How High Does The International Space Station Orbit Earth?
How High Does The International Space Station Orbit Above Earth?
Over the past decade, astronauts and cosmonauts on board the International Space Station have been orbiting the Earth between altitudes of roughly 220 to 248 statute miles, which is equivalent to around 350 to 400 kilometers.
The International Space Station is regularly boosted to its most efficient altitude of 248 miles or 400 kilometers above Earth.
Why Does The International Space Station Orbit At These Altitudes?
Fuel
By raising the station's altitude, the amount of fuel required to maintain its orbit can be reduced by more than half, which is the main advantage. Bill Spetch, from the Systems Engineering, Analysis and Integration Office of the space station program, stated that the primary reason for increasing the altitude is to conserve propellant over the long term as a precaution against increased solar activity. As the atmospheric density within the altitude range of the space station increases with rising solar activity, it experiences more drag, causing a more frequent need to raise the orbit.
Despite being located in the "vacuum of space," there are still enough atmospheric molecules to impact the surfaces of the station's solar panels, truss structure backbone, and pressurized modules, altering its velocity of around 17,500 miles or 28,000 kilometers an hour. The station's size, equivalent to a football field including the end zones, causes a slight but continuous decrease in its altitude due to the cumulative effect of these tiny particles. To counteract this effect, thrusters on the station or visiting vehicles like the space shuttle, Progress resupply vehicles, or ATVs are occasionally fired to "reboost" the station. However, this requires significant fuel or propellant launched from Earth.
Spetch clarified that flying higher means fewer reboosts are required to maintain the orbit, resulting in a reduced need for fuel.
Space Junk
The U.S. Space Surveillance Network currently tracks over 22,000 objects that are larger than 4 inches (10 cm). Of these objects, only around 1,000 are operational spacecraft, while the rest are classified as orbital debris. The estimated population of particles that are between .4 inches and 4 inches (1 to 10 cm) in diameter is approximately 500,000, and the number of particles smaller than .4 inches (1 cm) is likely in the tens of millions.
Most of the orbital debris is concentrated within 1,250 miles (2,000 km) of Earth's surface, and the concentration varies with altitude. The highest concentration of debris can be found at an altitude of 500-530 miles (800-850 km).
The debris that orbits in low Earth orbit (below 1,250 miles or 2,000 km) moves at a speed of between 4 and 5 miles per second (7 to 8 km/s). However, when colliding with another space object, the average impact speed is around 6 miles per second (10 km/s). As a result, even a small piece of debris can cause significant damage upon impact.
Radiation
Exposure to radiation can pose a significant long-term health risk for crew members, and as each crew member lives and works aboard the space station, doctors and researchers are able to gather more information on this critical topic. This information will be essential for future space exploration beyond low-Earth orbit, where the risk of radiation exposure will be even greater. This is due to the fact that Earth is shielded from much of the cosmic radiation by the Van Allen Belts, which lie outside the orbit of the station.
The Balancing Act Of Altitude
Spetch explained that the decision to increase the altitude of the space station involves weighing various factors such as the capabilities of vehicles to reach that altitude, cargo capacity, crew radiation exposure, and the risk of orbital debris. The goal is to fly as low as possible while still being supported by propellant resupply, and this altitude increases with solar cycle activity. However, the station's current low altitude was chosen to maximize the cargo delivered by space shuttles.
Currently, the space station requires approximately 19,000 pounds of propellant per year to maintain its orbit. At the new, slightly higher altitude, it is projected to consume only 8,000 pounds of propellant annually. This reduction in propellant usage will enable more food, water, clothing, research instruments and samples, and spare parts to be transported on cargo vehicles, ensuring the station's continued operation beyond 2020.
