How Spacecraft Currently Avoid Debris
MIKE LUNDY, HR & PROJECT MANAGEMENT ASSOCIATE
4.5 minute read
In humanity's never-ending quest for progress and advancement, it is inevitable that things are left behind. Throughout history, no matter where people have gone they’ve left their mark, usually evident by the trash or junk left behind. Space is no different. Since humanity first left the surface, humans have left everything from massive rocket bodies to small wrenches and tool bags soaring through our closest orbits at a mere 17,000 mph. Like most adventures of exploration and discovery, the impact of what is left behind is an afterthought and generally that problem belongs to tomorrow. Much like the discarded and no longer needed items of Earth, space is left with the unique problem of what to do with the leftovers traversing around our planet, further complicated by the rate at which those items are traveling. Of course, at one time it was believed that there was plenty of space, so what's the problem? The laws of physics interfered with this masterful plan of “plenty of space out in space” and orbital forces kept those pestering rocket bodies and debris fragments just floating around waiting for their next opportunity to shine. Those opportunities come in the form of very high-speed close encounters, and sometimes catastrophic collisions with other active or inactive satellites, spacecraft, or the International Space Station (ISS). Without diving head first into the ever-expanding, growing issue of space debris and the Kessler Syndrome it is important to understand the steps and actions that are carried out in order to avoid such collisions today.
Computer generated image of objects in Earth orbit that are currently being tracked (NASA ODPO)
With Earth’s orbits cluttered by tens of thousands of large debris objects and hundreds of millions of pieces of small debris, it is not a question of if, but when, a spacecraft needs to maneuver away from risk. For this reason, active spacecraft need to not only be capable of performing their intended missions but must also have procedures in place to ensure safe avoidance maneuvers are possible. The ISS for instance, needs to maneuver away from potential collisions a few times per year (37 times for debris alone across its operational lifespan). When the ISS maneuvers it is not like a car swerving on a highway to avoid a piece of road debris. The ISS would be notified of a potential for collision when the object is thousands of miles away. Think of the ISS being in San Francisco and the object in question being in Washington D.C. An adjustment of a single degree while they approach over the course of such a distance would put almost 43 miles of separation between the two objects and what would have been a collision course. In order to perform the avoidance maneuver the ISS can adjust its orbital plane and velocity with a relatively small action like a thruster burn that may only last 5 minutes. Due to the vast distance between them the adjustments seem small, but are still emergent and costly if not effective. Before jumping into what maneuver capabilities and avoidance look like, it is important to understand the current functions used for tracking and detection to accomplish such avoidance maneuvers.
Currently, the United States Space Force 18th Space Defense Squadron (18th SDS) operates the Space Surveillance Network (SSN) that tracks, catalogs, and identifies the Earth’s artificial resident space objects in orbit. The SSN uses radar and optical sensors from positions around the world for observation and tracking. The size of debris that is tracked is based on the heights of the orbits and ranges from softball sized objects in lower orbits to objects larger than a basketball in higher orbits. The sensors are used to determine and predict approaches, reentries, and collision probability. There are other international agencies that track and detect in a similar manner, but the Space Force’s 18th SDS is the United States’ primary agency and is focused on US satellites and also monitors the ISS for collision avoidance.
Satellite break-up (ESA/ID&Sense/ONiRiXEL)
Knowing where the debris is provides the owners/operators with some of the knowledge that is needed to avoid collisions. The spacecraft have to react on information that is known and ensure that they avoid collisions with operational and non-operational objects. For instance, the ISS operates at just about 250 miles above Earth’s surface, which is considered relatively low in orbit. Taking roughly 90 minutes to fully orbit Earth, its maneuverability is limited to maintain its trajectory. The ISS maneuvers by firing its thrusters, which are on the Russian segment and not always available on short notice, to raise orbital altitude with a velocity increment of less than one meter per second. This is executed once a month to maintain orbital altitude. In the original plans for the station, it was assumed that the monthly orbit boost would be sufficient for avoiding debris, as a debris avoidance maneuver could also serve to raise the orbit for that month. With the exponential growth of RSO and conjunctions, the ISS has had to maneuver more frequently and beyond what is necessary to only maintain its orbit. The use of tracking debris, thruster firing, and added protective sheets to external portions of the ISS all assist in the prevention of catastrophic damage.
For satellites and other spacecraft with onboard propulsion the avoidance techniques rely on similar concepts as the ISS. Changing orbital altitude based on tracked objects and the potential for collisions can be effective but comes with the cost of unpredictable wear and tear on critical components. The decision to maneuver is based on probability of collision and differs between manned and unmanned craft. Use of propulsion for thrust operations is utilized if available, but if the spacecraft lacks onboard propulsion then other methods such as alternating surface areas to increase atmospheric drag to change orbital altitudes can be used. Some other means of maneuvering are reaction control thrusters, magnetorquers, reaction wheels, and control moment gyroscopes or in the case of the ISS they can also use the engines of docked spacecraft.
In today’s space environment and in the Earth’s orbit, debris avoidance and maneuvering are built into the mission at a procedural level. Compounded by the amount of RSO in orbit, these operations have to be a major part of the mission and levels of precautions that are taken. With new active payloads entering the orbital environment by the thousands every year it is essential that actions be taken to solve the orbital debris problem and not just avoid it. Keeping Space Clear for All is not just a mission for KMI, but it is a mindset that needs to be shared everywhere for the continued use of space.
Recommended column to read next: ISS Debris Avoidance Over the Years