Spacecraft

Overview

A spacecraft is a vehicle designed to operate in space. Whatever its mission, it must solve the same set of problems — power, propulsion, thermal control, attitude, communication and data handling — under constraints that have no terrestrial equivalent.

• Mass is the dominant design constraint, because every kilogram must be accelerated to orbital velocity.
• There is no air in space, so all waste heat must be radiated — thermal control is a major subsystem, not an afterthought.
• Solar power becomes impractical in the outer Solar System, which is why distant missions use radioisotope generators.
• Redundancy and radiation tolerance drive component choices that look decades out of date by consumer standards.

The core subsystems

• Structure: carries launch loads, which are usually far more severe than anything encountered in flight.
• Power: solar arrays with batteries for eclipse periods, or radioisotope thermoelectric generators where sunlight is too weak.
• Propulsion: chemical thrusters for large manoeuvres, and increasingly electric propulsion for high-efficiency low-thrust operation over long durations.
• Attitude determination and control: star trackers, sun sensors and gyroscopes to know orientation; reaction wheels and thrusters to change it.
• Thermal control: radiators, multilayer insulation, louvres and heaters, to keep every component within its operating range.
• Communications: high-gain antennas for data, low-gain for robust contact in unfavourable orientations.
• Command and data handling: the onboard computer, plus storage for data acquired faster than it can be transmitted.

Thermal control, the underrated problem

In vacuum there is no convection and no conduction to the surroundings, so radiation is the only way to shed heat. A spacecraft in sunlight absorbs energy continuously while its electronics generate more, and the same vehicle in shadow can cool rapidly toward very low temperatures.

Managing this requires careful selection of surface coatings, multilayer insulation blankets, radiators positioned to face cold space, and often active heaters running on scarce power. Instruments have narrow operating ranges, and infrared detectors must be cooled far below ambient — JWST's sunshield exists to keep its optics near 40 kelvin, and it is the largest single structure on the observatory.

Why spacecraft electronics look obsolete

Flight computers routinely use processors that would be considered antique in consumer terms. This is deliberate. Radiation-hardened parts must tolerate charged particles that flip bits and degrade semiconductors, and hardening requires larger feature sizes and mature designs. Qualification and flight heritage take years, and a component with a long successful record is worth more than raw performance.

Redundancy compounds the effect. Critical systems are duplicated, sometimes with dissimilar designs, and the extra mass and power that costs must be justified against everything else on the vehicle. The result is machines that are slow by earthbound standards and extraordinarily reliable in an environment where repair is impossible.

Propulsion trade-offs

Chemical propulsion delivers high thrust for short durations and is required for launch, orbital insertion and landing. Its efficiency, measured as specific impulse, is limited by chemistry.

Electric propulsion — ion and Hall-effect thrusters — accelerates propellant electrically to far higher exhaust velocities, achieving several times the efficiency but at tiny thrust levels. It cannot lift anything off a surface, but firing continuously for months or years it can deliver a large total velocity change with very little propellant mass. Dawn used it to orbit two separate main-belt bodies in one mission, which chemical propulsion could not have achieved.

Continue in the data

Catalogues, hubs, and reference pages that hold the underlying records for this topic.

• Spacecraft systemsSubsystems and components as catalogued entities.
• Space engineeringDesign practice, testing, and qualification.
• RocketsLaunch vehicles, engines, and propellants.
• InstrumentsThe scientific payloads spacecraft carry.

Frequently asked

How do spacecraft get electrical power?

Most use solar arrays with batteries to cover eclipse periods. Sunlight intensity falls as the square of distance from the Sun, so beyond roughly Jupiter solar power becomes impractical for many missions, and spacecraft use radioisotope thermoelectric generators instead — converting heat from radioactive decay directly into electricity, as Voyager, Cassini and the Mars rover Curiosity do.

Why do spacecraft use such old processors?

Because radiation tolerance and flight heritage matter more than speed. Charged particles flip bits and degrade modern high-density chips, so radiation-hardened parts use larger, more robust feature sizes. Qualifying a component takes years, and a design with a long record of successful operation is more valuable than raw performance in an environment where repair is impossible.

How do spacecraft stay cool without air?

By radiating heat to space, which is the only mechanism available in vacuum. Designers use radiators facing away from the Sun, multilayer insulation, and carefully chosen surface coatings, together with heaters for components that would otherwise get too cold. Infrared instruments need additional active or passive cooling to reach the very low temperatures they require.

What is ion propulsion and why use it?

It accelerates ionised propellant with electric fields, achieving exhaust velocities several times higher than chemical rockets and therefore using far less propellant for the same velocity change. Thrust is tiny — comparable to the weight of a sheet of paper — so it only works over months or years of continuous firing, but for deep-space missions that trade is often decisive.