From the discovery of America to the exploration of the Poles, the desire to explore unknown territories has always been a distinctive trait of our species. Nowadays we are able to easily reach any place on our planet, so our curiosity leads us to look upwards, behind the horizon, towards the last frontier.
In the southern part of Cardiff Bay a small strip of land, the Cardiff Bay Barrage, connects the area of Porth Teigr to that of Penarth Marina. It is a very suggestive place for those who love the sea: on one side you have the view of the bay itself, with a generally calm sea full of small ships, and on the other side you can see the immense Bristol Channel, often lashed by a strong cold wind that creates large foamy waves on the surface. Strolling between these two seas, I occasionally see ships heading west out of the Channel. They are boats that may be carrying goods to Ireland, Europe or maybe even overseas. Seeing them, I often wonder how the crew, and particularly the captain, feel when they are still relatively “safe” inside the channel before heading out into the open sea. As they say, the ship is safer in the harbor, but that’s not what it was built for.
Today, thanks to GPS, a ship on the open sea can know at any time its position, the weather forecast in the area where it is located, and can adjust its course accordingly.
The GPS (Global Positioning System) uses a network of about thirty satellites in geostationary orbit, on board each of which are installed atomic clocks with very high precision. The satellites transmit radio signals containing information about their position and the current time, which arrive at GPS receivers on Earth, such as those in our smartphones. The receiver monitors the signals sent by different satellites: based on the time elapsed between the emission of the signal from them and its reception, and knowing the position of the satellites, with only four signals, the receiver is able to solve the mathematical equations necessary to determine precisely its position on Earth.
The GPS can calculate in a short time where the nearest port is, how far it is from its destination, and approximately how much more navigation time it will need before docking. The explorers of the past however did not have a GPS, and they had to scout the unknown beyond the horizon in order to navigate. If you wanted to reach distant places within a reasonable time, the best transport was a ship (the airplane would not be invented until the Wright brothers’ famous flight in 1903). Navigating the sea for several weeks without interruption, however, presented significant risks. Since there were no modern technologies, the biggest limitation for sailors was the orientation system. The most widely used instrument was the marine astrolabe, a variant of the original astrolabe, which made it possible to calculate the latitude of a ship by measuring the altitude of the sun or of a known star (in particular the North Star in our hemisphere, or the Southern Cross if sailing in the southern hemisphere). The astrolabe was a fairly accurate instrument and remained the main orientation device for those who sailed until the invention of the sextant in the 1700s.
The weather was another unpredictable element, as there was not the knowledge nor the tools to provide accurate forecasts. However, there was a minimum understanding of atmospheric elements: some winds were known and used, such as the trade winds, which blow from north-east to south-west in the northern hemisphere and which were also used by Christopher Columbus for his famous voyage towards the discovery of the Americas.Who knows what thoughts he had when, in August 1492, he set sail from Palos de la Frontera, in Andalusia, directing the prows of the famous three caravels towards the unknown. The last known port where they docked before venturing into the open and unknown ocean was the port of San Sebastian, in the Canary Islands, from where Columbus set sail on September 6th. The voyage was long and put the crew’s psychological resilience to test: for more than a month the three ships sailed the open sea without seeing the slightest sign of land. But when the bad moods were about to reach uncontrollable levels, finally a sentry on the lookout position spotted what was renamed by Columbus himself San Salvador. They landed on October the 12th 1492.
Thanks to the discovery of Columbus, the spirit of exploration underwent a great acceleration, with numerous naval expeditions across the oceans and as many land expeditions in the Americas, Asia, Africa and Australia, which continued until the end of the 1800s, concluding with the exploration of the polar regions in the early XX century.
Today we know every corner of our planet, but that ancient and innate human curiosity to explore the unknown is not yet sufficiently satisfied.
Walking toward the locks in the bay, which regulate incoming and outgoing maritime traffic, there is a particular bench on which I occasionally sit. It is a bench in the shape of a book, “The enormous crocodile” by Welsh writer Roald Dahl (known to most for writing “Charlie and the Chocolate Factory”). Next to the bench there is a very long plastic crocodile on which children often play, representing the protagonist of the book of the same name. Sometimes, while sitting here, I happen to see the moon in the sky reflected in the sea. In this we can find in some way the future and the past of exploration. Since there is now practically nothing left to discover here on Earth, our human curiosity leads us to look upwards, towards what is the last unexplored frontier.
As everyone knows, we reached the Moon for the first time in 1969. However, it has been almost 50 years since anyone has set foot on it. The last astronaut who walked on its surface was Eugene Cernan, with the Apollo 17 mission, back in 1972. The Moon remains, in any case, the first and easiest target to test or start a possible space exploration or colonization. In fact, due to its proximity, it could be reached in a relatively short time: from 3 to 7 days, depending on the approach and landing maneuver adopted.
Looking a little further ahead, one can’t help but think of Mars as a possible human colony on a larger scale. Here, however, things become more complex, as Earth and Mars have slightly different orbits from each other. While the distance Earth-Moon (384400 km) is practically constant, and therefore we could leave at any time without this distorting the duration of our journey, in the case of Mars it is appropriate to schedule an adequate launch window. This is because the minimum distance between the two planets is around 56 million kilometers, but when they are at maximum distance it exceeds 400 million kilometers. At present a trip to Mars could last from 6 to 10 months (depending on the launch window), even if it is assumed that with a suitable nuclear thermal propulsion system (still under study) it could arrive on the red planet in about 3 months. Fonte:Delbert
But why is it important to try to reduce the duration of an interplanetary trip as much as possible? On the one hand because a very long journey in deep space could have significant psychophysical consequences, generating a high stress load on the astronauts. Moreover, staying for a long time in an environment without gravity leads to physical consequences on the body, such as muscle atrophy and bone deterioration. However, the main reason is another: prolonged exposure to cosmic radiation.
Cosmic rays are highly energetic particles originated by various astrophysical phenomena such as supernova explosions, quasars, the fusion of two neutron stars or even from the Sun itself. Prolonged exposure to them can affect health, with an increased chance of developing cancer or tumor.
The Earth, like any other celestial body, is continuously bombarded by this type of radiation. Our lifeline is the Earth’s atmosphere, which like a huge shield protects us by shielding the cosmic rays and allowing the very existence of different forms of life on our planet. However, as a boat that ventures into the open sea loses the protection provided by the harbor, in the same way a spacecraft that leaves the atmosphere is no longer protected from the harmful effects of exposure to this radiation.
To understand how dangerous this radiation can be, a parameter called equivalent biological dose is used, whose unit of measurement is the milliSievert (mSv). It considers how much radiation a person has received and, at the same time, how much damage that particular type of radiation can cause. The greater the damage caused by a certain dose of radiation, the greater the value in mSv. In order to make a more quantitative comparison, just think that the amount of radiation that a person absorbs in a year due to natural terrestrial radioactivity is about 2.4 mSv (0.0065 mSv per day), while from two chest x-rays you absorb a little less than 1 mSv. On the other hand, astronauts who are in the International Space Station in 6 months can absorb between 80 to 160 mSv (0.44 to 0.88 mSv per day). If we think of a hypothetical mission to Mars lasting three years, we see that the numbers raise some legitimate concerns. In this case it has been calculated that the equivalent biological dose would be about 1200 mSv, more than 150 times the one due to natural terrestrial radioactivity. NASA itself today prohibits its astronauts to make further missions once they have exceeded a certain threshold of absorbed radiation accumulated during their career in space. Fonte:Nasa
Neither the Moon nor Mars have an atmosphere capable of protecting against cosmic radiation: hence, the development of buildings and spacesuits capable of appropriately protecting against it are a key prerequisite to establishing a possible human colony in these places. Looking back at the 2015 NASA Office of Inspector General report Fonte:Office we see that the situation still requires several steps forward:
“Although NASA continues to improve its processes for identifying and managing the health and performance risks associated with human spaceflight, we believe that given the current state of knowledge, the agency’s timelines for risk mitigation are optimistic, and that NASA will not be able to develop countermeasures for some of the risks of deep space navigation before the end of 2030s, at a minimum […]. As a result, astronauts selected for early deep space expeditions will have to accept a higher level of risk than those flying on the International Space Station today. We have also determined that NASA is currently unable to report in detail the costs of developing countermeasures for such risks.“
Despite the current safety and health limitations, it is possible that at some point in the future this problem will be solved, and we will have spaceships that can take us further away from Earth. It is therefore fair to ask: how far can we go beyond Mars?
If we imagine to be on a spaceship, from the command bridge we should first point our ship where we know there is an exoplanet, a planet outside the Solar System orbiting around a star. If we are in command of the Enterprise and we are driven only by the spirit of exploration, we could point to any exoplanet. But if we want to look for a base for a future colony, then our exoplanet must meet one more requirement: it must have the potential to host life.
There are three main ingredients required to satisfy this condition. Fonte:Balbi First of all we need a source of energy, such as a star, in order to provide the energy to develop the processes necessary for the formation of life and its maintenance. Then there must be some specific chemical elements, which are the basis of the chemistry of living systems: hydrogen, oxygen, carbon, nitrogen, sulfur and phosphorus. The last requirement is the presence of liquid water (not in other aggregation states), which is the most difficult of the three conditions to meet.
The liquid water requirement introduces the concept of the star’s habitable belt. It is defined as a ring-shaped zone around the star within which the planet might have liquid water. A bit more rigorously, the inner edge of the habitable belt is defined as the one where the water in oceans evaporates, and the outer edge is where the temperature is no longer such as to keep the water above freezing temperature.
The definition of habitable belt is very general and does not automatically imply that a planet can host life. In addition, the surface temperature of a planet also depends on the composition of its atmosphere. If we look at our Solar System, in the habitable belt of the Sun there are Venus, Earth and Mars, three planets very different from each other, and only one of them actually hosts life. Venus has a dense atmosphere full of greenhouse gases, with a very high surface temperature, while Mars has a very thin atmosphere that makes it a very cold planet.
It should be added that in some cases the presence of liquid water has been discovered even outside the habitable zone. Studying the moons of the gaseous planets of our Solar System, liquid water has been recognised on Europa and Ganymede (satellites of Jupiter) and Enceladus (satellite of Saturn), located well outside the habitable zone of the Sun.
The three essential ingredients are indispensable but not sufficient to give a planet the potential to host life. Other factors are also needed, such as the right surface gravity and the presence of an atmosphere suitable to shield the surface from cosmic radiation. The concept of habitable zone is therefore something that helps astronomers involved in the search for exoplanets suitable to host life to make a first general skim, focusing on those that seem to have better chances for habitability.
The surface gravity of a celestial body is the acceleration of gravity experienced by an object at the equator. For the Earth, this value, known as g, is equivalent to about 9.8 m/s2. Surface gravity is directly proportional to the mass of the celestial body and inversely proportional to the square of its radius.
Now that we know which kind of planet we are looking for, we can point our ship towards the outside of the Solar System, pass Neptune and enter the so-called Kuiper Belt. This belt extends up to 50 astronomical units from the Sun, where 1 astronomical unit is the distance Earth-Sun (about 150 million kilometers). In this zone there are minor bodies such as comets and asteroids, nothing too interesting for our exploratory spirit. We then go further and venture into the large Oort Cloud, and from here we continue into deep interstellar space.
At this point the closest exoplanet that has been discovered so far will appear on our radar. It is called Proxima b, Fonte:Menietti and orbits the star closest to our Sun, Proxima Centauri. We know that it is about 2-3 times the size of the Earth and it is located in the habitable zone, although, as we have seen, this does not automatically make it suitable to host life. Unfortunately it is subjected to a strong stellar wind, that is, a large amount of gas generated by Proxima Centauri. This wind is about 2000 times stronger than the one created by the Sun that arrives on Earth, Fonte:Garraffo and is enough to sweep away any form of atmosphere. To ensure adequate protection from cosmic radiation Proxima b5 would then need a strong planetary magnetic field, whose presence has not yet been conclusively established or excluded.
Additionally, it seems that Proxima b is in so-called synchronous rotation with its star, that is, it has the period of rotation equal to that of revolution. This means that one half of the planet would be always facing the star, and another half always in the dark, a bit like the Moon with the Earth. As a consequence most of the planet would risk being either too hot or too cold, with the possibility to host life only in those regions of the planet close to the so-called twilight zone, that is the limit between these two extreme areas.
Another problem for the habitability of Proxima b, in the absence of atmosphere, is constituted by the star itself around which it rotates. Proxima Centauri is in fact what is called a flare star. While our Sun is quite regular in its various cycles of activity, this kind of star is characterized by sudden and rapid increases in brightness, which can last from a few minutes to several hours. During these events Proxima Centauri releases a large amount of X-rays that, in the absence of proper protection, would be lethal to any living form residing on Proxima b.
There is also a not negligible detail to consider: the distance. We said that Proxima b is the nearest exoplanet that has been discovered, but it is also true that to get there we have made a very long journey. It is located about 4.3 light years from us, where a light year is the distance covered in a year by light; travelling at about 300000 km / s (or about 1.08 billion km / h), this corresponds to about 9460 billion kilometers. At the present we are very far from reaching speeds even remotely close to those of light. The maximum speed reached so far by an experimental aircraft is 12144 km/h, which would take more than 80000 years to reach Proxima b. In space things are slightly better. The Juno probe, for example, has reached a speed of 265000 km/h; however, even if we could keep up this speed, we would still need something around 16000 years to reach Proxima b.
Even in the hypothetical case in which we have super engines able to reach the speed of light, the duration of the trip would not be less than 4.3 years, with all the risks due to the prolonged exposure to cosmic radiation that we have already discussed. It could be argued that if we could travel, for example, ten times faster than light, at that point the trip would consist only of a few months. However, there is a fundamental limitation, physical rather than technological: nothing can go faster than light. Fonte:Balbi
The basic physical principle is that, if we want to bring an object of a certain mass m to travel at a certain speed v, we must spend a certain amount of energy. At speeds much slower than the speed of light, called non-relativistic speeds, the amount of energy to spend is given by a fairly simple relationship: E=mv2/2. If we take this equation and calculate how much energy is needed to bring to light speed (v=c, where c is the speed of light) an object of mass 10 grams, it turns out that the amount of energy required is that produced by a power plant in 5 days. It is a big number, but it does not seem impossible.
The problem lies in the fact that when we are dealing with a very large speed close to the speed of light, known as relativistic speed, things get complicated and the previous equation is no longer valid, but is replaced by a more complex one. The closer we want to bring our object to 300000 km/s, the more energy we have to expend, eventually requiring an infinite amount of energy to reach v=c. From this derives the fact that speed of light is a physical, insurmountable limit. Someone could argue that the equation is wrong or inaccurate, but it is not so. It has been correctly verified countless times, for example in particle accelerators, where particles reach relativistic speed (up to some very high fraction of the speed of light), and the energy spent each time to bring them to a slightly higher speed is always in agreement with the above equation.
For space exploration, this relativistic problem, means longer travel times than the 4.3 years assumed in the case of the hypothetical ability to travel at the speed of light.
But then, if the speed of light is a limit that cannot be crossed, what are the current technological limits with which we must confront? And what could be the possible approaches to push to the maximum the speed of future spaceships?
Earth is the cradle of humanity, but one cannot remain in the cradle forever.”
– Konstantin Eduardovich Tsiolkovsky
Today, rockets that launch satellites or carry astronauts to the International Space Station are accelerated to the speeds they need to travel by ejecting propellant. The faster the propellant is ejected, the greater is the acceleration of the rocket and the maximum speed it can reach. But couldn’t we eject it at a rate to have the necessary speed for interstellar travel?
Here, another physical limit comes into play. It is expressed by the so-called Ciolkovsky rocket equation, named after the Russian engineer and scientist who derived it (although it had been independently derived before). The maximum speed at which fuel exits from current rockets is about 5 km/s. Fonte:Balbi Suppose we wanted to reach a speed equal to one hundredth of the speed of light, which would allow us to arrive at Proxima Centauri in about 400 years. Unfortunately, this equation tells us that the mass of fuel that would be needed is greater than all the one of the observable universe (and this last one is huge number, equal to about 1053 kg, or if you prefer 100.000.000.000.000.000.000.000.000.000.000 kg). From this we can see that if we want to use rocket designs like those still in use today, we cannot increase the mass of fuel to be used but we must act on the other variable in the equation: trying to increase the propellant ejection velocity.
“Earth is the cradle of humanity, but one cannot remain in the cradle forever” Konstantin Eduardovich Tsiolkovsky
Some projects in the past tried to go in this direction, even trying to use different mechanisms. Fonte:Balbi The Orion project Fonte:Dyson foresaw, for example, the possibility to significantly increase the speed of the rocket through a sequential explosion of nuclear warheads. From the technological point of view it would have been a feasible project, and theoretical estimates predicted that it could bring the rocket up to one tenth the speed of light, allowing it to reach Proxima b in a few decades. However, international treaties categorically prohibit the use of atomic weapons in space. And secondly, all the radiation generated by the atomic explosions could have had consequences for the crew in the long term, especially considering that the exposure would have been very prolonged.
Another method studied in the past to reach relativistic speeds was to try to exploit nuclear propulsion, but based on nuclear fusion Fonte:Post instead of fission. Dedalus’ project tried to explore this possibility but it turned out that in this case the limitation was not physical but technological. In fact, although it is theoretically possible,currently we are not able to control a nuclear fusion process (but this does not preclude that we could do it in a not-too-distant future).
It is a thermonuclear process that takes place inside stars, in which two atomic nuclei fuse creating heavier nuclei and releasing energy. In order for fusion to occur it is necessary to obtain an extraordinarily high temperature, on the order of hundreds of millions of degrees.
It is a nuclear reaction in which the nucleus of an atom, usually a heavy element, splits into two or more nuclei that are lighter than the starting nucleus. The reaction releases a large amount of energy and, unlike fusion, it produces radioactive daughter elements (so-called radioactive waste).
Controlling the nuclear fusion process is the biggest challenge. One of the methods to control the plasma at such high temperatures is magnetic confinement: a huge magnetic field “encloses” the plasma inside the reactor, allowing fusion to take place in a controlled and continuous manner. To generate this magnetic field electromagnetic superconductors are cooled with liquid helium. To keep these magnets active requires a lot of energy, almost as much as that currently produced by experimental reactors. Currently, the most advanced experiments are able to keep the fusion process under control only for a few seconds, and the energy spent to do this is higher than the energy produced by the fusion itself.
Human creativity has been working to see if using a different propulsion principle could lead to something more concretely feasible. A first idea that came up was to try to use the so-called Bussard manifold. Fonte:Balbi The idea is to equip the spacecraft with a giant collector able to capture hydrogen present in interstellar space, which at that point could be used to power a fusion reactor. This approach does not have any physical limitation, but the problem in this case is that the density of hydrogen in space is very very low. It has recently been remeasured to 0.127 particles per cubic centimeter Fonte:Sandri or, to give a more practical idea, 120 hydrogen atoms in a space the size of a one-liter milk carton. The collector should therefore be gigantic, of several tens of km, and this cannot be overcome at the moment.
A collector is technically a stator reactor, that is a reaction engine. In the case of the Bussard collector, the collector would consist of a huge magnetic field that, like a sort of giant “spoon”, would collect the hydrogen needed to feed the nuclear fusion reaction. Fonte:Balbi
A fusion reactor is a system capable of activating and keeping under control a nuclear reaction based on the principle of nuclear fusion. It differs from modern power plants in the principle of operation, as at present the only way to efficiently produce nuclear energy is through the method of fission. To date there are no fusion reactors in use, and several experiments are underway to develop the necessary technology.
The other idea, much more feasible, are the so-called solar sails, or photonic sails. Fonte:Balbi The basic principle is that the particles of which light is composed, photons, carry energy and momentum. Consequently, if photons hit a surface, such as a sail, they transfer their energy and momentum to the surface by exerting what is called a radiation pressure. This pressure would represent the propulsion mechanism able to accelerate the sail (and the spacecraft connected to it). The sail should obviously have particular characteristics, including being extremely light and very large. This technology has already been developed and tested successfully: the most famous example is the Japanese probe IKAROS, which in 2010 landed on Venus and was the first ever to use the solar sail as a means of propulsion. Another example was the LightSail 1 mission in 2015, in which a small spacecraft was launched into orbit and successfully deployed its solar sail, and its follow-up LightSail 2 in 2019.
Electromagnetic waves carry energy and momentum (a physical quantity defined by the product of an object’s mass and velocity). When a body interacts with an electromagnetic wave, it is subject to a force, the value of which per unit area defines the pressure. The force per unit area generated on an object by an electromagnetic wave is called radiation pressure.
The technological feasibility of photonic sails and recent successes have prompted many minds to bet on this approach as the most viable for future space exploration.
The advantage of this technique is the absence of a need for fuel. However, even if the radiation pressure exerted by photons arriving on Earth is enough to move a photon sail, it may not be enough to guarantee a trip to Proxima Centauri in a reasonable time. Here comes into play the idea of replacing sunlight with an artificial source of high energy, such as a laser. The idea was already theorized in the second half of last century Fonte:Marx Fonte:Forward and now there is a concrete project, called the Starshot project. Fonte:Balbi It should be specified that for the moment we are talking about sending into space small spacecraft, not large manned spaceships. Nevertheless, the idea has a major potential, because it has no technological limits that could compromise its realization.
The challenge however is still enormous. It would involve using a huge amount of lasers at the same time, distributed over an area of a few square kilometers. The total power would be of the order of gigawatts (one billion watts) or more, roughly corresponding to that produced by one hundred million LED bulbs. Once the spacecraft would be in orbit it would deploy the photonic sail, which would be hit by the battery of lasers, thus receiving the acceleration necessary to arrive on Proxima b. However, the lasers would have to be aimed in an extremely precise way, because after that any direction corrections would be impossible (because of the huge distance between the spacecraft and the Earth). Once the spacecraft arrived in the vicinity of Proxima b, its telemetry would arrive on Earth with about four years of delay, and it would take the same time for any correction signal sent from Earth to reach the spacecraft.
It has been calculated that with this method it is possible to get a small spacecraft to Proxima b in about twenty years. The most optimistic estimates predict that the first launch can take place in twenty years: adding the travel time and the time of about four years for the return of the signal with the data sent, it will still take more or less half a century (in the best case) before we can properly test what is currently the most realistic possibility to send a spacecraft outside the Solar System.
If Proxima b is the closest exoplanet discovered so far, it does not mean that it is the only one nor the last. Before 2010 we knew about 400 exoplanets, but in the last decade an increasing number of them have been detected: in only the last five years we have discovered more than two thousand. Fonte:Exo Most of them do not meet any requirement of habitability, others instead might be very interesting. As an example, it has been observed that around a star called TRAPPIST-1 there are seven orbiting exoplanets, three of them in the habitable zone. From preliminary studies it seems that they have dimensions comparable with those of the Earth, and that their surface temperature may allow them to host liquid water. But from here to declare them suitable for future colonies it’s a long shot. TRAPPIST-1 is located about forty light years from our Sun, ten times farther than Proxima b. If already getting to Proxima b seemed titanic, getting to TRAPPIST-1 seems borderline impossible.
There is however good news, and it has to do with travel time for the astronauts. When we talked about getting to Proxima or TRAPPIST-1, the travel years mentioned referred to what observers on Earth would see. But for the crew aboard the spacecraft, since the speed of the craft is very high, time would flow more slowly, as Einstein’s Relativity teaches us (just think of the movie Interstellar when the protagonists are in the vicinity of a black hole). To give an example, to get to Proxima b the journey would be 4.3 years for those who see it from Earth, but just over three and a half years for the crew. To reach the center of the Milky Way, the journey would be 30000 years for those who remained to watch the ship on our planet, while for the travelers only 20 years would have passed after takeoff.
The physical and technological limitations we described should not deter our exploratory spirit. Looking back, only 2000 years ago we Europeans thought our entire world was Europe. Within two millennia humans have managed to explore the entire globe, to photograph it from space, and we have not stopped yet.
On one hand, the current limits on the protection from cosmic radiation and on the actual speed at which we can travel might induce us to give up interstellar travel. On the other hand, however, looking at how many important scientific and technological achievements we have achieved in the last two centuries should give us even a minimum of optimism for the future of space exploration.
Fifty years ago my grandparents took almost a whole day to travel from Calabria to Rome by train. Today in 12 hours by plane you can get from Europe to the Pacific coast of North America. With these two images in mind, I look up again from the sea to the sky, and I think that even if physics teaches us that nothing can go faster than light, the unlimited potential of human genius will allow us to travel to wonderful new worlds, sooner or later.