The International Space Station orbits the Earth at an altitude of about 230 miles. The space shuttles, over their combined 135 missions, usually orbited around 200 miles above the Earth, though the peak operational limit was around 600 miles. This is only slightly higher than the Mercury and Gemini missions that flew I the early 1960s; they usually orbited around 160 or 170 miles. But one mission, Gemini 11, went way higher. In September 1966, the mission set a record that has yet to be broke, orbiting the Earth at 850 miles. Why? In short, because the mission's commander Pete Conrad wanted to.
In its early days, there was talk of using the Gemini program to send men to the Moon. The proposal resurfaced in 1964, just months before manned flights began. The program looked so promising and Apollo, which started just a year after Gemini, was so wrought with problems that Gemini mission planners considered extending the program to include a lunar flight. When Pete Conrad heard about this proposal, he was intrigued.
A lunar Gemini mission, at least of the kind NASA was considering adding to the program in the mid 1960s, wouldn't land on the Moon – it wouldn't have the right hardware. But even going into orbit or just swinging around the Moon's farside would be a huge step for NASA in its race with the Soviet Union. It would also be a hell of a mission for the crew. Conrad wanted to be on that crew.
So he went to Congress and argued his case, seeking support for at least one Gemini mission to go to the Moon. He didn't get it. The rationale was (rightly so) that NASA had spent so much time and money developing Apollo already it wasn't going to throw that away and spend more money developing a lunar Gemini mission. Apollo simply was a higher priority.
But Conrad wasn't quite satisfied with that. He wanted to do something different on his Gemini 11 mission. If he and pilot Dick Gordon couldn't go to the Moon, they'd at least get a close as possible.
The Gemini Program's main goal was to train astronauts to rendezvous and dock with a target vehicle in orbit, in this case with the Agena vehicle. It was a tricky manoeuvre they were going to have to use on Apollo lunar missions, and while flights before Gemini 11 had managed it they'd all done so on later orbits. Conrad suggested he and Gordon rendezvous and dock with their Agena on their first orbit – make it more interesting. Then he suggested they use the Agena's engine to put Gemini 11 in a high orbit.
He enlisted the help of fellow astronaut and nuclear engineer Bill Anders – who would be on the first crew to go to the Moon on Apollo 8 – to prove that a high orbit wouldn't pose any risks to the Gemini spacecraft. At least, no more risk than was associated with an orbital spaceflight at the time. Finally, NASA gave him a 'go' for a record altitude flight.
The mission launched successfully on September 12 during its two second long launch window – the necessarily short window for the first orbit rendezvous and dock with the Agena that Conrad and Gordon managed. The crew then used the Agena's propulsion system to boost the Gemini 11 to its 850 mile orbit, and, keeping the two vehicles connected with a tether, spun them to produce a small amount of artificial gravity (though in such a small spacecraft they didn't get to enjoy the effect much). The crew achieved all their goals, and also got some of the most striking images of the Earth from orbit in NASA's history.