To Accelerate…With Light?
Light sails rely only on photons to reach up to 20% of the speed of light. Maybe they can even help us reach another star system.
One of the most challenging issues space exploration faces is efficient propulsion and reaching far distances. For the most part, we rely on chemical propulsion systems to get out of Earth’s orbit and to our destination.
Chemical propulsion systems use liquid or solid fuel to push a rocket out of Earth’s orbit. Then the rocket sheds off parts of itself, which we call dead weight. This way, the rocket is lighter and can travel farther and faster. However, the rocket runs out of fuel pretty soon, and after that, it relies on inertia (what goes in motion stays in motion unless acted upon by an external force) to reach its destination.
Relying on inertia means that the spacecraft is going to travel really slowly in terms of astronomical distances. For example, the New Horizons probe, which travels at about 53,000 km/h, would take 54,000 years to reach Proxima Centauri, the nearest star to the Sun.
But scientists really want to explore deep space. Not just the Kuiper Belt and Oort Cloud, but also interstellar space and possibly the Proxima Centauri Star System. So how do we get there?
Light Sails
There’s plenty of ways to reach far distances in space, but one of them is by using spacecraft called light sails. Light sails are huge reflective surfaces that accelerate using photons from a powerful light source, such as the Sun. Solar sails accelerate using photons from the Sun, while laser sails accelerate using photons from lasers.
Photons are massless particles of light. However, they have momentum. Thus, when a photon bounces off a light sail, they’re able to transfer their momentum and give the sail a little push. The pushes are infinitesimal, but when billions of photons are hitting the light sail each second, it actually gets a lot of push.
Light sails need to be really light in order to get a push from the photons, so they’re generally only a few microns thick and made from light polyesters or alloys. The spacecraft attached to the solar sails are also really small, like CubeSats. A 1U CubeSat is a tiny satellite that’s normally 10 cm x 10 cm x 10 cm. They’re very versatile though, and can easily be used to study deep space.
The best part about light sails is that they need very little to no fuel to travel at high speeds. Relying solely on photons lets them continuously accelerate to reach those speeds.
For instance, on the first day, a solar sail may only move at 160 km/h. At the end of the first year, it may move at 58,000 km/h. By the end of the third year, the solar sail will be moving at 160,000 km/h. In reality, this acceleration process depends on a multitude of factors, such as the power of the light source, distance from the light source, size of the solar sail, and weight of the solar sail. However, through this acceleration process, it’s easy for solar sails to travel far distances in a reasonable amount of time.
Do Solar Sails Exist Yet?
There have been a couple of missions that used solar sails, either to demonstrate the technology or for real research purposes. Many attempts to launch them failed, unfortunately.
In 2005, The Planetary Society’s Cosmos-1 was a 100 kg spacecraft was meant to use solar sails. 8 triangular blades were to be deployed from its central hub. Each blade was 15 m long, all of which had a total surface area of 600 m². The rocket to launch the spacecraft failed, so it never reached orbit.
In 2008, NASA’s NanoSail-D Demonstration Solar Sail, which was 10 m², was meant to demonstrate the practicality and usefulness of solar sails. The rocket that launched the sail also failed.
In 2009, NASA planned another demonstration of a solar sail that was developed and tested as part of a ground sail test program. This demonstration was canceled.
In 2015, The Planetary Society once again launched a solar sail, this time called LightSail-1. The 32 m² sail reached orbit, but suffered from technical malfunctions and stopped transmitting signals before deorbiting and burning up in the Earth’s atmosphere.
Despite the numerous failures, there have been two successful light sail demonstrations.
In 2010, JAXA launched IKAROS, a 196 m² solar sail into orbit. The sail, which was 7.5 microns (0.0075 mm) thick and made of polyimide resin, functioned properly within the first year of launch. The solar sail then headed to Venus to collect data, where would remain in operation for an additional 5 years before the end of its mission.
In 2019, The Planetary Society launched the LightSail 2, which made improvements based on the malfunctions that occurred during LightSail 1’s mission. The sail, which is 4.5 microns thick (0.0045 mm), is made of cobalt alloy and spans 32 m². The sails have rip-stop seams running through them to prevent tearing. A 3U CubeSat, about the size of a loaf of bread, lies in the center. Inside the CubeSat are operating systems, such as motors and momentum wheels. So far, it’s still operating in Earth’s orbit and sending data back to The Planetary Society.
Overall, solar sails have been around for a while, but are still in testing phases. Luckily, most of the missions that failed weren’t due to malfunctions directly related to the solar sails, but rather the launches. People still have hope for light sails, which is what led to the announcement of the Breakthrough Starshot Project.
The Breakthrough Starshot Project
In 2016, physicist Yuri Milner and renowned cosmologist Stephen Hawking announced the Breakthrough Starshot Project, whose goal was to send a fleet of nanocraft to Proxima Centauri (the nearest star system) using laser sails.
The project hopes to construct a working prototype by 2036 at a total cost of 10 billion dollars. Breakthrough envisions sending 1,000 stamp-sized spacecraft to Proxima Centauri, all with 16 m² laser sails. The photons accelerating the spacecraft would come from extremely powerful lasers on Earth, allowing the spacecraft to reach 15–20% the speed of light.
Proxima Centauri is about 4.2 light years away, or 25 trillion miles. Normal, chemical-propulsion-based spacecraft would take thousands of years to reach there. However, at the speeds the laser sails generate, the fleet could reach Proxima Centauri within 30 years.
Of course, there’s a huge list of challenges that scientists are facing for not only developing the Breakthrough Starshot Project, but light sails in general. For example, how do we generate such powerful lasers? How do we correctly position the lasers? How do we face space debris and dust? And, possibly the most important, how do we slow them down?
Currently, there are no known ways to slow down light sails, making it one of their greatest drawbacks. If the fleet were to reach Proxima Centauri and crash into one of the Earth-like planets there, it would be like bombarding it with a thousand Hiroshima nuclear bombs. Can’t really study the star system if the entire fleet is destroyed…
But, moonshot aside, solar sails are one of the most promising pieces of space technology today in terms of acceleration and reaching far distances since they don’t need chemical fuel. In fact, one of the most practical applications we’re hoping for light sails in the near future is to send a spacecraft next to the Sun that can regularly transmit data back to the Earth. This is most important for deadly events such as coronal mass ejections and geomagnetic storms.
Light sails hold a bright future in space technology, and may one-day be exactly what propels us to the next star system.
Have questions? Send me an email at chloewang.lv@gmail.com and I’ll be happy to respond!
