Docking In Space

ON OCTOBER 11, 2000, THE SPACE SHUTTLE DISCOVERY took off from Kennedy Spaceflight Center at Cape Canaveral. Two days later, it docked with the International Space Station, setting the stage for eight clays of construction work. At the moment of docking, the shuttle/station complex, including the three modules that made up the station at that time—Zvezda, Zarya, and Unity—weighed more than 160 tons and spanned a length of more than 150 feet. Bringing structures like this together in space seems almost routine today. Zvezda, the station’s first habitable module, docked automatically with the station in July 2000 with no problems, and as of last June American astronauts have docked the shuttle to the station nine times. Before that, shuttle and Soyuz dockings with the space station Mir happened in an almost humdrum manner.

Successes like these can make docking a space vehicle seem about as challenging as backing a car into a garage, but in fact it is vastly more complicated. A better analogy would be to a driver trying to attach his four-ton truck to the tow hitch of another 16-wheeler while both are racing down the highway at night, without lights. Even this greatly understates the difficulty, because in earth orbit the task happens in three dimensions and at speeds greater than 17,500 miles per hour. Moreover, the driver has to use a joystick instead of a steering wheel, and no help is available from friction.

Not surprisingly, the first successful docking took several years and some downright heroic acts to accomplish. Almost from the beginning of manned exploration of space, the need to dock spaceships together was considered an essential task. The entire U.S. lunar landing program depended on orbital rendezvous and docking. According to NASA’s plan, as announced in 1962, a lunar landing craft would detach from an Apollo capsule while in orbit around the moon and soft-land on the lunar surface. After the completion of surface exploration, this lunar module would blast off from the moon and rendezvous and dock with the Apollo capsule. To make this plan work, American engineers and astronauts had to learn the technique of orbital rendezvous and docking and make it totally reliable. And they had to do it fast if they were to get to the moon before the end of the 1960s.

When NASA’s space-docking effort was barely under way, it had to deal with a jarring piece of news. Just as had happened five years earlier with Sputnik , the Soviet Union caught the U.S. space program flat-footed and took a major step forward—or seemed to. On consecutive days in August 1962, the Soviets launched a pair of manned capsules, Vostok 3 and 4 . Soon after Vostok 4 reached orbit, it flew to within four miles of Vostok 3 , and its cosmonaut, Pavel Popovich, thought he could see the blinking lights of the other craft. He and Vostok 3 ’s cosmonaut, Andrian Nikolayev, spent the next three days orbiting the earth together.

The Soviets’ repeated references to the mission as a “group flight” sent many Americans and others into a panic. Kenneth Gatland of the British Interplanetary Society called the dual mission “a clear indication that the Russians intend to land a man on the moon within three or four years; once they have achieved orbital rendezvous, they have taken the vital step towards the lunar flight.” Or as a headline in the Washington Daily News proclaimed: A SPACE GAP! AND HOW!

NASA had done little that could compare to the Vostok flights. Its Mercury program was only two-thirds complete and had managed to put into orbit only two manned missions, each a mere five hours long. None of the Mercury capsules could make orbital maneuvers; all they could do was ride a rocket into space, circle the earth a few times, and come back. To the American public, the possibility that the Soviets had already developed orbital rendezvous—little more than a year after Yuri Gagarin had made the world’s first manned space flight—was frightening.

Yet, while the secretive Soviets tried to imply otherwise, the truth was that neither Vostok spacecraft could make orbital changes. The only reason they were so close together was the careful timing of their respective launches. After their initial meeting, the capsules never came as close again, slowly and inexorably drifting apart during the rest of the mission. The Vostok stunt was essentially equivalent to a showpiece steel plant or model collective farm created for propaganda purposes.

Meanwhile, as NASA was making plans for Apollo, it was also creating the Gemini program to test and develop orbital rendezvous and docking techniques and other technologies necessary to get men safely to the moon. Gemini was planned as io two-man missions (following a pair of unmanned test flights) to take place every two months, beginning in March 1965. Nestled in time between the novelty of Mercury and crowning drama of Apollo, the program is easy to overlook, yet its achievements were indispensable. In addition to docking and rendezvous, Gemini would test whether humans could survive in space for two weeks, whether they could work in space and perform space walks, and whether onboard computers could help guide them in rendezvous and re-entry.

The first manned Gemini mission, Gemini 3 , was launched on March 23, 1965. Crewed by the Mercury veteran Gus Grissom and the rookie John Young, it was a three-orbit shake—down flight whose main purpose was to demonstrate that the Gemini equipment and systems worked. It would also test their ability to change course accurately, a first step toward docking.

The capsule was cone-shaped, with an interior space comparable to that of a compact car. At launch it weighed a little more than 8,000 pounds. Its nose was a narrow cylinder, as long as the capsule, that contained parachutes and 16 small thrusters, plus fuel for steering during re-entry.

Attached to the capsule’s wide end were two disposable equipment sections. The inner section contained the retrorockets that would send the spacecraft back to earth; the outer section contained water, oxygen supplies, fuel, and, most important, 10 engines for steering the spacecraft while in orbit. Protruding from the rear of the cylinder were 2. large engines, each capable of producing about TOO pounds of thrust. Eight smaller engines, with 2.5 pounds of thrust each, were placed in pairs at four points ringing the cylinder’s outside.

Since movement in space is three-dimensional, the placement of the eight small thrusters on the cylinder’s outside allowed them to govern three different orientations: roll, pitch, and yaw. Roll is the capsule’s rotation around its long axis, pitch is the tilt up and down of that axis, and yaw is the swing of that axis to the pilot’s left or right. An astronaut using these thrusters needed to be able to control all three motions to within inches. To complicate matters even more, in space, changing position or velocity in one direction will automatically change the vehicle’s position and velocity in the other two.

The explanation goes back to Johannes Kepler, who first derived the laws governing the motions of planets, and Isaac Newton, who codified them in mathematical terms. Newton’s laws show that when a planet—or a spacecraft—is orbiting another body, increasing its speed will increase the size of the orbit, and vice versa. Similarly, decreasing its speed will decrease the size of the orbit, and vice versa. These laws are simple enough to be covered in most freshman physics courses, but their application in space proved very tricky.

No great problems presented themselves in the first manned flight. During their five hours in space, the Gemini 3 astronauts used the spacecraft’s thrusters to change their trajectory three times. In the first maneuver, they altered their orbit from oval-shaped, with a low point of 100 miles and a high point of 140 miles, to a much rounder 98 by 105 miles. Then they changed their orbital inclination, or tilt to the equator. Finally, they lowered their orbit to 52 miles high, an altitude low enough that the atmosphere would drag them back to earth if their retrorockets happened to fail. The ease with which Gemini 3 accomplished these tasks suggested that orbital rendezvous might not be that difficult. On the next mission, however, reality imposed itself.

Gemini 4 , piloted by Jim McDivitt and Ed White, reached orbit on June 3, 1965. In accordance with his flight plan, McDivitt immediately tried to maneuver close to the spent second stage of the launch rocket, which was drifting about 650 feet away. To his surprise, McDivitt found that he could not trust his earthbound instincts when it came to orbital dynamics. First of all, judging distances in space was extremely deceptive. Without any reference points or radar, McDivitt found it almost impossible to estimate the precise distance to the booster. Instead he had to ask Mission Control for the orbital data of both the booster and the capsule.

More significant, he found that simply pointing the craft toward the target and firing his rear attitude thrusters did not help him overtake the spent stage, as one would expect. Instead, the distance between them actually increased, as if he were pressing a car’s gas pedal in forward gear and the car was moving in reverse.

This was where Newtonian mechanics kicked in. By increasing his craft’s speed, he had increased its distance from the earth. In this new, higher orbit, the craft’s linear velocity, measured in miles per hour, was greater than before. But its angular velocity—the rate at which it was traveling around the earth, measured in revolutions per hour—was lower. As Kepler had pointed out, objects in low orbits will complete an orbit around the earth faster than those in high orbits, even though their linear velocity is lower.

Thus, by speeding up his spacecraft, McDivitt had made it circle the earth more slowly than the craft he was trying to catch up with. Mission Control had to call off the rendezvous, since McDivitt was using too much fuel. NASA engineers and astronauts extracted a valuable lesson from this mission: It was difficult, if not impossible, to steer a spacecraft merely by eye. The orbital dynamics are so counterintuitive that—combined with the lack of references for judging distances—no human could do the job without help from electronic sensors.

As Gemini astronauts brushed up on their physics, the program continued its march. Gemini 5 , launched on August 21, 1965, introduced the use of radar to solve the problem of judging distances in space. The plan was that on the second orbit, the astronauts—Gordon Cooper, who had flown in Mercury, and Pete Conrad, a rookie—would release a miniature satellite called the Radar Evaluation Pod. Radar in the Gemini capsule’s cone would detect a beacon in the pod and measure the distance and relative speed between the two spacecraft. The astronauts would then use the data to rendezvous with the pod.

Unfortunately, a failure in Gemini 5 ’s fuel cells early in the flight curtailed most of the mission’s experiments, including the pod rendezvous. As Mission Control scrambled to find a way to keep the eight-day flight in orbit with limited electrical power, the pod drifted and was lost. Unable to perform any rendezvous maneuvers while the pod was nearby and available, Cooper and Conrad could only test the radar, successfully measuring the pod’s distance and speed for half an hour.

Despite the loss of the pod, Mission Control suggested that the astronauts could still do a rendezvous test even if their target was imaginary. On the third day of their mission, they were given an orbiting position in space to aim for, and four maneuvers later, they reached this position. This phantom rendezvous, however, was really just a slightly more precise sequence of the orbital maneuvers that had been accomplished in Gemini 3 . No one had yet successfully maneuvered one spacecraft closer to another while both were in orbit.

NASA decided to go ahead with docking on Gemini 6 . To give the Gemini capsule a target, NASA would place an Agena target vehicle in orbit, using an Atlas rocket to get it there. The Agena, which weighed 7,100 pounds, was outfitted with a thruster engine on one end and a docking port on the other.

This port, five feet wide and four feet deep, consisted of a collar with a single V-shaped alignment notch. To achieve a docking, the astronaut would aim the Gemini capsule’s nose cone into the collar, aligning a two-foot-long post on its side with the notch. When the post pressed against the back of the notch, it would push the collar inward, forcing three latches in the port to grab the nose. The latches locked the spacecraft together while pulling an electrical plug forward into an outlet in the center of the nose. At that point the two spacecraft would be hard-docked, allowing the astronauts to fly them together as a single vehicle.

The flight plan for Gemini 6 , manned by Mercury’s Wally Schirra and the rookie Tom Stafford, was to lift off one day after the Atlas-Agena, chase it down, and then dock with it. On October 25, 1965, however, the Atlas rocket exploded over the Atlantic Ocean six minutes after launch. Schirra and Stafford now had nothing to dock with, and their launch was scrubbed.

To salvage the mission, NASA officials decided to reschedule Gemini 6 and have it rendezvous with Gemini 7 , a twoweek flight intended to prove that humans could live in space long enough to travel to and from the moon. Gemini 7 was launched on December 4, 1965, crewed by Frank Borman and Jim Lovell. For the next five years these two men would hold second and first place, respectively, for the most time spent in space. Borman later became the head of Eastern Airlines, while Lovell is best remembered as the commander of Apollo 13 .

Flying two different manned missions at the same time, however, was not that simple. NASA had only one launch pad capable of handling such flights, so immediately after Gemini 7 lifted off on December 4, workers scrambled to clean up the pad and reassemble the Titan rocket and Gemini 6 capsule, which had been taken apart and put in storage when its mission was scrubbed in October. After eight days of frantic effort, Gemini 6 once again was ready for launch. With Schirra and Stafford in the capsule, the countdown reached zero and the Titan rocket beneath them ignited.

And then, after 1.2 seconds, the rocket shut down. With brown smoke drifting across the launch pad and 150 tons of explosive fuel below him, Schirra had a split second to make a life-or-death decision. He could pull the emergency ejection cord, activating ejection seats to fling him and Stafford from the Gemini capsule; parachutes would then open and lower them to the ground, about 500 feet away. Or he could sit tight and hope that the Titan’s fuel did not explode. If Schirra pulled the ejection cord, he and Stafford would probably escape harm (though many questioned the safety of the ejection seats), but the damage to the Gemini capsule would likely mean that his mission would never fly.

Schirra sat tight. As he reported to Mission Control, “We’re just sitting here breathing.” For 90 minutes he and Stafford waited tensely as ground personnel drained the rocket’s fuel tanks and secured the launch pad. Then the two astronauts climbed down from the capsule while technicians swarmed over the pad, trying to figure out what had gone wrong.

The speedy inspection revealed two causes for the shutdown, both ridiculously simple to fix. First, a small electrical plug had separated from its outlet about two seconds too soon. Second, a dust cover, placed on an engine valve during maintenance work in July, had mistakenly been left in place. While the first problem was what had caused the Titan’s engines to stall in December, the second would have caused a stall in October as well.

The next launch window was three days later. While everyone waited, Borman and Lovell floated in orbit, going around and around the earth while confined in a space about the size of a Volkswagen Beetle. “We’re beginning to itch,” replied Borman when asked about their condition by a doctor in Mission Control.

“Scalp or all over?” the doctor asked, suddenly concerned.

“We’re just getting a little crummy,” Borman replied.

Finally, on December 15, Gemini 6 tried again and, on its third attempt, lifted off. As the rocket cleared the launch tower and drove the spacecraft skyward into a perfect orbit, Mission Control said, “ Gemini 6 , you are go!” In the capsule Schirra could not help replying, “Go! You hear that, man, go!”

Based on the experience of Gemini 3,4 , and 5 , Schirra and NASA had a better understanding of in-space maneuvering, and in six hours and four orbits, the astronauts easily steered Gemini 6 to within 10 feet of Gemini 7 . To accomplish this task required a complex series of steps, none of which are initially obvious or intuitive to earthbound humans.

Engineers call the high point in any orbit the apogee and the low point the perigee. These are located at opposite points in the orbit. If the orbit is perfectly circular, the apogee and perigee are identical. Most orbits are not circular but instead have the shape of an ellipse, with the earth at one of the foci. The more eccentric the ellipse, the greater the difference between the apogee and perigee, and the longer and thinner the orbital oval.

To change the altitude of apogee or perigee requires a change in speed, but unlike on earth, you can’t simply jump at the moment when you want to go higher. It turns out that just the opposite applies: To raise or lower the perigee, you wait until you are at apogee, and to raise or lower the apogee, you wait until you are at perigee. Changes can be made at other points in the orbit, but doing so is very wasteful of fuel.

Gemini 6 ’s initial orbit was 100 by 161 miles (i.e., a perigee of 100 miles and an apogee of 161 miles), compared with Gemini 7 ’s 185-mile-high circular orbit. Each engine burn, of which there were a total of eight, had to bring these orbits step by step into alignment. In fact, the rendezvous sequence began on the ground, with the careful timing of Gemini 6 ’s launch to bring it as close to Gemini 7 ’s orbit as possible.

The first in-space burn took place at the end of Gemini 6 ’s first orbit, when the capsule was at perigee. This burn raised the apogee to 168 miles. The second burn, when Gemini 6 reached its new apogee 43 minutes later, raised the orbit’s perigee from 100 to 139 miles. Though Gemini 6’s orbit, now 139 by 168 miles, was still lower than that of Gemini 7, Gemini 6 also trailed Gemini 7 as they circled the earth. The lower elevation allowed Gemini 6 to orbit faster than Gemini 7 in order to catch up.

Next came a slight adjustment to Gemini 6 ’s orbital inclination, changing the capsule’s tilt relative to the equator by a tiny 0.007 degrees. Making even a small inclination change like this requires a lot of fuel because of the capsule’s high angular momentum, just as it is difficult to tilt the axis of a rapidly spinning wheel. In some cases, adjusting the orbital inclination by the necessary amount is simply impossible. This is why spacecraft at Mir cannot travel to the International Space Station. Their inclinations are too different.

The fourth burn came at perigee at the beginning of Gemini 6 ’s third orbit. It lowered the apogee slightly, a precise adjustment that became necessary when ground calculations revealed that the first burn had raised it too high. When the craft reached this new apogee, 47 minutes later, another burn raised the perigee so that Gemini 6 was now in a near-circular orbit, 168 by 170 miles, about 17 miles below Gemini 7 .

For the next two hours, the two spacecraft orbited the earth, with Gemini 6 slowly creeping up behind Gemini 7 from below. During this time, Schirra made two more course corrections to raise his spacecraft’s orbit those last 17 miles. Finally, at an altitude of 185 miles, the Gemini capsules moved within a few feet of each other, completing the first successful rendezvous in space. For four hours, the spacecraft flew side by side, at one point easing so near that Schirra and Stafford could see the scruffy beards on Borman and Lovell, who had been locked in their tiny capsule for more than n days.

Near the end of the rendezvous, Tom Stafford suddenly announced, “We have an object in view. Looks like it’s in a polar orbit and in a very low trajectory, traveling north to south.” Ground controllers were startled by this announcement, as were Borman and Lovell. What could Stafford possibly be talking about?

He continued, “It looks like he’s trying to signal us. Stand by—we’ll try to pick this up.” There was a long pause. Then Wally Schirra began playing “Jingle Bells” on a harmonica, with Stafford accompanying him with a string of bells; it was io days before Christmas. At ground control the response was a laugh and the words “You’re too much.” One day later, Gemini 6 returned safely to earth, while Gemini 7 continued in orbit for another two days, completing its appointed fortnight in space.

Despite this success, no true docking had yet been accomplished. Because the Gemini 7 capsule had no docking port, the vehicles had never tried to link, and NASA’s docking mechanism remained untested. Moreover, while the eight small attitude jets that ringed the outside of the Gemini capsule appeared to work with the necessary precision, their ability to bring two spacecraft together safely could be proved only by an actual docking.

The next mission was Gemini 8 , launched on March 16, 1966, and piloted by Neil Armstrong and Dave Scott, both of whom would later land a lunar module on the moon. This time, the Agena target vehicle successfully reached orbit. This time, after six and a half hours and four orbits, Armstrong was able to guide his capsule into the Agena’s port and achieve the first hard docking in space.

Just before docking, with the spacecraft three feet apart, Armstrong paused to make sure that everything was in order. Then he eased the capsule forward, at a relative speed of less than a foot a second, letting the docking pin slide into the alignment notch on the Agena’s docking collar. The latches grabbed the capsule, the electrical systems linked, and Armstrong announced, “We are docked. And he’s a real smoothie,” referring to the ease of the whole operation.

The success did not last long. Less than half an hour later, Mission Control made the uneasy public announcement “We’ve encountered some trouble in the flight.” A short circuit in one of Gemini 8 ’s attitude-control thrusters had sent the docked vehicles into a tailspin.

At first the astronauts thought something in the Agena was the cause of the problem, so after 15 minutes of struggle trying to gain control, Armstrong undocked. That only made the tumbling worse. With the capsule spinning 60 times a minute, Armstrong and Scott were experiencing vertigo and blurred vision. Unless they did something soon, the stresses would tear their spacecraft apart.

In desperation, Armstrong shut down the entire orbital attitude system, correctly guessing that one of its eight thrusters was stuck in the on position. The spinning immediately eased. Without this control system, however, the astronauts could now orient the capsule only with the re-entry control system in the capsule’s nose, a set of thrusters that were normally reserved for use during the return to earth’s atmosphere. NASA was forced to abort the rest of the mission. After an emergency splashdown in the Pacific Ocean, Armstrong and Scott bobbed for three hours in their capsule while waiting for the destroyer USS Mason to pick them up. Armstrong’s cool handling of the emergency would make him an obvious pick to land the first spacecraft on the moon in 1969.

A little more than two months later, NASA tried again. As with Gemini 6 and 8 , the plan was for Gemini 9 to dock with an Agena target rocket. However, as with Gemini 6 , the Atlas rocket exploded over the Atlantic soon after launch.

This time NASA had a backup plan. A vehicle dubbed the Augmented Target Docking Adapter, or ATDA, replaced the lost Agena and was sent into orbit two weeks later on another Atlas rocket. Unlike the Agena, however, the ATDA had no engines. It could only provide the Gemini capsule with a docking port.

When Gemini 9 , manned by Tom Stafford and Eugene Cernan, reached orbit on June 3, 1966, the astronauts discovered that the shroud protecting the docking port on the ATDA was jammed in place, blocking the port. To Stafford, the two halves of the shroud resembled a pair of gaping jaws, making the ATDA look like an “angry alligator.” With the port blocked, the best Stafford could do was to duplicate what Schirra had done on Gemini 6 , piloting his capsule to within inches of the target. After almost a year of effort and five missions, NASA still had not been able to dock two vehicles in space and use them together as a unit.

Finally, Gemini 10 , launched on July 18, 1966, and crewed by John Young and Michael Collins, made it happen. In fact, these astronauts not only docked with their own Agena target, they also rendezvoused with the Agena rocket left behind by Armstrong and Scott, where Collins did a space walk, leaping across a 15-foot gap to recover an experiment package that Gemini 8 had been meant to retrieve.

Young and Collins also flew higher than any human had ever flown before, firing the Agena’s engines to rise to 475 miles, an altitude that only Apollo’s lunar astronauts and one later Gemini crew would exceed. The curve of the earth was quite pronounced as they flew on the edge of interplanetary space. “My God,” Collins remembered thinking, “the stars are everywhere, even below me.”

Two more Gemini missions followed. Astronauts and engineers used these final flights to refine their docking and spacewalking techniques. The program ended with the splashdown of Gemini 12 in November 1966.

In later years, both the United States and the Soviet Union would perfect orbital docking techniques. While the United States has always depended on the manned piloting of orbital spacecraft, the Soviets developed remote docking technologies with unmanned spacecraft. In keeping with the centrally directed Soviet economic and political system, engineers decided to create spacecraft that could be completely controlled from the ground.

A little more than two weeks after the completion of the last Gemini mission, the Soviets launched the unmanned Cosmos 133 , the first test flight of today’s Soyuz spacecraft. Ground controllers could fire its engines and adjust its attitude. A failure in its attitude-control system, however, prevented these maneuvering tests, forcing ground control to activate the selfdestruct command during re-entry, blowing up the spacecraft before it could hit the ground in neighboring China.

Over the next three years, the Soviets flew a number of manned and unmanned missions in an effort to achieve a hard docking in space. One mission, Soyuz 1 , on April 23, 1967, resulted in the death of its cosmonaut, Vladimir Komarov. The plan had been for a second Soyuz capsule to follow it up and dock with it, whereupon two cosmonauts would space-walk from Soyuz 2 to Soyuz 1 (the docking port in this early version of the Soyuz spacecraft had no internal linking tunnel) and return to earth with Komarov.

Before that could happen, Komarov’s flight was plagued with numerous problems, not all related to his rendezvous and docking equipment. One of his solar panels failed, depriving him of half his power. Then the automatic control system used by ground controllers failed as well. Since the spacecraft had not been designed for the cosmonaut to operate, Komarov had great difficulty keeping his capsule from tumbling wildly. Finally, upon re-entry, the launch of the second Soyuz having been scrubbed, Komarov’s reserve parachute tangled with his main chutes, preventing their deployment. He hit the ground at 400 miles per hour and was killed instantly.

It wasn’t until January 1969 that the Soviets accomplished their first docking in space, when two crewmen from Soyuz 5 transferred by space walk to Soyuz 4 . By that time, NASA was on the verge of landing on the moon.

In 1975 the United States and the Soviet Union used a docking in space, the Apollo-Soyuz Test Project, to demonstrate the possibilities of détente and peaceful cooperation. In this case, a Soyuz spacecraft carrying two cosmonauts served as the target for an Apollo capsule carrying three astronauts. To equalize the disparate atmospheres between the two capsules, the United States built a special docking module. In addition, the countries jointly developed an “androgynous” docking system, with interchangeable units on the two craft instead of one male and one female. A modified version of this design is in use today in the International Space Station.

All these dockings were as tricky and as challenging as the first Gemini dockings in 1966. As recently as June 1997, the difficulties of docking in space were made clear once again when a Progress freighter crashed into Mir during a docking test. In this accident, the cosmonaut Vasily Tsibliyev was using a new system for remotely controlling an unmanned Progress tanker, a system that had caused some problems in the past. In previous tests the system’s television camera had interfered with the radar that measured the distance and speed of approaching spacecraft. To avoid this problem, Mission Control decided to turn the radar off, telling Tsibliyev to judge distances and speed by eye and with a hand-held laser range-finder.

The freighter approached from above, so when Tsibliyev used its camera to find Mir, the station was backdropped by the earth, making it difficult for him to see Mir on his video screen. These handicaps, not greatly different from those faced by Jim McDivitt on Gemini 4 three decades earlier, led Tsibliyev to badly misjudge the approach of the tanker. It smashed into Mir, damaging one solar panel and causing an air leak that forced the crew to abandon and seal off one station module.

Today, 35 years after Gemini, we are still using the same docking techniques to build the first permanent human settlement in space, the International Space Station. Over the next few years, more than ioo components will be linked with the station, brought there by more than 40 missions. Moreover, the American shuttle and Russia’s Progress freighters and Soyuz spacecraft will dock there, bringing food and supplies.

In the future, we will use these same skills to assemble in orbit our first interplanetary spaceships. Getting humans to Mars and elsewhere will require large structures similar to the International Space Station, made up of many modules launched separately and linked together by rendezvous and docking. And if and when we finally launch our first interstellar spaceships, we will surely use the same techniques that were pioneered when an Agena rocket and a tiny Gemini capsule linked together 245 miles above the earth.