine for Rocket Vehicle Application) nuclear fission rocket engine. This system, being developed by NASA, consists of a fission reactor core with small tubes running through it. Liquid hydrogen is passed through the tubes and heated to about 1,700 °C (3,100 °F) to yield a high-thrust engine with twice the exhaust velocity of a chemical rocket.

Between 2020 and 2050, scientists may be able to use nuclear fusion (fusing atomic nuclei to release energy) to propel spacecraft. Nuclear-fusion vehicles could accelerate to 10 percent of the speed of light—about 30,000 kilometers (18,600 miles) per second. Their high speed would make fusion vehicles better-suited for interstellar (between the stars) missions than for flights within the solar system.

However, it is possible to slow down a fusion-powered spacecraft by adding a nonreactive material to the high-energy plasma (hot gas of ions) created during the reaction. This technique could make it possible to complete a round-trip manned Mars mission in three months, or a one-way robotic mission to Pluto in three years.

Even Farther In the Future

Also between 2020 and 2050, scientists may harness antimatter (matter identical to ordinary matter but opposite in electrical charge) to power spacecraft. The first attempts at this technology will probably involve using small amounts of antiprotons, the antimatter equivalent of protons (positively charged particles in the atomic nucleus), to trigger a tiny fission reaction. This fission reaction could in turn ignite a fusion reaction.

After 2050, scientists may be able to build a type of solar sail powered by laser energy. Instead of using the weak push of sunlight, these “laser sails” would be driven by light from powerful lasers in orbit around the Earth or sun. Using lasers with enough power, laser sails could accelerate to speeds approaching the speed of light.

Scientists will also probably have to wait until after 2050 before making significant progress on the ultimate method of space propulsion: matter-antimatter annihilation. Though still only a theoretical possibility in 2000, matter-antimatter annihilation would provide a much higher exhaust velocity than any other known type of propulsion system. In this type of propulsion, protons would be collided with antiprotons, and the two forms of matter would destroy each other with a tremendous release of energy. The annihilation reactions would also generate positively charged subatomic particles called pions, which could be deflected by magnetic fields to produce thrust with an exhaust velocity of about 33 percent of the speed of light. This is the type of performance required for fast interstellar missions. However, such missions would require tons of protons and antiprotons. In 2000, antiprotons were extremely difficult to produce and store, making this system little more than a future vision.