Flyby (spaceflight)

• 1. Introduction

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  A flyby is what a spacecraft called New Horizons carried out on the 31st of December 2018 and the 1st of January 2019. It swept past a small, distant body called Arrokoth in the Kuiper belt. At that moment, New Horizons sat 43.6 astronomical units from the Sun, the most distant flyby recorded to date. The spacecraft did not stop, orbit, or land. It gathered what it could during the pass, then kept moving outward into deeper space. What makes a fleeting pass like that worth an entire mission? And why would engineers choose one over settling into orbit or setting something down on a surface?

• 2. Defining The Pass

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  Mars and Earth stand out in the record as the two planets where these planetary flybys have actually occurred. A flyby, in the broadest sense, means a spacecraft passing near another body, usually the same body the mission set out to study. When that body's gravity is strong enough, the spacecraft can also pick up a boost from the encounter. That boost is called a swing-by, and it flings the spacecraft onward toward a second target.

  Asteroids are also described this way, even though they carry no engines or mission plan of their own. The word flyby gets used more loosely whenever a body such as an asteroid coasts past Earth. Two numbers decide how notable any of these passes becomes, the time of closest approach and the distance at that moment.

  Small Solar System bodies join planets and moons on the list of things a flyby can visit. A spacecraft flyby can target a planet, a natural satellite, or a smaller, non-planetary object drifting among them. Sometimes the difference between a landing and a flyby is decided after launch, not before it.

• 3. An Accidental Loop

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  An exploded oxygen tank aboard Apollo 13 left its crew with no way to land on the Moon as planned. Instead, the crippled spacecraft had to fly by the Moon and loop back toward Earth. This particular kind of pass has its own name, a lunar flyby, whenever the passing body is Earth's own Moon.

  Artemis 2 is the more recent mission on record to include a lunar flyby, this time as part of the plan rather than a fallback. Long before Artemis 2 ever flew, engineers had already been asking whether flying by an entire planet, not just a moon, might replace a landing.

• 4. Skipping The Landing

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  NASA's Manned Spacecraft Center evaluated a flyby rendezvous with Mars as a possible mission design in the 1960s. Engineers at the time had already sketched plans for a combined lander, short-stay surface habitat, and ascent vehicle. It was called the Mars Excursion Module, or MEM. The MEM's ascent stage was meant to rendezvous not with Earth, but with a separate spacecraft. That spacecraft had only flown past Mars, never entering orbit or touching down.

  MOR was the alternative mission architecture it was measured against, one that would have required a spacecraft to actually enter Mars orbit. A flyby rendezvous spared one spacecraft that need entirely. Fuel and supplies for the trip home were never carried in and out of Mars orbit. One related idea that grew out of these rendezvous studies, still discussed today, is called a Mars cycler.

• 5. Reading Mars From Afar

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  Mariner IV's flyby of Mars in July 1965 sent back atmospheric data far more accurate than anything gathered before. It also returned much closer views of the surface than any previous mission had managed.

  Mariner 6 and Mariner 7 flew by Mars in 1969 and delivered another leap in understanding. Their infrared radiometer readings showed the Martian atmosphere was made up mostly of carbon dioxide. The instruments also picked up trace amounts of water on the surface.

  Rosetta swung by Mars at a distance of only 250 kilometers, using the encounter for a gravity assist. That remains the closest flyby of Mars on record.

  Twin spacecraft called Mars Cube One, or MarCO, carried out a flyby in 2018 built specifically to relay communications during the InSight lander's descent. Both MarCOs had launched toward Mars riding the same cruise stage as InSight. Both reached Mars successfully. They relayed data through InSight's entry, descent, and landing on the 26th of November 2018.

  Tianwen-1's own deployable camera photographed the spacecraft during its journey to Mars in September 2020. The mission then made its flyby of Mars around the 10th of February 2021, based on the trajectory planned for it. Afterward, it headed into deep space or a solar orbit.

  None of these Mars passes matched the distance New Horizons would soon reach. In July 2015, it flew by Pluto at about 32.9 astronomical units from the Sun.

• 6. Titan, Again And Again

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  Cassini-Huygens launched in 1997. It spent from 2004 to 2017 in orbit around Saturn, flying by many of the planet's moons, Titan chief among them.

  October 2004 marked Cassini's first flyby of Titan, the earliest in a sequence that would eventually add up to 126 separate passes of that moon.

  The 22nd of April 2017 brought Cassini's final close flyby of Titan, coming just months before the spacecraft's retirement.

• 7. Comets And Close Calls

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  International Cometary Explorer, once known as ISEE-3, passed through the plasma tail of Comet Giacobini-Zinner. The pass, on the 11th of September 1985, came within 7800 kilometers of the comet's nucleus.

  The Deep Impact spacecraft, flying its EPOXI mission, carried out a flyby of comet Hartley 2 in 2010.

  This same loose sense of the word applied in 2014, when a passing comet made its own flyby of Mars, without any spacecraft steering it.

  P/2016 BA14 came within about 2.2 million miles of Earth during its 2016 flyby. Radar imaging at the time let scientists calculate the size of its nucleus, roughly 1 kilometer across.

  The short period comet 46P/Wirtanen made its closest approach to Earth on the 16th of December 2018. It came within about 7.1 million miles, one of its nearest passes ever. NASA researcher Michael DiSanti said the flyby would let scientists study the comet's composition in greater detail. More of its nucleus would be exposed to sunlight during the close pass.