On Sept. 4, 1962, NASA's Mariner 2 spacecraft performed the first-ever rocket maneuver in deep space as it did a quick mid-course correction on its way to Venus. It had launched just eight days earlier and needed to burn its engines to ensure that its trajectory would take it to Venus.

The midcourse maneuver was necessary because the Atlas-Agena rocket that launched Mariner 2 didn't launch with enough accuracy to get the trajectory right in one shot. When it was about 1.5 million miles (2.4 million km) from Earth, it fired its engines to help steer it in the right direction.

The entire maneuver required five separate commands sent from mission control, telling the spacecraft to roll, pitch, turn and burn its motor. One hundred days later, Mariner 2 flew by Venus and became the first spacecraft to successfully fly by another planet.

Why it mattered

Tweaking a spacecraft's course in deep space seems a bit routine these days (NASA's newly launched Roman Space Telescope just performed such a feat this week), but in 1962 -- when reaching another planet with a probe was still the stuff of fiction -- it was far from a safe bet.

Mariner 2 launched in Aug. 27, 1962 on what would eventually be the first successful misson to another world. But as stated above, the launch wasn't accurate enough to place Mariner 2 on a direct course for Venus.

That's where the mid-course correction came in. The mission team designed a short engine burn that directed Mariner 2 to roll itself about 9.33 degrees, pitch its nose around 139.83 degrees so that it could turn its rocket engine to face forward, firing the engine, as Time Magazine recounted. The entire process took up to 36 minutes.

Without the mid-course correction, Mariner 2 would have missed Venus entirely. After launch, Mariner 2 was on a heading that would carry it only within 238,600 miles (384,000 kilometers) of the planet's center, then speed on by, according to NASA's official Mariner 2 report. After the course correction, Mariner 2 was on a path to fly by at a much closer distance of 21,607 miles (34,773 km).

Today, such mid-course corrections are a regular part of space mission planning, though they are still critical moments since a mission's success can hinge on their results. In the decades since Mariner 2, space mission designers have achieved amazing feats in mid-course changes, including the now familar gravity assist flybys that allowed NASA to send Voyager 2 on its Grand Tour of Jupiter, Saturn, Uranus and Neptune. Many other missions have achieved such feats.

Even this week, on Sept. 3, 2026, the BepiColombo mission to Mercury performed its own mid-course event, with the mission's Mercury Transfer Module releasing its two spacecraft (one each for Europe and Japan) to begin their arrival at Mercury. The event came eight years after launch and followed a series of maneuvers, including nine planetary flybys to reach the planet.

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On Aug. 27, 1962, NASA launched Mariner 2, the world's first spacecraft to successfully visit another planet.

Mariner 2 flew by Venus after a 3.5-month voyage through space.

The mission launched just one month after the Mariner 1 mission failed during launch. After departing from Cape Canaveral, Florida, Mariner 2 almost met the same fate when an electrical problem with the Atlas-Agena rocket caused it to start rolling, which made it unable to respond to any guidance commands from mission control.

By some miracle, the problem fixed itself, and Mariner 2 was on its way to Venus!

Why it mattered

Mariner 2 was a watershed mission for NASA at a time when, in the early 1960s, the U.S. was trailing behind in the Space Race with the Soviet Union — which had already launched the first satellite (Sputnik 1 in 1957), the first living creature (Laika the dog on Sputnik 2 that same year), the first human in space (Yuri Gagarin in 1961) and sent a probe to the moon (Luna 1 in 1959).

NASA's big Venus first mission, Mariner 1, failed out of the gate as mentioned above during an attempted launch on July 22, 1962, but those fortunes changed with Mariner 2.

It was an electrical short that caused Mariner 2's rocket to roll during ascent, but the booster managed to correct itself after about a minute, NASA has said.

The trip to Venus was a bit rocky, too, with Mariner 2 suffering a solar panel power drop out two times, as well as glitchy attitude control gyroscopes. Mariner 2 also got surprisingly hot as it approached its Venus flyby, prompting worries it might be cooked before it even reached the planet.

Despite all that, Mariner 2 achieved the first U.S. "first" in space, with a successful Venus flyby on Dec. 14, 1962. The probe flew within 21,564 miles (34,675 kilometers) of the hellish planet, taking the first close measurements of Venus at the time.

Mariner 2 carried no cameras (surprisingly), but its observations were the first detailed measurements of the Venus atmosphere and Venus' temperature, proving to scientists that the planet experienced a runaway greenhouse effect in the past. The probe's radio signals were also used to calculate Venus' mass in extreme detail, NASA has said.

"There will be other missions to Venus, but there will never be another first mission to Venus," said Jack James, NASA's Mariner 2 project manager at the Jet Propulsion Laboratory in Pasadena, California, before his death in 2001.

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Human-built spacecraft have been exploring other planets for a full 60 years.

On Dec. 14, 1962, a NASA spacecraft called Mariner 2 flew past Venus in the first-ever planetary flyby. The maneuver gave the spacecraft 42 minutes to observe what scientists now consider Earth's hellish twin — whose hidden surface at the time was thought to be lush, tropical jungle or swamp.

Data from Mariner 2 instead showed that even the night side of Venus was scorching, with surface temperatures as hot as 421 degrees Fahrenheit (216 degrees Celsius), according to NASA. Scientists already knew about Venus' super-slow spin (a day on Venus lasts about 243 Earth days), but had expected the day and night sides of the planet to show a much larger temperature difference than Mariner 2 found.

At the time, agency officials wrote that the Mariner 2 data painted an image of "a glowing hot earth covered by thin, dark and cool clouds," according to the New York Times.

Related: Venus, once billed as Earth's twin, is a hothouse (and a tantalizing target in the search for life)

Mariner 2 followed its twin and predecessor to the launch pad just a month after the failed launch of Mariner 1 in July 1962. Range personnel exploded Mariner 1 less than five minutes after launch because the booster was off course and expected to crash, according to NASA. 

But the launch of Mariner 2 went smoothly, and despite hiccups along the way, the probe endured its three-and-a-half-month trek to Venus. In addition, that journey allowed Mariner 2 to confirm the existence of the solar wind, the constant stream of charged particles flowing off the sun, and observe a major solar flare, according to NASA.

During the Venus flyby, Mariner 2 used its seven instruments to study both the day and night sides of the planet. In addition to measuring the planet's temperature, Mariner 2 detected the thick layer of clouds in its middle atmosphere; scientists now know those clouds are mostly sulfuric acid.

Although Mariner 2 made the first planetary flyby, it wasn't the first spacecraft to zip past another object in our solar system. That honor goes to the Soviet Union's Luna 1 mission, which flew past the moon in 1959. 

Venus was a common destination for both the U.S. and the Soviet Union from the 1960s into the early 1980s before its popularity began to fade. Currently, only one spacecraft is dedicated to Venus, Japan's Akatsuki mission that has been in orbit around the planet since 2015.

But the solar wind has returned the favor, so to speak. Both NASA's Parker Solar Probe and the European Space Agency's (ESA) Solar Orbiter are observing Venus as they make flybys to adjust their trajectories and venture closer to the sun.

And Venus is back on the agenda in a major way. NASA has committed to the DAVINCI and VERITAS missions, and ESA to the EnVision mission, all of which will explore the planet in the 2030s.

Email Meghan Bartels at mbartels@space.com or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.

Fifty years ago today (Dec. 14), NASA's Mariner 2 spacecraft made its closest approach to the planet Venus, marking the first-ever flyby of another planet.

Mariner 2 zoomed to within 21,564 miles (34,675 kilometers) of Venus on Dec. 14, 1962, gathering a trove of data about Earth's hellishly hot sister planet. The probe took the first close-up measurements of Venus' scorching temperatures, for example, helping confirm scientists' suspicions that a runaway greenhouse effect had taken hold of the world.

Tracking Mariner 2's radio signals, researchers also calculated Venus' mass with unprecedented precision, NASA officials said.

The spacecraft's flyby also marked a proud moment for NASA and the United States, after a five-year stretch in which the Soviet Union had claimed all of the world's big space firsts. The Soviets successfully launched the first artificial satellite in 1957, sent a probe to the moon in 1959 and put the first human in space in 1961. [Video: Mariner 2's Venus Flyby]

"JPL has always attempted to do mighty things on behalf of NASA and our nation," Charles Elachi, director of NASA's Jet Propulsion Laboratory in Pasadena, Calif., which managed Mariner 2's mission, said in a statement. "Achieving America's first 'first in space' is among the lab's proudest achievements."

Mariner 2 had a bumpy ride. Shortly after launch on Aug. 27, 1962, an electrical short caused the probe's rocket to roll, rendering it unresponsive to guidance commands. But the short circuit mysteriously healed itself about a minute later, NASA officials said.

Things got dicey during Mariner 2's cruise to Venus, too. A solar panel on the spacecraft stopped working on two separate occasions, and attitude-controlling gyroscopes misbehaved. Further, the probe's temperature rose dramatically as it approached Venus, causing managers to worry that Mariner 2 might be cooked before it could complete its mission.

But Mariner 2 overcame all of this, and the rest is history. In addition to lifting Venus' veil, the spacecraft's observations also confirmed the existence of the solar wind and enabled scientists to refine the value of the astronomical unit — the distance from Earth to the sun, which is about 93 million miles (150 million km).

In the half-century after Mariner 2's close encounter, other spacecraft have further studied the second planet from the sun. The Soviet Union even landed a number of probes on Venus' surface, beginning with Venera 7 in 1970. But Mariner 2 will always have a special place in history.

"There will be other missions to Venus, but there will never be another first mission to Venus," Jack James of JPL, Mariner 2's project manager, said before his death in 2001.

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Mariner 2, which flew by Venus in 1962, was the first spacecraft to successfully fly by another planet. The mission not only taught NASA about Venus, but also how to operate a spacecraft far from Earth.

The spacecraft encountered some anomalies in its months-long journey to Venus, but mission controllers managed to keep Mariner 2 healthy enough to perform its mission when it arrived in December that year.

Among other achievements, the spacecraft measured the solar wind streaming from the sun, confirmed the lead-melting temperatures on Venus' surface, and revealed how different a world Venus is from Earth.

Redundancy in pairs

The early 1960s were the dawn of the space age. The first Soviet Union satellite, Sputnik, went into space in 1957, and the Americans followed them there with Explorer 1 in 1958. But both countries were just learning about spaceflight, and failures were common.

The Americans adopted a robust backup strategy to get around this situation. Instead of launching one spacecraft, they would launch an identical pair. If one of the spacecraft failed, the mission could still be carried out by the second one.

Mariner 1 and Mariner 2 would be the first set of "twins" in the Mariner series. Each spacecraft was about 3.2 feet (1 meter) across and 1.1 feet (0.36 meters) thick, and would be powered through solar cells. The spacecraft each carried a battery on board for supplemental power when they had a lot of work to do.

The spacecraft would wing their way through space, using Earth and the sun as references to keep their attitudes stable. There was gas on board in case the spacecraft needed to make any corrections to their trajectories while flying to Venus.

Mariner 1 lifted off on July 22, 1962. However, in the minutes after launch, it began veering uncomfortably close to a shipping area and inhabited zone close to its launch site at Cape Canaveral. The range safety officer detonated the rocket at 293 seconds after launch. Mariner 1's flight lasted less than five minutes.

A review board determined a typo in the computer code sent improper guidance controls to the spacecraft. This was exacerbated with problems in the on-board beacon equipment that was supposed to guide Mariner 1.

"This caused the computer to swing automatically into a series of unnecessary course corrections with erroneous steering commands which finally threw the spacecraft off course," NASA later wrote.

Happily, Mariner 2 launched to better success Aug. 27, 1962. About half an hour after launch, the spacecraft was in space and flying toward its ultimate target: the planet Venus.

Mysterious planet

Venus is a bright and easy target for a telescope, but the problem is you can't see anything of its surface from Earth. This is because the entire planet is shrouded in cloud.

With Venus' true nature hidden, science fiction writers of the early 20th century speculated it might harbor life. Edgar Rice Burroughs and Ray Bradbury were among a group writing stories that portrayed jungle life on Earth's closest neighbor.

In 1956, U.S. Naval Research astronomer Cornell Mayer led a team that pointed a radio telescope at Venus to measure radio waves the planet generated and sent into space.

To everyone's surprise, he measured a surface brightness temperature of 600 degrees Fahrenheit (315 degrees Celsius), about three times the boiling point of water.

Some astronomers suggested there might be a runaway greenhouse effect happening on Venus. But nobody would know for sure until a spacecraft arrived there. Scientists were hoping Mariner 2 could answer questions about this.

One notable absence from Mariner 2's suite of instruments: a camera. Astronomer and science popularizer Carl Sagan was among the designers of the spacecraft. Decades later, he wrote it was a wrong-headed decision to send the spacecraft without a camera.

"There were those who maintained that cameras weren't really scientific instruments, but rather catch-as-catch-can, razzle-dazzle, pandering to the public, and unable to answer a single, straightforward, well-posed scientific question," he wrote in his 1996 book Pale Blue Dot.

"I thought myself that whether there are breaks in the clouds was one such question. I argued that cameras could also answer questions that we were way too dumb to even pose ... at any rate, no camera was flown."

Glitches during the flight

Just over a week after launch – on Sept. 4 – Mariner 2 began a midcourse adjustment to its path to Venus, which it pulled off with little incident. Its first hiccup came four days later, on Sept. 8.

The spacecraft abruptly tilted and its cruise science experiments were turned off, for reasons NASA could never determine. One possible explanation: it was hit by a small object in space. The spacecraft recovered itself only three minutes after the incident occurred, though. The loss of attitude control and quick recovery repeated themselves on Sept. 29.

A NASA report shows Mariner 2's mission went relatively smoothly for the next month, until Oct. 31. A "partial short circuit" occurred in one of the spacecraft's solar panels, and the science cruise instruments were taken offline until the panel went back to normal power the following week.

However, the panel failed completely and permanently on Nov. 15. Mariner 2, though, was moving closer to the sun every day as it approached Venus, so there was enough power being generated through the one good solar panel to keep the spacecraft going.

The spacecraft also experienced glitches in its magnetometer (to measure magnetic fields on Venus). Additionally, in late September there was a power failure at NASA's Goldstone Deep Space Communications Complex – which was tracking the spacecraft – leading to the loss of about 1.5 hours worth of data.

Most importantly, though, the spacecraft was still more than healthy enough for the Venus encounter. Controllers made final adjustments on Dec. 14. Later that day, the spacecraft passed underneath Venus at a closest distance of about 21,607 miles (34,773 kilometers).

Science of Mariner 2

Mariner 2 only had a brief glimpse of the planet, but even that glance showed us a lot. It confirmed the planet was a hothouse; today we know the average temperature on the planet is 864 degrees Fahrenheit (462 degrees Celsius). That was too hot for the jungles that Burroughs and others wrote about.

Additionally, the spacecraft revealed the planet is under high pressure. Soviet spacecraft that later landed on Venus (including Venera 13, which beamed back color pictures) were crushed within minutes of arrival.

More mysteriously, Mariner 2 showed that the planet was rotating opposite to the direction in which Earth and other planets in the solar system rotate. A Venus day is very long, by Earth standards: 243 Earth days. Because Venus rotates so slowly, it cannot generate a magnetic field even though it has a metal core.

Mariner 2 also made fundamental discoveries of interplanetary space, showing that "the solar wind streams continuously, and the cosmic dust density is much lower than the near-Earth region," according to a NASA account of the mission.

The spacecraft's last transmission to Earth was on Jan. 3, 1963, but other Venus missions were already in the wings. NASA's next spacecraft to Venus, Mariner 5, did carry a camera. 

But Mariner 2's close-up view of Venus was an important starting point for all Venusian science that has followed.

— Elizabeth Howell, SPACE.com Contributor