An artist's concept of Genesis spacecraft deployed in space. Photo: NASA/JPL

When Genesis' solar arrays are extended in space, the spacecraft resembles an unbuckled wristwatch. The watch's face is the science deck, and the figurative straps are the opened solar panels. The framework of the spacecraft is composed mostly of aluminum, composite materials and some titanium. The use of composites and titanium, lighter and more expensive materials, is an efficient way of conserving mass while retaining strength. Genesis' structure is similar to that used in the construction of high-performance and fighter aircraft.

The Genesis spacecraft incorporates innovative, state-of-the-art technologies pioneered by other recent missions, and uses off-the-shelf spacecraft components, designs and, in some cases, spare parts and instrumentation left over from previous missions.

There are five major science elements in the Genesis payload: the science canister, a stack of collector arrays, the ion concentrator, the electron monitor and the ion monitor. The science canister includes a bulk collector array mounted on its lid, and also houses the collector array stack and the ion concentrator.

Sample Return Capsule
The sample return capsule is the shape of two blunt-nosed cones attached at their bases, and has a diameter of 162 centimeters (64 inches). It has five major components: a heat shield, backshell, sample return or science canister, parachute system and avionics. The total mass of the capsule, including the parachute system, is 210 kilograms (460 pounds).

A hinged clamshell mechanism opens and closes the capsule. The science canister -- housing the solar wind collector arrays and ion concentrator -- fits inside, with a central rotating shaft to extend the collector arrays into the solar wind. The capsule is encased in carbon-carbon heat shielding and a silicone-based ablative material called SLA-561 to protect the samples stowed in its interior from the heat of reentry. A parachute deployed by a mortar unit is carried inside the capsule and will be used to slow its descent.

The heat shield is made of a graphite-epoxy composite covered with a thermal protection system. The thermal protection system is made of a carbon-impregnated material manufactured by Lockheed Martin Astronautics, Denver, Colo., called carbon-carbon. The capsule heat shield will remain attached to the capsule throughout descent and serve as a protective cover for the sample canister at touchdown. The aeroshell is designed to dissipate into the atmosphere more than 99 percent of the initial kinetic energy of the sample return capsule.

The backshell structure is also made of a graphite-epoxy composite covered with a thermal protection system: a silicone-based material called SLA-561V that was developed by Lockheed Martin for use on the Viking missions to Mars and that is currently used on the Space Shuttle external tank. The backshell provides the attachment points for the parachute system, and protects the capsule from the effects of recirculation flow of heat around the capsule.

Illustration of Genesis with description of its components. Photo: NASA/JPL

The science canister is an aluminum enclosure containing the specialized and bulk collector arrays and the ion concentrator. On the inside of the lid of the science canister is a bulk solar wind collector array. The specialized collector arrays are rotated out from inside the science canister. Underneath the stowed collector arrays, the ion concentrator forms the bottom of the science canister. The canister is inside the sample return capsule, which is mounted between the backshell and heat shield on a set of support struts.

The parachute system consists of a mortar-deployed 1.6-meter (5.25-foot) drogue chute to provide stability at supersonic speeds, and a main chute 10 by 4 meters (about 33 by 13 feet) that is released at an altitude of about 6 kilometers (approximately 20,000 feet). The system incorporates the two parachutes into a single parachute canister.

Inside the parachute canister, a gas cartridge will pressurize a mortar tube and expel the drogue chute. The drogue chute will be deployed at an altitude of approximately 30 kilometers (about 20 miles) above mean sea level to provide stability to the capsule until the main chute is released. A gravity-switch sensor and timer will initiate release of the drogue chute. Based on information from timer and backup pressure transducers, a small pyrotechnic device will cut the drogue chute from the capsule at about 20 kilometers altitude (12 miles). As the drogue chute moves away, it will extract the main chute from the parachute canister. At the time of capture, the capsule will be traveling at about 5 meters per second (roughly 10 miles per hour).

Command and Data Handling
All of the spacecraft's computing functions are performed by the command and data handling subsystem. The heart of this subsystem is a RAD6000 computer, a radiation-hardened version of the PowerPC chip used in many models of Macintosh computers.

With 128 megabytes of random access memory and three megabytes of non-volatile memory, which allows the system to maintain data even without power, the subsystem runs Genesis' flight software and controls the spacecraft through interface electronics. Interface electronics make use of computer cards to communicate with external peripherals. These cards slip into slots in the computer's main board, giving the system specific functions it would not have otherwise. There are two identical strings of these computer and interface electronics, so that if one fails the spacecraft can switch to the other.

Communication with Genesis' sensors that measure the spacecraft's orientation in space, or "attitude," and its science instruments is done via another interface card. A master input/output card collects signals from around the spacecraft and also sends commands to the electrical power subsystem. The interface to Genesis' telecommunications subsystems is done through another card called the uplink/downlink card.

There are two other boards in the command and data handling subsystem, both internally redundant. The module interface card controls when the spacecraft switches to backup hardware and provides the spacecraft time. A converter card takes power from the electrical power subsystem and converts it into the proper voltages for the rest of the command and data handling subsystem components.

The command and data handling subsystem weighs 11.9 kilograms (26.2 pounds).

Telecommunications
Genesis' telecommunications subsystem is composed of both a radio system operating in the S-band microwave frequency range and, in the return capsule, a system that operates in the UHF range. The S-band system provides communication capability throughout all phases of the mission. It is used for communications between Earth and the spacecraft. The UHF system is used during the recovery of the capsule. It broadcasts to the ground the capsule's location during the later stages of entry based on information from a Global Positioning System receiver onboard the return capsule. The capsule also has a locator beacon.

The spacecraft's radio system communicates with Earth primarily through a medium-gain antenna. This antenna is spiral-shaped, about 10 centimeters (4 inches) in diameter, about 12 centimeters (4.87 inches) tall and weighs 105 grams (about 4 ounces).

The spacecraft also uses four low-gain antennas, located on the solar arrays. These are patch antennas, which sit on a coaster-sized square (10 by 10 by 1 centimeters (4 by 4 by 0.4 inches)). These have a much wider field of view.

The low-gain antennas will be used to make initial contact with the spacecraft after it leaves the Delta rocket's third stage, and afterwards only near Earth during the return or for emergencies. The medium-gain antenna will be used for most of the spacecraft's communication with Earth.

The telecommunication subsystem weighs 10.1 kilograms (22.3 pounds).

Electrical Power
All of the spacecraft's power is generated, stored and distributed by the electrical power subsystem. The system obtains its power from an array of standard silicon solar cells arranged on two panels on either side of the equipment deck. The two solar panel wings are fixed in place after being deployed. They hold grids of silicon cells which generate a total of 254 watts at Earth's distance from the Sun. A power distribution and drive unit contains switches that send power to various loads around the spacecraft. Power is also stored in a nickel-hydrogen battery that can deliver 360 watt-hours of electrical energy.

The electrical system also contains a pyro initiator unit which fires small explosive devices that configure the spacecraft following launch, performing such tasks as unlatching Genesis' solar arrays when they are deployed and opening covers on the electron and ion monitors. The pyrotechnic system also releases the sample return capsule.

The electrical power subsystem weighs 36.5 kilograms (80.5 pounds).

Guidance, Navigation and Control
Genesis maintains its orientation in space, or "attitude," by continuously spinning in space. The spacecraft's spin helps maintain stable pointing at the Sun. The attitude control system will keep Genesis spinning at a rate of 1.6 rpm during solar wind collection. During maneuvers, the spin rate will be increased to enhance stability. The axis of spin will point 4.5 degrees ahead of the Sun, so that collector arrays will face into the oncoming solar wind.

Genesis determines its orientation at any given time using a star tracker in combination with Sun sensors. The star tracker can track stars of third magnitude or brighter; Genesis then processes star tracker data in its main onboard computer to recognize any star patterns as they pass through the tracker's field of view. The spacecraft uses both the directions of the Sun and of stars as measured by the Sun sensors and star tracker, respectively, to determine its orientation in space. As long as the spacecraft is spinning below about 2 rpm, it can use stars and thus determine its orientation more accurately. During maneuvers when the spacecraft is spinning faster than 2 rpm, the spacecraft will use its Sun sensors to determine a sufficiently accurate orientation. There are two star trackers on board to back each other up, and the Sun sensors also back each other up.

The guidance, navigation and control subsystem weighs 10.0 kilograms (22.0 pounds).

Propulsion
The propulsion subsystem has two sets of thrusters. The larger are used to make major trajectory correction maneuvers, and the smaller to continually maintain the spacecraft's desired orientation and orbit.

Firing the thrusters changes the spacecraft's orientation. Two clusters of four small hydrazine thrusters each are mounted to the aft side (away from the Sun) of the spacecraft's deck, providing 0.88 newtons (0.2 pounds) of thrust each for small maneuvers to keep the spacecraft in its desired orientation and orbit, and to increase or reduce the spacecraft's spin rate. Four more thrusters are also mounted on the spacecraft, each providing 22.2 newtons (5 pounds of thrust) for major trajectory correction maneuvers. These thrusters are only used when the sample return capsule's lid is closed, so that the exhaust does not contaminate the solar samples.

In addition to miscellaneous tubing, pyro valves and filters, the propulsion subsystem also includes two 55-centimeter-diameter (22-inch) propellant tanks, each containing hydrazine, pressurized with gaseous helium.

The propulsion subsystem weighs 36.6 kilograms (80.7 pounds).

Structures
The structure of the spacecraft is composed of an equipment deck that supports engineering components and the science instruments. The medium-gain antenna is on the underside, and the low-gain antennas are mounted on the solar wings. Except for what is inside the sample return capsule, all the equipment is mounted directly onto the equipment deck.

The structures subsystem weighs 98.6 kilograms (217.4 pounds).

Thermal Control
The thermal control subsystem is responsible for maintaining the temperatures of each component on the spacecraft within their allowable limits. It does this using a combination of active and passive control elements. The active components are the heaters.

The passive components are black and white thermal paint as well as multilayer insulation blankets, some with an outer layer of carbon-impregnated black kapton, and some with an outer layer of indium-tin-oxide-coated kapton that has a gold color due to an aluminum backing that reflects light through the transparent yellow kapton.

The thermal control subsystem weighs 15.9 kilograms (35.1 pounds).

Mechanisms
The solar arrays must be stowed during launch and then released. During deployment, force from springs push the wings to rotate outward on hinges until two latches per wing engage and lock them in place.

The sample return capsule has three two-legged struts that hold it in place. The sample return capsule is mounted on its struts with its nose atop six spring-loaded cans. Following release of the struts, a ring between these cans and the nose gently shoves the capsule off its platform.

The sample return capsule's lid opens and closes on a main hinge, which is tethered to the deck. In order to keep the hinge from damaging the sample return capsule as it plunges through Earth's atmosphere, the hinge is retracted away from the capsule before reentry. Elbow joints at the top of the hinge have separation bolts and cable cutters that separate and retract the hinge assembly. The hinge carries with it the severed cables that allowed communication between the capsule and the rest of the spacecraft.

The ion and electron monitors each have a door mechanism that exposes their sensors by using pyrotechnics to expand small metallic balloons to open the doors.

Four mechanical latch/hook assemblies work to grab the lid of the sample return capsule and hold it in place throughout launch and reentry. The science canister mechanisms are: the lock ring device, canister lid mechanism and collector array deployment mechanism.

All of the mechanisms combined weigh 17.0 kilograms (37.5 pounds).

Flight Software
Genesis receives its commands and sequences from Earth and translates them into spacecraft actions. The flight software is capable of running multiple concurrent sequences, as well as executing immediate commands as they are received.

The software used during the data collection will determine solar wind conditions based on data from the ion and electron monitors. It will then command collection arrays to an appropriate configuration and adjust the ion concentrator's voltage.

The flight software is also responsible for a number of autonomous functions, such as attitude control and fault protection, which involve frequent internal checks to determine whether a problem has occurred. If the software senses a problem, it will automatically perform a number of preset actions to resolve the problem or put the spacecraft in a safe mode until the ground can respond.

Redundancy
Most systems on the spacecraft are fully redundant. This means that, in the event of a device failure, there is a backup system or function to compensate.

A software fault protection system is used to protect the spacecraft from reasonable, credible faults but also has resiliency built into it so that many faults not anticipated can be accommodated without placing the spacecraft in a safe state.