Dr Stuart Grey

Deep Space and the Impact of Space Debris on Communication Networks

Dec 03, 2019

Written to accompany my talk given at the University of Pennsylvania in March 2019

One of the defining moments of the last century was the live broadcast of Neil Armstrong’s first steps on the moon. Powerful live images were beamed back directly into the homes of an estimated 650 million viewers.

Of course, on closer scrutiny, this could not have been the case. Even today, an outside television broadcast requires a truck’s worth of equipment and every kilogram of weight on the lunar lander was already accounted for in equipment required to keep the astronauts alive and get them home.

In popular media to this day the assumption is that communication from and through deep space is instant and direct when this has never been the case. The infrastructure needed to broadcast the Apollo 11 landing to the world was complex and was one of the first examples of using space-based infrastructure to broadcast to the world.

The video was transmitted via a lightweight antenna on the top of the lander. The antenna was lined with 60 kilometres of fine gold-plated wire to broadcast the signal 400,000 kilometres back to Earth. The ground station intended to receive the signals was at Goldstone Observatory in California. Unfortunately Armstrong and Aldrin were too excited to take sleep as scheduled once their craft had landed on the moon and requested to leave the landing module ahead of schedule. Fortuitously the Honeysuckle Creek Tracking Station in Australia was in prime position to receive the signals. Once the signals were received they were uploaded to Earth-orbiting satellites, then transmitted back to NASA’s Manned Spaceflight Centre at Houston and, via further satellite links, to TV receiving centres around the world [1].

In order better to understand the nature of our space-based communications infrastructure, we have to understand where this infrastructure is located, what exactly it is, and with what other objects it is sharing its small slice of the space commons. To find the required context we have to go back to the dawn of the space age, the 1950s. In 1956 there were no man-made objects in orbit around the Earth, but the idea of space-based communications infrastructure had been discussed from the 1940s when the possibilities inherent in the technologies developed during the second world war became apparent.

Advances in rocketry during this period allowed for the possibility of infrastructure in space, but the limitations imposed by the physics of space flight meant that this infrastructure would look very different from that found on Earth.

The first limitation imposed is that for an object to spend a sustained period of time in space, it must be in an orbit. When we commonly talk of being in orbit or being in space, we are likely to be talking of distance. Although the distance from Earth is important, this is not in fact what makes an orbit special. What makes an orbit hard to achieve is the speed the object must reach. If we were to be shot out of a cannon straight up then we could, if we went high enough, reach space. The problem would be that we would very quickly fall straight back down to Earth. While a significant altitude is necessary, therefore, it is not sufficient for an orbit. To be in orbit we must also be moving sideways quickly enough that as we free fall, our curved path at least matches the curvature of the planet below us. If this is the case, our path is bent by the force of gravity but our speed means we never actually hit the ground.

In this way, the media’s use of the term ‘zero-gravity’, such as the example below from the film ‘Gravity’ is incorrect [2].

Gravity (2013) Script

RYAN
           Houston, I'm fine, it's just...
                          (SHE SWALLOWS)
           ...Keeping your lunch down in zero G
           is harder than it looks.

When objects are on orbit, they are in constant freefall and appear to be weightless in the same way you can feel weightless when going down a rollercoaster. Gravity is still very much present.

Once these concepts were understood, it became an engineering problem to get an object high enough and fast enough to maintain an orbit.

The first object to achieve this was Sputnik in 1957. It was the first human-made object in orbit and the first human-made communication infrastructure to have nearly world-wide coverage. It contained a short-wave radio transmitter that could be heard using amateur equipment on the ground whenever it passed ahead. Its orbit had a period of 96 minutes and it passed over the USA seven times a day, much to the consternation of the US establishment. The orbit itself is determined by the forces acting upon the satellite. The most dominant force by far is that of the Earth’s gravity bending the ballistic trajectory of the satellite into a closed orbit. There are, however, a number of other forces that determine the trajectory, or orbit, of a satellite. Variations in the gravity of the Earth itself perturb the satellites orbits. These variations are caused by non-uniform distribution of mass due to the highest mountain ranges and the deepest ocean trenches.

All tracked man-made objects in orbit around the Earth in 1960. Object size is not to scale. Object positions are to scale.

Satellite’s orbits are also affected by objects much further away. In particular, there are perturbations to the orbits caused by the gravitational pull of the moon, the sun, as well as Jupiter, due to its large mass, even though it is many of millions of kilometres away. The orbits of communication satellites are also affected by other forces, related to the mediums through which and by which they operate. One of the challenges of space-based communications is the fact that signals must be transmitted both into and out of the Earth’s atmosphere. The atmosphere causes signal degradations and attenuation but also poses problems for the orbit of the communications satellite themselves.

Earth’s atmosphere offers one of the few mechanisms for radically altering a satellite’s orbit. If a communications satellite has a low enough altitude, then its path will take it through the very top of the atmosphere. The atmosphere does not suddenly stop but attenuates exponentially from sea level. When a satellite passes through this extremely thin upper atmosphere, it experiences an incredible amount of friction because of the speed at which it is moving, over 22,000 kilometres per hour.

This physical interaction with the atmosphere slows the satellite down, reducing its altitude and causing it to slow further due to the increased density of the atmosphere as altitude decreases. This compounding effect leads to the swift demise of any satellite whose altitude drops low enough due to the object undergoing tremendous heating due to friction with the Earth’s atmosphere.

The destructive effect of atmospheric friction on objects in space is not, however, necessarily negative, especially when considered from the point of view of what can be done to manage the problem of space debris. Space debris is non-useful, man-made objects in orbit around the Earth and consist of number of different types of debris, from spent rocket bodies to fragments from collisions between objects.

For the destruction of obsolete or dangerous objects by burning them up in the atmosphere is one of the only factors that works to reduce the population of objects in space over time. Another force that can work in concert with the drag force of the atmosphere to clean up errant objects is that of photon pressure.

This force results from the continued impact of light, in the form of photons, on objects in orbit. While light, and thus photons, don’t have any mass, they do have momentum. Thus when photons strike an object, the collision results in a force being applied to the struck object as the photons reflect or are absorbed. We experience this force whenever light strikes us but its effect is so small as to be undetectable, yet in space, photon pressure acts to change the orbits of satellite and debris over time. Although slow, these changes in orbit can be enough to cause the orbit to graze the top of the atmosphere, starting the process of re-entry.

All tracked man-made objects in orbit around the Earth in 1970. Object size is not to scale. Object positions are to scale.

Photon pressure also has an effect particular to communication satellites. Space-based communication currently relies on the transmission of high frequency electromagnetic, or radio, waves. Like all parts of the electromagnetic spectrum, visible light included, these signals consist of photons of different energies, all of which exert a force on the satellite. In order for a space-based communication signal to be of use when it reaches the surface of the Earth, these photons must be of a high enough energy. This results in the interesting scenario of the very signal that the communication satellite creates causing a ‘recoil’ force on the satellite, pushing it into a higher and higher orbit (if its signal is aimed at the Earth). Unchecked by corrective manoeuvres, this would eventually lead the communications satellite to push itself away from the Earth simply by virtue of its communicating.

Even with all these different forces acting on communication satellites, their orbital paths, once put in motion, are dominated by the Earth’s gravity and thus can be stable if they are far enough away from the retarding influence of the Earth’s atmosphere. In the discussion of communication satellite and space debris in general, it is important to classify three distinct regions where their orbits can occur. The first of those regions is known as Low Earth Orbit or LEO. LEO is commonly defined as the region up to an altitude of 2000km above the surface of the Earth. It is important to note, however, that a satellite’s altitude is not always constant. Most orbits are in fact eccentric; that is, they are elliptical rather than circular, with the Earth situated at one of the foci of the ellipse. This means that, depending on how eccentric an orbit is, its altitude can be wildly different at different points of its orbit.

While satellites orbiting at the lower end of the LEO region (200-400km in altitude) have an orbital lifetime measured in days and months, satellites at the higher reaches of the LEO region are largely unaffected by the Earth’s atmosphere and have orbital lifetimes measured in centuries and millennia. This raises the key issue with the use of the near-Earth environment: objects that we put into orbit, both intentionally and unintentionally, can have lifetimes quite disproportionate to their utility. An extended orbital lifetime in of itself is not necessarily a bad thing, but every object in orbit around the Earth does pose a risk to our continued use of space.

This risk comes from the possibility of collision between objects. As stated earlier, the key feature of objects in orbit is their speed, and a collision between even very small objects has a tremendous amount of energy. For instance, an object the size of a fleck of paint or a small grain of sand, moving at orbital velocities in excess of 22,000 kilometres per hour, has the same amount of kinetic energy, and can therefore do the same amount of damage, as that of a high calibre rifle bullet impact back on Earth.

This extreme amount of energy means that even very small objects, including any debris objects created by collisions, can in turn cause a large amount of damage and create even more debris in the future.

All tracked man-made objects in orbit around the Earth in 1980. Object size is not to scale. Object positions are to scale.

The vast majority of operational satellites and debris can be found in the LEO region, including many communications satellites. Orbits close by the Earth offer reduced latency for communications due to the shorter distance that the electromagnetic signal has to travel. Communicating using electromagnetic signals requires line-of-sight between the transmitter and the receiver and this lower orbit comes at the expense of only being visible to a receiver on the ground for a short period of time. This limitation can be overcome to some extent by placing a number of identical satellites in complementary orbits so that some number are always visible to an observer or receiver on the ground.

Another class of communications satellites takes a very different approach to maintaining coverage over an area of the Earth’s surface. As satellites increase their altitude, their orbital period (the time it takes for them to complete one orbit) also increases. This can be argued intuitively by going back to our idea of an orbit being a free-fall around the Earth. If an object is further away then it needs a slightly reduced velocity to be sure it avoids the Earth, but its path will also be much longer than that of an object in lower orbit, meaning it will take longer to complete one revolution.

We can use this predictable relationship between altitude and orbital period to place satellites at a very specific altitude with a very specific orbital period. If we choose an altitude of 35,786km, for example, then the orbital period is exactly 24 hours. If this orbit is equatorial then the Earth and the satellite have the same angular rotation rate, making the satellite appear stationary in the sky to an observer on the ground. This orbit is called a geostationary orbit (or GEO) and it is exceptionally useful for communications. Because satellites in GEO do not move in relation to an observer on the ground then they can be used to broadcast communications signals to large swaths of the Earth.

Geostationary satellites tend to have long lifetimes, and as they can provide coverage for a large area (countries/continents) and space available at this orbit is scarce they themselves tend to be very large, extremely expensive and have lifetimes measured in decades. Yet the advantages of GEO for satellite communication system are checked by the fact that GEO’s debris environment is less understood than that of LEO, due to the fact that only optical sensors can be used to view objects at these altitudes, which puts severe limitations on what can be tracked. At LEO, radar can be used to track objects, resulting in better knowledge of the space debris population at these lower altitudes. The radar used for this purpose are nearly entirely military-owned due to the size and expense of the radar installations required in order to detect objects at many thousands of kilometres in altitude. The primary mission of these radar is in fact missile defence, with the detection of space debris as only a secondary mission. As such, the dissemination of knowledge about the space debris environment is almost entirely in the purview of the military. As with many space-based activities the knowledge and the technology that is required to produce it overlaps with national defence concerns. The fundamental knowledge however is in the public domain and it is important that media studies scholars contribute to the wider education of the general public as to the realities, limitations and risks to our communications and media infrastructure.

All tracked man-made objects in orbit around the Earth in 1990. Object size is not to scale. Object positions are to scale.

There is also a third regime, between LEO and GEO that contains satellites, that, while not used directly for communication, are fundamental to the continued operation of modern communications systems. The region with an altitude of about 20,000km is called “medium Earth orbit” or MEO. MEO is home to the navigation satellite constellations that have become, over the last two decades, vital to the operation of modern communication systems. While ostensibly tasked with providing positioning data to receivers on the ground, it is actually one of the navigation constellations’ secondary products, time (rather than position). that has had a more profound impact on how we live our lives.

In order to unpack why this is the case we first have to look into how a global navigation satellite system (GNSS) such as GPS, which is only one of a number of GNSS, works. A GNSS is comprised of a number of identical satellites that make up a constellation. These constellations are designed so that at least four satellites are in view from any point on the Earth’s surface (barring local obstructions such as buildings). Each satellite contains within it a set of complementary and, to an extent, self-correcting, atomic clocks.
The timing signal from typical GNSS satellites is accurate to within less than fifty Nano-seconds. This timing signal is used to broadcast signals that can be interpreted by receivers on the ground so that they can triangulate their position. These independent signals are also used extensively in communications to control the flow of information and data across the world. This information could be digital television broadcasts but is not limited to media communications. The global financial industry is heavily dependant on digital communication technologies and in particular the GPS timing signals are used as the source of truth when reconciling the billions of financial transactions that occur each day. Within the GNSS constellations themselves there is redundancy, but problems can and do occur within the ground segment of these constellations, that is the ground based infrastructure that monitors and controls the constellations. An example being the week long outage of the European Galileo constellation in July 2019. Countries can also lose access to certain features of these constellations due to political manoeuvring. One example being that the UK government led the development of the secure aspects of the Galileo system and fought for the inclusion of a clause in the foundational agreements that access to these secure aspects be limited only to EU members. The UK’s imminent exit from the EU has led to its exclusion from using the very infrastructure they helped to design. The global communication network comprises vital, interdependent space-based infrastructures in many orbital regimes and any significant loss of access to any of these regimes would mean fundamental changes to our way of life.

The primary danger posed to this space-based infrastructure is from other objects in orbit, that is, operational satellites and space debris.

All tracked man-made objects in orbit around the Earth in 2000. Object size is not to scale. Object positions are to scale.

Space debris itself encapsulates a number of different types of objects. One major group of debris is debris created during the launch and insertion of satellites into their orbits and through their normal operation. This class of debris includes spent rocket bodies, which are the remnants of the upper stages of the rockets used to launch the satellites. Once these upper stage’s fuel has been depleted, they are discarded but frequently have enough velocity to remain in orbit. These rocket bodies are relatively large in size, comparable in fact to the size of a school bus, which makes them relatively easy to track, but any collision between a rocket body and another moderate to large object would be extremely dangerous to other objects in nearby orbits due to the volume of new debris created.

This leads on to the other key source of new debris: collisions. The energies involved means any collision between two objects with orbital velocities will create many more new pieces of debris. The chances of collisions between large pieces are, however, low due to the the size of the volume of space in which these objects are distributed. Collisions with smaller objects, which are far more numerous, are much more common.

The final class of space debris is non-operational, or ‘dead’, satellites. Satellites, like any other technology, have a finite lifespan; but when they reach the end of their life, the laws of physics do not change and so they continue to follow the same orbital paths. This in itself means that the space debris population tends to rise over time as satellites cease to function and replacements are launched. This poses an increased risk of collision and debris proliferation in areas where there is a higher density of operational satellites, notably LEO and GEO. In geostationary orbits, satellites must have a very precise altitude and thus there is a competition for space above the areas of highest population density.

This allocation is managed in part through the international telecommunications union (ITU), although this organization is mostly concerned with the allocation of transmission frequencies to avoid interference. The spatial allocation is in fact done on a much more ad-hoc basis and relies heavily on the rewards of being a ‘good citizen’ outweighing any potential repercussions from bad behaviour.

One area of space citizenship is the safe disposal of communications satellites in geostationary orbits. Due to their distance from the Earth, dead satellites cannot be propelled towards the Earth’s atmosphere, so instead they are placed in a “graveyard orbit” at a higher altitude than GEO where they will not pose a danger to the highly lucrative operational satellites. While this strategy was well intentioned, it did not entirely take into account the changes that orbits can undergo over extended periods of time due to forces such as photon pressure. Non-operational satellites have been placed in graveyard orbits over a number of decades, but recently these dead objects have started to return back to the vicinity of the geostationary ring due to their orbits being perturbed over time. These so called ‘Zombie’ satellites and their associated debris objects pose a small but real risk to operational satellites at GEO.

All tracked man-made objects in orbit around the Earth in 2010. Object size is not to scale. Object positions are to scale.

A clear opportunity for bad behaviour and a vulnerability for all communications satellites in GEO is that all operational satellites at GEO are orbiting in the same direction to match the rotational direction of the Earth. An orbit of this type is called a prograde orbit. An orbit can be thought of as path through space and it can be travelled in both directions. This means that an object could be put into a retrograde orbit that matches the geostationary orbit but is travelling in the opposite direction. While no actor has undertaken this, such an action would, in matter of hours, destroy all satellites at GEO and a large swath of humanities communications infrastructure in one fell swoop.

The use of objects in retrograde orbits has, however, been used to destroy satellites intentionally, at LEO. Fengyun-1C was a defunct Chinese weather satellite in a sun-synchronous polar orbit at LEO. On January 11th 2007 Fengyun-1C was destroyed by an object put into a matching, retrograde orbit [3]. This object, colloquially known as a kinetic-kill-vehicle, was a Chinese weapon and the destruction of Fengyun-1C was an undeclared test of an anti-satellite weapon. The first that the wider world was aware of this weapons test was the appearance of hundreds of new objects in the radar tracking data shared by the US air force. Over the course of the next few weeks, analysts again used the technique of retrodiction (inferring from the present a state in the past based on the governing laws of the system) to determine that the object destroyed was in fact Fengyun-1C. As the debris cloud created by the weapons test dispersed, the true magnitude of the debris created became apparent. By the end of the 2006 over 2000 new debris objects were being tracked that were all created by the needless destruction of Fengyun-1C.

In reality, many more than 2000 new debris objects were created as there is a minimum size of object that can be detected by the radar systems. There is however another level of obfuscation in the dissemination of debris tracking data. Due to the fact that the US air force are the major source of tracking data, data for the smallest objects is not shared as this would allow competing powers to infer the performance of these military radar, which is highly classified information. As such the tracking data that is made available is for all objects with a size of 10cm across or larger so the Fengyun-1C missile test created 2000 objects of this size. For context, by 2008 there were 13,000 trackable objects in orbit around the Earth, of which 2000 were from the single Fengyun-1C weapons test. It should also be noted that the tracking data coming from military sources means that there are clear omissions form the catalogue of classified objects. There is however a burgeoning community astronomers who use back yard telescopes to track some of these objects and publish their data openly.

While greatly increasing the risk of collisions in LEO, the missile test was a singular event and was entirely within human control, should we choose to exercise that control. Much more worrying is the possibility of two large objects colliding with each other without any human intervention at all.

On February 28th, 2009, this precise scenario occurred. Two satellites, Cosmos-2251, a 2000lb defunct Russian military communications satellite and, Iridium 33, a 1000lb commercial telephony and data communications satellite, collided [4]. While a close approach of less than half a mile had been predicted for this day, the uncertainty in the tracking data lead to this prediction being incorrect. The two satellites collided with a combined velocity of over 40,000 kilometres per hour.

This collision again created a large amount of debris in LEO which spread from its initial orbit over the coming year. By the end of 2009, over 2000 new trackable debris objects had been created by this collision. The Cosmos-Iridium collision brought into focus the fact that while a collision between any two objects in orbit is very unlikely, the more objects we have in orbit and the longer they are there, the greater the risk of a catastrophic collision. The extra debris created from this collision creates an increased risk of future collisions, and the possibility of a run-away cascade of collisions in a given orbital regime must be considered.

The accuracy of the tracking data and the associated orbital predictions are improving over time. This means that if one or more of the objects in a predicted collision are actively controlled then that object can have the opportunity to manoeuvre out of the way of the debris object. Unfortunately, only about 1500 of the 20,000 objects in orbit are active satellites, so more collisions are inevitable, and we can only hope that they are rare enough events not to materially affect our ability to operate communication infrastructures in the various orbital regimes.

All tracked man-made objects in orbit around the Earth in 2019. Object size is not to scale. Object positions are to scale.

A new development in space-based communications hardware threatens to upset the status quo and radically change the risks posed to space based communication networks. So called mega-constellations are a new concept to allow for the blanketing of the surface of the Earth with high speed broadband internet connectivity. These mega-constellations will consist of a large number of satellites in LEO. In order to minimise the latency of these data connections, these constellations plan to use relatively low orbits. This has the knock-on effect that there must be a large number of satellites in a number of different orbital patterns to give constant coverage on the ground.

The issue with these planned mega-constellations is with the sheer number of satellites that will be put into orbit. For instance, the OneWeb constellation plans to place over 600 new satellites into LEO [5]. OneWeb is, however, dwarfed by the ambition of the Starlink project being developed by SpaceX [6]. The Starlink mega-constellation proposes over 12,000 new operational satellites. Considering there are only 1500 active satellites at the time of writing, the magnitude of this constellation cannot be underestimated. The first satellites in the Starlink constellation have been launched and the numbers will rapidly increase over the coming years. What remains to be seen is whether the increased risk posed by this large number of objects is manageable. With the current uncertainty inherent in predicting the nature of the whole orbital environment using sanitised military tracking data, this remains to be seen.

Dr Stuart Grey

3rd December 2019

Bibliography

[1] "FROM THE MOON TO YOUR LIVING ROOM: THE APOLLO 11 BROADCAST," 7 August 2019. [Online]. Available: https://www.scienceandmediamuseum.org.uk/objects-and-stories/moon-to-living-room-apollo-11-broadcast.

[2] A. Cuarón, Director, Gravity. [Film]. UK/USA: Warner Bros. Pictures, 2013.

[3] Celestrak, "Celestrak - Chinese ASAT Test," 6 June 2012. [Online]. Available: https://celestrak.com/events/asat.php.

[4] Celestrak, "Celestrak - Iridium 33/Cosmos 2251 Collision," 22 June 2012. [Online]. Available: https://celestrak.com/events/collision/.

[5] Federal Communications Commission, "FCC Grants OneWeb US Access for Broadband Satellite Constellation," 22 June 2017. [Online]. Available: https://www.fcc.gov/document/fcc-grants-oneweb-us-access-broadband-satellite-constellation.

[6] Federal Communications Commission, "FCC Authorizes SpaceX to Provide Broadband Satellite Services," 29 March 2018. [Online]. Available: https://www.fcc.gov/document/fcc-authorizes-spacex-provide-broadband-satellite-services.