The last few decades have witnessed a truly transcendent evolution in the field of space exploration. Ever since the first artificial satellite, Sputnik 1, was launched into orbit in 1957, we have sent humans to the Moon, established our presence on the International Space Station, and built multiple spacecrafts and rovers to explore our Solar System and beyond. Earth’s farthest space probe, Voyager 1, roams more than 23 billion kilometres away [1]. Space exploration has always been subject to great wonder and excitement; it is a source of new and insightful scientific discoveries that answer fundamental questions about the origins and evolution of the universe, and a practical application of the intellectual curiosity innate in humankind.

The future of space exploration promises a continuation of profound scientific discoveries; with the strong foundation of space telescopes dedicated to detecting exoplanets – most notably NASA’s Kepler mission and Transiting Exoplanet Survey Satellite – along with pioneering works on the radial velocity method performed by Nobel Prize-winning physicists Mayor and Queloz [2], we strive to search for extraterrestrial life and discover other worlds just like our own. This article, however, will also explore the substantial economic benefits which space exploration might bring, as new opportunities, particularly in the industry of asteroid mining, are created. In addition, with the inception of advanced propulsion technology and reusable rockets, the sci-fi fantasy of colonising other planets, expanding our civilisation, and ensuring the long-term survival of our species, may soon become a reality.

Search for Extraterrestrial Life

There has always been profound interest on the possible existence of worlds apart from our own. Some of history’s brightest minds have had vastly different perspectives on the matter: ranging from Aristotle’s belief that “there cannot be more worlds than one”, to Italian philosopher Giordano Bruno’s thought-provoking argument that “this space we declare to be infinite… in it are an infinity of worlds of the same kind as our own.”

From a scientific point of view, Galileo’s discovery of Jupiter’s moons was the first time that the possible existence of exoplanets was backed by factual evidence. Over three hundred years later, Wolszczan and Frail announced their discovery of two rocky planets orbiting PSR B1 257+12, a pulsar in the constellation Virgo. With dramatic improvements in observational techniques came the discovery of thousands of more exoplanets; as of 2023, there are more than 5000 confirmed exoplanets in more than 3500 planetary systems.

Whilst it remains a rather challenging task to search for evidence of life on exoplanets due to their long distance away from Earth, it is an area of intense research in which there have already been some promising discoveries. Another approach is to look for signs of life on planets and moons within our own solar system. For example, scientists are studying the atmospheres and surfaces of Mars, Europa (a moon of Jupiter), and Enceladus (a moon of Saturn) to look for signs of past or present life.

Detection Methods of Exoplanets

When it comes to detecting exoplanets, the fundamental issue is the fact that exoplanets are incredibly dark in comparison to their parent star, making direct imaging exceedingly challenging and only useful in a few circumstances. Nowadays, the most prominent methods of detection include radial velocity and transit detection.

The former recognises the gravitational attraction of an orbiting planet’s Doppler shift of the starlight’s spectral lines. This method relies on the fact that a star does not remain completely stationary when orbited around, and the fluctuations in radial velocity which the planet causes on its parent star can be used to determine its mass. 

With the latter technique, a light curve is produced when a planet periodically passes in front of its parent star, causing a periodic drop in light intensity as a result of some of the radiated light being blocked. This occurrence is known as a transit. The mass of a transiting object must be determined using the radial velocity approach in order to be positively identified as a planet. The transit method is more likely to detect transits of massive planets with a small orbital semi-major axis, such as a “Hot Jupiter”, than other planets.

Circumstellar Habitable Zone and Biosignatures

The study of the Circumstellar Habitable Zones is closely linked with the search for extraterrestrial life on exoplanets. The Circumstellar Habitable Zone (CHZ) is a region around a star within which the conditions are favourable for the emergence and maintenance of life, and is defined based on the ability of a planet to sustain liquid water on its surface [3]; it is no secret that the presence of liquid water is considered a key requirement for the existence of life, as it is a vital component for the emergence of biochemistry. The location of the CHZ is dependent on the star’s luminosity and the planet’s atmospheric conditions, and it is typically the region around a star where the equilibrium temperature of a planet falls within the range for liquid water to exist, given the planet’s atmospheric conditions. Thus, physical exploration of the planet is not necessary (and not possible due to the high temperature), being instead replaced by astronomical observations and theoretical models.

Upon the discovery of a star’s CHZ, we can further explore extraterrestrial life through biosignatures. A biosignature is defined as any trait that can be utilised to support the existence of past or present life. In addition to natural objects like leaves and feathers, it may also take the form of fossils that have been preserved in rocks, organic compounds produced by living things, or even variations in the chemistry of the atmosphere or a body of water. There are two main types of biosignatures: morphological and chemical.

The creation or structure left over from living organisms is referred to as a morphological biosignature. An example of this is stromatolites: layered crystalline structures formed by the accumulation of bacteria living together in slimy microbial mats in shallow water. Numerous forms of fossils, rock layers or etchings, and even direct views of cells or living things are just a few morphological biosignatures present on planets.

On the other hand, chemical biosignatures cover a wide range of potential methods by which life may leave its mark in the chemistry of rocks, bodies of water, and even atmospheres. This also includes biological macromolecules such as lipids, carbohydrates, nucleic acids, and proteins. Whilst it may not be applicable to search for DNA and RNA patterns in alien worlds, considering that they may utilise different information storage molecules as us, we may search for the lipids that are utilised to construct the membranes of live cells. Using this method, scientists have been able to reconstruct the kind of animals that were living on Earth long before humans arrived by using the lipids left over from long-gone life.

The ratio of chemical element isotopes serves as another type of chemical biosignature. According to studies, all life prefers to use lighter chemical element isotopes [4], as more energy can be used for metabolism and growth. Measurements of the lighter isotope ratios to the heavier isotope ratios within samples from nature can be used as biosignatures.

Exoplanet Missions

There have been a number of missions dedicated to exploring exoplanets, most notably the Kepler Space Telescope [5] and The Transiting Exoplanet Survey Satellite (TESS) [6]. The Kepler mission is a space telescope, which was launched in March 2009 and retired in 2018, with the goal of searching for exoplanets orbiting other stars in the habitable zone. The telescope is equipped with both instruments for transit method and radial velocity. Thus, the transit equipment is capable of detecting the small dimming of a star’s light as a planet passes in front of it. By measuring the amount and duration of this dimming, scientists can infer the size and orbital period of the planet. The telescope’s radial velocity instrument can also be used to infer the planet’s mass and orbital distance.

The TESS mission is a space telescope, led by MIT and developed by NASA, which was launched in April 2018; the main difference between TESS and Kepler is that TESS uses only the transit method. TESS is expected to survey over 200,000 stars during its two-year mission, and to discover thousands of new exoplanets. The telescope will observe nearly the entire sky, focusing on the brightest and closest stars to Earth, which are the best candidates for follow-up studies by other telescopes. TESS will also be able to detect the wobbles in a star’s motion caused by the gravitational pull of an orbiting planet, which will allow scientists to infer the planet’s mass and orbital distance. Most recently, on 10th January 2023, TESS discovered TOI 700 E, an Earth-sized planet orbiting its star’s optimistic habitable zone, possibly having liquid water on its surface.

Missions Within Our Solar System

In addition to studying exoplanets, scientists are also using other approaches to search for life beyond Earth. For example, NASA is looking for signs of life on other planets and moons within our own solar system, such as Mars and Europa. The Curiosity rover has been exploring the surface of Mars over the past decade, accompanied by a newer Perseverance rover which landed on Mars in 2021. The primary goal of the Curiosity rover is to investigate the potential habitability of Mars by exploring Gale Crater and studying its geology, whilst the Perseverance rover is searching for signs of past microbial life on Mars by collecting samples that will be returned to Earth in a future mission. The Perseverance rover is equipped with more advanced instruments, including a SHERLOC spectrometer that can detect water and minerals, and a MOXIE experiment that will produce oxygen from the carbon dioxide in the Martian atmosphere.

Another example of a mission focused on studying Europa is the Europa Clipper mission, which is currently being planned by NASA. The Europa Clipper mission is expected to launch in 2024, and it will be used to study the atmosphere and surface of Europa in detail. The mission is particularly interested in studying the ocean beneath the surface of Europa, which is thought to contain liquid water beneath its icy crust.

Resource Extraction from Space

With the demands on Earth’s natural resources continuing to grow, it is likely that we will need to look beyond our own planet to find the materials and energy that we need. This could involve mining asteroids for valuable minerals, or harnessing the power of the Sun to produce energy for use on Earth. There are already a number of companies and organisations working on these types of projects, and it is likely that space exploration will play a significant role in our efforts to sustain our way of life.

Space-based resource extraction comes with a plethora of potential benefits. For one, space is an almost limitless source of raw materials; there are millions of asteroids in our solar system alone, and many of them contain valuable minerals and other resources that could be used to meet the growing demands of our planet. These include platinum group metals (platinum, palladium, rhodium, etc.) which are in high demand for use in catalytic converters and electronics, rare earth elements (dysprosium, europium, etc.), and carbon rich materials such as graphite, diamond and fullerenes,. The presence of water on asteroids can be used as rocket propellant for long-term missions. In addition, space-based solar power could provide a virtually limitless source of clean energy, helping to reduce our reliance on fossil fuels and mitigate the negative impacts of climate change. There are also a number of challenges that need to be overcome in order to make space-based resource extraction feasible. One of the biggest challenges is the high cost of access to space. Currently, it is extremely expensive to launch payloads into orbit, especially due to rockets being around 90% fuel and thus increasing payload weight.

In the past, there have been many organisations actively pursuing space-based resource extraction as a way to meet the growing demands of our planet. However, some of the most visionary companies, such as Deep Space Industries (DSI) and Planetary Resources, backed by Google’s founder Larry Page himself, have failed to take advantage of the industry and instead filed for bankruptcy. Humanity has yet to commercially mine a single asteroid. The main reason for this was that space mining is a long-term undertaking and investors do not necessarily have so much patience to wait for return on investment.

In spite of these challenges, our efforts have not stopped, and there are many who still believe that space mining will become a reality. AstroForge, a company founded in 2022 by former SpaceX and Virgin Galactic engineers [7], aims to mine and process precious metals in space before returning them to Earth for sale. AstroForge will affix its refining payload to commercial spacecraft and launch those satellites on SpaceX rockets to reduce launch costs.

Colonisation of Other Planets

Perhaps the most exciting area of space exploration, heavily advertised in science-fiction, is the colonisation of other planets. For decades now, blockbuster movies like Interstellar and Avatar have depicted the dream of all humans to step foot and inhabit in a different world. With the development of new terraforming technologies, along with space missions such as NASA’s Artemis and SpaceX’s Starship, this dream may come true sooner than we realise.

Needless to say, there are a great number of benefits from the colonisation of other planets. Primarily, it could ensure the long-term survival of human civilisation should a catastrophic event happen on Earth, such as a natural disaster or a nuclear war. The existence of a new frontier could also help alleviate resource scarcity on Earth by providing opportunities to distribute our population over other planets , as our population continues to increase. Of course, it would also significantly aid our efforts to search for extraterrestrial life.

However, with great possibilities comes great challenges. First, most planets do not have a suitable atmosphere and protective magnetic field, making the surface inhospitable to human life. Combatting this problem would require the development of advanced terraforming technologies for creating a habitable environment, including the creation of artificial atmospheres, the manipulation of temperatures, and the protection against harmful radiation. Secondly, the long distance between Earth and other planets makes transportation and communication difficult and costly, hence requiring the development of advanced new propulsion systems and the construction of infrastructure for supporting human life on other planets.

Terraforming

One way of establishing human settlements on other planets is the use of terraforming technologies, which refers to the process of intentionally altering the environment of a celestial body in order to make it more habitable for humans or other life forms. This could involve a number of different strategies, such as releasing greenhouse gases into the atmosphere to increase the temperature, or introducing plants or other organisms to create a more Earth-like environment. Despite still being in early stages of development, this technology could be our best bet for permanent residence on another planet.

NASA is currently developing large reflective mirrors that can be deployed several hundred thousand kilometres from Mars and used to reflect the sun’s energy to heat the Martian surface, as part of a solar sail propulsion system [8]. Such mylar mirrors with a 250-kilometre diameter would be far too big to launch from Earth since they would weigh over 200,000 tonnes; the mirrors might, however, be made from components that can be found in outer space through resource extraction from asteroids.

A mirror of this size could increase Mars’ surface temperature by a few degrees if it were pointed at the polar caps on the planet, as it would melt the ice and liberate the carbon dioxide trapped inside; the increase in temperature would cause a cascade of unleashing further greenhouse gases like chlorofluorocarbons (CFCs).

Nuclear Propulsion

The development of nuclear fusion and fission will be necessary for deep space exploration, as systems for spacecraft propulsion that can utilise both fission and fusion are being actively investigated and developed. A liquid propellant, most often hydrogen, is pumped through a reactor core in Nuclear Thermal Propulsion (NTP) systems to operate. Fission occurs when uranium atoms split apart inside the core, producing heat. The propellant is heated during this physical process, turning it into a gas that is expanded via a nozzle to create thrust.

NTP rockets are twice as efficient and have a higher energy density than chemical rockets [9]. Specific impulse, or the amount of thrust you can obtain from a specific amount of fuel, is how engineers gauge this performance. A chemical rocket’s specific impulse, which is 450 seconds, is exactly half that of a nuclear-powered rocket’s first objective in terms of propellant efficiency (900 seconds). This is because lighter gases accelerate more readily. Chemical rockets burn to produce water vapour, a by-product that is far heavier than the hydrogen utilised in an NTP system. As a result, the rocket is more efficient and can travel further on less fuel. However, rockets will be first launched into space using traditional chemical fuel before NTP is turned on, since NTP systems are not designed to produce the amount of thrust needed to leave the Earth’s surface.

Conclusion

Simply put, the future of space exploration is endless. There are far too many possibilities left unexplored in such a short essay: space manufacturing, commercialised space tourism, space-based astronomy, just to name a few. The spacecraft industry, once dominated by government-funded national agencies such as NASA and ESA, is now moving towards increased privatisation by profit-driven commercial enterprises. The front-runners, SpaceX and Blue Origin, are owned and operated by tech billionaires with heavy funding from a mixture of private investments and government contracts. This trend of privatisation has led to more cost-effective and innovative solutions being produced, on account of the main motivation being profit, and results in greater risk-taking and freedom to choose which projects to pursue, thus allowing flexibility to react to changing market conditions. Indeed, many high-profile achievements have already been made by such companies, most notably the development of reusable rockets; in 2020, SpaceX successfully launched and landed a human-rated spacecraft, Crew Dragon. Reusable spacecrafts will be a part of our bright future in space exploration, and key in all areas explored in this essay.