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.
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