Space probes carry instruments into space to make measurements that cannot be made on Earth. Temperature, spectrum, magnetic fields and radiation comprise just a few of the types of phenomena measured. Space probes ride atop a rocket and then separate and set a course based on instructions from Earth-based operators and pre-programmed instructions on board. On arrival, the instruments start taking measurements and transmitting them back to Earth.
Space probes typically carry an instrument package based on the mission, communication equipment to transmit the data and a guidance system. On-board power comes from batteries or fuel cells for short missions, solar panels for researching the inner planets or a small nuclear reactor for travel to the outer planets. Once launched, space probes travel to their destination on their own momentum. In some cases, a gravitational slingshot can help accelerate the probe by using the attractive force of another body, such as a moon, to pull the probe toward its destination.
On arrival, space probes can fly by their destination, making observations as they travel, go into orbit or descend to the surface. Sometimes, a probe orbits for a period of time and then, as its mission ends, it crashes into the planet, transmitting data until it is destroyed.
Because they are unmanned, space probes have some real advantages. They can measure data in very hostile environments, such as the hot surface of Venus or the methane lakes on Titan, Saturn's largest moon. They can make very long voyages without critical life support resources, such as oxygen, water and food.
Early space probes launched in the 1950s took measurements around the Earth and moon. In the early 1960s, space probes explored Venus and Mars, giving Earth observers their first real data about these planets. Space probes to the large outer planets, launched in the 1970s, made a number of significant discoveries. Voyager 1, launched in 1977, first explored Jupiter and Saturn, and then was guided to explore deep space. It still transmits data as it travels beyond the solar system. In the 1980s, new space probes orbited the outer planets and their moons for extended study, sending back thousands of pictures along with other physical data for analysis.