Landing

When it reaches its destination, a probe may stay in orbit, radioing data and images back to Earth, or it may attempt to land a capsule on the surface. Landing on a planet many millions of miles away, under remote control, is always a challenge. The speeds of approach can be enormous. In 2003, the Galileo probe accelerated to 108,000 miles per hour (173,780 kilometers per hour) as it dived toward Jupiter.

Once on the target planet, a lander can use remote-controlled arms and scoops to collect samples of rock and soil. Its instruments analyze the samples and the gases in the atmosphere and measure temperature, pressure, and radi­ation levels. A few probes have released a small rover to explore the areas farther from the lander.

Some probes are sent to collect mate­rial and return it to Earth at the end of their mission. A reentry capsule drops down through Earth’s atmosphere, by parachute, for recovery on the ground or in the air using the “air snatch” tech­nique, by which an airplane scoops up the capsule before it hits the ground.