Mariner 10 was a spacecraft launched by NASA in 1973 and was the first to complete planetary flybys of Venus and Mercury.  

Mariner 10 Launch

Built by Boing in Seattle, Mariner 10 was shipped to JPL in Calinfornia where the spacecraft was tested.

The Mariner 10 mission launched on 3 November 1973 on an Atlas-Centaur rocket from Cape Canaveral, Florida.

During the first week, the on-board camera was tested by taking shots of Earth and the Moon.

Recombined image from the Mariner 10 on-board camera that shows the relative sizes of Earth and Moon.

Mariner 10 was the first spacecraft to use an interplanetary slingshot manoeuvre on its way to Mercury.

The three month journey to Venus was filled with setbacks and frustration. These required a lot of patches and course corrections to hit the mission objectives.

Mariner 10 Venus Flyby

On 5 February 1974, it flew past Venus at a distance of 5,768 kilometers (or 3,584 miles).

It was already the twelfth spacecraft to reach Venus, but the first one to send through images. here is a real color photograph:

Observations from Earth or with visbile light didn't reveal much about the planet's atmospheric mack-up.

An ultraviolet shot of Venus was a lot more revealing: a surprising patterns of clouds surrounding the planet.

During the encounter, the probe sent back 4,165 of Venus. Other findings were related to the composition of the atmosphere, temperatures and pressure.

First Mercury Flyby

After changing its course, the Mariner 10 flew by Venus for the first time on 29 March 1974 at a distance of 703 kilometer.

Here is what that looked like coming in:

And going out:

After looping once around the Sun while Mercury completed two orbits, Mariner 10 flew by Mercury a second time on 21 September 21 1974.

This time it passed the plant at 8,069 km (or 29,869 miles) below the southern hemisphere.

A mosaic of images taken during this flyby looked like this:

The third and final flyby on 16 March 1975 at a range of 327 km (or 203 miles) looked like this:

Mariner 10 took 2 800 photos of mercury's surface. This helped with the mapping of 40-45% of its surface.

The flybys also helped to find out that it's atmosphere mostly consisted of helium and that the planet has a large iron-rich core.

Other measurement suggested that Mercury has a night time temperature of −183 °C (−297 °F) and maximum daytime temperatures of 187 °C (369 °F).

After this last flyby Mariner 10's manoeuvring gas was depleted, putting it in an eternal orbit around the Sun.

Interesting fact

Often these spacecrafts had a double that would be launched in case something went wrong. But for budgetary reasons, a spare was developed but never launched. The Mariner 10 back-up was sent to the Smithsonian museum.