Welcome to Slow English FM. This is Today in History, where we explore one event from the past and the ideas around it. Today, we travel back to November 3, 1973, when NASA launched a spacecraft toward two neighboring planets.

NASA is the United States space agency. That day, it launched Mariner 10, a robotic spacecraft weighing about 500 kilograms, from Cape Canaveral in Florida. NASA’s Jet Propulsion Laboratory, which manages space missions, managed this mission from Pasadena, California. Mariner 10’s main goal was Mercury. No spacecraft had visited the planet, and its small size and closeness to the Sun made it hard to study from Earth.

There was a challenge: reaching Mercury directly would take extra fuel and a larger rocket. NASA approved another route in 1969. The spacecraft would first visit Venus. A gravity assist is a planned close pass by a planet that uses its gravity to change a spacecraft’s path. In this case, Venus could redirect Mariner 10 toward Mercury and help avoid the need for extra fuel and a larger rocket.

Mariner 10 reached Venus on February 5, 1974. It returned more than 4,000 images and information about the planet’s atmosphere. Then Venus’s gravity sent the spacecraft toward Mercury.

Mariner 10 carried cameras and instruments to measure temperature, particles and magnetic fields. On March 29, 1974, it made its first flyby of Mercury, passing within 703 kilometers. It returned more than 2,000 images. They showed a heavily cratered surface. Instruments also detected a weak magnetic field and a very thin atmosphere made mostly of helium. Across three flybys, the spacecraft eventually imaged nearly half of Mercury’s surface.

Italian scientist Giuseppe Colombo worked out that Mariner 10’s path could bring it back to Mercury about every six months. It flew by again in September 1974, then made its final, closer pass on March 16, 1975. Eight days later, after its attitude-control gas ran out, ground controllers turned off its transmitter. The spacecraft fell silent but continued in orbit around the Sun.

Mariner 10 showed that one spacecraft could visit more than one planet by using a planet’s gravity to change its path. It also gave scientists the first close-up spacecraft observations of Mercury. No spacecraft returned there for more than 30 years, until NASA’s MESSENGER mission arrived in 2011. That is the story of Mariner 10: a mission that used Venus as a stepping stone to explore a world still largely unknown.

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