The art of bringing spacecraft into contact without a collision has never been as simple as science fiction depicts it. This month we celebrate 50 years of space docking and look at the latest control techniques.
Today we take for granted that two spacecraft can easily locate each other in the vastness of the orbital realm and come together for a docking. As we mark the 50th anniversary of the first such union, it turns out that, having perfected this basic tool of the rocket business, European engineers are working on some clever new twists for the future.
Of course the story really begins on 25 May 1961, when US president John F Kennedy urged his country to land a man on the Moon “before this decade is out”. Fledgling space agency Nasa already had a scheme on paper, called Apollo, but its designers were startled to learn that they had just nine years to get the thing built and working. Nasa’s track record in manned rocketry up to that time was precisely one 15-minute suborbital arc in a cramped Mercury capsule, completed just three weeks before Kennedy’s famous speech.
Nasa soon began urgent work on Gemini, a substantial upgrade of the Mercury, with two seats and ‘gull wing’ doors to enable space walking (or escape by ejector seat in the event of a troubled launch). The craft also sported external steering thrusters that would test astronauts’ abilities as pilots, rather than just hapless passengers, in the space adventure. Gemini would also be the first spacecraft to carry a guidance computer.
Not that the hardware was up to much by modern standards. Hidden behind a wall of Gemini’s cabin, the computer weighed fully 26kg and had a memory of just 4096 words (each comprising three 13-bit ‘syllables’). In 2007, veteran Gemini astronaut John Young gave this an unflattering description of it: “Imagine a box with a lot of wire coat hangers bent in all directions. They were threaded through tiny electromagnets that served as memory nodes. It was very crude compared to today’s microprocessors.”
Yet this primitive machine, developed by IBM, enabled Gemini to calculate orbital changes and with help from a nose-mounted radar, locate other vehicles for docking. Science-fiction artworks and speculative Nasa depictions always took for granted that space hardware could be launched piecemeal and mated in orbit, but no one knew exactly how to do it for real. Nasa’s astronauts and their software developers needed to learn fast ahead of the Moon missions and Gemini was their teaching tool.
After two successful unmanned tests of the capsule, Gemini 3 took off on 23 March 1965, carrying Virgil Grissom and John Young. Mission Control was wary of the new computer and commander Grissom was instructed not to use it for re-entry as its on-board displays disagreed with Nasa’s predictions of where the capsule would splash down. Gemini 3 eventually hit the water more than 100km from the nearest reception teams. When Young and Nasa experts rechecked the figures, they found that if they had trusted the computer, it would have steered the capsule into the water nicely alongside the recovery ships.
Gemini missions over the next two years refined the business of bringing vehicles nose-to-nose in earth orbit, but were the Nasa astronauts ahead of the game, or had their Soviet rivals already beaten them to it?
In August 1962, cosmonaut Andriyan Nikolayev was launched in the same kind of Vostok capsule that had propelled Yuri Gagarin into the history books the previous year. The next day Pavel Popovich went up in another Vostok. For the first time, two people were in space simultaneously in different ships. The Russians timed the launches so the second Vostok would briefly approach to within 6.5km of Nikolayev’s craft, in a cosmic clay pigeon shoot that enabled the Kremlin spin doctors to claim the first ‘space rendezvous’.
It was really just a question of carefully timing the two launches. The Vostoks had no thrusters and quickly drifted apart. Even so, those unfamiliar with the physics of orbital navigation (essentially, most journalists at that time) were fooled into thinking that the Soviets had developed genuine rendezvous skills. Gemini astronaut Walter Schirra was dismissive (and colourful) while discussing this with space writer Francis French in 2002: “When a man looks across a street, sees a pretty girl and waves at her, that’s not a rendezvous, that’s a passing acquaintance. When he walks across the street and nibbles on her ear, that’s a rendezvous.”
The first attempt to ‘nibble on the ear’ of another spacecraft was made 50 years ago on 16 March 1966, by Neil Armstrong and his co-pilot Dave Scott, aboard Gemini 8. Their target was an unmanned Agena rocket stage, launched separately earlier that day. When the nose of the Gemini slipped smoothly into the special collar at the tip of the Agena, there seemed no doubt about this moment of triumph. The Soviets were some years away from being able to ‘hard dock’ two vehicles.
Spinning into disaster
The sense of triumph did not last. Capsule communicator (CapCom) Bill Anders strolled into Mission Control just as the docking was under way and colleague Jim Lovell handed the microphone to him, laconically saying, “It’s all pretty boring so far”. Moments later, Armstrong’s voice came on the line. “We’ve got serious problems. We’re tumbling end over end.”
Armstrong immediately pulled Gemini clear from the Agena. In September 2001, he explained his thinking to Nasa historians. “We first suspected that the Agena was the culprit. I was afraid we might lose consciousness, because our spin rate was getting pretty high and I wanted to make sure that we got away before that happened.” He was also concerned that the two vehicles could tear each other apart.
As it turned out, Agena and its positioning thrusters were blameless. A gas jet nozzle on the Gemini itself was jammed in the ‘on’ position. When the capsule’s spin rate reached a sickening one revolution per second, Armstrong took a bold decision and cut power to all the thrusters, hoping to kill the rogue unit in the process. It worked, but now the spacecraft was essentially crippled, because no one dared reactivate the thrusters and it was still spinning, although at least the rate was steady now.
Armstrong powered up a separate, smaller thruster array with limited fuel reserves, whose proper purpose was to orient the capsule in the last moments before re-entry. Back under control, but dangerously short of propellants, Gemini 8 prepared for an immediate return to Earth. They got away with it and Nasa senior managers noted the calm manner in which Armstrong had addressed a potentially terrifying crisis.
The control problems experienced by Gemini 8 were not repeated and Nasa became adept at the docking business. Soviet teams soon caught up, establishing an exceptional expertise in docking space components. Their equivalent of a ‘Gemini 8’ scare came in June 1997, when an unmanned cargo module conducted a practice docking with Russia’s Mir space station while remotely piloted by a crewman aboard the station, dangerously exhausted from previous overwork. The module came in too fast and crashed into a solar array. Mir narrowly avoided a fatal outcome. Overall, the international space community has the docking thing licked. So what comes next for this now standard routine?
Between 2008 and 2014, five uncrewed Automated Transfer Vehicles (ATVs) were launched by European Space Agency (ESA), carrying food, water and scientific equipment to the International Space Station (ISS). Each was targeted towards the station’s Russian-built Zvezda docking module. In the final approach phase, special reflectors on Zvezda bounced back laser pulses emitted by the approaching ATV. The pulse travel time between the two spacecraft was computed with help from 26 mirrors contained within a 25mm cube inside the ATV’s ‘videometer’.
This is an impressive piece of optical engineering that works well when both spacecraft are playing the same game, closing the gap between them along the same axis and neatly maintaining line of sight with each other. Yet what might happen if one craft is unstable?
Engineers at the European Space Agency (ESA) have thought about how to rescue crews from crippled vehicles, or dock unmanned hardware elements in deep space, where a radio time-lag of seconds, or even several minutes, would prevent mission controllers on Earth from taking over by remote control if anything goes amiss. The Laser Infrared Imaging Sensors (Liris) system was tested in August 2014 by Georges Lemaître, the fifth and last of ESA’s ATVs to dock with the space station. Liris ‘paints’ the entire front-facing surface of its target with light, using a fusion of laser and radar range finding techniques known as lidar. The Liris computer creates an almost real-time 3D model of ISS from which specific areas of high reflectivity can be identified and their range and relative orientation calculated. This enables the ATV to steer itself unaided using, essentially, a virtual model of the ISS.
If ISS or some equivalent structure happened to be in trouble, or its crew were unable to respond to instructions, Liris could be essential. According to ESA’s head of space transportation, Nico Dettman, “our ultimate aim is to develop the technology so that we can dock with an uncooperative target.” This could include not just space stations in trouble, but tumbling asteroids or “pieces of debris requiring collection”.
Liris was built by Airbus Defence and Space, with German manufacturer Jena Optronik providing the lidar components and France’s Sodern space instrumentation company responsible for infrared and visible wavelength elements. The tests were successful, although for safety’s sake, Georges Lemaître’s actual docking was conducted with older and more familiar video camera alignment systems, while Liris merely conducted a data-gathering parallel run. Even so, it’s clear that Liris and its derivatives will reshape what new steps can be added to an old and familiar space waltz.