Payloads

Ninety-nine percent of the mass of a rocket poised on the pad for launch is accounted for by the rocket itself. This mass consists mostly of propellant, but it also includes tanks, valves, communications and navigation instrumentation, stage separation mechanisms, and a fairing . The remaining 1 percent consists of the rocket's payload. Protected by the fairing from the supersonic airflow of rapid ascent, the payload reaches orbit altitude and velocity within one or two minutes of the launch initiation.

Many spacecraft are equipped to modify the orbit that the rocket carries them to. They might have propulsion onboard to raise their orbit or to trim it, or to escape Earth orbit altogether and head out to the planets or beyond the solar system. This onboard propulsion system—chemical, electric or even solar sail—is part of the launch vehicle payload, but in the design of the propulsion system, the rest of the spacecraft is its payload.

A spacecraft itself is an integrated suite of parts. The components that provide necessary services in orbit are known collectively as the spacecraft bus. They include the telemetry system (radios); the structure, including attachment to the launch vehicle; solar panels and batteries; enough computing power to accomplish onboard "housekeeping" tasks; the guidance system needed to navigate in space and control the spacecraft's attitude, and sometimes services such as data storage. The spacecraft bus is intended to provide all the services and resources that the science instruments, communications equipment, imaging or other remote sensing system, and any other onboard devices specific to the mission, require. These instruments are referred to by the spacecraft bus developer as the payload.

Humans as Payloads

The first spacecraft carried computers, cameras, and sometimes animals as their payloads. Gradually, humans began to take over many observation, experimentation, and control functions aboard spacecraft. Human payloads are also known as astronauts. The human payload imposes many special requirements on a spacecraft's design. Whereas all spacecraft must be highly reliable because they are out of human reach for servicing, the additional burden of ensuring flight safety for the crew is particularly demanding in regard to the design. A typical astronaut weighs 75 kilograms (165 pounds) or more, a mass exceeded by support material for the astronaut, including a breathable atmosphere, food, water, waste disposal, seating and viewing accommodations, an exercise facility, instrumentation, medications and first aid, and clothing and other personal items. On brief missions, such as the space shuttle, each human payload accounts for 300 kilograms (660 pounds) of additional mass to be carried into orbit.

Although the human ingenuity and dexterity of an astronaut are not yet replicable by machines, the majority of space payloads are electronic and electro-optical. Synthetic optics with very high resolving and light-gathering power perform imaging of Earth and astronomical objects across a broad range of wavelengths . Microwave, infrared , visible, ultraviolet , X-ray , and gamma ray sensors are flown routinely on small and large spacecraft. Many satellites are communications relay stations, and the payload consists mainly of high-powered transponders. The transponders receive signals from ground stations, for instance, digital television transmissions, and rebroadcast them to large areas where they can be received by consumers directly. Alternatively, the downlinks are carried through a smaller number of large dish antennas and distributed terrestrially. Some telephone and computer data are also relayed via satellite. The Global Positioning System (GPS) carries highly accurate rubidium clocks into orbit. These clocks are synchronized with a number of atomic standards on Earth to provide the highly precise time reference needed to locate objects precisely on or near Earth's surface.

The Use of Robotics and Small Satellites

In addition to human, electro-optical, radio, and precise timing payloads, some satellites now carry robotic payloads. The best known of these payloads are the small rovers that were released by a spacecraft that landed on Mars. As the science of robotics advances, the search for resources and signs of life on distant planets and moons will be carried out increasingly by rovers and other robots. Unlike a human, a robot does not need to return to Earth. Launching enough mass to the surface of another planet to support human crew members and then launching back off the surface to return to Earth requires launch vehicles larger than any that exist in the early-twenty-first century. However, a one-way trip for even a small swarm of rovers is within current capabilities, and a planetary exploration mission can be carried out more economically by a rover than can a weeklong human sojourn in low Earth orbit . Robots can withstand greater environmental extremes than humans and can "sense" the atmosphere around them.

Some satellite payloads are themselves very small satellites. These tiny spacecraft can be used to look back at the host spacecraft. Visible and infrared imagery, plus other radio diagnostics onboard the subsatellite, can be used to watch the major spacecraft in its deployment from the rocket and operation to help diagnose problems and restore operations. The space shuttle has demonstrated a small robotic spacecraft that is a precursor to the inspection craft that will be used in place of astronaut extravehicular activity (EVA) to monitor the condition of the exterior of the space station.

see also Crystal Growth (volume 3); Getaway Specials (volume 3); Payload Specialists (volume 3); Space Shuttle (volume 3).

Rick Fleeter

Bibliography

Fleeter, Rick. The Logic of Microspace. El Segundo, CA; Dordrecht: Microcosm and Kluwer Academic Publishers, 2000.

Mullane, R. Mike. Do Your Ears Pop in Space? And 500 Other Surprising Questions About Space. New York: John Wiley & Sons, 1997.

Wertz, James R., and Wiley J. Larson, eds. Space Mission Analysis and Design El Segundo, CA; Dordrecht: Microcosm and Kluwer Academic Publishers, 1999.