According to the ESA’s Space Debris Office (SDO), there are about 31,630 debris objects in orbit that are regularly tracked by space surveillance networks. However, this only accounts for the larger objects and doesn’t include the (literally) millions of tiny bits of “space junk” that pollute Low Earth Orbit (LEO). According to the SDO, this includes an estimated 36,500 objects greater than 10 cm in diameter (~4 inches), 1 million space debris objects measuring between 1 cm to 10 cm (0.4 to 4 inches), and 130 million space debris objects measuring between 1 mm to 1 cm.
These objects pose a regular threat to the International Space Station (ISS) and will only worsen as satellite “mega-constellations” are deployed to LEO and humanity’s presence there grows. To simulate the dangers these impacts will pose to future missions, a team of Canadian engineers developed an Implosion-Driven Launcher (IDL) that would accelerate magnesium projectiles to hypervelocity – up to 10 kilometers a second (36,000 km/h; 22,370 mph). This gun will effectively simulate the damage micro-objects could inflict on future space stations, spacecraft, and satellites.
The team consisted of mechanical and chemical engineers from McGill University (Montreal, Quebec) and the Royal Military College of Canada (RMC) in Kingston, Ontario. The study that describes their findings recently appeared online and is under review for publication in the scientific journal Shock Waves. As they describe in their paper, the IDL measures 8 mm in diameter and relies on shock-compressed helium gas – at five gigapascals (GPa) pressure – to launch 0.36 g magnesium projectiles. This is consistent with micro-objects ranging from 1 mm to 1 cm in diameter that are sped up by Earth’s rotation.
As Higgins explained to newsastronomy.com via email, the shielding on the ISS is sufficient to withstand collisions with objects smaller than 1 cm in diameter. For larger objects, tracking them in orbit allows for a degree of advanced warning so the ISS can perform avoidance maneuvers (which it does several times a year). The objects that range from 1 to 10 cm are particularly hazardous because they cannot be tracked and generate more impact energy than current shielding can withstand.
“There are likely tens of thousands of objects in this size range in Low Earth Orbit. When these impacts occur, the events are so extreme [and the] properties of materials are not well understood under these conditions. Thus, while computer simulations of hypervelocity impacts are performed, there is considerable uncertainty in how well they can be trusted (the “garbage in, garbage out” problem). So, experimental testing is necessary, particularly at velocities of 10 km/s and faster.”
While most debris objects in orbit are tiny, the nature of their velocity makes them very dangerous. In LEO, objects are sped up to 8 km/s (28,800 km/h; 17895.5 mph) due to Earth’s rotational velocity.
“Statistically, the most likely collisions will occur at around 11 km/s,” he added. “Although orbital velocity in LEO is 8 km/s (because various objects are in different inclinations), the most likely impact would be a side-on collision at 11 km/s. The requirement for testing orbital debris shielding at greater speeds has long been identified as critical, including by studies done by the U.S. National Academy of Science.”
Laboratory testing involving hypervelocity impacts and shielding has so far been restricted to launchers that can achieve about 8 km/s. Attempts to test objects at greater speeds have been difficult because of the extreme temperatures and pressures generated by the propellant gas launchers. The launchers are further limited since they can only fire projectiles that are less than 1 gram (0.035 ounces) in mass. These launchers, Higgins explained, are known as “light gas guns”:
“As high-pressure gas pushes a projectile down the launch tube, the gas expands and cools and eventually cannot push the projectile any faster: The projectile has outrun the gas. For this reason, scientific gas guns for testing at the greatest speeds use either hydrogen or helium as a propellant. Being a light gas, they have a high speed of sound and are able to keep up with the projectile, but even then, there is a limit.
“To get the greatest possible pressure, laboratory light gas guns fire a larger piston (using gun powder or another high-pressure gas) down a cylinder filled with hydrogen or helium, compressing the light gas to very high temperatures and pressures. This high-pressure light gas then acts to push the smaller projectile. Hence, a two-stage light gas gun [see video above].”
For their purposes, Higgins and his colleagues needed a launcher that could achieve speeds over 10 km/s with projectiles larger than 2.5 cm (~1 inch) in diameter. To that end, they built disposable launchers that used high explosives to “squeeze” the propellant gas to very high temperature and pressure conditions. In this design, an explosive-driven tube replaces the first-stage piston-driven “pump tube.” And while the launchers are single-use only, Higgins and his team know how to build them inexpensively, and all the metal is recycled after each test.
High-speed recording of a projectile launched by the IDL gun (click to see the full video). Credit: Andrew Higgins
Their launchers are more compact because of the high-explosive energy source, measuring about 1 meter (3.3 ft) in length compared to conventional light gas guns that can be tens of meters long. Higgens also provided a demonstration video (shown above) and provided newsastronomy.com with further technical details:
“A layer of explosive surrounds a tube filled with helium. When the explosive is detonated, a detonation wave sweeps down the tube and squeezes and compresses the helium, similar to how you would squeeze a nearly empty tube of toothpaste. The pressures and temperature in the helium can get to greater than fifty thousand atmospheres and 30,000 C, which is then able to push the projectile to speeds of 10 km/s. The explosives can even extend onto the launch tube, to help contain the pressure and continue to squeeze the propellant. Remarkably, the projectile can withstand these pressure and accelerations—approaching one million times Earth’s gravity—and emerge from the end of the launch tube intact.”
These studies are critical given the growing human presence in space, the commercialization of LEO, and the growing problem of orbital debris. In particular, there’s the dreaded “Kessler Syndrome,” where the prevalence of debris in orbit causes breakups and collisions that lead to a cascading effect. According to various models, even if we stopped launching satellites to LEO today, the situation is still projected to worsen. But at present, broadband internet satellite providers like Starlink and OneWeb have no intention of slowing or stopping and want to grow their constellations in the coming years.
The costs associated with space debris collisions are also a growing concern. In 2019, the Organization for Economic Co-operation and Development (OECD) published its first report titled “Space Sustainability.” As it states, “Space debris protection and mitigation measures are already costly to satellite operators, but the main risks and costs lie in the future, if the generation of debris spins out of control and renders certain orbits unusable for human activities.”
A piece of debris hit the Canadarm2 on the International Space Station. Credit: NASA/Canadian Space Agency
“So, this problem is not going to go away, and a space-faring civilization will need to adapt to the problem of orbital debris,” added Higgins. “How to avoid future collisions and—since collisions will happen—how to best protect spacecraft and minimize the generation of new debris in such collisions. Addressing these issues will involve testing in the lab, and this is where our implosion-driven launcher can contribute, at the greatest velocities and largest projectile sizes.”
The implosion-driven launcher Higgens and his colleagues built is currently being used to test samples of different materials that go into the creation of the Canadarm2. This 17-meter (~56-foot) robotic arm has been a key part of the ISS since it was installed in 2001 and was instrumental in the station’s assembly. It has since played a vital role in ISS operations, where it is chiefly used for docking/undocking arriving and departing spacecraft. Last year, the Canadarm2 suffered an impact from a tiny piece of space debris but took the hit like a champ and kept working!
Further Reading: arXiv
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