Asteroids: how are they helping researchers with space exploration?

Asteroids. Publicly feared, invaluable to science. They provide evidence for how our Solar System was formed, have been involved in shaping life on Earth, and will likely help us conquer space in the future. Every year, International Asteroid Day falls on 30 June. The date was first announced by the United Nations eight years ago and was not chosen randomly. The article below, originally as “Asteroids: Danger and opportunity” was first published in the CAS Magazine A / Věda a výzkum.

Asteroid Day refers to June 1908, when an astronomical object of approximately 50 metres in diameter crashed into an uninhabited central part of Siberia. On its descent, it could not withstand the atmospheric pressure and exploded several kilometres above the Siberian taiga, uprooting and setting fire to forests over the span of 2,000 km². The explosion of approximately 30 megatons was roughly 2,000 times more destructive than the Hiroshima atomic bomb and comprises the largest and most devastating impact event of a space object in our modern history.

Why do we commemorate the Tunguska catastrophe, as it was described by the contemporary press? International Asteroid Day is meant to raise public awareness of these celestial objects, to remind us that threats from space are not just the stuff of fantasy for the silver screen, and that it makes sense to put the effort and time into finding ways to protect our planet from such disasters in the future.

At the cosmic shooting range
Alien astronomical objects enter our atmosphere with surprising frequency. On average, it’s about once per month, typically celestial bodies one metre in size. “Most of the time, however, these are smaller objects that burn up once they enter our atmosphere,” explains Petr Pravec from the Astronomical Institute of the CAS. Researchers refer to them as bolides. When their more solid remnants survive the fall and reach the Earth’s surface, they are then called meteorites.

Petr Pravec is leading the team researching minor planets at the Astronomical Institute of the CAS. (CC)

On average, objects larger than 20 metres in diameter hit the Earth’s surface about once every 100 years, while even larger ones, up to 600 metres in diameter, appear once every 500,000 years – these can already have a damaging global effect. A truly massive body, such as the ten-kilometre Chicxulub asteroid, will appear on our planet’s horizon on average once every 100 million years.

A collision with a so-called near-Earth asteroid, such as we know from disaster movies, may be unlikely but certainly is not impossible. Even large astronomical bodies pass our planet almost constantly. At the distance of geostationary satellites, an object of ten metres passes by the Earth on average once every 70 days. And at a distance that equals that between our planet and the Moon, an asteroid one kilometre in length passes us once every 150 years.

Of course, this does not mean that there is an ongoing countdown since the last space catastrophe. The appearance of an asteroid is always guided by chance. However, it is advisable to keep an eye out for potential danger. That’s why NASA initiated the Asteroid Watch plan years ago to monitor and map 90% of near-Earth objects that are over 140 metres in size. Experts believe that objects of similar or larger sizes could pose a risk to our civilisation. In the past, small asteroids were often detected by the tracking instruments less than 24 hours before they passed in close proximity to Earth.

Social creatures
The vast majority of minor planets (or planetoids), which is a more accurate name for asteroids, are concentrated in a few regions of the Solar System, known as belts. The closest one, called the main asteroid belt, spans the space between the orbits of Mars and Jupiter, i.e., between two and four astronomical units from the centre of our system, with one astronomical unit equalling roughly the distance from Earth to the Sun. The shape of the main belt resembles a donut or an inflatable lifesaver about 150 million kilometres wide – also known as a torus. Estimates suggest that there may be as many as one billion objects in this belt. Some of them are no bigger than a pebble, but there are also millions of objects that have more than one kilometre in diameter.

Ceres, with its 945 kilometres, is the largest known inhabitant of the belt, followed by Pallas, Vesta, and Hygiea. Together, these four alone account for 62% of the total mass of all physical material in the belt. Ceres alone comprises ca 39% of the total mass. At the same time, if we placed all the minor planets here, dust, particles and all, on a pair of scales, they would still weigh less than 5% of the mass of our Moon.

This is because the belt is by no means solid. On the contrary: its mass is so thinly dispersed that if a person were standing on any asteroid in the belt, they would be no other object in plain sight. The average distance between them is about one million kilometres. Therefore, it poses no problem for the probes we send into space to fly safely through the main asteroid belt.

The dwarf planet Pluto is the largest object in the Kuiper belt.

Minor planets cluster not only in this main belt, but also in the much more distant Kuiper belt, which is beyond Neptune’s orbit. Its nearest point is about 30 astronomical units from the Sun. The Kuiper belt is about 20 times wider than the main asteroid belt and has up to 200 times its mass. Since its discovery in 1992, researchers have mapped about a thousand mostly large objects that are part of the belt, which is also estimated to harbour over 70,000 objects measuring over 100 kilometres. These are so-called transneptunian bodies and include the trio of dwarf planets – Haumea, Makemake, and Pluto, which is the largest object to be found in the belt.

They look like a star
The discovery of the first asteroid, the aforementioned Ceres, dates back to 1 January 1801. That is when, at around 8 PM, the monk and astronomer Giuseppe Piazzi, founder of the observatory in Palermo, Italy, made the momentous discovery. In one section of the Taurus constellation, he observed a small bright point, some sort of relatively faint star. He recorded its exact position and, as was his custom, the next day he looked again at the same part of the sky. However, he did not find his star again, at least not in the same place as before – it had moved. At first, he attributed the shift of the object to an error in his own records from the previous evening, but on the night of 4 January, as he observed the object kept travelling steadily across the sky, he concluded that he had discovered a new comet. That same day, the news spread across Italy and beyond.

Although Piazzi let the public know of the discovery, it took him some time before he decided to write to two of his foreign colleagues, renowned astronomers, one of whom he was personally friends with. It was to him that Piazzi confided his doubts in his letter: the comet, although close to the Sun, lacked a surrounding dust cloud, and what’s more, its movement was too slow and uniform for this type of object. He was not alone; many other experts also noticed the oddity and began to question the nature of the object: what was it that the Italian had actually seen through his telescope?

The German astronomer Johann Elert Bode had his own opinion about the enigma, suspecting that Piazzi’s comet was in fact a hitherto unknown planet in the Solar System located between Mars and Jupiter. He had been trying to prove this hypothesis for many years. According to the published records, he located the object in the sky and named it Juno.

However, the international astronomical community didn’t accept this name. The new celestial body ought to always be named by its rightful discoverer. Piazzi chose the name Ceres Ferdinandea. The first part of the name referred to the Roman goddess of harvest and patron saint of Sicily, while the second was in honour of the then ruler, Ferdinand III of the Kingdom of Sicily. However, the reference to the king was eventually removed by Piazzi due to objections; simply because the name was too long. And so only Ceres remained.

What shall we call them?
A year later, when the famous British astronomer William Herschel presented the latest advances in the observation of space objects to the Royal Society (of London for Improving Natural Knowledge), he carefully distinguished between the classical planets and a new type of astronomical body that appeared as bright as a star but did not have the characteristics of a large planet. He called it an asteroid, from the ancient Greek ἀστεροειδής (asteroeidēs), loosely translatable as “star-like” or “star-shaped”. He thus inserted into its name a reference to the way in which this celestial body was discovered. “Historically, the term ‘asteroid’ refers more to the observed properties of these bodies rather than the material and physical ones,” Pravec explains.

However poetic Herschel’s coining of new space objects may sound to us, it took more than a century for the term to catch on, and as late as the beginning of the second half of the nineteenth century, scientists were still using the terms “asteroid” and “planet” interchangeably.

The nomenclature then changed during the twentieth century. Like planets, asteroids orbit the Sun but are substantially smaller. Furthermore, they have much weaker gravity and zero atmosphere. These and additional differences led to the introduction of a more precise term: minor planets. So when we talk about asteroids, we are using a popular but actually somewhat outdated term. To be precise, we should call the celestial bodies minor planets.

Springing up like mushrooms
The first discovered Ceres did not remain by itself in the asteroid (or minor planet) catalogue for long. Soon the world learned of the existence of the minor planet Pallas. In 1804, Juno was added, and three years later, Vesta. The latter, thanks to its very bright surface, remains to this day the only known object of this type that humans can observe from Earth with the naked eye.

The dwarf planet Ceres is located in the main asteroid belt of the Solar System. It’s an impressive 945 kilometres long.

As the years went by, more observations were made, although sometimes nearly four decades passed between every discovery. By the end of the nineteenth century, however, scientists already knew of over three hundred minor planets. With the thousandth body discovered in 1923, we return to the beginning of our story: with its name, Piazzia pays tribute to the Italian astronomer who was the first to spot these cosmic objects.

Since the beginning of the twenty-first century, the study of minor planets has experienced a true renaissance, and since then the list has grown exponentially; most dramatically in the last ten years. In that time alone, researchers discovered more than half a million minor planets and, for a number of them, were able to describe their physical properties more precisely. To put this in perspective: in the early 1990s, we knew only of approximately 10,000 objects, and by the turn of the millennium, we knew of 20,000. At the time of writing this article (2022), the count of discovered asteroids was 1,113,527.

Space junk
The fact that minor planets cluster in the belts or gravitational fields of large planets gives astronomers a clue as to how they actually formed. In the very early days of the Solar System, cosmic dust and other material orbiting our central star accumulated thanks to gravity and coalesced into planets. However, not all of it was consumed, and a major asteroid belt formed in the broad region between Mars and Jupiter. Farther away from the centre, beyond Neptune, the Kuiper belt was formed. It consists of the residual material that makes up our Solar System.

Minor planets, or asteroids if you prefer, orbit the Sun in elliptical orbits, much like the planets of the Solar System. But unlike them, their gravity is too small to have been able to “clear” their orbit around them the way the big planets did. Main-belt asteroids, however, are under the influence of the gravity of the big planets and can be deflected from their safe orbits. “Minor planets don’t usually come close to Earth. But they can reach it due to gravitational disturbances,” Pravec explains.

Not only a danger, but an opportunity, too
For these reasons as well, it is important to study the mechanics of asteroids, their interactions, and the dynamics of their journeys through space. Teams of experts from the Astronomical Institute of the CAS are studying the behaviour of the most common small asteroids – so-called rubble piles. These are objects that are not “made of one piece”, but coalesced rock (small pieces), held together by gravity alone which, however, is relatively weak, because these are not very large bodies. Thus, for instance, if the rubble pile were to fly past a very massive object, this object could even alter the shape of such an asteroid.

Another important influence is solar radiation. It hits the surface and, because of the abovementioned structure, the object heats up unevenly. The heat energy is then radiated back into space – for instance, when the far side is cooling. But because this occurs anisotropically (the radiation is oriented in the same direction), it results in several different effects: the asteroid’s orbit may change, the orbit may become unstable, or the asteroid may even set off into a spin. And if it starts rotating too fast, gravity can no longer hold it together and the rubble pile breaks apart. This is how so-called asteroid pairs (or clusters) are formed.

Petr Scheirich works at the Department of Interplanetary Matter at the Astronomical Institute of the CAS. (CC)

That’s what the experts at the Astronomical Institute of the CAS in Ondřejov are working on, among other things. Such bodies travel separately through space, each following its own trajectory, but their movement is very similar due to their common origin. “Today we know for sure that such objects were formed either via the splitting of a larger parent body or the break-up of a system of orbiting asteroids,” explains Petr Scheirich, also from the Astronomical Institute of the CAS.

Last year, his colleague Petr Fatka and his team managed to discover and study a very young, until then unknown asteroid pair. The two bodies, designated 2019 PR2 and 2019 QR6, with similar orbits around the Sun, were formed about 300 years ago, probably by the break-up of a larger asteroid. The astronomers found that at least one of the objects must have exhibited comet-like activity in the past, and they revealed that volatiles, probably water and carbon monoxide, were present in its composition.

Storage of space chemicals
We talked about asteroids as space debris, but we cannot neglect the important role they play within the “cosmic ecosystem”. They have influenced not only the surface of our planet, but probably also its evolution. Researchers say that without asteroid impacts on Earth in the distant past, humankind wouldn’t be here. From earlier studies of the age of craters on the Moon, experts know that numerous asteroid impacts on the Earth’s surface had occurred during the time of the first life on our planet. It was these occurrences that probably became the source of life-giving water, volatile atmospheric constituents, and perhaps the driving force behind life-giving chemical reactions.

Nucleic acids, the basic building blocks of organisms, appeared on our planet some four billion years ago and may have been formed by chemical reactions for which asteroid impacts were the energy source. This hypothesis was proven by researchers from the J. Heyrovský Institute of Physical Chemistry of the CAS by experimental means. “Asteroid impacts, together with lightning discharge, high temperatures, and pressure. These are the conditions in which the first biomolecules were formed,” astrochemist Martin Ferus, who led the research team, describes the conditions of the experiment.

Martin Ferus heads the Department of Spectroscopy at the J. Heyrovský Institute of Physical Chemistry of the CAS, currently working on the Czech space mission SLAVIA. (CC)

On Earth, similar processes occurred as a result of the impact of billions of asteroids spanning over hundreds of millions of years. Obviously, the researchers did not have that much time at their disposal. “In the lab, we simulated such conditions with high-powered lasers in millionths of a second,” he adds.

The composition of asteroids not only reveals our planet’s past but gives us insight into the structure of our entire Solar System. Because asteroids contain remnants of the material that formed our system, they are a valuable source of information about places and landscapes that human telescopes and probes cannot reach. In the past, was there water, oceans, conditions suitable for life on Venus? A mission that could find the answer is something for the distant future; elemental analysis of the surrounding planets will offer it much sooner.

The future of astronautics
Researchers are interested in the composition of asteroids not only for the theory, but also for their possible practical uses in the years to come. On space missions, humans have no choice but to take all the materials they need into orbit. But in fact, many useful materials can be found in space – and as luck would have it, especially on asteroids.

Asteroids are composed mostly of silicates and metallic substances like iron and nickel. The most important raw materials for the space industry are those that are difficult to carry into space, and they are usually of relatively little value on Earth. These are titanium, chromium, and oxygen. The most valuable substance in space, however, is something quite common on Earth: water. According to European Space Agency (ESA) guidelines, water is the most limiting factor in further space exploration, the prospective colonisation of the Moon, and missions to Mars. The ESA’s long-term roadmap for exploration envisages the beginning of asteroid mining between 2050 and 2060. Rare metals will be made use of first, with gases and water coming later into play, when a way to transport them more efficiently will be found.

There has been an ongoing serious debate about the exploitation of asteroid mineral wealth since the 1950s, and since 2015, the matter has also been addressed legislatively. Companies are already working on research into mining technologies, but it is questionable whether the current generation will live to see this put into practice. “If it turns out that mining in space is more efficient than that on Earth and could replace the terrestrial mining industry, the interest of companies will bring us much closer to this goal,” Martin Ferus points out.

Czech traces in space
The first step is a detailed mapping of potential mineral deposits and the selection of suitable asteroids. Researchers from the CAS are involved in this important task via the SLAVIA project, which has the potential to become the first 100% Czech ESA mission.

The aim of the SLAVIA mission is to map the sources of raw materials in space. It should send two microsatellites into Earth’s orbit. (CC)

The goal is to send a pair of approximately 20-kilogram satellites into space to study fragments of asteroids entering our atmosphere. Among other things, they will measure their radiation in the UV spectrum. This cannot be measured from the surface of the Earth because of the planet’s ozone layer and cloud cover. This is the largest and most ambitious project of Czech origin since the Magion satellites were launched into space more than forty years ago.

At the beginning of 2023, the project was in the feasibility study phase. Experts in various fields are looking into the conditions under which the mission can be carried out and whether any compromises will have to be made. There are a number of unknowns regarding the instruments themselves on board and their weight.

In addition to a hyperspectral camera capable of analysing the elemental composition of meteors and a radio antenna for observing meteor plasma, the satellites will carry a mass spectrometer that will examine in great detail the chemicals found in the dust particles of interplanetary matter. At the end of the study, researchers will know not only the precise mission plan but also the exact shape and form of the two satellites.

Filmmakers usually only present the darker side of asteroids to us: either as doom-bearing space wanderers or as useless space junk without much use. While their usefulness may seem as fantastical as these scenarios of disaster movies, the technologies for extracting their mineral wealth, which are being developed by the SLAVIA mission, are already in high demand. It seems that the future of the previously neglected space debris has the potential to be unusually lively.

The article, originally as “Asteroids: Danger and opportunity”, was first published in the 4/2022 issue of the CAS Magazine A / Věda a výzkum.

4/2022 (version for browsing)
4/2022 (version for download)

Authors: Jan Hanáček, Markéta Wernerová, Division of External Relations, CAO of the CAS
Translation: Tereza Novická, Division of External Relations, CAO of the CAS
Photo: Shutterstock; NASA/JHUAPL/SwRI; NASA/JPL-Caltech/UCLA/MPS/DLR/IDA; Jan Malý for the CAS; Jana Plavec, Division of External Relations, CAO of the CAS

The text and photographs labeled CC are released for use under the Creative Commons licence.

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