Uncrewed spacecraft

Uncrewed spacecraft are vehicles designed to travel through space without humans on board. These machines can operate with varying levels of autonomy. Some rely on remote control or guidance from Earth. Others are fully autonomous, following pre-programmed instructions unless humans intervene. A robotic spacecraft used for scientific research is often called a space probe or a space observatory. These missions are often preferred over crewed flights. They involve lower costs and much lower risks to life.

Robotic spacecraft are essential for exploring hostile environments. Some planetary destinations are too dangerous for human survival. For instance, Venus is extremely hot. The vicinity of Jupiter is also too hostile for current human technology. Other planets, like Saturn, Uranus, and Neptune, are too distant to reach with crewed technology. Telerobotic probes are currently the only way to explore these outer planets. Robots also help prevent biological contamination. Spacecraft can be sterilized to remove Earth micro-organisms. Humans cannot be sterilized this way because we coexist with many microbes. These microbes are also very difficult to contain within spacesuits.

Designing a spacecraft requires many complex subsystems. The United States Air Force refers to the main structure as the bus or platform. The bus provides the physical structure and thermal control. It also manages electrical power and telemetry, which is the data sent back to Earth. One vital part is the data handling subsystem. This system manages the command sequence and maintains the spacecraft clock. It also collects mission data, such as photographic images. Another key part is attitude determination and control. This system maintains the spacecraft's orientation, or attitude, in space. It must fight disturbances like solar radiation and magnetic-field torques.

Landing on a planet is a very difficult process. It involves three main stages: entry, descent, and landing. First, the craft enters the planetary gravity field and atmosphere. Next, it descends through that atmosphere toward a scientific target. Finally, it must achieve a safe landing to protect its instruments. During this, the craft uses integrated sensing to stay safe. It uses cameras and algorithms to perform hazard assessment. This means the craft looks for cliffs or craters that could cause a crash. It also uses localization to know exactly where it is relative to the surface. Without these abilities, a craft might collide with the surface or waste too much fuel.

To move through the void, spacecraft use propulsion systems. Most modern spacecraft use rocket engines to generate thrust. This follows Newton's third law, which states every action has an equal and opposite reaction. When fuel and an oxidizer meet, they release energy and heat. This pushes gas particles out the back, propelling the craft forward. Some systems use monopropellant, which only requires a fuel line. In these systems, the oxidizer is chemically bonded into the fuel molecule. This makes the engine lighter and more reliable. Other ships use bipropellant systems, which use liquid fuel and liquid oxidizer. These two liquids combust spontaneously when they touch.

Some spacecraft use advanced ion propulsion for long journeys. This system generates thrust by accelerating ions. It works by shooting high-energy electrons at a propellant atom. This process removes electrons and gives the atom a positive charge. These ions are then guided through precisely aligned grids. The ions accelerate through a negatively charged grid at very high speeds. This momentum provides the thrust needed to move the spacecraft. To stay powered, ships use photovoltaic cells, also known as solar cells. They may also use a radioisotope thermoelectric generator to create electricity.

Space exploration has a long and competitive history. The first robotic spacecraft was launched by the USSR in July 1951. These early flights carried dogs named Dezik and Tsygan. In October 1957, the USSR launched Sputnik 1, the first artificial satellite. On November 3, 1957, Sputnik 2 carried the dog Laika into orbit. The United States responded by launching Explorer 1 in January 1958. Explorer 1 was a long cylinder that discovered the Van Allen belts. Later, Mariner 2 became the first probe to study another planet, Venus, in 1962. The Voyager 1 probe was launched in 1977 and entered interstellar space in 2012. Today, many nations like Japan, China, and India successfully launch their own satellites.