A project to send a spacecraft to Proxima Centauri has announced a milestone towards that goal, launching the world’s smallest spacecraft.

Called Breakthrough Starshot, the project is being funded by Russian billionaire Yuri Milner. Yesterday, they announced they had launched the smallest fully functional space probes ever, built on just a single circuit board, known as Sprites.

They measure a tiny 3.5 square centimeters (0.5 square inches) in size and weigh just 4 grams (0.1 ounces). Despite their small size, they are still able to pack in solar panels, computers, sensors, and radios.

They were launched into low-Earth orbit back on June 23 on an Indian rocket, the Polar Satellite Launch Vehicle (PSLV). The rocket, of course, had some weightier payloads, but these tiny spacecraft were attached to the outside of two larger German satellites.

Zac Manchester from Breakthrough Starshot, who came up with the Sprites idea, told IFLScience that at least one of the spacecraft was currently operational “and should remain so for a while”. A further four Sprites would hopefully be deployed from the second satellite soon.

“Right now we're at the early stage of just getting these sort of extremely small spacecraft to work on orbit,” he said. "We'll keep expanding their capabilities and pushing their size and mass smaller.”

These spacecraft might seem small, but the eventual spacecraft used to get to Proxima Centauri will be about four times lighter. Called Starchips, the idea is to attach them to giant and thin sails. By firing a laser at them from Earth, they can then be accelerated up to about a fifth of the speed of light, and make the trip to Proxima Centuari 4.2 light-years away in 20 years.

A number of problems still face this idea, such as how to design a sail light enough to survive the blast of the laser. Equally difficult is getting all the necessary components onto the tiny Starchip vehicle.

The launch of Sprites, then, will help Breakthrough Starshot perfect some of their ideas. The team will monitor how well their electronics perform in orbit, with radio communications monitored from ground stations in California and New York.

“The biggest challenge will be figuring out how to communicate the data from those sensors back to Earth over huge distances,” said Manchester. Hopefully, these tests will help address that issue.