We may require new engines if we ever want to travel regularly from Earth to Mars and other far-flung locations. Engineers are investigating ground-breaking new technologies that could allow humanity to travel through the Solar System in a fraction of the time.
The distance between Mars and Earth ranges between 54.6 million km and 401 million km due to their orbital trajectories around the Sun.
When the two planets get close enough, missions to Mars are launched. Chemical rockets, the most common source of propulsion, require nine months to get to Mars on one of these approaches.
That’s a long time to be on the road for anyone. However, engineers, including NASA, are collaborating with industry partners to discover faster ways to get us there.
So, which technologies appear to be the most promising?
Before a human journey to Mars, solar electric propulsion could carry cargo to the Red Planet. By Dr Jeff Sheehy, chief engineer in Nasa’s Space Technology Mission Directorate, this would ensure that equipment and supplies were ready and waiting for astronauts when they arrived utilising chemical rockets.
Large solar arrays unroll to catch sun energy, transformed into power in solar electric propulsion. This fuels a device known as a Hall thruster.
There are advantages and disadvantages. You’ll use significantly less fuel on the plus side, resulting in a lighter spaceship. However, your vehicle will take longer to arrive.
“It would probably take us two to 2.5 years to get there to carry the payload we’d require,” Dr Sheehy tells the BBC.
“You’d need a lot of cargo to establish the kinds of outposts we’d need to build on Mars for humans to be able to subsist for months, plus the vehicles.”
Aerojet Rocketdyne is developing a Hall thruster for the Gateway, a projected lunar orbital space station.
Executive director of Aerojet Rocketdyne’s space division, Joe Cassidy, argues, “Solar is the greatest because we know we can scale it up.”
“These are currently flying on communications satellites now.” We fly at a power level of 10-15kW (kilowatts) today, and our goal with the Gateway is to grow it up to something more than 50kW.”
Aerojet Rocketdyne’s Hall thruster, according to Mr Cassidy, will be far more fuel-efficient than a liquid hydrogen and oxygen rocket engine.
However, he says that having fewer launches would be a smart approach to reduce the cost of access to space.
“I believe that solar electric propulsion with xenon as the propellant is a really good technology.” “However, the time it takes to get there and the size of the solar panels are two huge limitations,” says Tim Cichan, a human spaceflight architect at aerospace firm Lockheed Martin.
Dale Thomas, a systems engineering professor and prominent scholar at the University of Alabama in Huntsville (UAH), agrees.
“Solar electric works great for smaller payloads,” he tells the BBC, “but we’re still having difficulties scaling it up.”
If the technical obstacles can be overcome, he believes it might become a significant alternative technology. However, he claims superior possibilities are available presently, such as nuclear thermal electric propulsion.
Thermoelectric nuclear propulsion
Another option is to launch chemical rockets from Earth and land on Mars. However, some engineers advocate adopting nuclear thermal electric propulsion during the middle portion of the journey.
Nasa’s Orion capsule might be used to transport astronauts to the Gateway. A transfer vehicle would then dock with the Orion crew capsule.
After connecting Orion to the transfer vehicle, a nuclear electric rocket would take the crew capsule and transport module to Mars. They would dock with a Mars orbiter and lander waiting in Mars’ orbit.
A tiny nuclear reactor warms liquid hydrogen in a nuclear thermal electric rocket. The element’s gaseous form expands and blasts out of the thruster.
“Cutting the transit time [to Mars] down by 30-60 days will improve the crew’s exposure to radiation,” Mr Cassidy argues. “Nuclear thermal is a vital technology for us since it allows for faster transit times.”
Dale Thomas and the University of Alabama at Huntsville have a study contract with Nasa to construct a space rocket with a nuclear-thermal engine. Nuclear thermal electric engines, he believes, are the closest new engine technology to being ready for application.
“We can reduce the transit time to three months in some of the trajectories we run in my lab,” he says. “It’s still a long journey, but it’s roughly a third of the time that chemical propulsion takes to bring us there.”
Boeing is wary of nuclear thermal propulsion because of the dangers that a nuclear reactor could provide to astronauts.
Mr Thomas disagrees, saying, “This is a frequent misunderstanding.” The hydrogen fuel provides excellent radiation protection.
“On one end of the vehicle, the crew will be the engine on the other end.” As a result, preliminary calculations show that cosmic rays will expose the crew to more radiation than the nuclear thermal engine.”
However, he acknowledges that one drawback of the technology is the impossibility to test it easily on Earth.
However, Nasa is working on a ground test device that washes exhaust to eliminate radioactive particles, allowing ground experiments.
Ion propulsion powered by electricity
Electric ion propulsion is another option. These create propulsion by using electricity to accelerate ions (charged atoms or molecules).
Satellites in space are already powered by ion propulsion. However, they only produce a small push – about the same as a hairdryer – and so have a slow acceleration. They can, however, achieve great speeds given enough time.
Ad Astra claims to be developing the Vasimr, a type of thruster that uses radio waves to ionise and heat a fuel, followed by a magnetic field to accelerate the resulting soup of particles known as plasma. The Casimir is built to create significantly greater thrust than a standard ion engine.
Electricity can be generated in different ways. On the other hand, the crew plans to deploy a nuclear reactor to carry humans to Mars. For smaller payloads, the Vasimr would utilise solar power.
Franklin Chang Diaz, president and chief executive of Ad Astra and a former Nasa astronaut, believes that crewed flights to Mars should arrive in less than nine months.
He claims that travelling to Mars is considerably more difficult than going to the Moon.
Mr Chang Diaz told the BBC, “The solution is to go fast.” “You can fly to Mars in 39 days with a spaceship that weighs 400-600 metric tonnes and has a power output of 200 MW (megawatts).”
Scaling up the Vasimr, according to Dale Thomas, will be tough, akin to going from a lawnmower to a space launch. However, the technology shows potential.
“If, or perhaps I should tell, when, Ad Astra can overcome Vasimr’s technical problems,” Mr Thomas writes, “it appears to be the greatest alternative for electric propulsion at the human-ferrying spacecraft scale.”
“Physics suggests it should work.” However, I must emphasise that Vasimr is still in the lab and is a long way from being ready for flight at any scale.”
Mr Chang Diaz sees no issue with scaling up; it’s simply that there isn’t a demand for a 10MW engine right now, so Ad Astra is standard with 200kW.
“We have a large market for the 200kW engine; there’s a number of activity moving cis-Earth satellites in low-Earth orbit and around the Moon,” Mr Chang Diaz said.
Lockheed Martin believes the Vasimr technology is also intriguing, although it focuses on solar electric propulsion.
Chemical rockets: a case for them
While the new technologies are intriguing, veteran space companies Lockheed Martin and Boeing believe liquid chemical rockets should form the foundation of any human expedition to Mars.
According to Lockheed Martin, we already can get to Mars, and chemical rockets are a tried-and-true technology that worked on all Apollo missions.
Mr Cichan, the former Orion system architect, argues, “We already have the technology to get us to Mars today.”