S3C10: Space Exploration

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a) Why and how

Astronomy is a vast source of data and discoveries, however, as impressive as its achievements are, the information which can be inferred from remote observation is naturally more limited than what instrumentation deployed onsite or in close proximity can provide. A probe can not only see from closer up, regions that would otherwise be obstructed from our long-distance peering, but it can also sample the atmosphere, the ground and any liquid, whether for onboard analysis or experimentation. And experience, as we know, is an indispensable step in the process of hypothesis making and falsification, the relentless pursuit of building a skyscraper of knowledge sturdy enough to withstand the harsh glare of reality and slowly allowing us to master energy and matter. We try and want to know more because it is useful and, it has to be said, for the sake of knowledge itself.

It is not about being better, it is about providing incremental information, so in that sense astronomy and space exploration are complimentary. I mentioned probe because, by and large, space exploration has been done via robots rather than humans, for reasons of costs, risks and technological capabilities. So, a typical unmanned mission consist of a probe, a mechanical device loaded with a bit of fuel and plenty of exquisitely top-of-the-art instrumentation that has taken years to develop for a specific mission. Historically, this hardware is then loaded as payload in a launch system, mostly expandable and therefore expensive to operate, comprising a hugely powerful rocket supplemented by boosters with burn time of 2 minutes or so. We will look a little more in depth at what it takes to launch a payload in space in section f); and for the record payload refers either to the cargo-carrying capacity of a launcher or another vehicle, often expressed in terms of weight rather than volume, or it can also refer to the cargo itself.

The probe is then released when it reaches the right velocity and from then on it will use its momentum, planetary gravity assists and its thrusters (mostly to course correct or for an approach), in a carefully plotted celestial course to reach its intended destination. At this stage, the mission can have one of three forms: #1 a fly-by with photographs and measurements taken for a very limited duration before continuing on, #2 orbiting followed by orbit ejection or a suicide de-orbiting and crash, or #3 landing – arguably the most challenging of the scenarios given the force exerted by gravity. Typically, a landing and sampling is not followed by a take-off, too much fuel required for this though it has been done several times for Moon missions and twice properly on asteroids (Hayabusa2 in 2019 and Osiris-Rex in 2020) with samples returned to Earth.

As for manned missions, there has been fewer of them, and even though samples have been returned and experiments carried out, the prime objective initially was very obviously not of a scientific nature.

b) Achievements so far

It all went very quickly, because it was not a matter of being first at making scientific discoveries, it was a flag planting, chest beating contest between the USA and the USSR to showcase technological supremacy and validate the superiority of an economic and political model over the other. Two superpowers trying to impress. And we have to give it to them, impress they did, in that particular domain. Few knew it was about to start despite the formal voicing of intentions to launch artificial satellites in 1955 but the space race was truly underway when Sputnik made it to low Earth orbit in 1957 followed by the first manned orbital flight featuring Gagarin in 1961 with Shepard following less than a month later. A matter of days afterward, it was already time for a probe fly-by of Venus, then came a Mars fly-by and the high watermark was reached by Apollo 11 delivering the first two humans on the Moon in July 1969. Human missions came and left but the space high was over for the public and therefore politicians so the focus turned scientific with the establishment of Mir and then the International Space Station (ISS) featuring longer stays and experiments.

The ISS is the result of multilateral cooperation involving Japan, Europe, the USA, Canada and Russia, it orbits Earth at 400km of altitude since end of 1998 and was expanded over time in a modular fashion. It is powered by large solar arrays and is routinely used to conduct experiments under low or no gravity and study the effects of long-term space missions on astronauts.

As technological prowess and the advancement of science go, space exploration has continued. The most famous probe duo is arguably Voyager 1 and 2 launched in 1977 within days of each other (Voyager 1 was actually launched 16 days after its sibling), making the most of an extremely beneficial planetary alignment and the gravity assist options this provides – for more on this topic, please refer to S1 Section 10.f. Voyager 1 did flybys of Jupiter and all four Galilean moons, then Saturn and several of its satellites before heading for interstellar space, which it entered after crossing the heliopause in 2012 at 121 AU; within a few years past 2025 it will no longer have enough power to operate any single instrument. As for Voyager 2, it also covered Jupiter and Saturn but included Uranus (1985) and Neptune (1989) to its itinerary; it also entered interstellar space in 2018.

As for unmanned probes, they usually carry an array of scientific instruments that can withstand the environment they will be in and are designed for particular observations and experiments. These include, inter alia, cameras and filters for imaging, spectrometers to measure the chemical composition of gases and the spectrum of light, particle detectors tuned to specific ranges of energy levels, magnetometers to measure the magnetic field of the Sun and planets, and radio systems to sense the features of a surface.

c) Next destinations

Well, where do we go from there? It is tempting to answer: “Ad Astra”, to the stars. Yet we don’t have what it takes, technologically speaking, and geopolitical imperatives have resurfaced so it is important to score some quick “victories” and plant the flag, further away this time around.

The USA intends to send humans back on the Moon under the Artemis program and establish a habitable base there. China has also been taking giant strides, landing on the far side of the Moon and paving the way for a crewed lunar mission before 2030. This is déjà vu…

The key difference with this new wave of space exploration is that it is closely shadowed by commercial interests and there are ongoing projects, not merely ideas being floated around, for mining the Moon and asteroids. The target is not limited to metals and rare earth materials but the broader idea is to extract resources in a location that doesn’t have the same environmental sensitivity or does not need to escape Earth’s gravity before being used for space operations and exploration. For example, water molecules can be split to form propellant. Even without colonizing a particular planet or asteroid, establishing manufacturing operations and forward bases from which to conduct missions as opposed to returning to or launching from Earth each time will quickly turn out to be financially attractive.

Eventually, once the box of putting human boots on Mars has been ticked, the matter of creating space colonies will be front and centre. I don’t want to diminish the technological accomplishment of a manned mission on Mars but, unlike the Moon, it has to be seen in a greater context, which is finding a habitable planet and terraforming it to a great extent, which I don’t think will be the end goal there. The first few Mars missions will no doubt reveal many unexpected challenges and risks, they will also provide a wealth of data on several aspects of space travel itself and how the human body copes, or doesn’t cope, with it. Even a partial terraforming of Mars would require the development of a magnetic shield, the creation of a pressured atmosphere with a composition suitable to us breathing, the ability to grow food, recycle CO₂, produce oxygen and water, etc. And to begin with, we will need to find a way to onboard sufficient fuel to slow down the descent onto the surface of the red planet whilst carrying significant payloads – parachutes and airbags won’t do.

Within the course of Chapter 5 we charted the inner Solar System and it is clear Mars is the only practical option this side of the asteroid belt, however we also detailed in Chapter 6 some of the other plausible worlds on which we could attempt to live. To be sure, a world where simple life forms may have emerged doesn’t automatically equate a sensible option for human settlement. Around Jupiter, the best alternative is arguably Callisto since it is relatively less exposed to the radiations emanating from the magnetic field of its parent. Nonetheless, the most intriguing possibility is Titan, one of Saturn’s moons. It has a dense atmosphere and an ocean, albeit not water but ethane and methane. The planned Dragonfly mission would be sending a rotorcraft there to know more about the local chemistry and ascertain the presence of life, however this would only take place in the mid-2030s since it would take several years to get there and in the near future Mars remains by far the favoured option for the development of an extra-terrestrial human colony.

Surely, you may ask, there must be more inviting planets elsewhere in the Milky Way, no?

d) Interstellar travel

And so, Ad Astra it will be. There is little doubt about that in my opinion. Easy to say, not easy to do. Crossing interstellar space is not quite the same challenge as circumnavigating the globe in the early 16^th century. There is little analogy besides the common use of the terms “exploration” or “travel”.

The very first obstacle in space travel is the distances involved. Alpha Centaury is over 4 light-years away, corresponding to over 250,000 times the distance separating us from the Sun. For comparison, at its current speed, the Voyager 1 probe will take about 18,000 years to cover a distance of one light-year. In all likelihood, after more targeted remote observation of exoplanets, our first destination will be further away than 4 light-years, perhaps 20 or perhaps 100. Either way, to make it work we need to be travelling at speeds that can be expressed as meaningful fractions of light speed, otherwise we are looking at either cryogenic travel, essentially hibernating for thousands of years, or being born and dying in a spacecraft in the hope one of our descendants will make it to a planet that can be terraformed successfully.

Furthermore, besides the conceptual issues around investing one’s life just to pass the baton to future generations, there is also the not-trivial issue of remaining alive during all this time. It won’t be possible to embark all necessary resources at take-off, and resupply will not be an option once in interstellar space, so food will have to be grown aboard and the entire ecological system required for human survival will have to be self-sustaining. However, we know such a theoretically closed system will in practice not be completely isolated, some energy will leak out, most likely in the form of thermal radiation. Accordingly, an external source of energy such as stellar light will need to be available at regular intervals to battle entropy. A lot more research and development will be required to achieve such levels of autonomy and resilience over long durations.

Resilience in the domain of food and energy, and in the psychological one as well. Cramped conditions and the inability to get away from most individuals within the crew will be taxing for all involved. Proper preparation and selection will be required, as well as clear protocols relating to decision making, from the petty day-to-day to survival-related matters. Otherwise, mutiny will be what ocean and space travels have in common and it is worth noting that ship crews did have clear and strict instructions and chains of commands, which oftentimes was insufficient to prevent onboard authority from being disputed.

Besides the psychological aspects, there are very clear indications long term space travel is deleterious for the human body. The top two issues are related to cosmic rays and a low gravity environment – low being in reference to the 1g we experience on Earth. Lower gravity leads to loss in bone mineral density, making them more fragile, to muscle atrophy and to a decrease in blood pressure as the stroke volume weakens. So some gym work will have to be scheduled and artificial gravity would have to be maintained (this can be done through rotation, thereby creating a centrifugal force). As for solar and cosmic rays, they have the nasty property of ionizing matter and can therefore cause damage to our cells and DNA, leading to cancer and other dysfunctions. We currently do not have the technology to properly shield us from those.

In addition, there exists a range of other issues to be mindful of. For example, gas pressure will need to be maintained at all times to ensure proper respiration and avoid decompression sickness. Studies have also shown spaceflight to have a negative impact on the human immune system, evidencing both a drop in the number of immune cells and various dysfunctions. And lastly, though this list is not exhaustive, vision and hearing seem to deteriorate with long periods spent in space. This is due to a range of factors including micro-gravity, the presence of dust, the narrow confines, etc. Clearly, it is one thing to spend 3 to 6 months in space, and another to spend years or an entire lifetime in the environment of a spacecraft.

One more reason we would want to go much faster, at relativistic speeds ideally, is because time dilation would shrink the experienced travel duration for the crew, bearing in mind that faster-than-light travel is not deemed possible, which was discussed in S3 Section 7.f . This is squarely a technological challenge, one about the type of propulsion and fuel we could and should be using in the future.

Currently, the technology underpinning thrust at launch and during cruising is jet propulsion: propel a fluid in one direction and the rocket moves in the opposite direction – this is Newton’s third law of motion in full display (refer to S1 Section 6.d). The propellants used to this day are chemical in nature and their combustion through oxidation releases energy which increases the temperature and pressure of the gas produced during combustion, and this gas is ejected from the rear to accelerate the rocket – I will detail the workings of a combustion engine in S5 Section 1.b. The main types of fuel used are either in a solid or liquid state, the latter featuring hydrogen or nitrogen with oxygen as the oxidizer. The issue with this propulsion system is that it is not very efficient from a specific impulse perspective, being the ratio of the change in momentum over the mass of propellant, so that an inordinate volume would be required both for the acceleration and the deceleration phases.

At the other end of the spectrum in terms of propulsion is the ion thruster or ion drive: good specific impulse but low maximum thrust. This is clearly problematic for take-off, less so for landing since landing consists in going down a “gravity well”, not up. Ion drives do not rely on a chemical reaction, instead a propellant is ionized (i.e. stripped of their electrons) and the electrically charged particles are then accelerated by way of an electric field and used to create thrust. The propellant of choice are noble gases such as xenon or krypton because they require comparatively less energy to be ionized. Such ion drives are in commercial use today and are the favoured option for missions where only low levels of thrust are necessary; this being said, they can provide moderate acceleration for a very long time so they are a better option than chemical rockets for interplanetary or interstellar travel although when too distant from light sources, the source of electrical power would need to be switched from solar panels to an alternative such as a nuclear “batteries” called RTGs, which stands for radioisotope thermoelectric generator and work by converting the heat released by the radioactive decay of specific materials into electricity.

Now, entering the domain of potential future technologies, nuclear fusion could be used to provide high levels of electricity generation for ion thrusters or directly as source of propulsion based on fuel such as helium-3 that could be mined from the gas giants. Besides having a nuclear reactor next to the crew for many years, an obvious challenge for this design would be the likely mass of the fusion reactor, which may partially offset the benefit of the very high specific impulse.

Another very effective and light-weight but probably underpowered technology would be the use of a solar sail that would rely on the pressure of solar radiation. Physical pressure, not the conversion of light into electricity. The best analogy would be the sails of a ship making use of air pressure. To go around the issue of how much force can be captured and the weakness of stellar light once in interstellar space, the radiation could be beamed by way of a laser aimed at the spacecraft. This beam-powered propulsion could also be based on microwaves and the conversion of their embedded electromagnetic energy to electricity.

I would be remiss if I did not mention antimatter before bringing this section to a close. The theoretical attraction is naturally the high energy density of a matter and antimatter fuel combo and the practical limitations are that we are not very good at creating and storing antimatter. In other words, I would not want to be anywhere around the place where the technology is being tested. Assuming we are able to master antimatter, the energy released could be used directly for propulsion, or indirectly by harnessing its heat or converting it into electricity to then power some of the technologies described above. Time will tell if this stays within the realm of science fiction or if it comes to pass.

e) Philosophical musings

I wrote earlier that, in my view, we are very likely to go to the stars, some would say it is our destiny. I am pretty sure there is no such thing as fate or destiny and, if anything, the complexity of surviving in one ecosystem shows we trying to live on different worlds is a stupid enterprise, provided one has the choice. Unfortunately, there will come a time when we may no longer have the choice, and I am not thinking here about the effects of global warming because, if we are unable to control our climate, then we stand little chance of being successful in terraforming one or several other planets at scale, i.e. settlements not restricted to a batch of scientific bases and small communities living under a few pressured domes. In fact, our success or failure in reversing global warming and rising sea levels will be a good test case both technologically and politically, and it is very clearly not looking good so far. How could we hope then to successfully colonize other worlds?

More fundamentally, in the first place, should we seek to do this, provided we are left with a choice? For the purpose of answering this question, space exploration purely for scientific discovery and research purpose is out of scope; I think a case can always be made for this.

First, there is the non-trivial emotional aspect of never seeing those left on Earth ever again given the travel duration involved. So unless all your relatives are coming with you, you are leaving somebody behind (and by definition some of the relatives of these relatives will not be coming, unless everybody is leaving). A lot of people may not mind so much and quite a few will be happy to. This is a pretty soft argument against, I agree, yet it deserves to be highlighted.

Second, and this concern should be paramount: it is incredibly dangerous from an immunity standpoint if there are existing life forms, including mere viruses and bacteria, on the planet being colonized, which is somewhat likely if the conditions to support human life are met. Even with all our technology, we may not be able to fight off local pathogens. The risk works both ways however and chances are we would be the ones wiping out existing species from the surface of the planet where they evolved, “their planet” to use a human phrasing. Not everybody cares about ethics but I do and our impact on the ecosystems we visit is something we ought to bear in mind and limit as much as possible. It is one thing to send an unmanned probe or set up a research base, though proper sealing off can never be guaranteed, and it is quite another to have millions of inhabitants. In any case, if not from pathogens, the local species will need to contend with the consequences of terraforming on their own ecosystem – these will ultimately depend on the form and extent of the terraforming so I will leave it at that.

Third, those with a utilitarian point of view would argue the more humans there are the more total positive experience there is and since living on a single planet is naturally limiting, we ought to colonize many other planets, as many as possible in fact. I am a utilitarian and I absolutely disagree with this reasoning because it assumes our overall individual experience is positive and that the best way to maximize total experience is to increase the size of the base. This is flawed: individual experience over a lifetime is not and will not always be positive, and the objective is to maximize the average happiness of existing and future beings, not an absolute amount. This means having more people is not the main path to success, caring about our society and our planet is. This should be the focus. And our society doesn’t only include humans by the way. Which brings me to the last argument.

Fourth, whether it is in science fiction or debates surrounding the necessity to colonize other planets, beginning with Mars, one often reads “ensuring the survival of the human species” as a core justification. This is obviously flagrant speciesism and it makes no sense, such a mindset hosts the same negative aspects as nationalism does. Let’s be very clear about this, species is a human concept, one used by scientists for classification purpose. Saying we should focus on human survival is as legitimate as saying we should focus on the survival of blonds, or mammals; this is a roundabout way to say it is preposterous. So what if the human species becomes extinct one day?

In fact, settling different planets will unavoidably result in the divergence of species from the rootstock that came from Earth. Conditions will be different, including gravity, local cultures will develop be and languages will start branching out, etc. After a few millennia, we will need to find another word for those human-like people living on planets other than Earth. And since they will care foremost about their own species, they will not necessarily behave in a friendly fashion with the inhabitants of their planet of origin.

All-in, this begs the question: what is the point of colonizing other planets unless we no longer have another option? To answer Fermi’s question, maybe those species who are more advanced than we, are also wiser and more ethical than we are, so they decided to stay home.

f) Trivia – Launching a space shuttle

Something a little lighter to lift the mood in conclusion of this third series. And lifting is indeed going to be centre stage.

Launch vehicles are designed to send payloads such as satellites or crews into space, they can be expandable or reusable, in which case we need to be able to control the descent and landing of the various parts. Typically, the launch will be done using a multistage rocket on which a space shuttle may be attached and the space shuttle will land back, aircraft style, at the end of its mission.

The reason chemical propellants are used is to provide the necessary velocity to the rocket, and therefore the payload before it can be delivered into orbit. We have seen that we could maybe use more efficient systems such as ion thrusters but those aren’t powerful enough to overcome the gravitational pull of 9.8m/s² experienced by the rocket at the surface of the globe. The catchy name for this is “escape velocity” and, as one would expect, it is a function of local gravity, so it decreases as one gets further away from the centre of mass of the body creating the gravitational pull. If the speed of the rocket crosses over the threshold, which is equal to 11.186 km/s (or just over 40,000 km/h), then it can turn off the burners and still continue to move away from Earth. Satellites remain on orbit for the same reason, they have enough horizontal or tangential speed.

The choice of launch site is driven by the desired orbit with a preference for being closer to the equator since it is best for equatorial orbit and the rotation of the Earth is comparatively faster there, it offers more assist in terms of speed. This allows for more payload at the same price or for a cheaper launch. Also, since most rockets launch towards the east (in the direction of Earth’s rotation), the site would need to have a clear zone in that direction so potential debris and the various rocket stages do not land on inhabited areas.

The reason most launches are based on multi-stage rockets is actually simple, it allows for the shedding of weight when part of the fuel has been spent, instead of having to carry all this now useless metal. Boosters burn until they have no more fuel and then they are released before the next stage starts its burn. Divide, jettison, and conquer.