Space Tales: Light Sails and Magnetic Shields

FROM THE LECTURE SERIES: A FIELD GUIDE TO THE PLANETS

By Sabine Stanley, Ph.D.Johns Hopkins University

Some of the technologies for space travel that are being investigated are beyond anything that has been attempted till now. Some of these ideas include making spacecrafts that use light as fuel, and magnets that can protect space travelers and even entire planets. But how are they supposed to work?

 A space ship in outer space.
Space exploration comes with its own set of challenges that requires unusual solutions. (Image: iurii/Shutterstock)

Sailing with Light

What about technologies for reaching faster speeds during space travel? One interesting idea involves using a light sail. The concept is similar to a sailboat, but instead of using the momentum inside an atmosphere, a light sail would propel a spacecraft using the momentum of little parcels of light—photons.

Typically, light sail concepts have considered using solar photons for the propulsion. Like the LightSail project, funded through public donations to The Planetary Society. A very different approach to using photons for travel is getting our photons from lasers. The Breakthrough Starshot project is researching laser-powered travel.

Tiny Satellite, Big Sail

Here’s how it’s envisioned to work. First, a very small satellite, centimeter-sized, would be launched into near-Earth orbit. This satellite would then unfurl an attached light sail, about 16 square meters in area. Then lasers from the Earth’s surface would shine on the sail, imparting momentum to it. The sail would accelerate at about 100,000 meters per second, and accelerate at that same rate for 10 minutes.

The final velocity attained would be around 20% the speed of light—that’s 60,000 kilometers per second, well over 100 million miles per hour! At these speeds, the spacecraft could even travel to our nearest neighboring exoplanet, Proxima Centauri b, which is about 4.4 light-years away, in about 20 years.

This is a transcript from the video series A Field Guide to the PlanetsWatch it now, on The Great Courses Plus.

But What About the Power?

Illustration of the unlit side of a half-kilometer solar sail, showing the struts stretching the sail.
A light sail is a way of getting a spacecraft to travel very fast, but there are some technical difficulties. (Image: NASA/Marshall Space Flight Center/Public domain)

So what are the technical challenges for this approach? First, these small spacecraft wouldn’t really be maneuverable, and may run into all sorts of dust and small debris in the solar system on their way to Proxima Centauri b. That means the odds of the spacecraft getting to its final location are kind of small. How do we overcome this? One idea is to launch a whole bunch of these small spacecraft—maybe a thousand of them. Then the odds improve that one of them will make it.

Another technological issue is that the amount of laser power needed is almost a gigawatt; about the power produced by a large nuclear power plant. Yet another challenge is you have to carefully aim that laser to stay in contact with the light sail for those 10 minutes of acceleration. And what sort of material can withstand such a high- powered laser beam without vaporizing? You’d need a material that is highly reflective or has very low light absorption. This leads researchers to consider novel materials like graphene. But can this research into novel materials lead us to resolve another problem?

Learn more about human futures in the solar system.

“Shields Up!”

The dream of a future where humans wander the solar system confronts a significant hurdle: how to protect ourselves from high-energy particles and radiation launched by the Sun in solar storms, as well as high-energy cosmic rays that come from outside the solar system. Science fiction sometimes handles this by having some sort of all-purpose protective force field.

You hear a Star Trek captain yell “Shields up!” as soon as there is a threat to the ship. And those shields seem to do everything: protecting the ship from high-energy radiation in a strong stellar flare, deflecting weapons fire, and even keeping a seal between the pressurized cabins and the vacuum of space.

Engineering a Magnetic Shield

But let’s start a little simpler and just think about protecting our astronauts from solar storms, as well as galactic cosmic rays. Cosmic rays are another mix of photons and ionized particles, but they reach us traveling much faster than particle storms from the Sun. Some cosmic rays are from distant supernova explosions. So cosmic rays are even harder to protect against.

Here on Earth, we are protected by Earth’s magnetic field. But human-crewed spacecraft or a base on the Moon or Mars doesn’t have this protection. Is there a way to engineer a magnetic shield similar to Earth’s magnetosphere?

One idea is to create a mini-magnetosphere that can be used when there is a giant solar storm. But you would need to somehow transport very strong magnets into orbit. But the stronger you want your magnet to be, the more magnet mass it’s going to need. And that increases the amount of rocket fuel you need to launch off the Earth.

But there are some ideas that it might be possible to exploit a turbulent plasma’s ability to shield against high-energy particles. But the technology needs a lot of work before it looks to be feasible. But let’s dream a little bigger with this idea.

A Shield for Mars?

At the Planetary Science Vision 2050 Workshop held in 2017, NASA scientists and collaborators proposed creating a magnetic shield to protect the entire planet Mars.

Concept image of the Magnetic shield on L1 orbit around Mars.
NASA has come up with an idea of using a magnet to protect Mars from the solar wind. (Image: NASA/Jim Green/Public domain)

The idea is to create a strong magnetic dipole shield in stable orbit between Mars and the Sun centripetal forces. Scientists have proposed that using a magnet rated 1 Tesla and located at the L1 Lagrange point would create a magnetosphere around the magnet that’s big enough for Mars to sit far down in the magnetotail of the magnet. This would shield Mars from the solar wind.

Learn more about water on Mars and prospects for life.

An Atmosphere for Mars

And once in place, this could do more than just protect astronauts on the surface of Mars from solar storms. For example, we know Mars lost most of its atmosphere long ago. Even today, Mars continues to have atmosphere stripped from the planet by the solar wind.

If we could stop or slow down this stripping, then the atmosphere could thicken over time as new volatiles are released from impacts or whenever summer time temperatures bring certain materials into a gaseous state. If the atmosphere gets thicker, perhaps temperature would rise, due to added greenhouse gasses in the atmosphere. Liquid water might be stable on the surface once again!

In essence, a magnetic shield might be able to turn the clock back 3 or 4 billion years and allow for the terraforming of Mars. But these ideas are only ideas yet, and have yet to be implemented.

Common Questions about Light Sails and Magnetic Shields

Q. What are the technical challenges to creating a light sail?

First, small spacecraft with light sails wouldn’t really be maneuverable, and may run into all sorts of dust and small debris, and not reach their destination. The other problems have to do with the enormous power required, almost a gigawatt, and the problem of aiming the laser properly.

Q. How can an artificial magnetic shield be created for Mars?

Using a magnet rated 1 Tesla located at the L1 Lagrange point between Mars and the Sun would create a magnetosphere around the magnet that’s big enough for Mars to sit far down in the magnetotail of the magnet. This would shield Mars from the solar wind.

Q. What would be the potential effect if Mars were magnetically shielded?

A magnetic shield might help Mars regain its atmosphere and allow for the terraforming of the planet.

Keep Reading Past Waterfalls and Lakes on Mars Exploring Mars, Traveling to It, and Its Global Storms How Did All the Liquid Water on Mars Disappear?