When you think about the orbit above Earth's atmosphere, you think of an empty space. But the truth is, we have been polluting it for more than half a century, and what lies above could one day make space travel impossible.
Almost 60 years have passed since the first artificial satellite, Sputnik 1, was launched into orbit in 1957. The satellite itself was a primitive device. It transmitted beeps that could be heard by amateur radio operators on the ground, and after three months, while descending, it burned in Earth’s atmosphere. However, the launch was a huge Soviet scientific and engineering breakthrough. It took the US, their Cold War rivals, by surprise. What followed was a Space Race between the two superpowers: Laika, the dog, was launched into orbit on Sputnik 2, and then Yuri Gagarin became the first man in orbit in 1962, and in 1969, the Apollo 11 crew members, Neil Armstrong and Buzz Aldrin, landed on the moon. The Space Race sparked a revolution in many areas, but the most important one was probably the satellite industry. Today, there are more than 1000 functioning satellites orbiting the Earth, around 2500 ones that no longer work, and the number is increasing rapidly. Artificial satellites have undeniably made our lives better, and it is almost impossible to imagine the modern life without them. They allow us to communicate faster, receive TV and radio signal easily, allow us to use the phone and the internet in places where no infrastructure exist (oceans, remote areas). They take photos of Earth to help us predict weather and monitor the environment; they help us determining our location and make real-time driving instructions possible. The list is a long one.
However, there is a darker side of the story, and this is the topic of this blog: space debris. Also called space junk or space waste, it is a collection of man-made objects that are orbiting the Earth (sometimes, even natural objects like meteoroids orbiting the Sun are considered space debris). Since they can collide with active satellites, rockets and spacecrafts, they pose a real threat. When you think of space above the Earth’s atmosphere, you think of an empty place, populated with the moon and a couple of satellites. But that is very far from the truth. NASA estimated that there are around 19,000 debris items larger than 10 cm, around 500,000 items from 1 to 10 cm, and tens of millions of items smaller than 1 cm. Most of the items are really small (a couple of centimeters or less) and one would think that the damage they could do to satellites is negligible. But the problem is the fact that these objects move fast (at speeds of around 10 km per second in Low Earth Orbit). That means they have a lot of kinetic energy, and the impact they could have is big - a small object with a mass of 20 grams travelling at 10 km/s has the same kinetic energy as a 2-ton truck travelling at 100 km/h. These objects are polluting the entire orbit, but the biggest danger is in the busiest one - the Low Earth Orbit.
Space debris (Credit: NASA's Goddard Space Flight Center/JSC)
So, how are these object created? Well, in a couple of ways.
They are sometimes just pieces that fall of existing satellites, sometimes they are spent rocket stages, and sometimes they are satellites that are no longer in use. Conditions in space are extreme and satellites can not work forever. Every satellite is built with an expiration date in mind, and it’s typically around ten years. After satellites stop working, they remain in orbit.
Although satellites and bigger pieces of debris are monitored from the ground, collisions between satellites can and do happen. In 2009, a collision occurred between an inactive Russian military satellite (Kozmos-2251) and an active communication satellite (Iridium 33) owned by the satellite phone operator Iridium. The expectation was that these satellites would miss each other by 580 meters, but since these calculations are not always 100% precise, a collision occurred. It happened on an altitude of 800 km and at a speed of 40,000 km/h. The collision destroyed both satellites and around 1,000 pieces of debris larger than 10 centimeters (in addition to many smaller ones) were created.
Another cause of space debris is something with a scary name: “kinetic kill vehicles”. These are just rockets that are launched by the military of a country to test if they can destroy an enemy satellite in a case of warfare. In 2007, the Chinese military performed an anti-satellite missile test. A rocket, flying at a speed of 8 km/s hit and destroyed the Fengyun-1C Chinese weather satellite. The crash is the largest known space debris creation event, until this day. Around 2,000 objects of trackable size (a couple of centimeters) were created, along with around 150,000 smaller pieces. The US performed similar tests in 1985, and there were probably a couple of others, but these tests are usually covered up, for obvious reasons.
So, with a bunch of space debris in orbit, and more and more satellite launches every year, what is the darkest possible scenario? The math here is very simple. Since there are more and more artificial bodies launched into orbit - they cause the amount of space junk to increase constantly. That means that the probability of satellite collisions is constantly increasing. That brings us to a scenario proposed in 1978 by a NASA scientist called Donald Kessler. He proposed that there is a point where the density of objects in Low Earth Orbit will be so large that collisions would cause a cascade effect (nicknamed “Kessler syndrome”). That means that every collision would create more debris and that debris would then cause more collisions and so on. This scenario, if not avoided, could make the use of satellites in the Low Earth Orbit impossible, and make rocket launches very difficult.
A couple of proposed solutions exist, but none of them solves the problem completely. As already mentioned, larger pieces of debris are monitored from Earth using radar, and this information is used to predict and avoid possible collisions (not always successfully, as in the case of 2009 satellite collision). There are techniques used to reduce the number of space debris pieces: one of them is to use a graveyard orbit. When the satellite’s end of life approaches, it is moved to a different orbit, and in that way, there is more room for new satellites and less probability of collisions. There are also materials and techniques used when building a satellite that are designed to protect it from collisions. One such technique is to use a special type of armor, called the Whipple shield. The armor got its name after its inventor Fred Whipple, and it is build to withstand collisions with space debris that is moving from 3 to 18 kilometers per hour. However, this can not protect the satellite’s solar panels. Some of the proposed solutions sound like they were taken from a science-fiction movie. In the 1990s, NASA proposed a solution to use lasers from the ground and push pieces of debris when they are on a collision course with a spacecraft.
The darkest possible scenario may never happen. Someone will maybe find a good and effective solution. We don’t know. But the fact is, it is a real threat, and if not solved, it will become an even bigger threat in the future.