Overcoming the Challenges of Space Probe Exploration
1.0. Space Exploration
Throughout the many millennia of human existence, the night sky has been a source of enduring intrigue and fascination. From the depictions of astronomical bodies and celestial beings painted onto cave walls by palaeolithic humans, to the advanced mechanisms for modelling and observing the solar system created by civilisations such as the ancient Sumerians, Egyptians and Greeks, humans have always been perplexed by the age-old question of ‘what is out there?’ A sense of wonderment at what exists beyond our terrestrial abode is, I believe, an integral part of what it means to be human. Yet, it has only been in the past 60 years that we have begun to venture away from the security of our blue planet and, with our rockets, astronauts, satellites and probes, explore the vastness of space which surrounds us.
The very first successful attempt at sending a man-made object into orbit was the Soviet Union’s Sputnik 1 mission of 1957. This satellite was by no means a complex device and carried no scientific measuring equipment. Nonetheless, by deducing the drag on its orbit, and by monitoring the propagation of its radio signals, scientists were able to gain entirely new knowledge about the density of the upper atmosphere and the nature of the ionosphere. Most importantly, the very act of putting a satellite into orbit was an incredible feat of engineering and was undoubtedly the event which ushered in the beginning of the Space Age. Since Sputnik 1, we have become much more adventurous in our space exploits. We have launched over 5,000 man-made objects into space, approximately 200 of which have left Earth orbit to visit other planets and bodies. Two of these have exited the solar system altogether (NASA’s Voyager probes), and a further three are currently well on their way to doing so. In the process, we have spent trillions of dollars on a plethora of space programs, prompting ongoing debate as to whether space exploration is worth the cost.
Is It Worth It?
We, as a species, are driven to explore the unknown, to push the boundaries of our scientific and technical limits, and then to push even further. Our curious and explorative nature, alongside our ability to innovate and engineer solutions to problems, has been the driving force behind human development for thousands of years. Human space exploration helps us to address fundamental questions about our place in the universe, it drives the accumulation of scientific knowledge, prompts the innovation of new technologies, and fosters peaceful connections between nations., Systems capable of probing the cosmos for valuable resources, habitable planets and the possibility of extraterrestrial life, while exploring new scientific phenomena and contributing to research at the frontiers of physics, represent one of engineering’s greatest goals in this modern age. Space exploration, in my opinion, is more than worth the cost involved.
Space Probes – The Forefront of Exploration
The most fundamental and currently un-answered questions about the universe can be summarised as follows: What exactly is dark matter and dark energy which make up 96% of the universe? What phenomena occur in highly chaotic systems such as beyond the event horizons of black holes, or during the collision of supermassive bodies? And finally, do there exist planets with sufficient resources and the right conditions to sustain life, and if so, is there any evidence of extraterrestrial life? Engineers have continually been at work on better, and cheaper ways of searching space for answers to these questions. Improved telescopes, as well as devices for detecting gravitational waves and incoming neutrinos make up an integral part of this investigatory arsenal. However, when it comes to quite literally probing for deeper answers to these questions, there exists no better solution than physically sending out space probes. Thus, an ongoing challenge in the field of space exploration is finding ways of effectively incorporating new measurement and detection technologies into space probes.
The term ‘space probe’ refers to any unmanned spacecraft which travels through space, collecting and transmitting scientific information back to Earth. Space probes are typically grouped into three categories: interplanetary probes, which fly by celestial bodies; orbiters, which orbit celestial bodies and examine them over an extended period of time; and landers, which land on celestial bodies to study their surfaces in greater detail. Regardless of what type of space probe is used, and what its ultimate mission may be, the same challenges arise in the process of sending out the probe to retrieve scientific information.
We have looked at why there is a need to explore space, and what role space probes play in helping us to do so. The focus of this essay will now shift to following the journey of a space probe from the time it is deployed, looking at some of the key challenges that arise, as well as present and future solutions to these challenges. These challenges fall into three main categories: i) Protection – finding ways of keeping a probe in functioning condition despite the hazards of space; ii) Power – finding efficient methods of ensuring the probe reaches its destination; and iii) Communication – improving on our current methods of communicating data back to Earth. I shall investigate each of these in detail.
2.0. Space Probe Protection
The cost of development of a space probe runs into the many hundreds of millions of dollars and therefore, keeping a probe functioning for as long as possible in order to collect the maximum amount of data is of critical importance. A space probe begins its journey as the payload of a heavy-lift rocket-powered launch vehicle and after the fairings are jettisoned from the rocket, the space probe activates its own thrusters in order to push itself into the right trajectory. In doing so, the probe becomes entirely exposed to the radiation and extreme temperatures of space.
Space probes, once beyond the protection of the Earth’s magnetic field, are exposed to a stream of energetic charged particles known as cosmic rays. Although equipped with protective shielding, they are not always able to withstand such intense influxes of high-energy particles which can disrupt and shut down embedded electronics and mechanisms. A famous example of this was when cosmic rays caused solid-state circuit breakers on the Cassini probe to erroneously trip, requiring them to be reset, which in turn caused losses in valuable data. Another incident occurred when cosmic rays forced Cassini to enter safe-mode as it was sending images back to Earth, temporarily halting the transfer of scientific data. Fortunately, the mission was not entirely compromised, and Cassini ultimately spent nearly 20 years in space providing important information about the dynamic behaviour of Saturn’s rings, amongst other phenomena. However, this goes to show that even if a space probe’s shielding is able to reflect most cosmic radiation, precautions in the form of back-up computers are vital to ensure that valuable data is not lost.
As well as being able to resist radiation, space probes must be able to withstand extremely high temperatures, sometimes in excess of 2000°C, even if they never enter the atmospheres of any planets.The reason for this comes down to tiny particles known as space dust, which hit the probe and then turn into plasma. This poses a great risk to space probes, travelling at tens of thousands of kilometres per hour, but it is a problem which has only been recognised fairly recently. Sigrid Close, a Stanford researcher on hypervelocity impact plasmas, linked this phenomenon to the cause of several previously-unattributed satellite failures as recently as 2013.
Materials Engineering Offers Innovative Solutions
Scientists and engineers are continually searching for solutions to the aforementioned problems. Their goal is to find materials which exhibit heat-shield properties, while also resisting intense radiation. The most promising solution for the future seems to be carbon nanotubes. By taking carbon nanotubes (tube-shaped carbon molecules 50,000 times thinner than human hair) and pressing them into sheets, a layer of material known as ‘buckypaper’ can be produced which is 10 times lighter and 500 times stronger than steel. Not only does this material perform extremely well in heat trials, but it also shows potential for the development of radiation-resistant materials.
When it comes to protecting sensitive electronics from Galactic Cosmic Radiation and extreme temperatures, metals are the worst defence to use, because they are conductors and their nuclei are prone to fission which produces secondary radiation. For the opposite reasons, light elements such as hydrogen and helium are superior because not only are they insulators, but they can absorb cosmic rays without the associated risk of producing secondary radiation. Nanotubes provide a way of incorporating these radiation-resistant properties into a layer of material. A lattice of carbon nanotubes can store hydrogen at high densities and can also be laced with atoms of other elements which are good at filtering out different forms of radiation. All the while, the carbon layer remains light and flexible: an ideal protective barrier.
Materials Engineering certainly offers huge potential when it comes to space probe protection. Exciting advancements in self-healing materials offer further innovative solutions. Long chain-like molecules called ionomers can react to penetrating objects – for example, micrometeoroids – by closing and reforming the material behind it. Integrating self-healing features into the components of a space probe is, I believe, one of the key challenges in creating the robust space probes of the future.
One further hazard which no amount of material engineering will be able to overcome, and which is simultaneously a growing issue in the field of satellite and space probe protection, is that of space debris. More than 500,000 pieces of debris are tracked as they orbit the Earth and included in this number are over 3000 defunct satellites. As we conduct more launches, collisions are becoming more and more frequent. In fact, the situation has become so pressing that the European Space Agency has commissioned the world’s first space debris removal program, ClearSpace-1, because the only way to stabilise the orbital environment is to actively induce large debris items to burn up in the atmosphere.
While Earth-orbiting space debris is not at present the greatest challenge in the face of space probe exploration (as the probes themselves do not orbit Earth), the situation we find ourselves in is an important warning of what might occur around other planets in the future. As the number of orbiter and lander probes around other planets, such as Mars, begins to grow, space junk being a serious hindrance to future missions is becoming a very realistic possibility. In recognition of this problem, a number of studies have been carried out, particularly over the last two years, on the topic of Space Sustainability in Martian Orbits.
Mars is the natural next step as we expand our exploration programs to further-out planets and it is already orbited by 8 defunct spacecraft. Retrieval of space debris from Mars, due to the distances involved, will undoubtedly be extremely costly and thus, implausible, so I believe that we will need to find ways of implementing disposal mechanisms into space probes. An example of such a solution is thrusters which would be automated to take the probe out of orbit when its computers fail. Doing so will go a long way in the protection of the next generation of Martian probes.
3.0. Space Probe Power and Travel
In order for a space probe to reach its destination or desired orbital path, numerous challenges need to be surmounted in the areas of power generation, navigation, and propulsion. Space probe missions can last for years, decades even, and the ability to have a constant supply of power feeding into the various systems within the probe (measurement, communication, thermoregulation and propulsion) is crucial. The most obvious source of power in space is the Sun, which is why nearly all probes operating within the bounds of Mars’ orbit have solar panels consisting of photovoltaic cells. Currently, the most efficient cells consist of several very thin layers of semiconductor materials, each capable of converting a large part of the electromagnetic spectrum into electric current. Such cells are able to transfer up to 47% of solar energy into usable power, which is a significant improvement on the 6% which 1960s satellites were able to achieve.
The performance of solar panels degrades under the action of several phenomena. Firstly, solar radiation causes heating of solar cells, which has a negative impact on their performance. The estimated decrease in performance is 1% per degree Celsius, a significant issue for a probe such as the Parker Solar Probe, which passes within 4 million miles of the Sun’s surface every few months, encountering solar radiation intensities as high as 650 kilowatts per square metre. The solution to this challenge was the first of its kind: a water-cooled Solar Array Cooling System (SACS). This involved mounting the solar cells onto sheets of titanium containing tiny grooves through which cooling deionised water can flow. The water is highly-pressurised so as to avoid it boiling and it radiates the heat it absorbs into space as it flows into the probe’s radiator.
However, by far the most damaging factor, against which no amount of cooling can protect, is the action of high-energy particles produced by solar wind, which progressively damage the crystalline structure of the solar cells. The solar panels of the Magellan probe orbiting Venus lost two-thirds of their functionality during their operational lifetime. This proves the need for alternate methods of power generation, even for probes operating in close proximity to the Sun.
The intensity of solar radiation is proportional to the inverse square of the distance of a probe from the Sun. For the Voyager 1 probe, the Sun is merely the brightest star in the sky, 7000 times dimmer than it is from Earth. For this reason, Radioisotope Thermoelectric Generators (RTGs) are used as significant source of power for most space probes, especially ones operating beyond the orbit of Jupiter. RTGs are comprised of two main elements: a heat source, fuelled by the decay of radioisotopes (usually plutonium as plutonium dioxide 238-PuO2), and a set of solid-state thermocouples which convert this thermal energy into an electric current. The direct conversion of heat into electricity is made possible by joining two dissimilar conductive materials kept at different temperatures into a closed circuit. The power output of the thermocouples is a function of the temperature difference of each junction, which in turn relies on the steady decay of the plutonium isotopes.
The greatest current challenge in this field is finding ways of minimising the weight load of the plutonium, while maximising the power generated. By nature, this process if very inefficient: less than 10% of the heat produced from the nuclear decay is converted into electricity. To improve on this, current research is being done into Advanced Stirling Radioisotope Generators (ASRGs) which use a piston in conjunction with a working fluid to generate power (instead of thermocouples). The fact that the design incorporates moving parts inherently means that the development of a flight-ready ASRG is a difficult task, hence why tests didn’t produce adequate results by the time NASA’s development contract ended in 2013. However, privately-funded development continues, especially as the energy conversion process in an ASRG is calculated to be up to four times more efficient than previous radioisotope systems. This would mean that a space probe with a functioning ASRG would only need to carry one quarter of the plutonium which current space probes carry, thus saving as much as 10kg of mass.
Radioisotope power sources can produce a lot of energy over a long period of time. Indeed, the generator on the New Horizons probe is able to provide a stable power supply of 200W for more than 50 years, which is more than enough for the upkeep of its measurement and communication systems. However, the space-faring multi-purpose probes of the future will undoubtedly require more power than this 1) to carry out numerous braking operations for entry into orbit, 2) to make course corrections and orientation control manoeuvres, and 3) to accelerate to the required velocity for inter-planetary travel.
When it comes to high-power energy sources, nuclear fission (in the near future), and nuclear fusion (in the more distant future), are promising candidates. A fission reactor would be capable of fuelling the electric propulsion of a space probe beyond the inner solar system. Fission is a long-duration source, able to provide megawatts of power for performing sophisticated scientific investigations, high data rate communications, and complex operations. Going one step further, fusion, which releases energy by combining rather than splitting atoms, could in principle supply gigawatts of power. However, due to the extreme temperatures and pressures required, the development of commercial fusion reactors is currently a rather distant reality. Besides this, the first fusion reactors used for propulsion are likely to be very big, requiring a vehicle the size of the space station, and thus will be implausible for space probes until their size can be reduced.
Presently, ion thrusters are the primary method of turning power into propulsion, as they have a high specific impulse (see figure 1). They work by using a potential difference to strip electrons from stable gases, such as xenon or krypton, thus creating a stream of positively charged ions. This is accelerated using the principles of electrostatic forces, exiting the probe in low volumes, but at extremely high velocities – a very light-weight and efficient method of propulsion.
As with any propulsion method involving the expulsion of mass to generate a forward force, the motion of an accelerating space probe using thrusters can be modelled with the Tsiolkovsky rocket equation. This equation relates the maximum possible change in the velocity of a probe (\Delta v) to the mass ratio (a measure of how much propellant needs to be expelled relative to the mass of the probe). With reference to figures 2a and 2b, it is evident that the mass ratio increases exponentially with the required final velocity. Therefore, in order to continue pushing the boundaries of space exploration with ever-faster and further-travelling space probes, we would need to find propulsion systems with specific impulses exponentially greater than those of our current technologies.
Alternate Methods of Travel
Rather than pinning all of our hopes on future nuclear-fusion-powered probes, a solution to this challenge might be as simple as using methods of travel which don’t involve carrying any fuel source, altogether avoiding the limitations imposed by the rocket equation. In fact, we already do this, in part, by taking advantage of gravitational slingshots to provide ‘free’ acceleration to interplanetary space probes. By approaching a planet in the direction of its motion (i.e. from behind), entering its gravitational sphere of influence to gain some of its orbital energy, and then diverging from the planet’s orbital path, a space probe is able not only to change its direction of travel, but to gain a significant boost in velocity.
Solar sails work along the same lines of using natural phenomena to aid space probe travel. Photons are packets of energy which make up electromagnetic radiation and they have momentum associated with them. Therefore, when reflected in their billions, they impart a force on the reflective ‘sail’ and this can be used to gradually but steadily accelerate a space probe. The first probe to make use of this technology, IKAROS (Inter-planetary Kite-craft Accelerated by Radiation of the Sun), was a resounding success in demonstrating the practical uses of previously-discovered physical phenomena. However, it is worth noting that over the entire course of its 5-year mission, the solar sail only changed the probe’s velocity by an estimated 400 metres per second. In addition, solar sails can only be used for travelling away from electromagnetic radiation sources, which is not always helpful.
In summary, space probe power and propulsion technologies have come a long way, but there are still a great number of challenges to overcome before we can meaningfully increase the speed of travel and thus reduce the time taken by space probe missions.
4.0. Communication in Space
NASA’s Voyager 1 probe was launched in 1977 and it would go on to become the most-distant man-made object ever sent out into space. In 1990, when it was 6 billion kilometres away, it took and sent back the now-famous ‘Pale Blue Dot’ image, a stark reminder about how inconceivably vast distances in space really are. When it comes to space probe exploration, communication is clearly of immense importance, as without it, probes cannot send back any of the valuable data they collect. Space communications at the simplest level rely on the ability of a transmitter to encode a message using the modulation of electromagnetic waves (i.e. changing properties of the wave in order to represent data). These waves travel through space toward a receiver, which demodulates them in order to obtain the digital data.
The two greatest limiting factors on the transmission of data through space are the degradation of signal and the ever-increasing energy demands. Signal degradation is caused primarily by the long transmission distances, as well as the potential for waves to be scattered by Earth’s atmosphere. According to the inverse-square law, the power of any signal falls with the square of the distance travelled hence why energy becomes the critical factor in data transmission. A careful and difficult balance is required in maintaining enough energy to reach the receiver whilst keeping the energy required to a manageable limit. Radiation from other missions, the Sun, or clouds of ionised gas can further interfere with the quality of transmissions. Detecting such interference and the resulting errors, and then correcting them is thus vital. Solutions to this problem involve using ever-more advanced computer algorithms that interpret noisy transmissions and convert them into usable data.
Communication is of particular importance when it comes to space probe navigation. For space probes to navigate effectively over the vast expanse of space, they need to make calculations based on their velocity and position relative to other bodies in the solar system. NASA’s solution to this challenge is the Deep Space Network (DSN). The DSN is an array of giant radio antennas, strategically located at 3 sites around the world which are separated by roughly 120 degrees so as to permit constant communication with probes as the Earth rotates. The DSN supports space probe missions by transmitting radio signals which the probes receive and return with a slight frequency shift due to the Doppler Effect. By computing the time and frequency difference between the transmitted and received signals, the probe’s distance and velocity can be determined with great accuracy.
Optical Communication Methods
As space exploration missions become increasingly more sophisticated, the amount of data that probes gather and transmit to Earth is increasing rapidly. To accommodate this need, it is necessary to transmit using higher-bandwidth portions of the electromagnetic spectrum. The higher frequencies mean that a signal can be changed more rapidly and more data can be transmitted in any given time interval. However, there is no current method of using radio communication to carry higher data rates without enormously increasing the radio transmission power.
The most promising solution to this problem is the development of Deep Space Optical Communication (DSOC) methods. This makes use of lasers which have much higher frequencies than radio-waves and are thus able to transmit data at rates up to 10 times higher. The systems which transmit and receive these signals use comparable mass and power to current radio frequency technologies.
As is always the case with embracing new solutions, there are many challenges associated with optical communication. In particular, when transmitting from millions of miles away, the direction of transmission must be incredibly precise because the beam must be narrow to avoid compromising its intensity. However, as is the case with every other challenge mentioned in this essay, engineers are well underway with developing solutions. One such solution is shown in Figure 3: a robotic pointing mechanism for optical communications.
5.0. The Future of Space Exploration and Communication
The Next Step: The Interplanetary Internet
As we place more and more probes into orbit around other planets, it seems logical that we should internetwork space. Over the course of the next few centuries, engineers and scientists on Earth will need to be able to receive streams of data from probes (and people) at different locations in the solar system, manipulate sophisticated experimental instruments remotely, and share data to numerous locations in the solar system at once. In order to fulfil these needs, we will require an Interplanetary Internet: a computer network consisting of nodes in the form of orbiters and landers located on and around various bodies in the solar system. A space network will allow scientists anywhere in the solar system to receive, share, and send data via any of the nodes. This would overcome numerous present challenges, such as the fact that space communications require a line of sight.
We are curious beings, and it is curiosity that drives us to explore the unknown, expand our knowledge about the universe, and extend our influence into the solar system and beyond. The Interplanetary Internet is the natural next step in the process of exploring space, as it will allow us to transmit data and share new discoveries across the solar system more efficiently than ever before. More importantly, it will be a feat of engineering that extends the Internet, a symbol of communication and information in this modern age, into space, thereby signifying the beginning of humanity’s transformation into a truly multi-planetary species.
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Images and Diagrams
: Source: ResearchGate. Credit: Fatih Aydogan
: Credit: Krishnavedala on Wikipedia. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation
: Credit: The University of Texas Centre for Electromechanics https://www.cem.utexas.edu/content/canfield-joint