The Debris Situation

The launch of the first satellite “Sputnik” in 1957 marks the advent of the space age. Since then, around 5,000 rockets were launched into space and an even greater number of satellites has been placed into orbit. The utilization of space by humans has left its traces in the form of space debris. Space debris – some say space junk – consists for example of remaining rocket bodies, disused satellites and fragments of breakups, explosions and collisions. Around 20,000 objects larger than 10 cm are being tracked today. It is assumed, however, that a much larger number of smaller debris pieces is orbiting planet Earth. Estimates for the debris population go as high as 700,000 objects with a size larger than 1 cm.

Even very small objects can pose a serious risk to space infrastructures such as satellites or the International Space Station as these objects travel at very high relative speeds. Collisions with space debris can lead to grave damages or even the destruction of operational satellites. Moreover, collisions cause the breakup of new fragments which in turn can collide with other objects creating even more fragments. This snowball effect is called the “Kessler syndrome”. Once a critical mass of space debris is reached in a particular orbit region and, thus, the Kessler syndrome of cascading collisions takes effect it will quickly render this orbit region uninhabitable for satellites or human spaceflight.

Today, space utilization is not anymore limited to a few powerful countries. A large number of countries, international organizations and private corporations now operate their own satellites. While this is a clear sign of how space technology creates benefits for the people on Earth, it also highlights the issue of space debris. Intensified space usage will almost certainly aggravate the debris situation. Therefore, solutions to the space debris issue are urgently needed and can reach from regulatory incentives to technical solutions.

The Risk from Space Debris

Space-based infrastructures have become major assets for our economy and “space” has become part of our daily life. Meanwhile, the growing space debris population jeopardizes the use of space. The risk of a spacecraft colliding with debris depends on three factors:

  1. Debris density in a particular orbit region,
  2. Spacecraft cross section,
  3. Relative velocity of the objects.

However, the precise calculation of the collision risk of a given spacecraft over a certain period of time is a lot more complicated than it looks at first sight. Today, tracking the orbits of space debris is limited to larger pieces which represent only a small fraction of the entire debris population. As orbit determination for space debris is subject to inaccuracies, collision projections are a matter of probability calculus. As some debris experts say: You can only be 100% certain about a collision, once it has happened.

The most densely populated orbit regions are low Earth orbits and geostationary orbits. Today, satellites in low Earth orbit receive dozens of so called conjunction warnings every year. Only a few warnings will lead operators to initiate actual avoidance maneuvers. Avoidance maneuvers consume propellant and, hence, reduce a satellite’s lifetime. Defunct and uncontrolled spacecraft are a potentially dangerous source of space debris. Every collision creates even more new debris. In a worst case scenario, a cascade of collision events and a quickly growing number of debris pieces could render space utilization as we know it impossible. Luckily, such a situation can be avoided if we take the necessary steps. The risk from space debris can be managed. Reducing the creation of new debris by removing defunct and uncontrolled spacecraft is a major step towards a sustainable space environment for future generations.