On November 3, 1973, the American spacecraft Mariner 10 was launched from Cape Canaveral on a course toward Mercury, and in March 1974 it became the first craft to reach the planet nearest the sun. The design was unusual. It used the gravity of Venus to bend its path, and it passed close to that planet in February 1974 and took pictures of its clouds on the way past, and the manoeuvre let a single craft meet Mercury three times rather than once.
The path was worked out by a mathematician at the Jet Propulsion Laboratory who saw that a craft could be sent past Venus and then on to Mercury with very little fuel, and the idea allowed a small and inexpensive mission to do work that had been thought to need several craft. The saving was large. The craft carried cameras and instruments for measuring the magnetic field and the charged particles around the planet, and it had a shield to protect it from the heat of the sun.
The three meetings with Mercury in 1974 and 1975 showed a surface like that of the moon and covered with craters, and found a weak magnetic field, and measured a day that lasts about fifty-nine earth days, and took about half of the surface in pictures. The craft could not stay in orbit and flew past each time. The heat was severe. Its instruments began to fail as the sun warmed them, and the mission ended in 1975 when the fuel ran out.
The results changed the picture of Mercury from a small dull body into a world with a history of its own, and they set the questions that later missions took up. The American craft MESSENGER went into orbit around the planet in 2011 and mapped the whole surface before it was allowed to fall onto it in 2015, and a European and Japanese mission was launched in 2018. The method was copied. The first look came from a craft built to last a few months that worked for more than a year.