Artificial Satellite

C

Medium Earth Orbit

Medium Earth orbit (MEO) satellites orbit at an altitude about 10,000 km (about 6,000 mi) and balance the benefits and problems between LEO and GEO. The most common uses of MEO are by navigation and communication satellites. The U.S. navigation system NAVSTAR Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS), and Odyssey, a private U.S. communications satellite program, all use MEO.

D

Polar Orbits

Satellites in polar orbits orbit around Earth at right angles to the equator over both the North and South poles. Polar orbits can occur at any altitude, but most satellites in polar orbits use LEOs. Two polar satellites belonging to the U.S. National Oceanic and Atmospheric Administration provide weather information for all areas of the world every six hours. The satellites also map ozone levels (see Ozone Layer) in the atmosphere, including the level over the poles. Landsat is a U.S. government remote-sensing satellite system that operates in polar orbit. Scientists often use Landsat to view agricultural phenomena such as deforestation and crop blight. Transit, the first satellite-based navigation system, used polar orbits in order to support navigation around the world, especially for submarines in the polar regions.

E

Sun-Synchronous Orbits

A satellite in a Sun-synchronous orbit always passes over a certain point of Earth when the Sun is at the same position in Earth’s sky. A Sun-synchronous satellite has a retrograde orbit (it moves clockwise around Earth), orbits in a low Earth orbit, and orbits at a specific angle with respect to Earth’s equator (about 98°). The satellite crosses each latitude about 1° east of where it crossed the latitude the previous day. Thus, the satellite stays synchronized with the location of the Sun relative to Earth. Sun-synchronous orbits are useful for satellites photographing Earth, because the Sun will be at the same angle each time the satellite passes over a point on Earth.

VII

The First Satellites

The first artificial satellite to orbit Earth was Sputnik 1. Built by the Soviet Union and launched on October 4, 1957, Sputnik had an elliptical orbit, ranging in altitude from 225 to 950 km (140 to 590 mi). Sputnik broadcast a steady signal of beeps for 21 days and burned up in Earth’s atmosphere upon reentry on January 4, 1958.

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The Soviet Union also launched the first living creature, a dog named Laika, into space on November 3, 1957. Laika flew inside a pressurized chamber aboard the satellite Sputnik 2. She died from overheating and panic after a few hours in orbit. Sputnik 2 reentered Earth’s atmosphere and burned up on April 14, 1958.

The United States launched its first satellite, Explorer 1, on January 31, 1958. Explorer 1 had a highly elliptical orbit, ranging in altitude from 360 to 2,500 km (220 to 1,600 mi). Scientists discovered the Van Allen radiation belts using data transmitted back to Earth from Explorer 1.

On August 10, 1960, the United States launched a surveillance satellite, Discoverer 13, that carried the first artificial object ever retrieved from space. While Discoverer 13 remained in orbit it ejected a capsule earthward, which was then recovered by a team from the U.S. Navy. Later satellites carried cameras that photographed parts of Earth and then ejected recoverable containers of the exposed film toward Earth.