[Reading Science] What Are the 6 Worst Space Debris in the 'Worst-Case Scenario'?

"Be cautious of the six most dangerous space debris pieces expected in the worst-case scenario."

In 1978, NASA scientist Donald Kessler warned that Earthlings could become trapped by space debris they themselves created. This is known as the 'Kessler Syndrome.' Abandoned satellites and spacecraft collide with each other, creating thousands to tens of thousands of pieces of space debris. Through repeated collision and diffusion processes, the debris increases exponentially, filling the orbit. The scenario predicts that no satellite or spacecraft can navigate through. Space debris travels at an ultra-high speed of 70,000 km/h, making defense impossible with current technology. This was also the theme of the popular South Korean movie Gravity (released in 2013).

On the 7th (Eastern Time, USA), the Russian 'killer satellite' Cosmos 2499, which broke into 85 pieces, became a representative case. Scientists fear the 'worst-case scenario' if larger defunct satellites or launch vehicles collide. This is also why startups in Europe, the USA, and Japan have begun commercializing space debris removal, satellite repair, and recycling. In this context, caution has been raised about six existing or potential space debris pieces currently orbiting. These include the Hubble Space Telescope, which boasts the greatest astronomical achievements, and Cold War-era reconnaissance satellites.

Experts have identified six existing or potential space debris pieces currently orbiting Earth as subjects of concern. First, old large space launch vehicles launched from the Cold War era to the early 2000s are expected to be problematic. The SL-16 rocket, used as the upper stage of Russia's Zenit launch vehicle, is a representative example. The American space company LeoLabs analyzes that 18 SL-16 rockets are currently orbiting at an altitude of around 840 km. They are as large as a big school bus. In other words, these large space debris pieces are 11 meters long and weigh 9.9 tons, so if they collide, an enormous number of fragments could be generated. Given their current orbit, it may take hundreds of years for them to re-enter Earth's atmosphere. Other Russian upper stages, such as the SL-8 rockets, and secret reconnaissance satellites used during the Cold War era are also troublesome. The SL-8 rocket weighs 1.54 tons and was used to launch a total of 145 Russian Cosmos satellites (for reconnaissance and communication) into a 975 km orbit from the 1960s to the 1990s, after which they were abandoned. Additionally, the 145 satellites launched at that time have reached the end of their lifespan and orbit Earth weighing about 800 kg.

Fragments generated by China's anti-satellite missile test in 2007 are also a major headache. At that time, China used an anti-satellite missile to destroy a malfunctioning communication satellite (weighing 750 kg) orbiting at 865 km altitude at a speed of 29,000 km/h. The debris created by this explosion scattered into billiard ball-sized pieces, contaminating space over hundreds of miles between 350 km and 1700 km altitude. The number of fragments detectable by ground radar is about 3,500, of which about 2,800 are uncontrollable and threaten everything in their path while still orbiting today. Of the 400,000 close approach warnings sent by LeoLabs to spacecraft and satellite operators last year, 15% were due to debris generated by this satellite explosion test.

A hole in the space shuttle Endeavour. In 2007, during a round-trip mission to the International Space Station (ISS), it collided with space debris, creating a hole approximately 5.5mm in diameter on the radiator panel. (Source: NASA)

The large defunct satellite Envisat, launched by the European Space Agency (ESA), is also a subject of concern. Envisat has been orbiting at 800 km altitude since 2012 and is very close to the Russian SL-16 and SL-8 rocket upper stages and the 145 Cosmos satellites, making the situation even more precarious. Weighing 8.8 tons, it is one of the largest pieces of space debris in orbit.

NASA is also responsible for launching dozens of Landsat satellites, which are among the space debris of concern. Of the Landsat satellites weighing between 1 and 3 tons launched since the 1970s, only two are still operational. They orbit between 700 and 900 km altitude, but none have yet successfully re-entered the atmosphere. The US National Oceanic and Atmospheric Administration (NOAA) has also placed 24 polar orbiting observation satellites in orbits between 700 and 900 km since the 1990s. Of these, 17 have become space debris weighing 1 to 3 tons and continue to orbit dangerously alongside other debris.

The Hubble Space Telescope, considered one of the greatest astronomical observation tools to date, will also become a problem when its lifespan ends soon. Weighing 12.4 tons, it is larger than Envisat or the SL-16 rocket. However, it currently orbits at a low altitude of 535 km, so it would take only a few years to re-enter the atmosphere after its lifespan ends. Nevertheless, because it is such a large object, there is a risk it will not burn up completely and could impact the Earth's surface. Experts advise that the Hubble Space Telescope's operators should reserve enough fuel to guide its fall into the ocean upon deorbiting.

[Image source=Yonhap News]

The recent surge in the number of small satellites launched is also a major threat. Since the Soviet Union first launched Sputnik 1 in 1957, about 10,000 satellites have been launched into low Earth orbit over 50 years. In the subsequent 14 years, during the New Space era, launches for space internet construction by companies like SpaceX surged, more than doubling the number. As of the end of 2022, the number reached about 21,000. SpaceX alone operates 3,500 satellites as of February and performs avoidance maneuvers whenever an object with a collision probability of 1 in 100,000 appears. This is a much more proactive defense than NASA, which performs avoidance maneuvers at a 1 in 10,000 probability. SpaceX plans to launch up to 42,000 Starlink satellites, and other companies like OneWeb and Amazon are also expected to join.

Jonathan McDowell, a researcher at the Harvard-Smithsonian Center for Astrophysics, told the space news outlet Space.com, "In the next 5 to 10 years, the number of satellites in Earth's orbit is expected to increase by 20,000 to 100,000. Whether all these satellites can be operated safely is doubtful. If even one (satellite operator) company inputs incorrect information, all calculations could be wrong, leading to collisions."