KEY POINTS:
China's new 'Star Wars' weapon recently blew up a satellite for target practice. That one was disused, but there are hundres of others we couldn't live without.
WEATHER SATELLITES
Orbit level low (around 800km above Earth)
How many are there?
About 40. The satellite recently destroyed by China was one of the thousands of disused (or "sleeping") weather satellites in low-Earth orbit.
Satellite-borne instruments have allowed researchers to track weather patterns, changes in sea levels and the amount of carbon dioxide in the atmosphere. When the Orbiting Carbon Observatory lifts off in 2008, it will be the first Nasa spacecraft designed to make precise measurements of atmospheric carbon dioxide from an Earth-orbiting satellite. Ability to track retreating polar ice, shifting patterns of drought, winds and rainfall and other environmental changes is "at great risk" because of failures to replace satellite-borne sensors.
DATA TRANSFER & COMMUNICATIONS
Orbit level high (up to 36,000km above Earth)
How many are there?
More than 500.
Almost two-thirds of active satellites are used for communication. "Because most communications satellites orbit up to 36,000km above the Earth, they would be more difficult to target than those with medium-Earth orbits," says Dr Laura Grego of the Union of Concerned Scientists. Many commercial high-orbiting satellites are built to last. "The loss of those would be economically crippling," Grego adds.
NATURAL DISASTER MONITORING
Orbit level low (800km above earth)
How many are there?
Unknown.
Within hours of Hurricane Katrina hitting New Orleans in August 2005, images taken by satellites known as the Disaster Monitoring Constellation (such as that pictured) were delivered to help rescue efforts. The constellation is five remote-sensing satellites launched by the Algerian, Nigerian, Turkish, British and Chinese governments. DMC satellites provided vital imaging of the after-effects of the 2004 tsunami in Indonesia.
BROADCAST
Orbit level high (35,000km above earth)
How many are there?
Thousands.
If the Earth were flat, you could pick up broadcast television thousands of miles from its source. Satellite television solves the problems of range and distortion by transmitting broadcast signals from space. Television satellites lie in geosynchronous orbit, meaning that they keep pace with the speed of the Earth. Most TV satellites are approximately 35,000km away. If you have access to a yacht in the Caribbean, close to the Equator, they are visible, like small shooting stars.
SPACE EXPLORATION
Orbit level high (up to 1.6 million kilometres above earth)
How many are there?
Unknown.
Since the launch of Hubble in 1990, it has become one of the most important instruments in the history of astronomy. Seventeen years on, its future is uncertain; it has already surpassed its 10-year life expectancy and its technology is failing. While nobody wants to see Hubble go, space technology is advancing rapidly and a number of ground-breaking telescopes are scheduled for launch in the next few years. The James Webb telescope is one. Named after a former Nasa administrator, it is scheduled for launch in 2013 and will use its infra-red technology to peer through dusty clouds in search of the first galaxies that formed after the Big Bang. Equipped with a 6m mirror, the telescope will orbit about 1 million miles up.
EMERGENCY COMMUNICATIONS
Orbit level low (800km above earth)
How many are there?
17 large groups.
News journalists and militant groups might well agree on the importance of satellite communication; from the mountains of Pakistan to the streets of Baghdad, international calls can be made at the touch of a button. There are two major types of emergency communications satellites, Thuraya and Iridium. The Iridium constellation is a group of 66 satellites. The Iridium network is unique in that it covers the Earth, including poles, oceans and airways, although United States management means calls to and from North Korea, Iran, Cuba, Libya and Sudan are embargoed. Foreign correspondents travelling to remote parts of Africa and the Middle East tend, therefore, to rely on Thuraya, a single geostationary communications satellite above the Pacific Ocean.
MILITARY
Orbit level low to medium (500 to 13,000km above earth)
How many?
Hundreds.
Reconnaissance satellites, or keyholes, have been orbiting the Earth for over 30 years. Used to take photos for military operations, they circle the globe around 500km above its surface. Little is known about the full power of these satellites, as the governments that operate them usually keep information about them classified. During the fall of Baghdad, 83 per cent of communications between United States forces were sent via satellites. "It would have a huge impact if a country could destroy those satellites," says Pat Norris, chairman of the Royal Aeronautical Society's space group.
NAVIGATION
Orbit level medium (19,300km above earth)
How many are there?
35 to 50.
GPS, short for the Global Positioning System, operates via a constellation of 27 Earth-orbiting satellites, of which 24 are active and three serve as back-up. Each solar-powered satellite circles the globe at an altitude of about 19,000km, making one complete rotation every 12 hours. The orbits are arranged so that at any time, wherever you may be on Earth, there are at least four satellites serving you in the sky.
EARTH OBSERVATION
Orbit level low (700km above earth)
How many are there?
Unknown.
Thanks to Google Earth, QuickBird, a high-resolution commercial spacecraft launched in 2001, has become one of the most famous Earth observation satellites. It can capture the largest images and has the greatest on-board storage capacity of any public-domain satellite, and takes colour shots in high enough definition to pick out the detail on the Great Wall of China. TopSat, a micro-satellite launched last year, flies at a height of 700km and circles the Earth once 1h 38.5m. At $14,000, such satellites are affordable for even small firms. Not only do they store satellite photographs, but they may be used in the future to locate mineral and oil deposits and assist relief teams during natural disasters, and even to spy on those who haven't paid their council tax.
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