In 1957, the Soviet Union launched the very first artificial satellite into orbit. This mission was known as Sputnik-1 ("satellite" in Russian), a simple satellite designed to broadcast radio pulses. In the sixty-three years that followed, space agencies and commercial launch providers have made countless launches to space.
To date, the vast majority of these launches have been for the sake of deploying satellites to Low Earth Orbit (LEO). The purpose of these satellites has ranged from telecommunications, scientific research, Earth observation, weather tracking, military operations, and navigation.
Initially, these satellites were used only by government organizations and armed forces, who relied on the satellites to provide vital services. But in recent decades, satellite deployment has become dominated by the commercial space sector (aka. NewSpace), which has been taking advantage of new technologies and lower costs to launch greater numbers of satellites.
Naturally, this has led to growing concerns that the orbital space lanes are becoming too crowded. This is due in no small part to the growing number of inoperable satellites and orbital debris (aka. "space junk") floating around up there.
And with so many new satellite constellations (or mega-constellations) planned for the future — like Starlink and other broadband internet satellite services — the problem is only going to become more pressing over time.
Some Sobering Numbers
Satellites and other objects in orbit are monitored by various organizations, like the European Space Agency's (ESA) Space Debris Office (SDO) — located at the European Space Operations Center (ESOC) in Darmstadt, Germany.
NASA is also responsible for monitoring orbital debris through the Astromaterials Research & Exploration Science (ARES) Orbital Debris Program Office. It also tracks potential hazards in conjunction with the Department of Defense's (DoD) Joint Space Operations Center (JSpOC).
There's also the Inter-Agency Space Debris Coordination Committee (IADC), which was created in 1993 as a means to allow members to coordinate on matters pertaining to space debris. Today, it includes NASA, the ESA, Roscosmos, and the national space agencies of China (CNSA), Canada (CSA), Japan (JAXA), South Korea (KARI), India (ISRO), and Ukraine (NSAU).
- 34,000 objects larger than 10 cm (~4 inches) in diameter
- 900,000 objects measuring between 1 cm to 10 cm (~0.4 to 4 inches)
- 128,000,000 objects measuring between 1 mm to 1 cm
The sheer amount of debris is quite staggering. And while some might think that objects measuring no larger than a speck of dirt are nothing to worry about, they may want to think twice. Because of the rate of Earth's rotation, orbital debris can reach speeds of up to 56,000 km/h (34,800 mph).
At this speed, even the tiniest bit of junk can inflict serious damage on an operational spacecraft, space station, or satellite. Researchers at MIT recreated what this looks like for astronauts by shooting microparticles against a hard surface and recording the results with a high-speed camera.
What they found was that particles as tiny as 10-micrometers in size (0.01 mm) and traveling at speeds of 3600 km/h (`~2240 mph) would cause tiny cratering to occur on a metal surface, including melting and erosion.
Meanwhile, collisions between larger objects can lead to another major hazard - catastrophic break-ups that result in even more debris. This phenomenon is known as...
Also known as the Kessler Effect, this phenomenon takes its name from Don J. Kessler, an American astrophysicist and former NASA scientist who specialized in the study of space debris. In 1978, he published a paper titled "Collision Frequency of Artificial Satellites: The Creation of a Debris Belt."
In this paper, Kessler observed that once space debris reached a certain critical mass in orbit, a catastrophic chain reaction could occur - where collisions give rise to more debris, which will give rise to more collisions, and so on. This would make LEO far too hazardous an environment for operational satellites and space exploration.
In 2009, Kessler commented on how the phenomenon he predicted would actively unfold:
"Aggressive space activities without adequate safeguards could significantly shorten the time between collisions and produce an intolerable hazard to future spacecraft. Some of the most environmentally dangerous activities in space include large constellations such as those initially proposed by the Strategic Defense Initiative in the mid-1980s, large structures such as those considered in the late-1970s for building solar power stations in Earth orbit, and anti-satellite warfare using systems tested by the USSR, the US, and China over the past 30 years. Such aggressive activities could set up a situation where a single satellite failure could lead to cascading failures of many satellites in a period much shorter than years."
Time has certainly proven parts of Kessler's prediction to be correct. According to the ESA, debris levels in LEO have increased by 50% in the last five years alone. Since 1957, there have been around 550 fragmentation events, with just over 50 ocurring in 2020 alone.
The CubeSat Revolution
Part of the problem is that satellites have become much more prolific in Low Earth Orbit (LEO). As the cost of launching payloads has decreased, space exploration and space-based research have become a great deal more accessible. This is true not only for smaller space agencies but also for commercial entities and non-profits.
One of the major drivers for this is the advances that have taken place in electronics, digital technology, and circuitry, which have allowed satellites to become much smaller and cheaper in recent decades. As a result, a new type of small satellite has emerged known as the CubeSat.
The name refers to satellites whose dimensions are 10 x 10 x 10 cm (1000 cubic cm) - or roughly 60 cubic inches (4 x 4 x 4 in) - and weigh about 1 kg (2.2 lbs). Because of their diminutive size and mass, CubeSats can be used on their own or stacked together to create larger satellites that can carry out more tasks.
Prior to 2013, the majority of CubeSats were launched on behalf of academic institutions, and used to conduct scientific research. However, commercial entities and private organizations have come to account for more than half of the market in recent years.
This is largely due to the fact that they are much cheaper to send to space and can take advantage of rideshare programs (where they are launched as part of a larger payload). Unfortunately, the accessibility that CubeSats and other advancements have made possible is also adding to the overall problem of orbital clutter.
In the long-term, having more participants in space will mean that the amount of junk left behind will become greater, making the Kessler Effect all the more likely. This problem cries out for a one-two punch solution, meaning that there need to be strategies at both the mitigation and removal end of things.
In terms of removal, a number of options are currently being explored. These include everything from satellites with harpoons and nets that could snare space junk and deorbit it, tousing magnetic space tugs and even lasers!
At the other end of the spectrum, scientists are working to create next-generation satellites that can avoid collisions and remove themselves from orbit. In this case, strategies include novel propulsion systems and/or tools that allow them to deorbit themselves once they are no longer operable.
The UN Office for Outer Space Affairs has also established debris mitigation guidelines, which include planning for end-of-life disposal of orbiting satellites. The number of countries and agencies signing on to these guidelines is growing. On top of this, Around 88% of small payloads launched into LEO will automatically adhere to space debris mitigation measures because of their low altitude, meaning they will break up in Earth’s atmosphere at the end of their lifetime.
Between 30-60% of all satellite mass (that not involved in human spaceflight) is estimated to adhere to end-of-life guidelines for the same reason.
Options for Propulsion
Traditionally, satellites have been equipped with the same kind of propulsion technology as spacecraft — chemical thrusters. These systems rely on reactive chemicals (solid or liquid) that are then ignited to produce a high-temperature expanding gas. This gas is then channeled through nozzles to generate thrust.
This method has been used since the dawn of the space age to provide attitude control and maneuvering capability. The main benefit of chemical propulsion is its simplicity. In such a system, there are very few moving parts, and the energy requirement is quite small.
However, they also have their share of drawbacks, which include a limited fuel capacity and the fact that propellants are often toxic. This becomes a problem when satellites and spent rocket stages become defunct before they have had a chance to exhaust their fuel supply.
When these objects collide with others in orbit, explosions can occur, causing breakups and large clouds of debris. Hence, why efforts are underway to develop alternative means of propulsion.
Cold Gas Thrusters:
Cold gas thrusters rely on an inert gas (like nitrogen) in a pressurized tank, which is then released through a nozzle to produce thrust. The immediate advantage of this is that it relies on propellants that are non-explosive and (in most cases) non-corrosive or toxic.
This method also has the advantage of being the simplest form of propulsion technology since it relied solely on cryogenically-preserved gas and (usually) a single valve. Unfortunately, they have their share of drawbacks as well, the most obvious of which is low performance (when compared to chemical propulsion).
This method relies on electricity to accelerate propellants, which usually consist of inert gases. Examples include ion thrusters, Hall-effect thrusters, pulsed plasma thrusters, and electrospray thrusters. In all cases, these systems use electric energy to charge (ionize) an inert gas.
The resulting ionized particles are accelerated by a magnetic chamber to generate thrust. Typically, ion engines have relied on xenon as a source of fuel, but there are newer concepts that make use of other elements as well (like iodine).
This technology is advantageous because of its extreme fuel efficiency and ability to generate a high specific impulse over time. However, it also requires a great deal of power, which calls for large solar cells or batteries and large propellant tanks.
Solar sails (aka. light sails/photon sails) rely on pressure from solar radiation from stars to push sails of highly-reflective and ultra-thin material. The immediate advantages of this method are that they require no propellant, and the force they generate is scalable (based on the size of the sail itself).
On the other hand, solar sails need to be larger than the satellite to generate a useful degree of propulsion. Deploying and storing these sails also requires that they come equipped with a complex mechanical apparatus, adding to the satellites' overall size and mass (and adding the possibility of mechanical failure).
Water Propulsion to the Rescue?
Since the 1960s, NASA and other space agencies have relied on a combination of liquid hydrogen and oxygen for propellant. This has given rise to proposals for engines that rely on water, either by vaporizing and expelling it to generate propulsion or through a process known as water-electrolysis.
Electrolysis comes down to applying an electrical charge to separate water into hydrogen and oxygen. NASA has been researching the potential of this technology for powering spacecraft, but it is only in recent years that it has become feasible. This is largely due to the introduction of small satellites (aka. CubeSats) that require much less thrust to maneuver.
For years, NASA has pursued the development of this technology through their Small Business Innovation Research (SBIR), their Small Spacecraft Technology (SST), their CubeSat Launch Initiative (CLI), and their Pathfinder Technology Demonstrator (PTD) program.
In 2017, NASA demonstrated a water-based propulsion system developed by scientists at Purdue University. The system was incorporated into the Operations and Data Transmission Optical Communications and Sensor Demonstration (OCSD) mission, which consisted of two CubeSats performing a coordinated maneuver in LEO on June 21st, 2019.
The ostensible purpose of the OCSD mission was to test new optical transmission methods for relaying data back to Earth more efficiently. However, the mission also demonstrated the effectiveness of the propulsion system involved, which came down to four onboard thrusters that relied on solar energy to convert small amounts of water into steam.
In 2020, the first viable water-electrolysis engine for small satellites was developed by the Washington-based company Tether Unlimited Inc. (TUI). A "Water Electrolysis Thruster" is technically a chemical propulsion system since it burns hydrogen and oxygen.
One of the immediate advantages of such a system is that it can provide its fuel in-situ, harvesting ice in space to replenish its propellant. Earlier this year January, NASA announced that TUI's thruster would be included in the first mission of their PTD program (PTD-1), which launched on January 24th as part of a six CubeSat package destined for orbit.
Another recent innovation is the ThermaSat engine developed by Howe Industries, an Arizona-based company specializing in nuclear technologies, thermal systems, and space propulsion. Compared to water-electrolysis engines, the ThermaSat engine is an example of the "thermal rocket" approach.
This engine relies on an optical surface to convert solar radiation to heat, which is used to turn water into superheated steam an instant before it is shot out of a rear nozzle. This allows it to perform fast burns that will allow satellites to deploy quickly and make rapid changes in their orbit.
Since it relies on water and solar energy, it does not incorporate materials or fuels that are corrosive, toxic, or explosive. Therefore, this system offers a number of advantages over conventional engine technology, not the least of which are cost-effectiveness and versatility.
It also has the benefit of having only two moving parts, which makes it simpler and more compact than other systems. Nevertheless, it can deliver 1,800 Newton-seconds of total impulse (or 203 lbs/s of specific impulse) with just 2.2 lbs (1 kg) of water. This is enough to maintain a CubeSat in LEO for up to five years.
The technology is also beneficial when it comes to the mitigation of orbital debris. As Dr. Troy Howe (Ph.D.), founder and CEO of Howe Industries, told Interesting Engineering via email:
"The ThermaSat concept is attractive to addressing space debris concerns for two major reasons. The first is that the system is so universal and easy to use that it can provide propulsive capabilities to nearly every mission in orbit. With the ability to eventually de-orbit at the end of the mission, most of the debris can clear itself out when finished operating.
"The second capability is the fact that the ThermaSat can provide high impulse burns for collision avoidance. Many of the larger pieces of debris can be tracked or might be seen coming towards the satellite ahead of time, and minor course corrections can be made to avoid interception. It may not solve the problem of space debris altogether, but it can at least keep that specific mission safe."
These and other systems will not only allow satellites to avoid colliding with each other. They will also permit satellites to deorbit themselves as they reach the end of their service period. When at last their instruments break down or the mission is nearing the last of its fuel supply, satellites equipped with propulsion systems will be able to initiate a "Swan Song" protocol.
This will involve lowering their altitude significantly, effectively diving into Earth's atmosphere so they experience more air friction and their orbit decays faster. In no time at all, they will burn up on reentry and pose no threat to future missions. Kind of poetic, if you think about it!
"Necessity is the Mother of Invention"
Many of the innovations we are seeing today are driven by pressing needs. Between climate change, population growth, and the prospect of having to feed more people with fewer resources, researchers are coming out with new technologies and ideas that offer solutions.
When it comes to avoiding potentially catastrophic chain reactions in orbit, there have been a wide variety of proposals — ranging from the inventive to the deceptively simple. David Mayer, the PTD-1 project manager at NASA’s Ames Research Center said:
"We have a driving need for small spacecraft propulsion systems. The need is for many reasons: to reach a destination, maintain orbit, maneuver around other objects in space, or hasten de-orbit, helping spacecraft at end-of-life, to be good stewards of an increasingly cluttered space environment."
As Dr. Howe added, this situation demands innovative solutions, which include equipping satellites with engines so they are able to avoid collisions:
"Generally speaking, the laws of space are not as well developed as the laws on Earth. Low Earth Orbit (LEO) is shared between all nations that can place equipment in it, and in the past, there have been very few civil matters requiring litigation. But as this area becomes more popular, space debris, communications interference, privacy, and other issues become more prevalent.
"We believe that having a propulsion system onboard every satellite will give operators the ability to control their spacecraft and introduce some level of responsibility to space development. A satellite without propulsion is akin to a car without a steering wheel, and it would probably be best for everyone if every vehicle had one."
In the next few years, the small satellite market is expected to grow extremely fast. Thanks to miniaturization and improvements in digital technology, opportunities for space-based research, exploration, and commercial ventures will become even more common.
Add to that the multiple multinational corporations proposing to launch constellations of internet satellites and the growth of the telecom and commercial space sectors, it's clear that the satellite industry (and Low Earth Orbit) are going to become very crowded very soon!
- CSA - What is a CubeSat?
- USCUSA - UCS Satellite Database
- ESA - Space debris by the numbers
- ESA - The Kessler Effect and how to stop it
- NASA - ARES Orbital Debris Program Office
- NASA - Pathfinder Technology Demonstrator
- NASA - Ion Propulsion: Farther, Faster, Cheaper
- The Planetary Society - Lightsail: Flight by Light for CubeSats
- NASA Spinoff - Water-Powered Engines Offer Satellite Mobility