Short answers, pulled from the story.
Mariner 10 was an American robotic space probe launched by NASA on the 3rd of November 1973. It was the first spacecraft to visit Mercury and the first to use a gravity assist from one planet to reach another.
Mariner 10 flew past Venus on the 5th of February 1974, using the planet's gravity to bend its flight path and slow its heliocentric velocity from 82,785 mph to 72,215 mph in a four-hour window. This reshaped its orbit so that its closest point to the Sun matched Mercury's orbit. The maneuver was inspired by the orbital mechanics calculations of Italian scientist Giuseppe Colombo.
Mariner 10 found that Mercury has a tenuous atmosphere consisting primarily of helium, a magnetic field, and a large iron-rich core. Its infrared radiometer measured nighttime temperatures of -183 degrees Celsius and maximum daytime temperatures of 187 degrees Celsius. The surface appeared Moon-like, with heavy cratering.
Mariner 10 flew past Mercury three times: on the 29th of March 1974 at a range of 703 km, on the 21st of September 1974 at a range of 48,069 km, and on the 16th of March 1975 at a range of 327 km. The same side of Mercury was sunlit during each pass because the spacecraft's orbital period was almost exactly twice Mercury's orbital period.
Mariner 10 mapped 40-45% of Mercury's surface across its three flybys, taking more than 2,800 photographs. The geometric constraint of the spacecraft's orbital period meant the same hemisphere faced the Sun on every approach, leaving the other side unobserved.
NASA set a strict limit of US$98 million for Mariner 10, the first time the agency applied an inflexible budget constraint to a mission. The mission finished roughly US$1 million under that cap. The seven scientific experiments alone cost US$12.6 million, about one-eighth of the total budget.