Mariner 10 in the context of Mariner 9

A planetary flyby is the act of sending a space probe past a planet or a dwarf planet close enough to record scientific data. This is a subset of the overall concept of a flyby in spaceflight.

The first flyby of another planet with a functioning spacecraft took place on December 14, 1962, when Mariner 2 zoomed by the planet Venus.

MESSENGER was a NASA robotic space probe that orbited the planet Mercury between 2011 and 2015, studying Mercury's chemical composition, geology, and magnetic field. The name is a backronym for Mercury Surface, Space Environment, Geochemistry, and Ranging, and a reference to the messenger god Mercury from Roman mythology.

MESSENGER was launched aboard a Delta II rocket in August 2004. Its path involved a complex series of flybys – the spacecraft flew by Earth once, Venus twice, and Mercury itself three times, allowing it to decelerate relative to Mercury using minimal fuel. During its first flyby of Mercury in January 2008, MESSENGER became the second mission, after Mariner 10 in 1975, to reach Mercury.

A gravity assist, gravity assist maneuver, swing-by, or generally a gravitational slingshot in orbital mechanics, is a type of spaceflight flyby which makes use of the relative movement (e.g. orbit around the Sun) and gravity of a planet or other astronomical object to alter the path and speed of a spacecraft, typically to save propellant and reduce expense.

Gravity assistance can be used to accelerate a spacecraft, that is, to increase or decrease its speed or redirect its path. The "assist" is provided by the motion of the gravitating body as it pulls on the spacecraft. Any gain or loss of kinetic energy and linear momentum by a passing spacecraft is correspondingly lost or gained by the gravitational body, in accordance with Newton's Third Law. The gravity assist maneuver was first used in 1959 when the Soviet probe Luna 3 photographed the far side of Earth's Moon, and it was used by interplanetary probes from Mariner 10 onward, including the two Voyager probes' notable flybys of Jupiter and Saturn.

Mercury's magnetic field is approximately a magnetic dipole, apparently global, on the planet of Mercury. Data from Mariner 10 led to its discovery in 1974; the spacecraft measured the field's strength as 1.1% that of Earth's magnetic field. The origin of the magnetic field can be explained by dynamo theory. The magnetic field is strong enough near the bow shock to slow the solar wind, which induces a magnetosphere.

The geology of Mercury is the scientific study of the surface, crust, and interior of the planet Mercury. It emphasizes the composition, structure, history, and physical processes that shape the planet. It is analogous to the field of terrestrial geology. In planetary science, the term geology is used in its broadest sense to mean the study of the solid parts of planets and moons. The term incorporates aspects of geophysics, geochemistry, mineralogy, geodesy, and cartography.

Historically, Mercury has been the least understood of all the terrestrial planets in the Solar System. This stems largely from its proximity to the Sun which makes reaching it with spacecraft technically challenging and Earth-based observations difficult. For decades, the principal source of geologic information about Mercury came from the 2,700 images taken by the Mariner 10 spacecraft during three flybys of the planet from 1974 to 1975. These images covered about 45% of the planet's surface, but many of them were unsuitable for detailed geologic investigation because of high sun angles which made it hard to determine surface morphology and topography. This dearth of information was greatly alleviated by the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft which between 2008 and 2015 collected over 291,000 images covering the entire planet, along with a wealth of other scientific data. The European Space Agency's (ESA's) BepiColombo spacecraft, scheduled to go into orbit around Mercury in 2026, is expected to help answer many of the remaining questions about Mercury's geology.