Six Sprites, "the world's smallest spacecraft," have entered low-Earth orbit, a small milestone for Breakthrough Starshot's plan for interstellar voyage.

“Space is big,” reads a line from Douglas Adams's Hitchhiker's Guide to the Galaxy. “REALLY big.” So big, one wonders if a spacecraft could ever reach even the nearest star system, Alpha Centauri.

One plan named Breakthrough Starshot wants to bridge that gulf to the nearest star within the century. Now, just a little over a year after the plan was announced, the initiative has achieved a small first, as a number of its small Sprite prototype spacecraft piggybacked into low-Earth orbit.

The idea behind Breakthrough Starshot is simple: accelerate a gram-scale spacecraft to 20% the speed of light (that's about 37,000 miles per second, fast enough to reach the Moon in less than 7 seconds) using ground-based lasers for the first interstellar mission. Yuri Milner announced the Breakthrough Starshot project as part of the larger Breakthrough Initiatives program in April 2016.

The $100 million of funding is a drop in the bucket compared to what a full interstellar mission would cost — it's basically a starter grant to fund research toward a proof of concept, to demonstrate that a tiny spacecraft could be accelerated to a fraction of the speed of light.

What the most recent test accomplished was a tiny but important milestone. Six postage-stamp-sized spacecraft, named Sprites, made it to orbit as supplementary payloads. Two are mounted on the larger Venta and Max Valier educational satellites built by European space company OHB System AG. Another four of the Sprites are still inside the Max Valier and awaiting deployment as independent spacecraft. The satellites launched aboard Polar Satellite Launch Vehicle from Satish Dhawan Space Center in southeast India on June 23, 2017.

From Sprites to Starchips

Each Sprite is built as a single circuit board 3.5 centimeters on a side and weighing just 4 grams (for comparison, a U.S. nickel weighs 5 grams). The femtocraft contain solar cells (which generate all of 100 milliWatts of power in direct sunlight), a magnetometer, gyroscope, and an antenna to communicate with Earth.

Both of the larger satellites have achieved stable orbits, but while ground stations in California and New York as well as amateur radio operators have detected Sprite signals, the team hasn't determined yet that they've detected two separate signals. Thus far, the Max Valier satellite is itself having trouble communicating, halting the release of the four Sprites still carried in its cargo container.

The ideas behind Breakthrough Starshot aren't without their own challenges, as the team readily acknowledges, but the scientists involved maintain that there aren't any dealbreakers.

Solar sail propulsion, long a staple of science fiction, is attractive in that the spacecraft wouldn't need to carry its own fuel. More fuel means more mass, which then means more fuel and, well, you get the idea. Robert Forward proposed the microwave-propelled Star Wisp mission in 1985, though it never took off. The Planetary Society lost its first solar sail mission in 2005, when its submarine-launched Cosmos-1 failed to reach orbit. Japan's Ikaros spacecraft finally became the first successful solar sail mission to fly in space, hitching a ride with the Venus-bound Akatsuki mission launched in 2010.

Once accelerated to high speeds, more challenges face the spacecraft. How would it avoid dust particles when it's zipping along that fast? A tiny grain of dust, though rare in the space between stars, would pack a mission-ending punch when a spacecraft is moving at 20% the speed of light. Then there's the issue of braking on the other end — at least on the first try, we may have to settle for a brief 1- or 2-day flyby of the Alpha Centauri system after a 20 year long trip.

There's also the issue of communication. For example, New Horizons used a 2.1-meter dish to talk to Earth after its flight past Pluto and Charon in July 2015, and the distance reduced its data rate to that of a bad dial-up connection, at 1 kilobit (125 bytes) per second. Enabling something the size of a penny to talk to Earth from 4 light-years away is a non-trivial issue that the Breakthrough Starshot team will need to address.

We can always hope that Moore's Law holds true and computing power will continue to double every 18 months. Breakthrough Starshot's vision anticipates this, as well as the continued miniaturization of technology and solutions to current technical challenges. Hey, we have to at least try as a species if we ever want to reach the stars, right?