The International Space Station (ISS) orbits the Earth at an altitude of about 220 miles, but this distance can change due to atmospheric drag, solar activity, and space junk. LEO is preferred for manned space stations, but objects in this region must have their orbits adjusted from time to time. At an altitude of 220 miles, the ISS uses approximately 19,000 pounds of propellant each year, while at its planned 248-mile orbit, it would need only 8,000 pounds of fuel.
Most of the time, the International Space Station (ISS) is orbiting the Earth at an altitude of about 220 miles (354 km), which places it in low Earth orbit (LEO). This distance can change, however, and has ranged from 205 miles (330 km) to a planned maximum of 248 miles (400 km). Even at these altitudes, there is a small amount of drag from the extremely thin atmosphere, slowing the station’s orbital speed and bringing it closer to the surface. This effect is greater for the space station than for other satellites due to its relatively large size. Sometimes, the altitude is adjusted upward to compensate for this effect or in anticipation of increased drag due to the interaction between solar activity and the Earth’s upper atmosphere.
Low earth orbit
The low-Earth orbit zone extends to about 1,243 miles (2,000 km) from the planet’s surface. The main advantage of placing something in this area is that travel times to and from the satellite are short and less fuel is used in transporting materials or crew back and forth. The downside is that there will be some drag from the atmosphere that extends into this region of space, albeit in a very subtle form. This means that objects in LEO must have their orbits adjusted from time to time, otherwise they will slow down, fall steadily closer to Earth and eventually crash. Satellites that do not need to be visited will normally be placed in a higher orbit, but LEO is preferred for manned space stations or other objects that require frequent manned or unmanned visits.
Finding the Right Orbit
Keeping an object in LEO at a given altitude requires the use of fuel, or propellant, to maintain its speed or to bring it to a greater altitude if it has descended slightly. Closer to Earth, less propellant is needed on trips to the space station, but atmospheric drag is greater, so more fuel is used to keep it on track. Further from earth, the ISS uses less fuel to maintain its orbit, but more is used to keep the station fueled and transfer crew. A balance must be found between these factors.
At an altitude of 220 miles (354 km), the ISS uses approximately 19,000 pounds (8618 kg) of propellant each year by adjusting its orbit. At its planned 248-mile (400 km) orbit, the station will require fewer adjustments and would need to use only 8,000 pounds (3629 kg) of fuel. This means that refueling rockets, while using more fuel, can carry less to station, leaving more room for other items.
Other factors affecting the altitude of the International Space Station are solar activity, solar and cosmic radiation, and space junk. When solar activity is high, it increases the density of the atmosphere at very high altitudes, increasing drag on the station, so its orbit must be adjusted upward. Since the station carries a human crew, however, the effects of increased exposure to ionizing radiation from the Sun and cosmic rays, which limit the increase in orbital distance, must be considered.
Space junk, made up of discarded fragments from rockets and other debris, is also very common in LEO and should be avoided. Ideally, the station’s orbit will keep it away from known objects, but the ISS can be moved slightly higher or lower to avoid debris if it is detected early and deemed a threat. Other times, crew members may temporarily transfer to the Soyuz spacecraft in case debris strikes the station and causes major damage.
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