What to do if you run out of food in space? Maybe you can "eat" asteroids!

Produced by: Science Popularization China

Author: Earth's Gravity

Producer: China Science Expo

Editor's note: In order to expand the boundaries of cognition, the China Science Popularization Frontier Science Project has launched a series of articles on the "Unknown Realm", which provides an overview of the exploration results that break through the limits in deep space, deep earth, deep sea and other fields. Let us embark on a journey of scientific discovery and get to know the amazing world.

Where does astronauts get their food?

At present, long-term space travel is still very difficult for humans. In addition to the fact that the confined space may cause psychological damage to astronauts, how to strike a balance between personnel, water, air and food within the limited load of the spacecraft is also a big problem.

If these substances necessary for sustaining life are completely replenished by the earth, it will not only mean huge costs, but also mean that our exploration of space is doomed to not go far... For example, according to a scientific study, the food requirements of six astronauts on a mission to Mars will weigh 12 tons (excluding the net weight of packaging), and even for SpaceX, which has a lower transportation cost, the cost per kilogram of payload is as high as US$2,720.

If manned exploration of Mars is still something we can achieve with a little effort, then the cost of exploring more distant planets such as Jupiter and Saturn, or even the outer solar system, in the future will be so high that it will be completely unaffordable, and the timeliness of supplies will be very low.

Because of this, people have never stopped researching space food. For example, the space farm that is easiest to implement under current conditions - growing and harvesting plants directly in a space station or spacecraft - has been experimented many times in two space stations in China and the United States, and a variety of vegetables have been cultivated in space. American astronauts have even eaten vegetables such as lettuce, carrots and peppers that they grew in space.

Vegetables grown on China's space station

(Photo source: CCTV)

In addition to vegetables, people are also trying to grow algae, mushrooms, and insects in space. However, these breeding or planting equipment require complex design and long-term maintenance to simulate the Earth's ecosystem so that animals and plants can grow normally in the equipment. In addition, if we want to achieve food self-sufficiency in space, the current equipment output is far from enough, and we need to increase the size and number of equipment, which will undoubtedly take up a lot of space in the spacecraft.

Veggie, a vegetable-growing device on the International Space Station

(Image credit: NASA)

Peppers grown on the International Space Station

(Image source: ISS Research)

In order to find a simpler method that saves space and cost, some scientists have turned their attention to asteroids - mining organic matter from asteroids, feeding it to bacteria after simple processing. The bacteria will digest them and form organic matter that is edible to humans.

No kidding, there are organics in asteroids!

Humans have been studying asteroids for hundreds of years - although we couldn't obtain samples directly from asteroids at the time, this did not prevent them from coming to us on their own - every year a large number of asteroid fragments fall to the earth, which we call meteorites. According to estimates, the number of meteorites falling each year is as high as 17,000.

Through long-term research, scientists have divided meteorites into three categories according to their composition: stony meteorites, stony-iron meteorites, and iron meteorites. Based on the specific structure and composition of meteorites, these three categories of meteorites are further divided into more meteorite groups. For example, under stony meteorites, they are divided into chondrites and achondrites according to their structure. Chondrites refer to the type of meteorites with spherical particles. According to statistics, 86% of the meteorites that fall on the earth are chondrites.

Chondrules from the Allende meteorite

(Image source: Wikipedia)

The chondrules in these meteorites are believed to have been formed by direct cooling of nebula materials at the beginning of the formation of the solar system. After their formation, the small chondrules combined to form small asteroids, which then collided and grew to form the rocky planets in the solar system today. Those asteroids that failed to form rocky planets are concentrated in the asteroid belt.

Due to the high proportion of chondrites falling on Earth, it can be inferred that the composition of most asteroids in the asteroid belt is similar to that of chondrites. And because chondrites are one of the oldest solid substances in the solar system, they are of great significance for studying the early history of the solar system. Scientists are very concerned about the study of chondrites and have conducted detailed analysis of their composition and structure.

They divided chondrites into 15 different meteorite groups (CI, CM, CO, CV, CK, CR, CH, CB, H, L, LL, EH, EL, R and K), among which those starting with C are classified as carbonaceous chondrites. These meteorites contain very high concentrations of organic compounds, and in some meteorites, organic matter may account for about 5% of their weight.

Among them, the carbonaceous chondrites that have been studied in depth include the Murchison meteorite and the Tagish Lake meteorite. Scientists have discovered a variety of small-molecule organic matter in them, including ketones, alkanes, carboxylic acids, amino acids, methane, and polycyclic aromatic hydrocarbons. However, most of these organic matter are large-molecule organic matter. The main organic components of the Murchison meteorite and the Tagish Lake meteorite are shown in the figure below.

Figure: The main organic components of the Murchison meteorite and the Tagish Lake meteorite (Image source: Homemade)

When these organic substances were first discovered, they naturally sparked global cheers, which made people believe that there was life on other planets, and even that life on Earth came from other planets, because people believed that only life processes could form organic substances. However, detailed studies soon found that chemical reactions could also form organic substances in natural conditions. For example, the formation of amino acids only requires some simple inorganic substances:

Chemical reaction equation for the formation of amino acids (rightmost chemical formula) from simple inorganic substances

(Image source: Wikipedia)

In addition, other evidence that these organic substances are formed by natural chemical reactions includes: there are differences in molecular structure between meteorites and the same organic substances on Earth, and many are isomers (same molecular formula, different structure); different chirality, the organic substances in meteorites have both left-handed and right-handed, but the organic substances formed by life on Earth only have left-handed structures.

Schematic diagram of the chirality of amino acids. Their structures are mirror-symmetrical, but they cannot be superimposed together by translation.

(Image source: top, Wikipedia, bottom, author)

And with the advancement of science and technology, scientists have also begun to find signals of organic matter in distant nebulae. All this tells us that organic matter is widely present in the universe. Today, the existence of organic matter in meteorites and the universe has become common knowledge in the scientific community.

So the question is, how can we "eat" asteroids?

Since the organic matter with the largest content in these meteorites are some macromolecular organic matter similar to plastic, it is definitely unrealistic to eat them directly, so scientists have borrowed from a recently realized microbial treatment experiment of plastic. In this experiment, people heat plastic (400-900℃) to pyrolyze, so that the macromolecular long-chain organic matter is broken down to form a series of low molecular weight hydrocarbons, and then use bacteria to treat these hydrocarbons. It was found that the bacteria can digest these hydrocarbons normally and reproduce in large numbers.

Scientists believe that in the future, astronauts will also use pyrolysis to process mined asteroid minerals rich in carbonaceous spherules, and then use bacteria to digest these substances. Since bacteria grow extremely fast, they will continuously provide astronauts with sufficient food.

In addition, just this year, scientists discovered through experiments that if meteorites are directly crushed into powder, under oxygen-deficient conditions, some bacteria of the Pseudomonas family can even directly use these meteorite powders to survive and reproduce for a long time.

These experiments all prove that using bacteria to "eat" asteroids and humans to eat the biomass produced by the bacteria may be a promising space food solution.

**In order to find out how much organic matter asteroids can provide, scientists used the asteroid Bennu (101955 Bennu) as an example for calculation. **The asteroid Bennu is one of the two asteroids that humans have landed on and retrieved samples. The other is Ryugu (162173 Ryugu). Bennu has a diameter of less than 500 meters and a mass of 77.6 million, and its composition is similar to that of carbonaceous chondrites.

Asteroid Bennu

(Image source: Wikipedia)

After calculations, they found that the biomass produced by the asteroid Bennu alone, under the lowest efficiency conditions, is enough to feed 631 astronauts for one year, and under the highest efficiency conditions, it is enough to feed 17,000 astronauts for one year.

After conversion, scientists found that under the lowest efficiency scenario, in order to supply an astronaut's food needs for one year, about 160,000 tons of asteroid minerals would need to be processed. Under the highest efficiency scenario, only 5,000 tons of asteroid minerals would need to be processed.

The minimum (orange box) and maximum (red box) amounts of food that the asteroid Bennu can provide, calculated

(Image source: Reference 1)

Although this research seems promising, wouldn't it be a bit tragic if future astronauts really had to survive on bacteria while performing long-term missions?

More exploration is needed in the future

Discovering the potential value of asteroids as a food source for future astronauts is not only an innovation in the traditional space food supply system, but also an important step for humans to adapt to extreme environments and realize the dream of interstellar travel.

Of course, there are still many technical challenges and ethical considerations in converting asteroids into food sources. Let us continue to move forward on this road of space exploration full of unknowns and miracles with an open mind and unremitting efforts. Perhaps in the near future, asteroids will no longer be just lonely travelers in the universe, but close partners of human exploration of the universe.

References:

1.Pilles E, Nicklin RI, Pearce J M. How we can mine asteroids for space food[J]. International Journal of Astrobiology, 2024, 23: e16.