How Things Work: Satellites
A satellite, by definition, is any object that orbits another object in space.
When one thinks of a satellite, the picture that comes most readily to mind is that of a man-made satellite — a contraption of steel and electronic devices with large, signature solar foils extending from each side.
In fact, the average human being interacts with a dozen different satellites everyday.
From Internet and television to cell phones and Global Positioning System (GPS) trackers, many devices rely on satellites to operate.
Satellites are a relatively recent invention. The first man-made satellite, Sputnik, was launched by the Soviet Union 50 years ago, an event that marked the beginning of the space race between the United States and the Soviet Union.
Today, there are over 560 known operational satellites in orbit. These satellites perform various functions, including weather surveillance, navigation, communication, and environmental monitoring.
These satellites do not include military satellites, which are mostly classified for national security purposes.
Satellites are usually specialized, meaning that different satellites are equipped to perform different functions.
Imaging satellites, for instance, are equipped with cameras and other imaging tools, such as infrared scanners or radar. These satellites relay images of Earth and space back to Earth.
Imaging satellites are used for a variety of functions, including astronomy, mapping, and weather prediction. Digital maps, such as Google Earth, rely heavily on data from imaging satellites.
The Hubble Space Telescope is perhaps the most well known imaging satellite. This telescope captures images of stars and galaxies.
Communication satellites, on the other hand, relay communication signals back to Earth.
Global positioning satellites, which are part of GPS, return only one piece of information to Earth — the distance between the satellite and the receiver. This distance is calculated by measuring the amount of time taken between the transmission of the signal and reception of the reply.
A GPS receiver usually communicates with three separate satellites. These satellites place the receiver’s location at the intersection of three coordinate spheres that are superimposed onto Earth’s surface.
These three spheres constrain the receiver’s location to a maximum of two different points on the Earth’s surface. One of these points is usually located within Earth, and so is eliminated.
In general, most satellites operate largely on solar power, as evidenced by the signature solar panels on satellites. Rechargeable batteries are used to power the satellite when the satellite passes behind Earth, which blocks sunlight.
Excess solar power, which is gathered when the satellite receives sunlight, is then used to recharge the satellite’s batteries.
Many satellites are also equipped with small thrusters, which are small rockets attached to the sides of the satellite. These thrusters provide a small amount of maneuverability in space.
This allows satellites to reposition themselves to perform various functions, including focusing on different locations on Earth, altering their orbits, and avoiding collisions with space debris. Satellites orbit the earth in various ways, the most common of which are polar orbits.
Satellites that are in polar orbits pass over the North and South poles.
Satellites in geosynchronous orbits, on the other hand, rotate around the earth every 24 hours, and hence return to the same location above Earth at the same time each day.
A Molniya orbit is a special type of geosynchronous orbit that places the satellite roughly above the same location on Earth at all times. This type of orbit is commonly used for communications satellites.
Launching a satellite into orbit requires careful calculation of the satellite’s velocity.
The required minimum speed for a stable orbit is above 17,000 miles per hour, but the maximum velocity that an object can travel without escaping Earth’s gravitational pull is around 23,500 mph. This is called Earth’s “escape velocity”.
If the satellite’s velocity climbs above Earth’s escape velocity, the satellite leaves Earth’s orbit altogether and flies off into outer space. But if the satellite’s velocity falls below the stable orbit velocity, the satellite reenters Earth’s atmosphere and burns up.
Hence, the satellite must travel at a velocity that is greater than the stable orbit velocity but less than Earth’s escape velocity.
During launch, the satellite “piggybacks” on a launch vehicle, such as a space shuttle.
The launch vehicle ascends to the desired height and enters orbit around Earth. Once the launch vehicle enters orbit, the satellite is gently released into space, and its thrusters are fired to detach it from the launch vehicle.
To maintain a geosynchronous orbit, a satellite must be released roughly at the height of 22,000 miles.
On Feb. 3, 2006, astronauts in the International Space Station launched a spacesuit with a radio inside into orbit around Earth. The satellite was named SuitSat, and it was part of an experiment to measure the conditions experienced by a human-sized body in orbit.
The space suit orbited for over 7 months before reentering Earth’s atmosphere.