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A planetary flyby is the act of sending a space probe past a celestial body close enough to record scientific data. Most deep space missions feature one or more planetary flybys, often for gravity assists to their primary target. Though the use of purely scientific flybys have declined in favor of more scientifically valuable orbiter or lander missions, scientific instruments may still be activated during gravity assists for gathering scientific data, testing, or calibration.

Orbiter and lander missions typically gather more scientific data about any specific body, but flyby missions can be more valuable for their expenditure given their lower mission and spacecraft complexity, which results in decreased cost. Their simplicity allowed the visitation of other planets before technologies had been perfected to place spacecraft into orbit around those planets, and every planet except Earth was first visited by a flyby mission, before an orbiter or lander. Flyby missions also have the potential to visit multiple bodies using less resources, as done in the Voyager program.

Flybys typically follow an approximate hyperbolic trajectory, only significantly deviating when interacting with the gravity of an additional large body such as a natural satellite preventing simplification into a two-body problem. An example of this would be a spacecraft performing a flyby of the Earth-Moon system and flying close enough to the Moon that its gravity acts significantly on the spacecraft relative to the gravity of the Earth, requiring consideration of a third gravitational body and creating a three-body problem.

History
Before a flyby of another planet in the solar system had been attempted, flybys and an impact of the Moon had already been conducted. Despite this, the first flyby of another planet occurred about five years before the first orbit of the Moon by Luna 10, when Venera 1 flew by Venus in 1961. At this point in the Space Race, similar numbers of missions were being planned to Mars and Venus as those to the Moon. Due to complexities associated with interplanetary spaceflight, flybys remained the focus of Mars missions until Mariner 9 entered orbit around Mars in 1971, and a critical component of missions to Venus throughout the Venera and Vega programs in the late 1970's and 1980's. In modern times, planetary flybys are often used in the form of gravity assists and all planets in the solar system have been flown by, including the dwarf planet Pluto.

Early attempts to flyby another planet were met with failure with two identical Mars 1M spacecraft, designed and launched by the Soviet Union. Nicknamed Marsnik 1 and Marsnik 2 by western countries, each craft was launched separately and both suffered unrelated launch vehicle failures in October of 1960. These failures did not prevent the Soviet Union's Venera program from launching its first probe, the Venera 1, in February of 1961. This probe launched successfully and began a course towards Venus, although contact with the spacecraft was lost only a week after launch the probe did continue on to become the first man-made object to fly past another planet. No other interplanetary probes were launched for over a year, until July of 1962 when the United States attempted the launch of Mariner 1, which was aborted mid-launch. An exact copy of the spacecraft, Mariner 2, was launched about a month later and flew by Venus on December 14th, 1962. Mariner 2 proved the United States' technical ability to perform a planetary flyby, and made temperature measurements of Venus' atmosphere and surface among other achievements. While Mariner 2 was traveling to Venus in late 1962, the Soviet Union launched two more probes intended to fly past Mars. The first of these, Sputnik 24, was unable to leave Low Earth Orbit (LEO) due to a launch vehicle failure. In November, Mars 1 was launched and flew by Mars in June of 1963 but only after a communication failure several months earlier rendered the spacecraft worthless, although it did return some data about the Earth's radiation belts and the interplanetary space.

Launch windows prevented missions to Venus until 1964, with the launch of the Venus-bound Zond 1 on April 2nd of that year. Zond 1 marked the first mission of the Soviet Union's Zond program, and its second mission, Zond 2, launched in November with plans to fly past Mars. Both Zond 1 and 2 failed despite eventually reaching their target, with communication to each probe lost during travel. Similar to the Mariner 1 and Mariner 2 probes intended to go to Venus, the United States launched a second pair of Mariner probes the same month as the Soviet Zond 2. Mariner 3 and Mariner 4 launched on November 3rd and November 28th respectively, and similar to the previous Mariner duo launched in 1962, the first of these suffered a launch vehicle related failure. This left Mariner 4 alone on its journey to Mars, and on July 15th, 1965 the probe successfully flew close by Mars with all systems functioning and becoming the first spacecraft to ever do so. Unlike Mariner 2, Mariner 4 was fitted with a camera to allow it to return the first close-up images of Mars. The probe also recorded the first basic data about the planet, reporting the surface temperature, lack of a magnetic field, and the approximate surface pressure.

In November of 1965, the Soviet Union began their final purely scientific flyby mission of Venus with the launch of Venera 2. The probe successfully encountered Venus while fully functioning, but to conserve power the spacecraft's communications had to be deactivated. The system was designed to reestablish communication with Earth-based ground stations after completing the flyby, but it failed to do so, this rendered the mission a complete failure, with no scientific data returned. About a year and a half later, the United States would launch its final attempt at a Venus flyby, as well. Mariner 5 was originally designed to replace Mariner 4 in the event of a failure, but after some minor changes the decision was made to send it by Venus. Successful flyby was achieved in October of 1967, just a couple days after the successful and first confirmed landing on Venus by Venera 4. After Mariner 5, focus shifted to landing on Venus, and flybys of Venus would only be conducted for the purpose of gravity assists or for relaying communications from Venus landers. NASA launched its third duo of Mariner spacecraft in 1969, with Mariner 6 and 7 launching in February and March of 1969. These spacecraft ended a nearly five year absence of Mars missions, and for the first time, both spacecraft of a Mariner pair successfully made it to the target. Although they were launched over a month apart, the two probes reached closest approach only a week apart. Like Venera 2, Mariner 7 marked the last purely scientific flyby of Mars, and focus would switch to Mars orbiters and Mars landers. Future flybys of Mars would occur for gravity assists, communications relays for landers, or as failed attempts to orbit or land on Mars.

Until Mariner 10, all planetary flybys were for used science, communication, or occurred unintentionally. Mariner 10, launched in November of 1973, became the first mission to use a planetary gravity assist to reach its target, Mercury. Additionally, Mariner 10 completed the first three successful flybys of Mercury, beginning in March of 1974 and lasting about a year. Two cameras allowed color images to be taken, providing the first close images of Mercury and later in color.