When we think of interplanetary travel, most of us picture sleek bullet-shaped rockets blasting through space.
In fact, this pretty much describes the two mega rockets headed for Mars – Boeing & NASA's Space Launch Systems, and the Space X Interplanetary Transport System.
Their powerful engines are designed to burn a mixture of liquid fuel and oxidizer, to generate enough thrust to push through Earth's gravity and propel them toward their destination. It's how we sent our first humans to the Moon and back.
However, chemical or fuel-based engines aren't the only method of propelling a spacecraft. Another method that hasn't received all that much attention, but which I think is an underestimated solution to getting us to Mars, are solar sails.
Imagine now a spacecraft equipped with giant reflective sails, drifting and tacking through space.
Its solar sails look and operate much like that of a traditional sailing ship. But instead of air or wind, this space ship harnesses the power of sunlight.
Science fiction? Not at all. Japan's Aerospace Exploration Agency successfully tested solar sail technology back in 2010; and its IKAROS spacecraft has since (as of a year ago) traveled over 110 million kilometers from Earth.
Other players developing this technology include The Planetary Society, a non-profit space group led by Bill Nye (the 'Science Guy'), and of course, NASA.
How it works
Put simply, light particles (or photons) have energy. And energy, as we know from our high school physics, can be transferred.
In this case, streams of photons emitted from the sun 'bounce' (or reflect) off a very thin and highly reflective sail, thus transferring some of their energy (and momentum) to the sail.
The resulting pressure is very small. But with a large enough sail, a spacecraft could be accelerated by 0.02 meters per second squared (m/s2). (To put that into context, a car moving from 0 to 100 km/h in 10 seconds would be accelerating at 2.8 m/s2…at 140 times greater acceleration.)
So what's the big deal about solar sails?
Firstly, unlike other forms of chemical or electrical propulsion, solar sails need no propellant. Reducing the need for fuel/propellant would, of course, significantly reduce the weight (and cost) of a space mission.
Secondly, solar sails can keep accelerating continuously -- at least while in contact with the sun's rays.
Not impressed with the 0.02 m/s2 acceleration we talked about earlier? Just wait. By the end of day one, our space ship would have accelerated 1,728 m/s (or 6,220 km/h). By the end of the week, its speed would have increased to 43,545 km/h -- faster than any humans have ever traveled (Apollo 10 astronauts hold the record at 39,8987 km/h).
Talk about the tortoise beating the hare!
What's the hold up then?
One reason why solar sails haven’t taken off yet is because we need a very large sail (the size of football fields) to generate the amount of thrust needed to move any decent sized payload.
Pro tip: You can use this online calculator to work out the sail size needed to get, say, a 2,000 kg payload to Mars. (You’d need a sail with a diameter of 400 meters.)
You could also work out the speed of a spacecraft with, say, a 100 Kg payload and a 2 km diameter solar sail. (Answer: 50 km/s. At this speed, it would take 26,000 years to get to the nearest star.)
Due to its reliance on sunlight, distances beyond Mars might also be a challenge if you're planning a return journey to Earth.
However, there are ways to mitigate the problem. Solar power works at the square of the distance from the sun. So if you want a bigger push, just start your journey closer to the sun.
Using the example above, if the same 100 Kg spacecraft were to 'take off' near the orbit of Mercury, it could attain a final speed of 83 km/s and get to that nearest star in 15,500 years. (Want an even bigger push? Fire a laser beam at it...really.)
Scientists reckon we could achieve close to 10% of the speed of light using a combination of these technologies. If so, then our journey could be cut down to a mere 40 years.
ONE innovative way to get to Mars
As you may know, most of the energy needed to get to Mars is used at launch to escape the Earth's gravitational pull (which extends way past Low Earth Orbit or LEO).
Most Mars mission plans envisage us sending a number of rockets to LEO for assembly and refueling before leaving for the Red Planet.
But what if we could reduce the number of launches overall by reducing the amount of propellant needed to get there?
We could do this by setting up a 'base' in High Earth Orbit (HEO), at special points around the Earth and Moon - called Lagrange points - where the gravitational pull of the Moon and Earth counterbalance each other.
At these gravitationally stable points, a spacecraft could be left 'parked' in the same place in relation to the Earth and Moon, without needing any propulsion to maintain its place.
More importantly, the propulsive force to get to Mars from a Lagrange point would be much smaller -- as low as 10% of a LEO launch.
Many space engineers have considered basing Mars and Moon missions at Lagrange points. However, with normal propulsion systems, you'd need almost as much energy to get there from LEO as you'd need to get to Mars.
A solar sail could change that equation...
Why not use a solar sail to slowly tug payloads up to a Lagrange point from LEO? Sure, it might take 6-9 months, but you would be able to do it propulsion free. The same could be done for the Mars Transfer Habitat, Landers, booster and fuel for the return trip.
During the final launch, the astronauts would take a small but fast rocket to the Lagrange point, hook up with the rest of the Mars ship, and give it a small rocket boost towards Mars (as opposed to the huge boost it would need from LEO).
The propellant savings could be in the order of 200-300 tonnes, equivalent to 3 SLS rockets or 6 Falcon Heavy flights (worth, oh, a few billion dollars).
Rockets, sails, and more
It is unlikely that solar sails will be the only technology that will take us to Mars and beyond, particularly given the time, resources and commitment already made to our mega rockets.
However, solar sails do work. And I believe they could be a viable backup technology for any Mars architecture plan; and a game changer when successfully scaled up for deep space missions.