Asteroids, meteorites and difficult issues to solve


April 13, 2023
By Giancarlo Elia Valori

Mankind is ready to start exploring space resources. Such resources seem unusually attractive today, but what are the real possibilities of space travel for their extraction?

The most sought-after mineral resource in space is water, which will be the source of oxygen and hydrogen needed for astronauts’ breathing and for rocket thrusters. An equally valuable resource is the Moon as a territory outside our planet, where manned stations with fully controlled living conditions can be located. Astronautics can begin to develop these resources in the near future.

As seen above, small bodies in the solar system – mainly the numerous asteroids – are becoming increasingly attractive as sources of raw materials as space technology develops. Meteorites sometimes fly to Earth from the depths of space, some of which consist of a pure iron alloy with nickel and cobalt.

Spectrophotometric observations of asteroids, however, show that the nature of the reflection of sunlight on their surface is almost the same as that from the surface of meteorites, which are further interesting celestial bodies. Hence we can infer that the composition of asteroids must be identical to meteorites. If this is true, then meteorites, along with asteroids, can also theoretically contribute to the development of the Earth’s depleted mining industry.

The annual world production of iron is currently estimated at one billion tonnes. The same amount of iron – some people say half as much (albeit a considerable amount) – can be contained in an asteroid about 300 metres in diameter. In the solar system, the number of asteroids of this size is estimated at 7,500 – hence the expanses of space look to many like a modern-day pristine Klondike. The presence of precious metals for industry, which have a high commercial value, should be added to this.

Most asteroids in the solar system are located in the so-called main asteroid belt, situated between the orbits of Mars and Jupiter. But some of the asteroids have orbits that approach Earth’s orbit and pass through it at a distance of several million kilometres. These asteroids may be of particular interest as they are substantially more accessible than the asteroids in the main belt.

The National Aeronautics and Space Administration pays great attention to the study of asteroids. It has carried out a number of successful missions that have laid the foundations for contact methods for their study. NASA has also initiated the development of projects to intercept asteroids in deep space and tow them into near-Earth space for industrial exploitation. However, there is still no certainty about the economic feasibility of using asteroids as raw materials in space.

Information on the physical and chemical properties of asteroids is scarce and needs to be updated. Remote methods for studying celestial bodies – systems developed in astronomy – make it possible to study the optical properties of the surface of asteroids. In particular, all hypotheses on the chemical composition of asteroids are based on a comparison of spectra obtained from asteroids and meteorites, the composition of which is reliably known.

It is important to remember that from the spectra of only about 70% of the asteroids analysed, it is possible to find traces of the meteoritic substance, but these are only indirect signs of their similarity. If we adhere to the widespread idea among astronomers that asteroids are constantly struck by meteorites and micrometeorites, it is easier to assume that the surface properties of asteroids are characteristic of the meteorites accumulated on it, and not of the main body of the asteroid itself. Therefore, the opinion based on indirect evidence that an asteroid consists of pure iron or platinum requires careful verification and even the sending of a reconnaissance mission, not to mention the solution of the technically more expensive and more complex task of intercepting an asteroid and delivering it to near-Earth space.

The second circumstance, which greatly complicates the design of devices for studying the contact of small solar system bodies, is associated with the inconsistency of today’s scientists’ ideas about the structure of asteroids. It is widely believed that asteroids that had suffered numerous collisions with each other (as evidenced by the many impact craters on their surfaces) should have been destroyed as a result of the aforementioned impacts and are now instead “piles of stones” once again joined into a single body by the mutual gravitational force of the fragments. This more than controversial view, based on only a few indirect observational data, is also persistently applied to explain the surface structure of the asteroids examined in detail.

If we agree with this widespread idea among astronomers that asteroids are constantly hit by meteorites and micrometeorites, we can assume that the surface properties of asteroids are characteristic of the meteorites accumulated on it, and not of the main body of the asteroid itself.

The Japanese researchers’ claim that asteroid 25143 Itokawa (with an average diameter of about 0.33 km) is also a “pile of stones” seems very strained and far-fetched. To do so, they were forced to assume that the entire small fraction of “fragments” covered a part of the asteroid in a uniform layer and were not preserved in other parts of it. They came to the conclusion that the asteroid surface is entirely solid and does not contain regolith. This is a blanket of unconsolidated, loose and heterogeneous surface deposits covering solid rock. It includes dust, broken rocks and other related materials and is present on Earth, the Moon, Mars, some asteroids and other planets and their moons.

In this respect, studies of the Moon surface have clearly shown that the lunar regolith layer is very thin, even under conditions of much stronger lunar gravity, and dust particles should also be ejected from the surface of small asteroids by micrometeoroid impacts.

From the viewpoint of astronautics, the property of asteroids whereby there is a completely negligible gravitational force on their surface is very important. With such acceleration in free fall, even a small force can propel the entire apparatus out of the asteroid’s region of attraction. The developers of the European Space Agency’s (ESA) Rosetta mission faced a similar problem when they landed the Philae module on the surface of 67/P Churyumov-Gerasimenko, which is a periodic comet in our solar system with an orbital period of 6.45 Earth years. The moderate landing speed of the module proved sufficient for the elastic forces to launch it off the surface of the cometary nucleus, and only after several jumps did Philae stop at a completely different spot where its soft landing was planned. This meant that the ESA designers had estimated the strength of the dust layer on the surface of the cometary nucleus differently.

Therefore, for the same reason as the low gravity of and on asteroids, we should be wary of the very idea that asteroids destroyed in many fragments as a result of mutual collisions can reassemble into a single body under the influence of mutual attraction.

This is because the force of attraction between the individual stones is fully negligible, and its action simply could not be sufficient to slow down the flying fragments. The asteroids of the main belt, instead, are most probably monolithic bodies due to melting and solidifying in remote times.

These examples clearly show that an accurate knowledge of asteroid surface properties is a prerequisite for conducting research missions to them, not to mention solving the problem of towing an asteroid. This implies that some plans for using the space resources available in asteroids are premature.

The outcome on the uncertain docking – rather than landing – of Philae of the Rosetta mission shows that in order to conduct real research missions to asteroids and comets, it is necessary to create a docking system for a small-mass celestial body, which would be equally effective for a monolithic asteroid and a loose comet nucleus, or for a hypothetical pile of rocks.

In preparation for missions to the Moon and Mars, a method was developed for landing containers with scientific equipment (and drillers) at a speed of hundreds of metres per second. Scientists have gone even further and should have found a way to safely bring the most delicate scientific equipment to the surface of the bodies studied after the containers landing hard at a cosmic speed of up to several kilometres per second.