Microbes carried by laser-propelled sails could serve as interstellar probes that can build communications stations to phone home from Alpha Centauri, suggests a scientist known for wanting to resurrect extinct woolly mammoths and use DNA to detect dark matter.
This concept from George Church, a geneticist at Harvard University, builds upon efforts to greatly speed up spaceflight. Current spacecraft usually take years to make trips within the solar system; for example, NASA's New Horizons probe took nearly 10 years to reach Pluto.
In theory, spacecraft using conventional rockets would require thousands of years to complete an interstellar voyage. For instance, Alpha Centauri, the nearest star system to Earth, is located about 4.37 light-years away — more than 25.6 trillion miles (41.2 trillion kilometers), or more than 276,000 times the distance from Earth to the sun. NASA's Voyager 1 spacecraft, which launched in 1977 and reached interstellar space in 2012, would take about 75,000 years to reach Alpha Centauri even if the probe were headed in the right direction, which it's not.
The interstellar challenge
The problem that all rocket thrusters face is that the propellant they carry with them has mass. Long trips need a lot of propellant, which makes spacecraft heavy. This, in turn, requires more propellant, making them heavier, and so on.
Previous research has suggested that "light sailing" might be one of the only feasible ways to get a spacecraft to another star within a human lifetime. Although light does not exert much pressure, scientists have long suggested that what little pressure it does apply could have a major effect. Indeed, many experiments have shown that "solar sails" can rely on sunlight for propulsion if the spacecraft is light enough and has a big enough sail.
Indeed, the $100 million Breakthrough Starshot initiative, announced in 2016, plans to launch swarms of microchip-size spacecraft to Alpha Centauri, each of them equipped with extraordinarily thin, incredibly reflective sails propelled by the most powerful lasers ever built. The plan has them flying at up to 20% the speed of light, reaching Alpha Centauri in about 20 years.
However, Starshot faces many technical challenges. These include building lasers powerful enough for propulsion and creating sails that can withstand extraordinary forces and stay on track to their targets.
George Church, Ph.D., is a genetics professor at Harvard Medical School and the Founding Core Faculty and Lead for Synthetic Biology at the Wyss Institute of Harvard University. He is also a Professor of Health Sciences and Technology at Harvard and the Massachusetts Insitute of Technology and serves as Director for both the U.S. Department of Energy Technology Center and National Institutes of Health Center of Excellence in Genomic Science.
In addition, even if Starshot successfully launches "space-chips" at Alpha Centauri, without another laser at that destination, there is no way for them to slow down. This likely limits Starshot missions to flybys instead of landings.
Any Starshot probe attempt to land would likely prove catastrophic. Although the spacecraft are designed to be extraordinarily lightweight — each just 0.035 ounces (1 gram) or so — when traveling at 20% the speed of light, they would each pack as much energy as one-eight the atom bomb dropped on Hiroshima in World War II, Church noted.
Instead, Church suggested using probes a billion times lighter. If they did make impact, it would only pack as much energy as half a food calorie, he noted.
"A probe that lands is tremendously more valuable than one that flies by at great distance and for a very brief time," Church told Space.com.
Picogram interstellar probes
How might such an incredibly light probe prove useful? If they carried genetically modified microbes, they could build themselves equipment upon landing, Church suggested.
Previously, Church has made a number of radical proposals that sound like science fiction. For example, he suggested DNA could help detect dark matter, the invisible and largely intangible substance that researchers suggest makes up about five-sixths of all matter in the universe. He also wants to resurrect extinct beasts such as the woolly mammoth.
However, Church is also a pioneering biologist. In 1984, he developed the first direct genomic sequencing method, which resulted in the first genome sequence, that of Helicobacter pylori, a bacterium normally found in the human stomach. He also helped initiate the Human Genome Project in 1984 to completely map the roughly 3 billion letters contained in human DNA.
Church noted that he became interested in this new idea because of how he grew up in Florida in the shadow of Cape Canaveral rocket launches, and because he teaches a course at MIT called "How To Grow Almost Anything." As such, he was "looking for projects that push that envelope," he said.
Previously, scientists have suggested creating interstellar "von Neumann" probes that can replicate themselves and equipment. The concept is named after mathematician John von Neumann, who proposed the idea of self-replicating machines in 1948, Church noted.
Church based his new proposal both on his experience in biology and the pioneering research conducted for Starshot. Since his probes are only about one-billionth the mass of Starshot craft, he suggested that a billion of his probes could be launched for a similar cost to a single Starshot mission.
Starshot also calls for a 100-gigawatt laser array, which would be by far be the most powerful laser humanity has ever constructed. Since Church suggested extraordinarily tiny probes, a relatively modest laser might suffice, he said. For example, a mothership about 0.0014 ounces (40 mg) in mass with a 1.3-foot-diameter (0.4 meter) sail that carries many tiny probes might only require a 2-gigawatt laser array.
Starshot's probes often call for a sail about 108 square feet (10 square meters) in size with a mass of less than 0.035 ounces (1 gram). In comparison, given how a typical bacterium has a mass of about 1 picogram, or one-trillionth of a gram, it would only require a sail about 15 millionths of a square inch (0.0001 square centimeters) in size with a mass of about 7.6 picograms, Church said. He added that light sails about 8.8 millionths of an ounce (0.25 milligrams) in mass have already been tested in vacuum and in microgravity.
"Deceleration is hard even for picogram scale, but not even under consideration for gram scale," Church said.
Interstellar probes would likely experience impacts that could cripple or destroy them — from dust grains, or even hydrogen atoms. However, the fact that one could launch a billion or so microbial probes for the cost of one Starshot craft means that losing probes might not prove a major setback.
A living probe with 'biolaser'
After the probes reached their destination, Church suggested that genetically modified microbes could build themselves communications modules. One strategy to communicate might be bioluminescence, with which microbes could emit light using the kinds of molecules found in fireflies or other naturally bioluminescent organisms. Although this light might be relatively dim, Church noted that given no predators and ideal growing conditions, microbes could cover an entirely planetary surface in just 124 hours.
For a more compact approach, Church suggested a living probe might create a "biolaser" capable of converting starlight into a communication beam. He noted that the gold beetle (Aspidimorpha tecta) can build reflective surfaces potentially useful for creating such an organic device, although Church conceded that building it "would be an interesting laboratory challenge."
Church suggested the communications array these probes build could transmit flashes back at Earth. These beams could encode data about the destination site such as temperatures, pressure and pH.
It might prove difficult to find places for these interstellar seeds to grow. "This is why we want millions of shots on millions of target sites," Church said. Scientists could also rely on so-called "extremophile" microbes known to survive extremes of temperature, pH, pressure and other conditions on Earth, Church said.
Church noted that one potential target might be the closest known exoplanet—Proxima Centauri b, a rocky world in the Alpha Centauri system. However, it receives only 3% of the kind of light useful for photosynthesis, which could make it difficult for any microbial probes to thrive there. It could also potentially experience 10,000 more flares from its star capable of stripping off any atmosphere, making it a hostile place to try and live.
Other potential targets include worlds that may exist around the sun-like stars Alpha Centauri A and B in the Alpha Centauri system. These may not be rocky planets—instead, they may be more similar to Uranus and Neptune, and covered in water and ammonia. However, there are microbes on Earth that could survive in such locales, such as bacteria found in deep-ocean hydrothermal vents.
One major concern would be planetary protection issues — in this case, making sure that Earth microbes do not inflict damage on any alien life that might exist at destinations. Probes can be designed to "aim for strictly limited amount of growth," Church suggested. An "absolutely high priority" would be testing any potential interstellar probes at targets within the solar system first to see how well they perform, he added.
Church detailed his idea (opens in new tab) Dec. 6 in the journal Astrobiology.