Directorate-General for Research & Innovation logo Horizon: the EU Research & Innovation magazine | European Commission logo
Receive our editor’s picks

Electric deep-space engines could bring humankind to Mars

Astronauts need faster spacecraft, better radiation protection and heat shields before they can enjoy the Martian landscape in person. Image Credit: NASA/JPL

Sending astronauts to Mars poses several large challenges, among them a long journey filled with life-threatening radiation from cosmic ray exposure and solar flares. Not to mention the fact that we haven’t yet worked out how to get them back again.

Now European researchers are addressing these problems by developing faster engines, solar forecasts and better shielding.

According to the European Space Agency (ESA), radiation levels are up to 15 times higher in space than on earth, so for astronauts to avoid excessive exposure while travelling to Mars, they will need to get there as fast as possible.

‘It would take six months with classic chemical propulsion (hydrogen and oxygen fuel),’ said Frédéric Masson from France’s National Centre for Space. ‘Nuclear Electric Propulsion (NEP) can get to Mars faster than other methods, if the reactor is powerful enough.’

NEP works by changing nuclear thermal energy into electrical energy which then powers a propulsion system. It is one of the most promising technologies for space exploration as it’s cost-effective and has a much greater fuel-efficiency than classic chemical propulsion meaning there is no need to carry large amounts of rocket fuel.

However using NEP to get to Mars is some way off, as the technology is still at an early stage. Masson is working on the EU-funded DEMOCRITOS project that aims to improve the scientific understanding of NEP propulsion so that a demonstrator can be developed.

He says that using nuclear energy to power spacecraft is safe because the reactor would be contained in a protective shell. This would most likely be made from graphite as it’s mechanically strong and able to withstand high temperatures.

Thank to its fuel-efficiency, the lower weight-to-capacity ratio of an NEP-powered spacecraft means there would also be more room available for scientific instruments and plants that provide a food source for the astronauts, even compared to other emerging alternatives such as solar cells.

It would also help to address a major challenge in a crewed mission to Mars – getting home.


Scientists are still working out how to ensure a spacecraft has enough chemical propulsion to get back into space from the planet’s surface. Some solutions are to send the fuel separately or to generate it on Mars, but the most secure option is to bring the return propellant with you, which an NEP spacecraft can do.

‘It is more efficient and safer to send people to Mars with one spacecraft,’ said Masson. ‘You can make our [NEP] designs bigger and more powerful very easily, there is no change in the technology.’

While DEMOCRITOS is not working on developing the technology to such an extent that it could be used for a manned mission to Mars, project coordinator Dr Emmanouil Detsis from the European Science Foundation says the technology could offer capabilities for other missions.

‘Nuclear Electric Propulsion can get to Mars faster than other methods, if the reactor is powerful enough.’

Frédéric Masson, National Centre for Space, France

‘NEP is really the only viable method that can deliver a heavy spacecraft to the outer moons of Jupiter.’

Dr Frank Jansen, senior scientist at the German Aerospace Center (DLR) who is also working on DEMOCRITOS, agrees.

‘The project is a technology study for different missions. If we realise this project in the 2030s, or earlier, this will be comparable to the Apollo missions or the ISS.’

Even with faster engines, however, extended radiation exposure would remain a critical threat to astronauts’ health.

‘Solar energetic particles are the biggest headache to any manned mission to anywhere,’ said Professor Ioannis A. Daglis, a space radiation expert from the University of Athens, Greece.

Solar energetic particles are surges of intense particle radiation that come from sudden eruptions, or solar flares, on the sun's surface. They race through space with such speed that astronauts caught in their path would have little chance of escaping unharmed.

‘We need to be able to forecast solar flare occurrence and the propagation of solar energetic particles in interplanetary space so we can estimate the levels of radiation,’ said Prof. Daglis.

Solar minimum

One solution is to send a human mission during ‘solar minimum’, which is the time during the sun's 11-year activity cycle where solar flares are less frequent.

‘I would expect a manned mission to Mars a few years after the next solar maximum, sometime around 2030,’ said Prof. Daglis, who was also the project coordinator of MAARBLE, an EU-funded project which modelled the earth’s radiation belts.

Radiation belts are the regions of enhanced particle radiation surrounding certain planets, like earth and Jupiter, where energetic, charged particles accumulate under the influence of the planet's magnetic field.

‘Our detailed knowledge of the belt structure and dynamics ... helps us find the best path and the best time to pass through it safely. This has been done for the Apollo missions and would also be done for other interplanetary missions,’ said Prof. Daglis.

A crewed mission to Mars would likely travel through the safest path in earth’s radiation belts during a time of ‘solar minimum’, but even so, the risk of solar energetic particles would still be there.

‘There are several ongoing efforts, but we are not yet in a position to predict solar flares,’ said Prof. Daglis.


Avoiding a solar flare by the time a mission reaches Mars would come as a huge relief, but the astronauts wouldn’t celebrate just yet.

A spacecraft enters an atmosphere at such high speeds that it creates a ‘bow shock’, a curved, stationary shock wave that can heat gases close to 10 000 Celsius, causing a dangerous amount of degradation of its wall material or heat shields.

There would also be effects specific to Mars. Dr Thierry Magin leads the AEROSPACEPHYS project, funded by the EU's European Research Council, which ran simulations of planetary atmosphere entries to determine the needs of a spacecraft’s heatshield.

‘During entry to the Martian atmosphere, you would create carbon monoxide which is known as being a strong radiator,’ he said.

At Belgium’s von Karman Institute for Fluid Dynamics, Dr Magin is helping organisations such as ESA to develop heat shields to protect spacecraft from the immense heat, by creating a test environment that mimics the entry conditions for different planetary atmospheres.

‘It is extremely difficult to account for all these atmospheric entry phenomena so what we did was partly reproduce them in our ground facilities,’ said Dr Magin.

This includes a plasmatron, a machine that can recreate the heat conditions for different planets, which vary because of the differing atmospheric gases.

Dr Magin combines data from the plasmatron with results from the ‘Longshot’ wind tunnel, which tests the aerodynamic forces when entering a planet’s atmosphere.

Surprisingly, one of the most promising materials for a Martian heat shield is cork. ‘We study the description of trees in California after forest fires because essentially the structure of the bark is very similar to the heat shield of a spacecraft,’ said Dr Magin. ‘Together with NASA we are now applying our simulation tools to study spacecrafts.’

  • American engineer Robert Goddard launched the first successful liquid-fueled rocket. Powered with liquid oxygen and gasoline, the rocket, dubbed "Nell", rose just 41 feet during a 2.5-second flight and then crashed in a cabbage field. This first flight was regarded as the birth of modern rocketry.
  • Inspired by Goddard’s published work, Werner von Braun, a German rocket engineer, successfully test-launched the first ballistic-guided missile, known as the V-2, in 1942. Two years later, the V-2 was the first manmade object to cross from the earth’s atmosphere into space and was later used in World War II. The V-2 laid the foundation for the Saturn V launcher which would take Neil Armstrong and Buzz Aldrin to the moon in 1969.
  • In Russia, engineers launched the first satellite, Sputnik 1, on board a two-stage liquid-fueled rocket known as the R-7, marking the start of the space race between the US and the USSR. The R-7 was originally designed to be the world’s first intercontinental ballistic missile, but it was never deployed because the warhead continually failed to survive re-entry into the atmosphere. On the other side of the world, the American Vanguard rocket was hopelessly failing to launch, earning it the nickname of Stay-Putnik. 
  • Thanks to America’s Operation Paperclip, Wernher von Braun was hired by the US to work on its space program. Von Braun designed the Redstone rocket, a modified ballistic missile, which in 1958 took Explorer 1, the first satellite with an onboard telemetry system, into orbit.
  • Yuri Gagarin became the first person in space with a 108-minute flight on Vostok 1, launched by the liquid-fueled Vostok-K rocket. American Astronaut Alan Shepard reached low-earth orbit less than a month later atop the Redstone launcher, another missile-derived rocket. Subsequent Mercury missions used another ballistic missile: the liquid-fueled Atlas.
  • The first crewed Apollo mission, Apollo 7, was launched on a Saturn 1 rocket, the first dedicated space launcher. The heavy-lift rocket was not derived from any missile, although it was later transformed into a ballistic missile with a 25-ton weapons charge. The more powerful liquid-fueled Saturn V rocket was then used to send astronauts to the Moon in a much smoother launch than the previous missiles they were riding on.
  • Mariner 9 became the first spacecraft to orbit Mars, launched on an Atlas Centaur rocket on a direct ascent to the red planet. The Atlas Centaur is the first rocket to use liquid hydrogen as well as liquid oxygen as propellant, which enabled the probe to reach Mars in just 167 days.
  • NASA launched the first deep-space mission powered by an ion engine. The Deep Space 1 ship was designed to test a dozen new technologies, most importantly its revolutionary engine. The thruster used electrons to change the charge of the propellant particles, and then shot the ionised propellant out of thruster at extremely high speed, moving the spacecraft forward. The Deep Space 1 probe proved the soundness of ion engines and exceeded its mission objectives.
  • The European ExoMars mission was launched by a Russian Proton M rocket from the Baikonur cosmodrome. The Proton rocket had initially been designed to launch a 100 megaton thermonuclear weapon over 13 000 km. The first Proton rocket used as a launch vehicle dates back to 1965, making it one of the most successful heavy booster rockets in spaceflight history. The ExoMars Mission has taken advantage of the positioning of Earth and Mars during the launch window, which will limit its journey to about seven months.
  • Although its implementation is a long way off, NASA engineers are already working on a prototype warp drive spacecraft. Thanks to a loophole in the theory of relativity, NASA is designing the ICS Enterprise, which has technology that makes it capable of expanding space-time behind it and contracting space-time in front of it, allowing the ship to travel faster than the speed of light. Another high-speed booster under investigation is photonic propulsion technology, where light particles are used to gradually accelerate a spacecraft.

More info