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We've learned that Interstellar Objects (ISOs) are not strangers to our Solar System. Many have visited, and many more will in the future. The Vera Rubin Observatory is expected to find hundreds each year. Scientists are keen to learn more about them, and a swarm of spacecraft on standby might be the way to do it.

On a basic level, an ISO is simply an object unbound to any star. The two we know of are 'Oumuamua which was detected in 2017, and Comet Borisov, detected in 2019. ISOs typically have very high velocities, follow hyperbolic trajectories that show they don't orbit the Sun, and have unique compositions that set them apart from Solar System bodies. 'Oumuamua, for example, could be a hydrogen iceberg, though this is just one possibility.

Scientists are eager to examine these objects closer and understand their compositions and origins. Unfortunately, their high velocities make them elusive, and we can only glimpse them with ground-based telescopes. What's needed is a way to visit one. The best way to do that is to have a spacecraft waiting to catch up with one as it passes through the inner Solar System.

Or even better, a whole swarm of spacecraft that don't require explicit instructions to rendezvous with an ISO.

Hiroyasu Tsukamoto is with the Department of Aerospace Engineering in the Grainger College of Engineering at the University of Illinois Urbana-Champaign. He and his colleagues developed Neural Rendezvous, a deep learning-driven guidance and control framework that can autonomously guide spacecraft to ISOs. Their work is in a paper titled "Neural-Rendezvous: Provably Robust Guidance and Control to Encounter Interstellar Objects" and published in Aerospace Research Central.

Artist's illustration. ISOs like Oumuamua only come through once, making them difficult targets for rendezvous.

Image Credit: NASA

"A human brain has many capabilities: talking, writing, etcetera," Tsukamoto said in a press release. "Deep learning creates a brain specialized for one of these capabilities with a domain-specific knowledge. In this case, Neural-Rendezvous learns all the information it needs to encounter an ISO, while also considering the safety-critical, high-cost nature of space exploration."

"Our key contribution is not just in designing the specialized brain, but in proving mathematically that it works," Tsukamoto added. "For example, with a human brain we learn from experience how to navigate safely while driving. But what are the mathematics behind it? How do we know and how can we make sure we won't hit anyone?"

The system is based on the "contraction theory for data-driven nonlinear control systems." Contraction theory is a rigorous mathematical framework which can place limits on the effects of disturbances and uncertainties in complex linear systems. Basically, it can provide stability in a complex situation that changes nonlinearly over time.

The Neural-Rendezvous system uses available data to predict a spacecraft's best actions to intercept an ISO. This complexity is necessary because ISOs are unbound and high-speed targets with poorly restrained trajectories.

"We’re trying to encounter an astronomical object that streaks through our solar system just once and we don’t want to miss the opportunity," Tsukamoto said. "Even though we can approximate the dynamics of ISOs ahead of time, they still come with large state uncertainty because we cannot predict the timing of their visit. That's a challenge."

The Hubble space telescope captured this image of Comet 2l/Borisov at perihelion in December 2019.

Image Credit: NASA, ESA, and D. Jewitt (UCLA)

ISOs only pass through the Solar System once. The usual method of observing an object like an asteroid or comet and determining its orbit doesn't work. According to the researchers, it's critical that ISO interceptors can "think" on their own.

"Unlike traditional approaches in which you design almost everything before you launch a spacecraft, to encounter an ISO, a spacecraft has to have something like a human brain, specifically designed for this mission, to fully respond to data onboard in real-time," Tsukamoto said.

There's no way to orbit an ISO. Oumuamua and Borisov were travelling at ~88 and 45 km/s relative to the Sun, so an intercepting spacecraft would need to travel at similar speeds. With our current technological level, a spacecraft would have to carry a prohibitively large volume of propellant to enter into orbit around one of these objects. Fast flybys are likely the only realistic mission architecture.

However, relying on a single spacecraft is like putting all your eggs in one basket. What if the spacecraft is unable to get a clear view of the ISO? Without a good look at the object, scientists won't be able to learn much about its surface and composition. This has led some researchers to consider multiple spacecraft.

Tsukamoto worked with two other researchers on "a novel multi-spacecraft framework for locally maximizing information to be gained through ISO encounters." Their work is presented in a separate paper titled "Information-Optimal Multi-Spacecraft Positioning for Interstellar Object Exploration." Along with Tsukamoto, the other authors are Arna Bhardwaj and Shishir Bhatta. The authors presented it at the 2024 IEEE Aerospace Conference.

"Because of the speed and uncertainty, it's challenging to obtain a clear view of an ISO during a flyby with 100 percent accuracy, even with Neural-Rendezvous," Tsukamoto said. "Arna and Shishir wanted to show that Neural-Rendezvous could benefit from a multi-spacecraft concept."

"Interstellar objects (ISOs), astronomical objects not gravitationally bound to the Sun, could present valuable opportunities to advance our understanding of the universe's formation and composition," the authors write in their paper. "In response to the unpredictable nature of their discoveries that inherently come with large and rapidly changing uncertainty in their state, this paper proposes a novel multi-spacecraft framework for locally maximizing information to be gained through ISO encounters with formal probabilistic guarantees."

Their framework involves a swarm of spacecraft, called deputy spacecraft, and one designated as chief. The swarm would be located around an ellipsoid representing the space through which an ISO will travel. The ellipsoid consists of multiple points of interest (POIs) that would be covered collectively by the deputies and the chief employing the Neural-Rendezvous system. This method can maximize the information gained from the encounter. In simple terms, it guarantees multiple views of the ISO.

"Now we have an additional layer of decision-making during the ISO encounter," Tsukamoto said. "How do you optimally position multiple spacecraft to maximize the information you can get out of it? Their solution was to distribute the spacecraft to visually cover the highly probable region of the ISO's position, which is driven by Neural-Rendezvous."

This simple drawing illustrates the deputy spacecraft in different positions in an ellipsoid an ISO is expected to pass through.

The number of spacecraft in the swarm would depend on the size of the uncertainty ellipsoid. The team ran simulations to examine the optimal number of spacecraft while keeping the cost down. With infinite resources, the swarm could be large enough to guarantee success. However, that's not how things work.

In three trials, they determined that five spacecraft delivered the best results when balancing all factors.

This diagram shows the terminal positions of a five-spacecraft system and the POIs in view and not in view.

At the moment, Neural-Rendezvous is largely theoretical. However, the work done by Bhardwaj and Bhatta illustrates how it could be employed practically to maximum effect.

As unwitting messengers from other solar systems, ISOs have a scientific value that could surprise us. They could hold clues to how solar systems form and evolve that are found nowhere else. An autonomous swarm of spacecraft could help scientists collect these clues.

When we think about searching for alien life, most people imagine Earth-like planets orbiting distant stars, bathed in the warmth of their suns and perhaps hosting oceans of liquid water. But what if we’ve been looking in the wrong places all along?

A growing number of scientists, including world-famous astrophysicist professor Avi Loevb believe that we should be expanding our search—not for life, but for technosignatures, the telltale signs of advanced technology. As Professor Loeb explains, some of the best candidates might be frozen worlds, far from the traditional habitable zone.

We typically assume life must exist in a “Goldilocks zone”—not too hot, not too cold—where water can remain in a liquid state. But nature rarely sticks to our expectations. Saturn’s moon Titan, for example, is frigid, with surface temperatures hovering around 90 Kelvin (roughly -180°C). Yet NASA’s upcoming Dragonfly mission, launching in 2028, will explore this icy moon for signs of life in its methane lakes.

Even colder worlds—icy moons and rogue planets drifting without a star—might host liquid water oceans beneath their frozen crusts. But here’s the twist: if intelligent life ever arose in such hidden oceans, it might never look up to see the stars. As Loeb puts it, without a sky, would they ever dream of building rockets?

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Technological life doesn’t need comfort

While biological beings like us need warmth, water, and protection, artificial life doesn’t. Robots and probes don’t care about temperature, atmosphere, or even sunlight. This is exactly why some scientists suggest that the best places to look for advanced civilizations may not be hospitable planets—but the uninhabitable ones.

Loeb says we should think about it: a civilization far older than ours might have sent autonomous machines across the galaxy. These machines could land on any solid surface, including icy rogue planets or barren moons, and start building. They wouldn’t need a breathable atmosphere or a warm climate. All they’d need is raw material.

And because light takes tens of thousands of years to travel across the galaxy, these probes would need to operate independently—like scouts establishing a technological foothold for civilizations that may no longer even exist.

Where should we look?

Here are some prime targets in the search for technosignatures:

Rogue planets: These are planets that drift through space without orbiting a star. They’ve been found using gravitational microlensing and may actually outnumber stars in our galaxy.
Icy moons: Moons like Europa, Enceladus, and Titan may have subsurface oceans and mineral-rich surfaces—ideal for machines to operate or harvest resources.
Airless rocky planets: Planets like those in the TRAPPIST-1 system, with multiple rocky worlds packed into a small space, could offer plenty of landing spots for robotic explorers.

Technosignatures could be anything from artificial radio bursts to unusual surface structures, energy emissions, or even planetary surfaces altered by mining or construction. Think of them as cosmic archaeological sites—clues left behind by visitors from long ago.

And if we find such a clue? We might not understand who sent it, or why, but like detectives at a crime scene, we could begin piecing together a story from what was left behind.

What if the senders are long gone?

It’s possible that we’re looking not for neighbors—but for ghosts of civilizations past. We might uncover robotic artifacts or technological ruins on dead planets, long after their creators vanished.

But even then, the discovery would change everything. It would tell us that we’re not alone, that someone else reached the stars before us, and perhaps offer a glimpse of what lies ahead in our own future.

In the words of astrophysicist Avi Loeb, exploring these possible robotic visitors might feel like stepping out of Plato’s cave—trying to understand the world beyond by studying only the shadows it leaves behind. If we ever do stumble upon a relic, it may not just reveal an ancient traveler—it may inspire us to become one.