Pioneering settlers on a distant world will require food, fuel and shelter if they are to survive, but bringing bulky supplies from Earth is far too costly. Synthetic biology offers another option. Microbes weigh precious little, and would take up next to no space on a spacecraft, but once the mission lands - on Mars, say - they could multiply by feeding on the materials available there. The products of their labour could provide the building blocks essential for a human settlement.
NASA has already begun research to realise this dream, says Lynn Rothschild at the Ames Research Center in Moffett Field, California. Rothschild is leader of NASA's new Synthetic Biology Initiative, which aims to build designer microbes for future crewed space missions. She shared her vision at last week's BioDesign Forum in Cambridge, UK.
Synthetic biology lies at the crossroads of biology and engineering. Its practitioners have built a biological toolkit consisting of chunks of genes, called biobricks, each of which performs a specific function - making a bacterium generate natural antifreeze molecules, for example. Biobricks can be inserted into other microbes to give them that function.
Using the approach, a microbe with the potential to survive on an alien world can become one that could sustain human life there.
Take the need for energy. Many earthly microbes would die in extraterrestrial atmospheres rich in carbon dioxide and nitrogen - the two main constituents of Martian air. An ancient cyanobacterium called Anabaena thrives in those conditions, though, metabolising both gases to make sugars. "As long as it has warmth and some shielding from ultraviolet light radiation, it should do well on gases in the Mars atmosphere," says Rothschild.
Naturally enough, Anabaena uses most of the energy it produces from CO2 and nitrogen, but synthetic biologists can encourage the cyanobacteria to share its supplies. Last year, at a synthetic biology competition - International Genetically Engineered Machines (iGEM) - a team from Brown University in Providence, Rhode Island, and Stanford University in California showed how inserting genetic machinery from E. coli makes Anabaena excrete more of its energy as sugar. The team even showed that they could support colonies of other bacteria on the sugar. In theory, such microbial colonies could make oil, plastics or fuel for the astronauts.
The team, led by recent Brown graduate André Burnier and advised by Rothschild, has also come up with a way to supply human settlers on Mars with bricks and mortar. They began with a bacterium called Sporosarcina pasteurii, which, unusually, breaks down urea - the principle waste product in urine - and excretes ammonium. This makes the local environment alkaline enough for calcium carbonate cements to form.
The idea is that the waste produced by astronauts could feed the microbes. The microbes, in turn, would help cement together fine rocky material on a planet's surface to create bricks.
As a proof of principle, Burnier's team confirmed in experiments that loose material can be cemented together in about two weeks to create a house brick with the compressive strength of concrete. They also managed to isolate the cement-building genetic component of the bacterium, creating a biobrick that they have inserted into E. coli to give this hardy bacterium the same cement-enabling properties.
The proposals are compelling, says Jim Haseloff, a synthetic biologist working on plants at the University of Cambridge.
"Every gram delivered to Mars or other planets translates into huge additional costs and energy demands," says Paul Dear at the MRC Laboratory of Molecular Biology in Cambridge. "Biology rather than physical engineering is the only realistic way to do things on a planetary scale."
Dear cautions that it would be cavalier to introduce earthly bugs into alien environments before we know whether such planets have, or have ever had, microbes of their own. But it will be decades before a bug box is used by astronauts, says Rothschild, making the contamination point moot for now.
"The most appropriate way forward would be tests on robotic missions," she says. Only after they've been tested successfully by the robots would bug boxes be considered for crewed missions.
Dear agrees with the robot-first approach. "It takes a lot of faith to trust your life to a bacterium."