What a Swarm of Probes Can Teach Us About Proxima Centauri B

You’ve likely heard of the Breakthrough Starshot (BTS) initiative. BTS aims to send tiny gram-scale, light sail picospacecraft to our neighbour, Proxima Centauri B. In BTS’s scheme, lasers would propel a whole fleet of tiny probes to the potentially water-rich exoplanet.

Now, another company, Space Initiatives Inc., is tackling the idea. NASA has funded them so they can study the idea. What can we expect to learn from the effort?

Proxima b may be a close neighbour in planetary terms. But it’s in a completely different solar system, about four light-years away. That means any probes sent there must travel at relativistic speeds if we want them to arrive in a reasonable amount of time.

That’s why Space Initiatives Inc. proposes such tiny spacecraft. With their small masses, direct lasers can propel them to their destination. That means they must send a swarm of hundreds or even one thousand probes to get valuable scientific results.

This is much different than the architecture that missions usually conform to. Most missions are a single spacecraft, perhaps with a smaller attached probe like the Huygens probe attached to the Cassini spacecraft. How does using a swarm change the mission? What results can we expect?

“We anticipate our innovations would have a profound effect on space exploration.”

Thomas Eubanks, Space Inititatives Inc.

A new presentation at the 55th Lunar and Planetary Science Conference (LPSC) in Texas examined the idea. It’s titled “SCIENTIFIC RETURN FROM IN SITU EXPLORATION OF THE PROXIMA B EXOPLANET.” The lead author is T. Marshall Eubanks from Space Initiatives Inc., a start-up developing 50-gram femtosatellites that weigh less than 100 grams (3.5 oz.)

Tiny probes like these can only do flybys. They’re too tiny and low-mass for anything else. When designing a mission like this, the first consideration is whether the probes will operate as a dispersed or coherent swarm. In a dispersed swarm, the probes reach their destination sequentially. In a coherent swarm, the probes are together when they do their flyby. Both architectures have their merits.

In either case, these tiny solar sail probes will be very thin. But thanks to technological advances, they can still gather high-resolution images by working together.

The image below shows 247 probes forming an array as they fly by Proxima b. Together, they have the light-collecting area of a three-meter telescope. This arrangement should enable sub-arc-second resolutions at optical wavelengths. Spectroscopy should be equally as fine.

“While both erosion by the Interstellar Medium (ISM) and image smearing will degrade imaging, we anticipate these systems will enable sub-arcsecond resolution imaging and spectroscopy of the target planet,” the authors write.

These tiny spacecraft could do some course correction, but not much. So, getting the navigation right is critical. Unfortunately, our data on Proxima b’s orbit is not as well-understood as the planets in our own Solar System. It all comes down to ephemeris.

Ephemeris tables show the trajectory of planets and other objects in space. But in Proxima b’s case, the ephemeris error is potentially quite large.

Added to that is the distance. If the probes can travel at 20% of light speed, reaching the planet will take over 21 years. The authors calculate that if they can restrict Proxima b’s ephemeris error to 100,000 km and send 1,000 probes, at least one will come within 1,000 km of the planet. “Meeting this ephemeris error goal will require improved astrometry of the Proxima system,” the authors write.

The probes would perform science observations on their way to Proxima b. As they travel, the swarm would have dozens or even hundreds of opportunities to use microlensing to study stellar objects. A stellar mass microlensing event requiring one month on Earth would only take one hour.

“It is now possible to predict lensing events for nearby stars; BTS probe observations of dozens or hundreds of predicted microlensing events by nearby stars will offer both a means of observing these systems and a novel means of interstellar navigation,” the authors explain.

The swarm would be only the third mission to leave our Solar System. The Voyage spacecraft left the heliosphere, but only inadvertently. So, the swarm could observe the interstellar medium (ISM) during its 20+ year journey. One of the questions we have about the local ISM concerns clouds. We only have poor data on the nature of these clouds, and scientists aren’t certain if our Solar System is in the Local Interstellar Cloud (LIC.)

“In situ observation of the properties of these clouds will be a primary scientific goal for mission science during the long interstellar voyage,” the researchers write.

Opportunistic science during the voyage is great, but arrival at Proxima b is the meat of the mission. One day before the probes arrive, they would still be 35 AU away. At that point, the mission could begin imaging. Proxima b would still only be several pixels across, but it’s enough to see any visible moons.

“At this point, it would be worth turning some probes to face forward and begin imaging the Proxima system to search for undiscovered planets, moons and asteroids in the system, and to begin a Proxima b approach video,” the researchers explain.

Upon arrival at Proxima Centauri b, a one-meter aperture telescope 6,000 km away from the planet could attain a six-meter resolution on the surface. That’s an idealistic number, as not all of the planet’s surface could be imaged at that resolution. PCb is also tidally locked to its star, meaning one side is in darkness. Because of that, the mission should be designed to gather low-light and infrared images of the night side. “Night-side illumination imagery might also be the most conclusive technosignature from an initial Proxima mission,” the authors write.

As probes pass through Proxima b’s shadow, they could use the light from the star to perform spectroscopy. Probes passing behind Proxima b could use the Earth laser system for spectrometry, and if the probes are in a coherent swarm, they could use the lasers from pairs of probes on either side of the planet.

“Transmission spectroscopy, which for Proxima b cannot be done from Earth,” the researchers explain, “will likely provide the best means of determining the existence of a biology or even a technological society on Proxima b through the search for the spectral lines of biomarkers and technomarkers.”

As humanity’s first mission to Proxima Centauri b, the swarm would face some hurdles and uncertainties. But in a coherent swarm architecture, the mission could also be almost too successful. “A BTS mission, especially with a coherent swarm, may collect more data than can be returned to Earth,” the authors write. If the data returned has to be selected autonomously by the swarm itself, that could be more demanding than deciding what data to collect in the first place.

Scientists have many questions about Proxima Centauri b. Should the swarm ever be launched, any amount of data it returns will be valuable. Even though it’ll take over four years for the data to be sent back to Earth.

Scientists don’t know how hot the planet is. They’re not certain if it even has liquid water. It looks like the planet is just over one Earth mass and has a slightly higher radius. But those measurements are uncertain. Scientists are also uncertain about its composition. The star it orbits is a flare star, which means the planet could be subjected to extremely powerful bursts of radiation. That’s a lot of uncertainty.

But it’s the nearest exoplanet, the only one we could feasibly reach in a realistic amount of time. That alone makes it a desirable target.

There’s no final plan for a mission like this. It’s largely conceptual. But the technology to do it is coming along. NASA has funded a mission study, so it definitely has merit.

“Fortunately, we don’t have to wait until mid-century to make practical progress – we can explore and test swarming techniques now in a simulated environment, which is what we propose to do in this work,” said report lead author Thomas Eubanks from Space Initiatives Inc. “We anticipate our innovations would have a profound effect on space exploration, complementing existing techniques and enabling entirely new types of missions, for example, picospacecraft swarms covering all of cislunar space or instrumenting an entire planetary magnetosphere.”

Eubanks also points out how a swarm of probes could investigate interstellar objects that pass through our inner Solar System, like Oumuamua.

But the main mission would be the one to Proxima Centauri b. According to Eubanks, that would happen sometime in the third quarter of this century.