Ever since Pluto was reclassified as a dwarf planet, the first planet of the solar system — Mercury — has also become its smallest. Mercury might be the smallest planet orbiting the sun, but studying it is no mean feat. In fact, it is quite the opposite, as studying Mercury is quite a challenge.
Did you know that it actually takes more energy for a spacecraft to get to Mercury, than it does to reach Pluto, which is at the outer end of the solar system? This is because Mercury is deep within the sun’s gravity well. As a result, a spacecraft that approaches Mercury also gets closer to the sun and hence speeds up because of the sun’s gravity. The spacecraft would thus need a vast amount of energy to be able to slow down. In addition to fuel, missions to Mercury employ gravity assists to help get the spacecraft to the first planet of the solar system.
The power of the gravitational slingshot
Gravity assist or a gravitational slingshot is a technique by which a spacecraft uses a planet’s gravity and orbital velocity to change its speed and trajectory, thereby saving on fuel. Think of it like a ball bouncing off a moving train.
While the technique had been theorised for decades, it hadn’t been used before it was employed by the Mariner 10 for its mission to Mercury. The manoeuvre brought with it a number of advantages. It not only cut the costs involved — be it for the additional fuel or the larger rocket that would have been necessary for launching a heavier spacecraft — but also provided a way to reduce the time involved to get to the final destination.
In 1969 — the year in which human beings landed on the moon — NASA finalised its plan for sending a spacecraft to Mercury using Venus for a gravity assist. First known as the Mariner Venus Mercury, the mission was renamed Mariner 10 and became the last of the Mariner series of robotic probes.
A view of the Mariner 10 spacecraft, while it was still known as Mariner Venus Mercury.
| Photo Credit:
THE HINDU ARCHIVES
Getting to Mercury
The solar system’s innermost planet remained largely unknown up until Mariner 10 completed its mission. A combination of factors were at play here in addition to the fact that no spacecraft had visited it before. The planet’s small size, its distance from Earth and its closeness to the sun all contributed to make terrestrial observations from Earth rather difficult. Mariner 10’s primary goal was to study Mercury’s atmosphere, if any, and provide close-up photography to reveal the surface and physical characteristics of the planet.
Launched on November 3, 1973, Mariner 10 returned stunning visuals of Earth and its moon soon after leaving Earth orbit. During its three-month cruise to Venus — which included many malfunctions that were overcome thanks to the ground control team — Mariner 10 made ultraviolet observations of Comet C/1973 E1 Kohoutek in January 1974. This made it the first spacecraft to return data on a long-period comet.
On January 21, Mariner 10 approached Venus for the gravity assist manoeuvre that would send it towards Mercury. Mariner 10 made its closest flyby of Venus at a distance of 5,768 km on February 5. With the gravity assist turning out to be successful, it sped on its way to Mercury thereafter.
Following a course correction on March 16, it made its first flyby of Mercury on March 29 at a distance of 703 km. Mariner 10’s trajectory meant that the spacecraft made one revolution of the sun for every two of Mercury. This didn’t escape the notice of Italian scientist and mathematician Giuseppe “Bepi” Colombo, who realised that this would make it possible for the spacecraft to flyby Mercury every six months.
And so it was that six months later, on September 21, Mariner 10 made a second flyby of Mercury, this time at a distance 48,069 km. By the time it arrived for a third flyby, it was nearly out of gas for attitude control. Nevertheless, the third and final flyby turned out to be the closest, as Mariner 10 went within 327 km of Mercury on March 16, 1975. The gas supply was soon exhausted, and the spacecraft made final contact on March 24, before sailing on in its solar orbit.
A view of Venus as seen by Mariner 10.
| Photo Credit:
THE HINDU ARCHIVES
What did we learn?
The many firsts that the Mariner 10 mission achieved meant that it contributed immensely to our understanding of the inner solar system. During its flyby of Venus, it sent back information about that planet’s atmosphere. It sent a total of 4,165 pictures of Venus, including some of the first close-up images of the planet.
As far as Mercury was concerned, the first flyby provided plenty of photographs that revealed the innermost planet’s surface to be heavily cratered, not unlike that of our moon. Some of the instruments mounted on the spacecraft were also able to detect a weak magnetic field and that the planet’s atmosphere was very thin.
Overall, Mariner 10 sent more 2,700 photographs of Mercury during the course of its three flybys. In addition to covering almost half of the planet’s surface, some of these images carried immaculate detail, capable of showing detail as small as 100m wide on occasions.
This image provided by NASA shows the planet Mercury. This 2008 picture was made by the MESSENGER spacecraft at a distance of approximately 27,000 km.
| Photo Credit:
AP / NASA
Missions to Mercury
Did you know that only two missions have ever made it to Mercury so far, with a third currently on its way?
The Mariner 10 mission that we’ve looked at here in detail was the first to study Mercury. Following Mariner 10’s final flyby in 1975, no spacecraft visited the planet for over 30 years. When NASA’s MESSENGER — that’s short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging — eventually made it, it became the first to orbit Mercury. Orbiting Mercury from 2011-15, MESSENGER gave us a more complete picture of the planet.
The third mission that is currently on its way is BepiColombo. Yes, you guessed it right. It is named after the Italian who calculated Mariner 10’s trajectory, suggested using gravity assist to get it to Mercury, and ensured that it made three flybys. He was also the first to explain that Mercury rotates three times for every two orbits around the sun. A joint European-Japanese mission, it brings together the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA). Launched in 2018, it has completed six Mercury flybys between 2021-25 to reduce its speed and is set to enter into orbit around the innermost planet in November this year. Watch out for that!
Many firsts of Mariner 10
First to study Mercury
First to use gravity assist to change flight path
First mission to explore two planets
First mission to return to its primary target for another look
Published – March 29, 2026 07:23 am IST
