What is Breakthrough Starshot?

The closest star system to our own Sun is Alpha Centauri, and nearly 4.5 light-years away from the Sun, they consist of three stars: Alpha Centauri A and B, who happen to form a binary star system as they orbit around each other in a cosmic dance. In Alpha Centauri's speed, commonly referred to as Proxima Centauri, it is a small and faint red dwarf star. Proxima Centauri is the closest star in the system to the Sun, at just 4.24 light-years away.

In August 2016, astronomers made a remarkable discovery about Proxima Centauri: they discovered that the star has at least one possible Earth-like planet orbiting it, what we know today as Proxima b. It orbits Proxima Centauri in what is called the habitable zone, a region of space where the distance from the star to the planet allows for liquid water to exist on the surface. The discovery of Proxima b was a huge victory for humanity and could perhaps be the destination for our first interstellar journey as a species.

In 2015, NASA's New Horizons spacecraft completed its near 5 billion kilometers journey to Pluto, a journey that took about nine years. Traveling at a speed of about 84,000 kilometers per hour, or about 23 kilometers per second, which is extremely fast, at that speed, the craft could circle the Earth in just under 30 minutes, or about one laundry cycle. Later, if we were to form a crew of astronauts to make the first journey to Proxima b, we can place them on a replicated New Horizons spacecraft. The crew could depart from Earth and reach Proxima b in only 54,000 years.

With humanity's current state of technology, it is just impossible to reach other stars in a timely fashion. However, scientists are working on a spacecraft that could make the journey to Proxima b in just 20 years. It is known as Breakthrough Starshot. The idea behind Breakthrough Starshot was heavily influenced by a design proposed by Robert L. Forward in 1985. This craft was known as Star Wisp and was theorized to be a 100-meter wide carbon fiber probe that would be powered by microwave lasers in orbit.

The telecraft, or probe, would only have a mass of about 1.08 kilograms, not much more than a one-liter bottle of soda. The microwave lasers would propel the spacecraft up to a speed of about 10 percent of the speed of light, about 30,000 kilometers per second. This is effective but would still take over 40 years to reach Proxima b. By that time, the crew of scientists working on the program would be nearly at the end of their lifetimes. We need something better, something faster.

This is where Breakthrough Starshot comes in. But before that, what even are solar sails? Light sails or solar sails are powered by, well, just that: light. It’s similar to the way a sailboat moves, with the light exerting a force on the reflectors of the spacecraft. It is the same as the ship's sail being blown by the wind.

The New Horizons probe was 478 kilograms at launch and about 400 kilograms once in space. The spacecraft, as previously mentioned, travels at about 84,000 km/h, which is fast but clearly is nowhere near fast enough to make interstellar journeys. The advantage of using a solar sail is the truly immense speeds that can be reached when launched. Solar sails require zero fuel or onboard propellant, which reduces the mass of spacecraft significantly.

Let's say we wanted to accelerate the New Horizons spacecraft to a significant portion of the speed of light. At the 400-kilogram dry mass, it would take about 7.5 times ten to the seventeen joules of energy to accelerate the craft to just 20 percent of the speed of light. So, to reach 99 percent of the speed of light, it would take 2.2 times 10 to the 20 joules.

For comparison, an average American household uses about 39 billion joules of energy per year. At this rate, it would take 5.7 billion years for the average American household to use that much energy. This amount of energy is just too unrealistic for mankind to be able to obtain as of 2017.

Now, most people around the world have heard of the equation E equals mc squared. It is perhaps the most famous equation in the world. Multiplying the mass of any object by the speed of light squared gives you the amount of energy within that object. This idea, most notably credited to Albert Einstein, is crucial to understanding the power and effectiveness behind solar sails.

But wait a minute, there’s the problem here. See, light behaves both as a particle and a wave at the same time. Waves like microwaves, gamma rays, x-rays, stingrays, tsunami waves—you get the idea—they don’t have any mass. So if we were to plug in the mass of light, which is nothing, into the equation E equals mc squared, we get zero, which obviously isn’t true. Light has energy.

That’s because the equation E equals mc squared has a little bit more to it than meets the eye. E equals mc squared only describes objects with mass that aren't moving, much like a box sitting on a table. The full equation is E squared equals mc squared squared plus pc squared. The new variable P represents the momentum of the specific object in question.

With this big equation, it can be a little daunting at first. It’s much easier to view this as a right triangle with legs pc and mc squared, and a hypotenuse of E. Using the Pythagorean theorem, a squared plus b squared equals c squared, you can deduce the formula as such. When calculating the energy of the aforementioned box on the table, not moving at all and thus having no momentum, P equals zero, and this entire leg of the triangle has a length of zero.

And there we are. We're left with good ol’ E equals mc squared. Likewise, since light has no mass, as previously mentioned, this entire leg of the triangle has a length of zero, and we are left with E equals pc.

So let’s focus on this example as it’s crucial for solar sails. Normally, momentum is defined as an object’s mass multiplied by its velocity, but wait! Light doesn’t have mass, so wouldn’t p equal zero? Surprisingly, no! To put it simply, light's momentum is determined by its frequency. The higher the frequency, the higher the momentum. So because blue light has a higher frequency than red light, it also carries a higher momentum.

Speaking of light, a photon of white light hitting you in the face—you wouldn’t really feel it. That’s because a single photon of visible light has almost zero momentum and is basically nothing. The Sun emits about 10 to the 45 photons every second. For reference, the Sun emits about one-six million times more photons in just one second than there are stars in the entire universe.

So even with all this power spewing out of the Sun, how can it move a solar sail? When you kick a ball across the ground, it eventually rolls to a stop due to the friction of it rubbing across the surface of the planet. But in space, there is no friction. Thus, the energy that goes into accelerating the sail is never lost.

And that’s the thing about solar sails: the acceleration is slow, but the high speeds are insane. This whole time, we’ve been referring to low mass spacecraft as those with less than 50 kilograms of material or so. What’s our Starshot plan to shatter that? With ultra-low mass nanosails weighing in at just grams, all of its computing processes, cameras, and communication devices will be stored from a small star chip on the craft.

The sail that will propel the craft towards Alpha Centauri will be four meters in width and height, but only a couple hundred atoms thick. Using pure sunlight will take even a nanosail like Starshot thousands of years to reach Alpha Centauri. We need something faster!

Rather than being powered by pure solar power, Starshot will be pushed via visible-light lasers built back on Earth. A field of lasers will produce a 100-gigawatt beam focused on the single Starshot craft to push it to 20 percent of the speed of light in less than an hour, reaching Proxima B in just 20 years.

Well, building a huge laser for one small spacecraft seems like a waste, right? Rather than just sending one craft to Alpha Centauri, Russian billionaire Yuri Milner, the lead behind the Breakthrough initiatives, plans to send hundreds or even thousands of these crafts at a time. There’s a small problem here though; a bunch of the technology for this mission isn’t exactly developed at this point.

For a solar sail only a couple hundred atoms thick, it must be extremely durable if it's going to get to 20 percent of the speed of light in under an hour. The crazy accelerations that would be experienced would be enough to rip your head clean off. Also, at these high speeds, hitting an interstellar pebble the size of a marble would likely destroy the craft instantly. These are just two of the many problems that need to be resolved.

But this hasn't stopped progress from being made. In 2016, Milner put a 100 million dollar investment into research and development to begin. Alongside that, Stephen Hawking and Mark Zuckerberg hopped on board with the Starshot program, which is huge. This proves that the Starshot program is legit.

We’re in an age where private space companies and programs are becoming a reality. Milner and the team plan to launch the first batch of Starshots within the next 20 years. This means that, if all goes according to plan, we could have actual pictures of interstellar worlds within the next 50 years. And maybe, just maybe, proof of life other than our own. Who knows what we’ll find at the other end?