Tiny gram-scale interstellar probes pushed by laser light are probably the only technology capable of reaching another star in this century. A laser beamer powerful enough (~100-GW) to boost a few grams to relativistic speed will probably be available by 2050 as well as laser sails robust enough to survive launch, and terrestrial light buckets (~1-sq.km) big enough to catch the optical signals. The proposed representative mission, around the third quarter of this century, will be to fly a large autonomous swarm of 1000s of tiny probes past the potentially habitable world Proxima b, our nearest stellar neighbor.
Given the extreme constraints on launch mass (grams), onboard power (milliwatts), and coms aperture (centimeters to meters), only a large swarm of many probes acting in unison can generate an optical signal strong enough to cross the immense distance back to Earth. The 8-year round-trip time lag eliminates any practical control by Earth. The swarm must possess an extraordinary degree of autonomy in order to prioritize which data is returned to Earth. Coordinating the swarming of individuals into an effective whole is the dominant challenge for the representative mission. Coordination in turn rests on establishing a mesh network via low-power optical links. Probes’ on-board clocks must be synchronized with Earth and with each other to support accurate position-navigation-timing (PNT).
The representative mission will begin with a long string of probes launched one at a time to two tenths of the speed of light. Following launch, the drive laser is used for signaling and clock synchronization, providing a continual time signal like a metronome. Initial boost is modulated so the tail of the string of probes catches up with the head (“time on target”). Exploiting drag imparted by the interstellar medium (“velocity on target”) over the 20-year cruise will keep the group of probes together once assembled. An initial string of 100s to 1000s probes spread out over hundreds of millions of kilometers dynamically coalesces itself over time into a lens-shaped mesh network about 100,000 km across. This is sufficient to account for ephemeris errors at Proxima, ensuring at least some probes pass close to the target.
A swarm of probes whose members are in known spatial positions relative to each other, having state-of-the-art microminiaturized clocks to keep synchrony, can utilize its entire population to communicate with Earth. The swarm will periodically build up a single short but extremely bright contemporaneous laser pulse from all of them. Operational coherence means each probe sends the same data but adjusts its emission time according to its relative position. This will result in all pulses arriving simultaneously at the receiving arrays on Earth. This effectively multiplies the power from any one probe by the number of probes in the swarm. Thus there will be orders of magnitude greater data return.
A swarm of probes could tolerate significant attrition during their trip, mitigating the risk of “putting all your eggs in one basket.” This will enable close observation of Proxima b from multiple vantage points. It is not necessary to wait until mid-century to make practical progress. Exploring and testing swarming techniques can now take place in a simulated environment. It is anticipated that resulting innovations would have a profound effect on space exploration. They will complement existing techniques and enable entirely new types of missions. For example, picospacecraft swarms covering all of cislunar space, or instrumenting an entire planetary magnetosphere. Well before mid-century there should be a number of such missions, starting in Earth or lunar orbit, eventually extending deep into the outer Solar system. Such a swarm of tiny probes could explore the rapidly receding interstellar object 1I/’Oumuamua or the solar gravitational lens. These missions would both be precursors to the ultimate interstellar mission, but also scientifically valuable in their own right.