With a mass of approximately 900,000 pounds when it is complete,the International Space Station (ISS) will be the largest, mostcomplex object humans have ever built in orbit (figure 1). Comprisedof modules, nodes, truss segments, resupply vessels, solar arrays,thermal radiators, and thousands of other components requiredto make it work, the Station, unlike any other before it, mustactually be assembled on site in orbit.

The United States' previous space station experience was Skylab.With a mass of about 165,000 pounds (slightly less than one-fifththe ISS mass), Skylab was the converted third stage of a SaturnV rocket: the kind that took humans to the Moon. Skylab was launchedinto space, needing no assembly.

The Russian Space Station Mir was not a single element like Skylab.Begun with a "base block" module launched in 1986, theMir Station has gradually added modules: Kvant in 1987, Kvant2 in 1989, the Kristall in 1990, Spektr in 1995, Priroda scheduledfor 1995. In addition, Soyuz crew transfer vehicles have deliveredcrew and returned them home, while various types of Progress cargovehicles have resupplied the station as required. Mir has grownover the years, using docking as the mechanism for building thestation.

The International Space Station is truly different from previousstations. It will require 44 spaceflights on five different typesof launch vehicles over a five-year period to haul into orbitall the components of the Station. Once in orbit, robots remotelycontrolled by astronauts will maneuver massive elements weighingthousands and tens of thousands of pounds on Earth. That's becausehumans can't safely handle such masses unassisted. Spacesuitedastronauts will handle final connections of cabling, switches,wires, tubes, pipes, fittings and any other connections that linkall Partner elements. No space vehicle has ever included so manyPartners: United States, Canada, Japan, Russia, and nine of theEuropean Space Agency (ESA) member countries: Belgium, Denmark,France, Germany, Italy, the Netherlands, Spain, the United Kingdom,and Norway.

The Space Station 's initial orbit is approximately 190 nauticalmiles high (217 statute miles). But by 2001, a year before assemblycompletion, it will have been gradually and steadily boosted 60nautical miles further out (285 statute miles). The extra altitudeis needed because the Sun 's 11-year sunspot cycle will be peakingthen, resulting in an increase of very short-wave ultravioletrays. These ultraviolet rays heat the atmosphere, causing it tobulge and swell in low Earth orbit where the Station would orbit.Although the expanded atmosphere is very tenuous, friction betweenit and the Station would still be enough to slowly drag the SpaceStation into a lower orbit. To minimize the drag, the Stationwill always operate within a 190-250 nautical mile altitude band,and it will always have sufficient propellant to remain in orbitfor at least a full year without any new propellant deliveries.Normal reboosts will occur every three to four months, usuallyone week after a Space Shuttle visit.

The Station also will orbit at a high inclination, i.e., at alarge angle to the Earth's equator. Most United States spaceflightsoccur at an inclination of 28.5 degrees. But because of rangesafety and the high latitude of the Russian launch facility atBaikonur, the lowest inclination Russian spacecraft can launchto is 51.6 degrees. Therefore, the Station will fly at an angleof 51.6 degrees. One benefit of this "high ground" isthat it will provide a clear view of at least 85 percent of theEarth's surface.

However, there is a negative, too. Space Shuttle Orbiters burnthousands of pounds of propellant to climb to this inclination.Therefore NASA is upgrading the Orbiter fleet to reduce propellantconsumption. Reducing the expendable fuel tank's weight and improvingShuttle engines' thrust make more propellant available to launchpayloads to orbit. Thus more Space Station components can be carriedaloft.

The 44 flights carrying all the Station elements to the orbitalconstruction site comprise Phase II and III of the program. PhaseI began in February 1994 with the first flight of a Russian cosmonaut,Sergei Krikalev, on a United States Space Shuttle (STS-60). Thetheme of Phase I Shuttle-Mir missions is familiarization of NASAand the Russian Space Agency (RSA) with each other's operationsand equipment. Phase I is expected to end with STS-86 in September1997.

Phase II starts in November 1997 when the first Station elementis launched on a Proton rocket from the Russian launch site atBaikonur. Phase II's primary goal is to deliver the United StatesLaboratory Module (Lab) and bring it to full operational status.This takes two flights. Assembly flight 6A delivers the Lab. Utilizationflight 1 (UF-1) delivers the Lab equipment and payloads. A secondgoal is to make the Station fully sustainable, i.e., capable ofbeing maintained inside and out. Flight 7A's delivery of the airlockin March 1999 meets this goal. Phase II includes seven Russianflights, seven United States flights, and one combined flight.

Phase III begins in May 1999, when a Shuttle flight delivers thefirst segment of the Station's backbone, the truss that supportsthe solar and thermal arrays. The goal of this phase is to completeSpace Station assembly. It includes 21 United States Space Shuttleflights, including five research or utilization flights. Russiawill launch Station elements into orbit seven times during PhaseIII on Proton, Zenit, or Soyuz rockets. France will launch theESA's Attached Pressurized Module (APM) aboard an Ariane V inSeptember 2001. The last flight of Phase III occurs in June 2002,when outfitting for the United States Habitation Module is deliveredby Space Shuttle.

In addition to the milestones of Phases II and III, assembly asan engineering process has three distinct principles. The firstis that the order of assembly flights should realize the earliestmaximum Station capability. That is, the Station will functionas a scientific laboratory even while assembly continues. A secondprinciple of Space Station construction is that every step shouldbe planned in advance and tested. Such planning ensures that themany complex steps needed to build the Station will proceed smoothly,even across a five-year period. The third assembly principle isthat no Shuttle should be launched without a full cargo bay, becausethe number of Shuttle launches, the space in the cargo bay, andthe mass a Shuttle can lift at any one time are all limited. Thussome elements will be brought to the Station before they can beused. They will be stowed on-orbit until activation or use.

Phase II begins with the launch of the first Station element inNovember 1997, the Functional Cargo Block (FGB). The pressurizedFGB Module is the foundation for the Space Station, since it providesthrust and attitude control for the first three assembly flights.The United States purchased and owns the FGB, but it is beingbuilt and launched by Russia; thus the first flight is a combinedflight.

One month later, a Space Shuttle Orbiter brings up United Statesnode 1. With its six connecting ports, - one at each end and fouraround the cylindrical body, - the node is a pressurized 14.6-foot-diametermultipurpose connector. It can simultaneously link up to six Stationelements and/or space vehicles if they have enough clearance.Node 1 is launched with two pressurized mating adapters (PMAs)attached, one on each end. These adapters connect the node tothe FGB on the aft (rear) side and to the Orbiter Docking System(ODS) on the forward (front) side. The ODS is an L-shaped pressurizedlink to the airlock. The docking system, node, and pressurizedadapters provide a pressurized passageway from the Space Shuttleto the FGB.

Next, the first pair of United States solar arrays (118 feet longby 38 feet wide) and the complementing heat-expelling thermalradiator (45 feet long by 11 feet wide) will be temporarily installedabove node 1 (figure 2). Before launch, the solar arrays and radiatorare attached to a 39-foot truss segment, Port 6 (P6). These forma permanent unit which, once on orbit, is temporarily stackedonto truss segment Zenith 1 (Z1). Zenith 1 is in turn permanentlyattached to the node (figure 3). From here, the arrays will providepower for early Station activities. Later, in Phase III, the entiresolar array-radiator-P6 unit will be disconnected, rotated 90degrees, and installed out on the port side of the main trussboom. This evolution of the P6 unit from its temporary site toa later permanent one illustrates the first assembly principle- realize maximum capability as soon as possible.

The November 1998 delivery of the United States Laboratory module(Lab) illustrates the second assembly principle - plan ahead forthe intricacies of construction. For this delivery, a Space Shuttlewill dock to a third pressurized mating adapter on the bottomof the node (Earthside). From there, an astronaut will commandthe Shuttle Remote Manipulator System (SRMS)-the Shuttle's robotarm-to reach up from the cargo bay, remove the second mating adapterfrom the Node and temporarily stow it on truss segment Z1. Next,the arm will remove the Lab from the cargo bay and berth it inthe spot the second mating adapter previously occupied. The laststep will be to again grasp the second mating adapter and berthit to the end of the Lab (figures 4A-4C). While all assembly activitieswill be planned and simulated far in advance, such planning isespecially crucial for complex maneuvers as this one.

Plans for the Lab's delivery illustrate the third assembly principle-getStation mass into orbit as soon as possible, even if it cannotbe used right away. Fully outfitted, the 3,812-cubic-foot Labwould mass more than a Space Shuttle can carry. So it is launchedas soon as possible without a full set of equipment racks, eventhough it can't be used immediately. One month later a Space Shuttledelivers the rest of the racks, bringing the Lab to full operationalstatus and fulfilling a prime Phase II goal before the end ofthe Phase. The next Phase II United States flight is a utilizationflight. It will deliver the first set of Lab experiments. Utilizationflights-there are five more in Phase III-are devoted solely toresearch during assembly.

The last Phase II United States flight, in March 1999, deliversthe airlock and high-pressure gas canisters. All internationalastronauts can go for spacewalks or extravehicular activity (EVA)through the United States airlock. With the delivery of the airlock,the Phase II goal of sustainability is met because the crew canconduct external maintenance.

The 3,531-cubic-foot Russian Service Module is launched in April1998, assuming propulsion and attitude control duties from theFGB. The Soyuz Crew Transfer Vehicle (CTV) follows the ServiceModule up one month later. These two flights, the third and fourthSpace Station flights, immediately give the Space Station a permanentthree-person capability. While the Service Module provides theliving space, it is the presence of the Soyuz which enables permanentresidence since it provides transportation to Earth in case ofemergency.

The third Russian flight delivers the Universal Docking Module(UDM). Like the United States nodes, the Docking Module has sixdocking ports. Unlike those on the nodes, the Docking Module'sports are not distributed symmetrically. Instead, it has one porton one cylindrical end-where it will permanently dock to the ServiceModule-and five at the other node end. The Russian Life SupportModule, Research Modules, Docking Compartment, and Crew TransferVehicle (Soyuz) will all dock there. The fourth Russian flightdelivers the Docking Compartment-essentially a second Russiannode. While the Docking Module has multiple docking ports, thecylindrical, 459-cubic-foot Docking Compartment has one at eachend. The Docking Compartment functions as a docking port for Soyuzand Progress vehicles. It can also function as an airlock forRussian EVA when no spacecraft are docked at the free end.

The next three Russian flights deliver additional solar arraypanels for the Service Module and science power platforms (SPP)1 and 2. The power platforms include truss segments, solar arrayjoints, the Russian central heat rejection system, and gyrodynesand integrated thrusters to assist with attitude control for theStation. Eight solar arrays will reside on the power platforms,providing electrical power to the Russian segment.

When the Russian and United States flights of Phase II are complete,the Space Station will have a three-person permanent crew. Withthe Russian Service Module, the equipped and outfitted UnitedStates Lab, the node, Universal Docking Module, Docking Compartment,and airlock, the crew will have more than 16,000 cubic feet ofspace for living and conducting research. The Russian ServiceModule and gyrodynes and United States control moment gyroscopesprovide guidance, navigation, and propulsion. All other essentialsystems-power, cooling, communications, and others-will be inplace as well, and the crew will have transportation home viathe parked Soyuz Crew Transfer Vehicle. The United States Labwill be fully operational and the Station will be fully sustainable,meeting two Phase II goals. However, Space Station assembly isless than half completed when Phase II closes. Phase III, withnearly twice the number of flights as Phase II, will completethe Station.

In May 1999, two months after the airlock is delivered, a SpaceShuttle inaugurates the Station's third phase of assembly by deliveringthe central segment of the Station's truss-S0 (Starboard-zero).This 43.3-foot long by 14.2-foot wide segment is lifted from thecargo bay by the Shuttle's arm and then transferred, or handedover, to the Space Station robot arm. The Station arm berths thetruss to the Module Truss Structure-a support resting on the UnitedStates Lab (figure 5). S0 is launched with a number of Stationelements stowed in its hexagonal aluminum frame. One is the MobileTransporter (MT), the rolling platform the Space Station robotsride on. Another is the Portable Work Platform for EVA astronauts.Others are as-yet-unconnected switches for Station electricaland video systems, Global Positioning System antennas, cords orumbilicals for connecting systems from truss to modules, and manyothers. With S0's installation, the construction of the Station'struss backbone has begun.

Nine more truss segments are delivered on seven flights. Eachcarries additional elements inside the hollow frame. Starboard-1(S1), for example, carries one part of the Crew and EquipmentTranslation Assembly (CETA), which is actually a cart. The crewrides in the cart, which moves them and their equipment down thetruss faster than they could move without it. S1 also includesthree thermal radiators-crucial to keeping the Station from overheating:the starboard half of the Station's thermal control system (TCS),and one S-band communications antenna. S1's installation is plannedto follow three months after S0.

In November 1999, the Port-1 (P1) truss segment is installed onthe port side of S0 (figure 5). It carries the same equipmentas S1, except for a UHF instead of an S-band antenna. In January2000, the port mid-section truss segment is delivered, along witha solar array. This mid-section truss contains the solar alpharotary joint-the mechanical joint that turns solar arrays so theycan track the sun. This truss segment supports a solar array onthe outside of the frame. Inside, it houses the photovoltaic systemwhich converts the sun's light into electricity, the batterieswhich store the electrical energy, and avionics (aerospace electronics).The next port segment arrives in February 2000, followed by thestarboard mid-section truss segment, which mirrors the port mid-section'sfunctions.

Recall that back in Phase II, P6 was temporarily installed onNode 1 with a set of solar arrays to provide power to the earlyStation. After flight 13A, astronauts on board will end this temporarysituation by moving P6 to its permanent position as the outermostport truss section. After 13A, three sets of the Station's mainsolar arrays are deployed and generating power. The penultimatetruss segment arrives in August 2001 and the final segment arrivesin January 2002 along with the final solar array pair.

Though the truss is crucial to supporting the solar and thermalarrays, its real function is to provide sufficient clearance betweenthe arrays and the Station modules. Unlike previous arrays onSkylab and Mir, the Space Station arrays are too large for attachmentto modules. But the 310 total feet of truss have additional uses.The truss serves as a rack for experiments designed for exposureto the space environment. It also supports antennas, cameras,space station robots, electrical, thermal, and other system hardware,and the CETA carts.

Node 2 is brought up in October 1999 on Flight 10A. The same flightdelivers the windowed cupola, which has a specialized workstationfor robotics operations. Logistics equipment will also be deliveredon 10A, along with outfitting for the internal Station volumes.A Centrifuge Accommodation Module, used for materials and lifesciences experiments, will be delivered on 14A. The 3,812-cubic-footUnited States Habitation Module (identical in size to the Labmodule) will be delivered in February 2002 of 16A to serve asliving quarters for Space Station astronauts. Assembly flightsend with the last United States flight, 19A, which carries outfittingand systems equipment for the Habitation Module.

Five utilization flights will take place during Phase III. Likethe first utilization flight in Phase I, they will be dedicatedto scientific research.

Seven Russian Phase III assembly flights occur from May 1999 toMarch 2002. Russia will launch each of these flights from itsBaikonur launch site, using a Proton, Zenit, or Soyuz rocket.

During these flights, Russia will complete its set of eight solararrays. Russia will also add three Research Modules and one LifeSupport Module to its Station segment. Each module encloses about1,700 cubic feet. As the names imply, the Research Modules willbe used for Russian science experiments and research, while theLife Support Module recycles air and water and provides livingquarters and hygiene facilities.

A Docking and Stowage Module will also be added to the Russiansegment. It will provide a docking port for Soyuz and crew transfervehicles. Russian cosmonauts can also store items there for futureuse or for return home.

Progress vehicles will visit the Station, resupplying propellantto the Service Module. While a Progress is docked to the aft portof the Service Module, it will provide all the thrust that movesthe Station, while the Service Module and integrated thrusterstogether provide the smaller thrusts for attitude control.

In February 2000, for the first Japanese flight, a Space Shuttlewill launch the pressurized section of the Japanese ExperimentModule's (JEM) logistics module and some of its outfitting. Onemonth later, another Orbiter delivers the JEM itself and the JEM'srobotic arm (attached outside to the end of the module). The finalJapanese flight occurs in March 2001, when a third Shuttle bringsup the open-to-vacuum section of the logistics module and theexposed facility (EF), a kind of "back porch" that extendsfrom the JEM.

The JEM has an approximate volume of 4,500 cubic feet. Combinedwith the logistics module's pressurized section, Japan's segmentof the Station totals about 6,000 cubic feet. The JEM allows researchersto conduct experiments and perform research in a shirtsleeve environment.The Exposed Facility is an experiment platform in vacuum, exposingexperiments to the space environment. Positioned above the exposedfacility, the JEM robot can manipulate the exposed payloads. Thelogistics module has both pressurized and unpressurized sectionsso samples gathered in both environments can be returned to Earth.

Japan may launch some portion of its segment aboard the H-II launchvehicle, but currently all Japanese flights are planned to flyon United States Space Shuttles.

Installing the European Space Agency's approximately 2,700-cubic-footAttached Pressurized Module (APM), or Columbus Orbital Facility,requires one unusual spaceflight. Columbus will be launched onan Ariane V rocket. Once in orbit, the module will be transferredto the Station by an Automated Transfer Vehicle (ATV)-a smallspace "tugboat." Once the transfer vehicle approachesthe Station, the Space Station arm will grab it and hoist Columbusto its permanent location.

Canada's Station element includes two remotely-controlled robotsand their supporting hardware and software. One robot is the SpaceStation Remote Manipulator System (SSRMS). It installs every largeStation element except those in the Russian segment or installedby the Shuttle arm. Without the large Station arm, it would bevery difficult to assemble the Space Station, since many Stationelements mass tens of thousands of pounds. The Station arm isdesigned to handle those masses-human beings are not.

The second robot is the smaller Special Purpose Dexterous Manipulator(SPDM). It performs maintenance work on the Station and some smallassembly tasks. With SPDM handling these tasks, the crew neednot suit up to go outside. This saves valuable time which canbe used for scientific research.

The Mobile Remote Servicer Base Structure (MRS BS) is a work platformfor the robots. It also serves as a storage area for payloads,a source of electrical power to attached payloads, and a toolbay for the large Station arm.

When Phase III ends in June 2002 after 29 total flights, the Stationwill have 43,000 cubic feet of living and working space. Manyexperiments will also be positioned on the truss. Although assemblywill just have been completed, four years of research will havealready occurred, begun in 1998 with the arrival of the RussianService Module and the United States Lab.

Not all the discoveries to be made aboard the Space Station inscience and engineering can be predicted. However, new discoveriesin materials, pharmaceuticals, life sciences, Earth observations,and large-scale space engineering can be expected on the basisof previous Space Shuttle and Mir flights.

Years of space operations experience have also shown that if thecrew's time is to be maximized for research in science and engineering,they will need assistance from the ground. At the mission controlcenters in Houston and Moscow, command and control personnel willbe ready for the task.