What Causes Space Junk?

Debris in orbit can come from many sources:

• Exploding rockets - This leaves behind the most debris in space.
• The slip of an astronaut's hand - Suppose an astronaut doing repair in space drops a wrench -- it's gone forever. The wrench then goes into orbit, probably at a speed of something like 6 miles per second. If the wrench hits any vehicle carrying a human crew, the results could be disastrous. Larger objects like a space station make a larger target for space junk, and so are at greater risk.
• Jettisoned items - Parts of launch canisters, camera lens caps, etc. 

Items initially placed into high orbits stay in space the longest.

The European Space Agency tracks more than 7,500 orbiting items with a width of 4 inches (10 cm) or more. Space debris may also be a reason why space shuttles typically orbit with their windows to the rear. This protects the astronauts onboard, at least to some degree. 

A special NASA satellite called Long Duration Exposure Facility (LDEF) was put in orbit to study the long-term effects of collisions with space junk. The LDEF was later brought back to Earth via a space shuttle for analysis. 

What is AMSAT?

AMSAT is a non-profit organization of ham radio operators worldwide that uses its own membership-supported satellites. The official name for AMSAT is the Radio Amateur Satellite Corporation. Hams that belong to AMSAT participate in:

• The actual development and assembly of over 40 satellites to date
• Ground control after the satellite is in orbit
• Conversations using the satellite and listening to others using the satellite as a radio relay link

AMSAT satellites can often be heard by use of a short-wave receiver or a radio scanner. Ham operators make use of the satellites during natural disasters when terrestrial links and cell phone systems may be down or overloaded.

The AMSAT-built satellites "hitch" a rocket launch on a "payload-space-available" basis. The first AMSAT satellite orbited in 1961 and was called OSCAR (Orbiting Satellite Carrying Amateur Radio). Tracking software is available for personal computers. Various AMSAT satellites have a combination of data, image and voice capabilities.

How Can I See an Overhead Satellite?

This satellite tracking Web site shows how you can see a satellite overhead, thanks to the German Space Operations Center. You will need your coordinates for longitude and latitude, available from the USGS Mapping Information Web site or at the website Topozone. 

• Satellite-tracking software is available for predicting orbit passes. Note the exact times.
• Use binoculars on a clear night when there is not a bright moon.
• Ensure that your watch is set to exactly match a known time standard.
• A north-south orbit often indicates a spy satellite!

How Much Do Satellites Cost?

Satellite launches don't always go well, as shown by this story on failed launches in 1999. There is a great deal at stake. For example, this hurricane-watch satellite mission cost $290 million. This missile-warning satellite cost $682 million.

Another important factor with satellites is the cost of the launch. According to this report, a satellite launch can cost anywhere between $50 million and $400 million. A shuttle mission pushes toward half a billion dollars (a shuttle mission could easily carry several satellites into orbit). You can see that building a satellite, getting it into orbit and then maintaining it from the ground control facility is a major financial endeavor!

Major U.S. satellite firms include: 

• Hughes
• Ball Aerospace &Technologies Corp.
• Boeing
• Lockheed Martin

Satellite Altitudes

Looking up from Earth, satellites are orbiting overhead in various bands of altitude. It's interesting to think of satellites in terms of how near or far they are from us. Proceeding roughly from the nearest to the farthest, here are the types of satellites whizzing around Earth:

80 to 1,200 miles - Asynchronous Orbits

Observation satellites, typically orbiting at altitudes from 300 to 600 miles (480 to 970 km), are used for tasks like photography. Observation satellites such as the Landsat 7 perform tasks such as:

• Mapping
• Ice and sand movement
• Locating environmental situations (such as disappearing rainforests)
• Locating mineral deposits
• Finding crop problems

Search-and-rescue satellites act as relay stations to rebroadcast emergency radio-beacon signals from a downed aircraft or ship in trouble.

The Space Shuttle is the familiar manned satellite, usually with a fixed duration and number of orbits. Manned missions often have the task of repairing existing expensive satellites or building future space stations.

Teledesic, with the financial backing of Bill Gates, promises broadband (high-speed) communications using many planned low Earth orbiting (LEO) satellites.

3,000 to 6,000 miles - Asynchronous Orbits

Science satellites are sometimes in altitudes of 3,000 to 6,000 miles (4,800 to 9,700 km). They send their research data to Earth via radio telemetry signals. Scientific satellite applications include:

• Researching plants and animals
• Earth science, such as monitoring volcanoes
• Tracking wildlife
• Astronomy, using the Infrared Astronomy Satellite
• Physics, by NASA's future study of microgravity and the current Ulysses Mission studying solar physics

6,000 to 12,000 miles - Asynchronous Orbits

For navigation, the U.S. Department of Defense built the Global Positioning System, or GPS. The GPS uses satellites at altitudes of 6,000 to 12,000 miles to determine the exact location of the receiver. The GPS receiver may be located:

• In a ship at sea
• In another spacecraft
• In an airplane
• In an automobile
• In your pocket

As consumer prices for GPS receivers come down, the familiar paper map may face tough competition. No more getting lost leaving the rental car agency at an unfamiliar airport!

• The U.S. military and the forces of allied nations used more than 9,000 GPS receivers during Operation Desert Storm.
• The National Oceanic and Atmospheric Administration (NOAA) used GPS to measure the exact height of the Washington Monument.

22,223 Miles - Geostationary Orbits

Weather forecasts visually bombard us each day with images from weather satellites, typically 22,223 miles over the equator. You can directly receive many of the actual satellite images using radio receivers and special personal-computer software. Many countries use weather satellites for their weather forecasting and storm observations.

Data, television, image and some telephone transmissions are routinely received and rebroadcast by communications satellites. Typical satellite telephone links have 550 to 650 milliseconds of round-trip delay that contribute to consumer dissatisfaction with this type of long-distance carrier. It takes the voice communications that long to travel all the way up to the satellite and back to Earth. The round-trip delay forces many to use telephone conversations via satellite only when no other links exist. Currently, voice over the Internet is experiencing a similar delay problem, but in this case due to digital compression and bandwidth limitations rather than distance.

Communications satellites are essentially radio relay stations in space. Satellite dishes get smaller as satellites get more powerful transmitters with focused radio "footprints" and gain-type antennas. Subcarriers on these same satellites carry:

• Press agency news feeds
• Stock market, business and other financial information
• International radio broadcasters moving from short-wave to (or supplementing their short-wave broadcasts with) satellite feeds using microwave uplink feeds
• Global television, such as CNN and the BBC
• Digital radio for CD-quality audio