Space debris, or space junk, is a collection of man-made stuff in Earth’s orbit that no longer serves a purpose. It includes old satellites, spent rocket stages, and fragments resulting from disintegration, erosion, and collisions.
The term “space junk” may evoke an ugly image of a cloud of randomly distributed garbage floating above the Earth. This scenario, as ugly as it would look, would not be particularly dangerous. Space is big, and the space around Earth is vast enough to accommodate any reasonable amount of litter generated from Earth. Yet, space junk is a real threat to space operations and space travel, not just something undesirable and unaesthetic.
Computer-generated image of space debris. The objects are not to scale, so reality is much less threatening than this picture suggests.
Why Is Space Debris A Problem?
To really understand why space debris is a problem we need to clarify three aspects of space travel.
First of all, objects in space around Earth are not stationary: they all travel along orbital paths whose shape and altitude are determined by the laws of orbital mechanics (a branch of physics) depending on their velocity. The typical orbital velocity is 25,000 km/h or higher, and when two objects travelling at that speed collide, their combined impact speed is larger than 36,000 km/h, a massive kinetic energy that can shatter the two objects into thousands of smaller debris, each one travelling faster than a bullet.
Second, objects tend to stay in orbit for a very long time because there is very little atmospheric drag to reduce their speed. The higher the altitude, the longer it will take for orbital debris to re-enter Earth’s atmosphere. Debris left in orbits below 600 km normally fall back to Earth within a few years. At altitudes of 800 km, the time for orbital decay is measured in decades. Above 1,000 km, orbital debris will keep on circling the Earth for a century or more.
Third, while the space around Earth is incredibly vast, satellites are mostly concentrated in a few strategic orbits characterized by unique properties. The greatest concentration of debris is found at a 750-800 km altitude, an orbital belt crowded by huge and massive (6 to 8 tons) remote sensing satellites. These kinds of satellites travel into polar orbits, meaning that they all cross the poles from a variety of angles multiple times a day, increasing the probability of high-speed collisions. Moreover, at that altitude it takes several decades of natural decay for an object to re-enter, making the issue even worse.
Satellites in polar orbits face higher probability of fatal orbital collisions above the Polar Regions.
The combination of three factors creates a serious problem that is growing more serious with the projected increase in launch rate. And while the probability of an impact is relatively low, the consequences are catastrophic because they lead to creation of thousands of new debris, with a consequential exponential increase of the problem.
Still Not Convinced?
Every day satellites cross path with pieces of space junk, sometimes at very close distance. Satellite operators try to sort out these conjunctions and decide which ones may result in a collision. Based on this evaluation, they perform collision avoidance maneuvers to preserve their operational spacecraft. Unfortunately, the data is incomplete because it is not materially possible to constantly track all the debris orbiting Earth, so the data is updated using statistical models.
When you have to deal with hundreds conjunctions per day, you need to find the right compromise. If the probability of a collision is not high enough, you must consider other factors, like how much time it will take to change course, how much fuel it will take, how much it will disrupt the service, and so on.
On February 10, 2009, an operational Iridium satellite collided with a nonfunctioning Russian Kosmos 2251 satellite. The satellites were traveling along two intersecting orbits at a 789 kilometers altitude. The speed of the impact was 42,120 km/h, enough to turn them both into hyper-speed confetti, large parts of which are still orbiting the Earth, threatening the safety of various space asset including the International Space Station.
Computer-generated simulation of the Kosmos-Iridium collision of February 2009.
Credits: Analytical Graphics, Inc (www.agi.com)
Operators of the Iridum constellation were aware of the conjunction but they had estimated that the two satellites would miss each other by 584 meters, with a risk of collision of one in 50 million. This estimate was based on public data coming from NORAD, the highest authority on debris tracking, and it shows that this kind of accidents are bound to happen every once in a while unless we work on reducing the debris population.
While there is not much we can do for existing space debris population, we can significantly improve the situation by making sure that every satellite that goes up will come back down in a direct and controlled way as soon as its mission is over.
This is what we are doing with D-Sat. Join our KickStarter campaign, and help us make this possible.