Space Exploration Technologies

Exploring the Solar System with Swarms of Microprobes

It’s satisfying to sit back and take stock of all the places in the Solar System that we’ve explored. The Moon came first, then over the following decades, we’ve sent spacecraft to Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and even distant Pluto. We’ve also explored some of the asteroid belt’s inhabitants and even several comets.

That’s an impressive list, but it’s still dwarfed by the number of objects we haven’t visited. Could swarms of microprobes help us expand our reach? New research shows that tiny, solar sail microprobes could complete a round trip to asteroid Bennu faster than OSIRIS-REx did.

A team of researchers at the University of California, Berkeley, says that fleets of solar-sail-powered microprobes could be the next big leap in space exploration. These tiny probes would weigh only 10 grams (0.35 oz.) and would be powered by only the pressure from the Sun.

The details are in a new paper in Acta Astronautica. It’s titled “BLISS: Interplanetary Exploration with Swarms of Low-Cost Spacecraft.” The lead author is Alexander Alvara, a mechanical engineering doctoral student at UC Berkeley.

“The Berkeley Low-cost Interplanetary Solar Sail (BLISS) project is intended to demonstrate that cell phone technologies and other miniaturization via technological advancements enable unprecedented capabilities in space,” the authors write in their paper.

Even though these tiny probes are solar sails, they don’t get their energy from the solar wind, the stream of charged particles coming from the Sun. Instead, solar sails get their energy from the radiation pressure from the Sun. The Sun exerts that pressure across multiple wavelengths. Solar pressure sails work, as The Planetary Society’s LightSail proves.

The LightSail vehicles were test missions that only orbited Earth for a short time before re-entering the atmosphere, as planned. During LightSail 2’s mission, it successfully demonstrated that its solar sails were able to raise its orbit.

Photo of LightSail 2’s sail deployment. During its flight, it successfully raised its orbit on solar radiation pressure alone and took some images, including this one. Credit: The Planetary Society

But sending a fleet of tiny solar sail spacecraft out into the Solar System is a more demanding endeavour. But the ongoing miniaturization of electronics is making microprobes more and more attractive and feasible.

Kristofer Pister, professor of electrical engineering and computer sciences at UC Berkeley, leads the BLISS effort. In an interview with UC media, Pister and lead author Alvara explained the motivation for the BLISS program and talked about the advantages solar sails have over other types of spacecraft, including their cost.

The BLISS program is motivated by Near-Earth Asteroids (NEAs.) There are about 1,000 NEAs larger than 1km in diameter, but we only have pictures of about 10 of them, and most are not great pictures. “We were excited by the idea that you could potentially take an iPhone camera, orbit around one of these things, take a thousand high-resolution colour photographs from a very close distance and then beam that information down,” Prof. Pister said.

This is a typical image of a near-Earth asteroid. It’s called 2006 DP14, and it’s about 400 meters long. The image was captured by the Arecibo Observatory. Image Credit: By from http://www.jpl.nasa.gov/news/news.php?release=2014-060, Public Domain, https://commons.wikimedia.org/w/index.php?curid=39100503

In their paper, Pister and the other authors write that “The mission goal for this work is to deploy 100s–1000s of low-cost small solar sail spacecraft to NEOs for imaging with the goal of identifying asteroids that might have the prospect of being deterred from earth impact trajectories and those with the capacity for harbouring organics and life within our solar system.”

This schematic of the BLISS spacecraft shows some of the details. A shows the carbon fibre control rods, B shows the spacecraft’s body, C shows the mylar main sail, and D shows the mylar roll sail. In the upper right, E is the radiator fin, F shows the MEM motor actuators, G shows the battery, and H shows the VoCore 2 CPU with embedded Linux. In the bottom right, I shows an iPhone camera, J shows the transmitter/receiver, and K shows the solar panels. Image Credit: Alavara et al. 2023.

To fulfill that mission, solar sails have one primary advantage over other spacecraft: they have no engines, which makes the spacecraft lighter and cheaper. It also means they don’t need fuel. “Unlike other spacecraft, solar sails can travel around the galaxy, or, more specifically, our Solar System, without having to carry any fuel or worry about refuelling,” Alvara said.

Another advantage of the BLISS microprobes is their tiny size. “A smaller size allows the spacecraft to be more agile. We don’t have to worry about buckling of the sail, which is just one square meter,” said Alvara. A larger spacecraft needs a larger sail, and larger sails need more support to prevent them from buckling when unfurling and changing orientation. Solar sails need to be unfurled after they’re released from a launch vehicle, and that requires some complex equipment. Since the microprobes are only 10 grams, the sails only need to be one square meter (10.7 sq. ft.) Compare that to LightSail 2’s sail, which was the size of a boxing ring: 32 m2, or 340 square feet.

These microprobes are also much less expensive than larger spacecraft, a critical feature in spacecraft design. “If we do everything right, the cost of the solar sails will be a thousand dollars or less,” said Pister. “We could then put thousands of these tiny spacecraft in a little package, the size of a small satellite, and launch them into space.”

That means that for the cost of a single launch, a swarm numbering around one thousand probes can be launched to explore near-Earth asteroids.

NASA’s “Eyes on Asteroids” site maps the known Near-Earth asteroids (NEAs) and shows the population of these objects. There are over 1,000 NEAs larger than one km in diameter. Courtesy NASA.

A key feature of BLISS is a type of microelectromechanical system (MEMS) called the inchworm motor. Inchworm motors use piezoelectric actuators to turn electricity into force. This is how the BLISS spacecraft moves its carbon fibre rods to change the sail’s orientation and make course changes. “It turns out that’s what you need to navigate — just like on a sailboat,” said Pister. “You pull on the lines and change the attitude of the sail through the wind, and that affects direction.”

To navigate, BLISS will use its camera to image stars. Then, it’ll compare those images to onboard images to determine its location. This is called the Lost in Space Algorithm. “The idea is that you map the stars that you can see, then compare them to the pixels of the images that you can get from your on-board cell phone camera,” said Alvara.

Communications are still possible in spacecraft so small. The researchers say that the BLISS spacecraft can stay in communication with each other at one million km separations and will be able to pass data around. BLISS will also communicate with Earth via a geostationary satellite. The authors point out that they could make even smaller spacecraft if they returned to Earth orbit to communicate.

Once they reach their destinations, the tiny spacecraft will image their surfaces. They could also collect samples of dust from comet tails, although the samples would obviously be tiny. Details on how these samples would be collected, stored, and delivered to Earth are scant.

To illustrate how BLISS could work, the researchers used the NEA Bennu and the OSIRIS-REx mission as a comparison. They say BLISS could get to Bennu more quickly than OSIRIS-REx did. While OSIRIS-REx took more than seven years to complete its round trip, BLISS would need just a little more than five years. There are obvious differences between the two. OSIRIS-REx has an entire suite of scientific instruments and took extra time to inspect Bennu and select a touchdown sampling site.

But OSIRIS-REx visited only a single asteroid. BLISS could visit hundreds of them for a relatively small monetary investment.

How soon could BLISS microprobes be ready for launch?

“We could feasibly do it in a few years,” Alvara said. Some of the theories are sound, and some of the motors have been tested.

“But there are six other systems and all kinds of software still needed, so it would be an undertaking,” Pister said. “But I’m hopeful that we can obtain funding for further research.”

Evan Gough

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