Philip Lubin and Joel Rothman, both professors at the University of California, Santa Barbara, firmly believe that one of the defining features of humanity is our drive for exploring. Whether it was crossing a mountain range during our hunter-gatherer days or traveling to the outermost regions of the solar system, we seem to have always been seeking to push the boundaries towards new frontiers. In a new study, the pair of researchers argue that interstellar travel is the next big obvious step, but before humans may undertake such a monumental journey, we’d better learn more about the challenges by sending tiny scouts, such as roundworms or the famously sturdy tardigrades.
Solar sails
Interstellar travel is multiple orders of magnitude more challenging than interplanetary travel through our solar system. It took the two Voyager spacecraft more than 40 years to travel 12 billion miles from Earth and exit the bubble surrounding our Solar System, known as the heliosphere — largely regarded as the boundary where our solar system ends and interstellar space begins.
If we were to rely on the chemical and nuclear propulsion of the Voyager missions, which allowed the spacecraft to cruise at speeds around 56,000 km/h (35,000 mph), it would take us more than 80,000 years to reach Alpha Centauri, the closest star system.
Most scientists seem to agree that neither chemical nor nuclear propulsion strategies have the necessary energy per mass needed to achieve spaceflight at relativistic velocities. The only two options that are theoretically available are antimatter and photon propulsion. The former is currently outside of our technological prowess, but photon propulsion is a realistic option.
“Driven by a wide variety of needs, photonics, like electronics, has grown exponentially in capability and dropped exponentially in cost with a doubling time in capability and cost reduction of about two years,” the researchers wrote in a recent study that appeared in the journal Acta Astronautica.
This photonic propulsion strategy is supported by NASA as part of its Starlight program, which aims to “use large scale directed energy to propel small spacecraft to relativistic speeds to enable humanity’s first interstellar missions.”
It looks something like this: small, microchip-sized probes equipped with tiny but reliable sensors, communication hardware, and scientific instruments would be propelled up to 20%-30% of the speed of light using light itself. A laser stationed on Earth or possibly the moon would fire onto a ‘solar sail’, enhancing its velocity like a gust of wind.
A beam of photons such as that fired by a laser or even sunlight actually exerts physical pressure. As you might have guessed this pressure is incredibly tiny. But because there’s very little to no resistance in space, a light sail-powered spacecraft will gain more and more momentum as it travels and an increasing number of photons bounce off the sail. As long as photons hit the sail, the spacecraft will endlessly continue to accelerate until it hits the universal speed limit set by Einstein — at least in theory. So, even though you still need a rocket to lift such a spacecraft into orbit, a solar sail can reach speeds that a chemical spacecraft could never ever attain.
Unlikely astronauts
And besides sensors and scientific instruments, Rothman and Lubin have proposed sending life forms on some of these light-powered spacecraft. Some candidates include the roundworm Caenorhabditis elegans, one of the most widely studied animals and already space veterans, being the subject of ongoing experiments onboard the International Space Station, as well as tardigrades, extremophiles capable of suspended animation that protects them from ungodly high temperature and pressure ranges.
Thousands of these tiny creatures could be sent to nearby star systems in tiny spacecraft called StarChips and put in a state of suspended animation in which their metabolism is slowed nearly to a halt. They could be awakened once the creatures arrive at their destination, many decades or even centuries later, and the effects interstellar travel had on them would be relayed back to Earth so scientists know what to expect for interstellar missions involving humans. This kind of data would be not possible to gather during Earth-based research or even trips to Jupiter or other far-flung worlds within the solar system, which is why the authors believe such a mission is warranted.
“We can ask how well they remember trained behavior when they’re flying away from their earthly origin at near the speed of light, and examine their metabolism, physiology, neurological function, reproduction and aging,” Professor Rothman said.
“Most experiments that can be conducted on these animals in a lab can be performed onboard the StarChips as they whiz through the cosmos.”
“The effects of such long odysseys on animal biology could allow the scientists to extrapolate to potential effects on humans.”
“We could start thinking about the design of interstellar transporters, whatever they may be, in a way that could ameliorate the issues that are detected in these diminutive animals.”
A scalable design: from handheld-sized spacecraft to kilometer-sized
The Starlight laser-phased array is a modular system, so it can be scaled from handheld to kilometer-scale. A small spacecraft accelerated to 25% the speed of light would require a directed energy power level on the order of 100 GW, but only for a few minutes per launch. The same technology could be used to power and propel satellites in geostationary Earth orbit, as well as in planetary defense applications.
This kind of mission has a completely opposite approach to the guiding principles of modern spaceflight programs, which go to great lengths to avoid contaminating Mars and whatever alien environments we plan to visit with microbes from Earth. In fact, the purpose is to contaminate outer space with Earth-based life — and this brings about a number of ethical questions.
“I think if you started talking about directed propagation of life, which is sometimes called panspermia—this idea that life came from elsewhere and ended up on the earth by comets and other debris, or even intentionally from another civilization—the idea that we would purposefully send out life does bring up big questions,” Lubin said.
Suppose that, unfeasible as it may sound, some of these tardigrades arrive on an alien planet and seed life there. Should this concern us? Not at all, the researchers say. Any such probe nearing an alien planet would surely be destroyed by atmospheric entry or impact with the surface. As such, the risk of forward contamination is nearly zero.
Humans in interstellar space is a great plotline for movies, but in reality it is likely hundreds of years away. For now, traveling to other worlds outside our solar system remains a distant dream, but if we’ve learned anything about human nature, it’s that we can’t stay put in just one place.
“I think we shouldn’t, and won’t, suppress the exploratory yearning that is intrinsic to our nature,” Rothman said.
