Nuclear technology for future space missions

These are some of the conclusions from a panel of international experts that participated at the webinar "Atoms for Space: Nuclear Systems for Space Exploration" held by the International Atomic Energy Agency (IAEA).

The international experts agreed on the need to continue advancing in nuclear fission and fusion in order to make voyages to deep spece (beyond our Solar System). Nuclear energy could provide electricity for onboard systems and instrumentation; it could also make it possible to have sustainable human presence in other planets of the Solar System.

The experts described technologies that use both nuclear fission and fusion, with three main goals: spacecraft propulsion, energy generations in missions on extra-terrestrial surfaces, and power for onboard spaceship systems.

Spacecraft propulsion

In the foreseeable future, ships launched into space will still depend on fossil fuels for their propulsion. Once in orbit, however, the nuclear engines could take the control and create propulsion to speed up the ship.

There are two key nuclear technologies for propulsion: nuclear thermal propulsion (NTP) and nuclear electric propulsion (NEP).

Nuclear thermal propulsion uses a fission reactor to heat up a liquid propellant, such as hydrogen. The heat converts the liquid into gas, which expands through a nozzle to provide thrust and propel the spacecraft. One of its main advantages is that space flights would need to lift less fuel into space, and the NTP engines would reduce trip times. A trip to Mars would be reduced by 25% compared to traditional chemical rockets. Moreover, reduced time in space also means a reduced exposure to cosmic radiation for astronauts.

With NEP, the thrust is produced by converting the thermal energy from a nuclear reactor into electrical energy. With this type of technology the thrust is lower but continuous, and the fuel efficiency far greater, resulting in a higher speed and potentially over 60% reduction in transit time to Mars compared to traditional chemical rockets. 

The spaceship company Ad Astra Rocket Company is building a NEP system: the Variable Specific Impulse Magnetoplasma Rocket (VASIMR). It is a plasma rocket in which electric fields heat and accelerate a propellant, forming a plasma. When the plasma is ejected from the engine, magnetic fields firect it in the right direction and the thrust iscreated. The VASIMR design would make it possible to process large amounts of power while retaining the high fuel efficiency that characterizes electric rockets.

In the short term, according to Ad Astra, plans are to use the VASIMR engine for a wide array of high-power applications, from solar electric energy in cislunar space to nuclear-electric energy in interplanetary space. In the longer term, VASIMR could preclude future fusion rockets which are still in the conceptual stage.

Fusion rockets

Fusion rockets such as the Princeton Field Reversed Configuration (PFRC) reactor being developed at the Princeton Plasma Physics Laboratory could produce a direct fusion drive (DFD), which directly converts the enenergy of charged particles produced in fusion reactors into thrust for the spacecraft.

The possibilities of DFD technology, according to Princeton Satellite Systems, open the door to interstellar space, to human missions to Mars and to a stable supply of energy for a future lunar base. Other advantages are their reduced size and the need for very little fuel. With just a few kilograms they can power a spacecraft for ten years.

Power for extra terrestrial surfaces

Nuclear reactors could also be used to provide a reliable source of surface power for extended exploratory missions, which would facilitate sustainable human presence in other planetary bodies. The designs of surface power fission reactors are microreactors that could provide electric energy in the range of tens of kW for decades. The current focus is on using low enriched uranium fuels or high-assay low enriched uranium fuels.

According to Anthony Calomino, Space Nuclear Technology Portfolio Manager at NASA, "NASA's priority focus remains on designing, building and demonstrating a low enriched uranium fission surface power system that has broad applications for the lunar surface initiative as well as our eventual mission to Mars with humans, scalable to power levels above 100 kWe, and has the potential to advance NEP system needs."

Power for onboard spaceship systems

Spaceships not only need electric energy power for thrust, but also to maintain life support systems, communications and other hardware. During the webinar, the experts made special emphasis on Radioisotope Thermoelectric Generators (RTGs), which have powered the Voyager spacecraft for decades far away from the Sun thanks to their potential to provide heat and electricity to onboard spaceship systems for long periods of time in the cold temperatures of space.

Future nuclear solutions like DFD technology could also simultaneously provide electricity. According to NASA studies, a direct drive fusion-powered rocket energy can produce both power and thrust with the best performance, generating electric power and propulsion form a single engine.

With the support of nuclear power, future space missions could have a much larger number of applications. In the words of  Mikhail Chudakov, head of the Department of Nuclear Energy at IAEA, "our pathway to the stars runs through the atom."

Sources: NucNet and IAEA