Space may be big, really big, but Earth orbit is getting a bit crowded as space debris accumulates and threatens operational spacecraft. Riding aboard the last Dragon cargo mission to the International Space Station was part of the answer to this problem. NASA's Space Debris Sensor (SDS) will be installed on the outside of the station, where it will spend the next two to three years monitoring debris between 5 mm to 0.5 mm in diameter to learn more about their characteristics.
Currently, there are over 1,400 operational satellites orbiting the Earth at distances from a few hundred to tens of thousands of miles above the planet. Over the past 60 years since the first Sputnik, our civilization has become completely dependent on these satellites. Without them, everything from the internet to commerce to national defense would be crippled overnight. Today, we depend on satellites for much of our long-distance communications and data transmission, weather monitoring, navigation, defense reconnaissance and many scientific investigations.
The problem is that over the decades many of these satellites have died and the rockets that launched them into space are also often in orbit, along with a great deal of miscellaneous space debris. This miscellaneous junk is the result of spacecraft breaking apart, including booster upper stages shredding themselves after over-pressurizing their tanks or the propellant inside vaporized, satellites blowing up when their discharged batteries generate volatile gases and droplets of solidified metal coolants from old Soviet nuclear-powered satellites.
According to NASA, there's over 170 million items of debris ranging from discarded boosters to flecks of paint. In all, there are over 5,500 tonnes of debris circling the Earth with 98 percent made up of 1,500 objects in low Earth orbit. These include not only dead satellites and boosters, but a glove lost by US astronaut Ed White during his historic Gemini 4 spacewalk in 1965, a camera lost by Michael Collins on the Gemini 10 mission in 1966 as well as another lost by Sunita Williams of STS-116, a thermal blanket lost during STS-88 in 1998, 15 year's worth of rubbish bags from the Russian Mir space station, a pair of pliers, a US$100,000 tool bag and the ashes of Star Trek creator Gene Roddenberry.
The potential damage from this debris as it travels at about 22,000 mph (35,400 km/h) is huge with even particle as small as 3 mm posing a threat to manned and unmanned spacecraft.
"Debris this small has the potential to damage exposed thermal protection systems, spacesuits, windows and unshielded sensitive equipment," says Joseph Hamilton, the SDS project principal investigator. "On the space station, it can create sharp edges on handholds along the path of spacewalkers, which can also cause damage to the suits."
Beyond degrading systems, there's also the possibility of a catastrophic collision. In 2009, the deactivated Kosmos 2251 crashed into the operational Iridium 33, creating a cloud of debris that makes up a large percentage of low Earth orbit junk. The fear is that a similar strike between two satellites could result in the Kessler syndrome, which is a chain reaction of collision debris causing more collisions at an exponential rate.
However, it's not all doom and gloom. The amount of debris in orbit remains constant due to debris entering the atmosphere at a rate of between one and three objects a day, depending on solar activity. This means that, at the very least, much of the problem would solve itself so long as we don't send up more debris.
This is essentially where things stand at the moment. Though there is no international treaty governing the control of debris, there are traffic rules similar to those used at sea and in the air to avoid collisions and many spacefaring powers have unilaterally adopted measures to control the problem.
For example, spacecraft and boosters are now designed to be much tougher and less prone to self destructing or breaking up if struck by debris. New satellites are designed to either move into disposal orbits in deep space at the end of their lives or to plunge into the Earth's atmosphere using thrusters, solar sails, or drag mechanisms. Many scientific missions are placed in eccentric orbits that decay rapidly at the end of their missions, while boosters and batteries include countermeasures to keep them from exploding.
Due to its size and human occupants, protecting the ISS from debris is a high priority for NASA. To protect the station, the modules are wrapped in Kevlar mats and incorporate Whipple shields, which are thin, outer layers that causes debris to break up and disperse on impact without damaging the main hull. Meanwhile, larger pieces of debris over 3 mm in diameter are tracked from Earth and the station uses thrusters to change its orbit to avoid a collision.
To better understand the nature of the problem and come up with better solutions, NASA's SDS will be mounted on the outside of the Columbus module, where it will provide near-real-time impact detection. It does this using a three-layered acoustic system.
The top two layers are identical and equipped with acoustic sensors and resistive lines that are 0.075 mm wide. If a piece of debris hits the first layer, it cuts one or more of these lines before heading for the second layer, then the particle is collected by the third layer made of high-impact Lexan plastic. Using the data from these three layers, scientists will be able to determine the debris particle size, density, and velocity on impact. It does this using a simple triangulation algorithm plus timing and location data.
"The backstop has sensors to measure how hard it is hit to estimate the kinetic energy of the impacting object," says Hamilton. "By combining this with velocity and size measurements from the first two layers, we hope to calculate the density of the object."
The space agency hopes that the data from the SDS will help scientists map the debris in orbit and help in the design of future sensors to be used beyond low Earth orbit.
"The orbital debris environment is constantly changing and needs to be continually monitored," says Hamilton. "While the upper atmosphere causes debris in low orbits to decay, new launches and new events in space will add to the population."
The video below outlines the SDS project.
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