The primary objective of Project Asterius is to characterize the surface of Europa. The secondary objectives of Project Asterius is to look for signs of life, to ascertain Europa's usefulness for future exploration, and to further our understanding of not only Europa, but of the Earth, the Solar System, and the whole Universe.
Asterius is physically divided into three distinct modules: the Orbital Operations Module, the Surface Operations Module, and the Water/Ice Scientific Probe. The Orbital Operations Module (OOM) is contains everything that will not be needed on Europa's surface, mostly the braking rockets used for Jupiter and Europa insertion. It will be jettisoned in Europan orbit before Asterius lands. The Surface Operations Module (SOM) is the part of Asterius that lands on the surface; most of the spacecraft components reside here. The Water/Ice Scientific Probe (WISP) travels to the surface with the SOM. It is deployed, and melts its way though Europa's crust, carrying several scientific instruments. The WISP communicates with the SOM acoustically: it carries a large sonic transducer to transmit data.
Asterius will begin its journey to Europa on March 23, 2004. It will ride a Titan IV/Centaur booster into geosynchronous transfer orbit, from which the Centaur upper stage will propel it on a trajectory to the Planet Mars. After a gravity-assist flyby of the Red Planet, Asterius will head for Jupiter, finally arriving on February 23, 2009. After insertion into the Jovian system, Asterius will spend about a year performing flybys of the Galilean moons to reduce its apojove. Finally, it will insert itself into orbit around Europa.
Asterius will spend several months in Europan orbit, scanning the surface with radar to determine the thinnest spot in the ice. Because it uses its high-gain antenna for radar, it will spend intervals with its antenna pointed toward Europa, scanning the surface, and other intervals with its antenna pointed toward Earth, transmitting data. Once the scientists on Earth decide on a landing site, Asterius will jettison its OOM. It will use its landing engines to transfer directly to the surface, using only two engine burns: one to initiate the transfer, and one to slow down to zero velocity once it reaches the surface.
Once on the surface, Asterius will begin its scientific mission in earnest. It will take light, infrared, and ultraviolet photographs using its solid state imager ``EuropaCam.'' It will take acoustic measurements by emitting pulses of sound and measuring the echo, and it will monitor the seismic activity. These give clues about the structure of the crust. It will test the composition of the atmosphere with its metastable ionization detector. The dust detector and energetic particle detector measure the flux of their respective subjects reaching Europa, as dust and high-energy particles seem to affect Europa's surface in important ways. Finally, local temperature and pressure measurement are taken at every moment.
Also, the lander will release the WISP into the ice to take subsurface measurements. The WISP will attempt to reach the conjectured ocean beneath the surface. As it descends, the WISP will take black and white photographs of the surrounding ice with its microscopic camera ``MicroWISPCam.'' It will take seismic measurements (to be compared with the seismic measurements taken from the lander). It will use electrodes to measure the resistance of the ice. Finally, it will use an Alpha-Xray-Proton spectrometer, similar to that used by the Mars Pathfinder, to measure the chemical composition of the ice. The WISP carries inflatable balloons to keep it afloat once it reaches liquid water.
The structures subsystem of Asterius is the backbone and foundation of the craft. The structures subsystem was designed for optimal strength, yet governed by the constraints of mass conservation and cost during the design effort. The result of this formula is a craft more than capable of delivering the best performance for the mission to Europa. The use of advanced materials like composites and ceramics make the craft state of the art, but doesn't sacrifice basic requirements and lessons learned from former deep space missions. The Asterius craft is essentially made up of four modules: the OOM, the SOM, the WISP, and finally the the launch vehicle adapter. These four modules are in turn composed of six major structures: the SOM skeletal structure, the landing gear, the lander adapter, the propulsion support assembly, the engine mounting unit, and finally the launch vehicle adapter structure itself. In addition to the major structures, many minor structures are then added on to these to complete Asterius' structural subsystem.
With the current design, Asterius not only promises a successful mission to Jupiter's frozen moon, but its unique look and innovative structures of design is hoped to set precedent for the future.
For attitude sensing, Asterius contains a wide field-of-view star tracker, the Ball Aerospace CT-631. This wide field-of-view tracker requires a smaller star catalog, consumes less power, weighs less, and provides better error handling than the narrow field-of-view sensor, but it is somewhat less accurate. A digital sun sensor provides additional attitude sensing capability. There is a second sun sensor for redundancy. Finally, three solid-state rate gyros provide higher-precision attitude sensing. Any two are sufficient to sense attitude correctly, the third rate gyro is for redundancy.
Attitude control is maintained with the reaction wheel assembly. There are four reaction wheels, only three of which are necessary to orient the spacecraft. 16 Olin MR-111 thrusters are used for momentum dumping during attitude maneuvers. The fourth is for redundancy. The crude baseline estimate for the required moments is 2.616 N-m and 0.046 N-m about the longitudinal and axial axes, respectively.
Asterius' propulsion subsystem enables the craft to maneuver, brake, land on the moon's surface, and maintain proper attitude during the descent. Asterius makes use of two Kaiser-Marquardt R-4D engines as the craft's main deep space engines. The attitude control propulsion system implements 16 Olin MR-111 thrusters which have thrusters aligned along the positive and negative z-axis and thrusters along the positive and negative x- and y-axes. The landing phase of the SOM is handled by four powerful Kaiser-Marquardt REA 20-4 engines which produce enough thrust to gently set the lander on the surface of Europa without incident and with accuracy.
Two antennas provide communication with Earth. The 2.2-meter high-gain antenna operates at 32 GHz, and has a data rate of 50,000 bps when communicating with the DSN's 70 m antennas. It's data rate is 10,000 bps when communicating with the DSN's 34 m antennas. The 0.2-meter low-gain antenna, which is located inside the high-gain antenna, operates at a frequency of 2.2 GHz, and has a data rate of 10 bps with the 70 m DSN antennas.
The low-gain antenna is used mostly during transit to Jupiter, when the use of the high-gain antenna is impractical due to thermal and pointing considerations. When Asterius reaches Jupiter, it will use the high-gain antenna. On Europa's surface, the high-gain antenna is designed to gimbal so that it points almost horizontal. This allows it to track the Earth for almost its entire distance across the Europan sky.
Communication between the WISP and the SOM is acoustic. It is mostly one-way, with the WISP operating autonomously, and continually transmitting data to the SOM. The WISP contains a powerful sonic transducer that emits on the order of 100 W of sound power. When the wisp is 10 km deep, the sound intensity at the SOM is about 40 dB, still a detectable signal.
Asterius uses a bus architecture for internal communication between its components. The various nodes that the bus connects are the on-board computer, the two solid-state recorders, the instrument interfaces, the housekeeping devices, and the timer. The on-board computer is capable of executing 24 million instructions per second. Also, the computer requires 24 megabits of RAM. The solid-state recorders are one gigabit each. The solid-state recorders were chosen over magnetic tape because of their superior qualities; we felt it outweighed the degradation due to the high radiation environment of Jupiter. The interfaces connect other components to the bus. Most instruments use a serial RS-232; the high-speed instruments use faster interfaces. Housekeeping devices (thermostats, fuel gages, strain gages, etc.) send information on the spacecraft's condition to the on-board computer. Finally, the timer keeps accurate real time. It has a watchdog circuit built in, capable of resetting the computer in an emergency.
Asterius uses a Dynamic Isotope Power Source (DIPS) as its main power source. The DIPS is separated from the SOM on a boom to reduce thermal flux into the spacecraft. The DIPS generates 600 W, which is enough to power Asterius at its peak power usage, with a margin. The DIPS was chosen for its high efficiency, and the subsequent reduction of necessary plutonium.
The radioisotope heat source used in the WISP is also the power source. It is an RTG, capable of delivering 140 W of electric power for the WISP. (It produces 2000 W of heat.) Most of the 140 W necessary is for the acoustic transducer. While the WISP is still attached to the SOM, the RTG's 140 W are at the disposal of the whole spacecraft. In times of peak power usage, the RTG can provide extra power if needed.
The RTG in the WISP was sized from the heat considerations. It was determined that about 1500 W of heat energy was needed to melt through 10 km of ice in one year. With a generous margin of 500 W (due to the conjectuaralness of the model used), the 2000 W heat source was chosen.
Asterius active maintains thermal balance throughout the mission in
three ways: using a radiator covered with louvers to selectively
radiate excess heat, selectively absorbing or rejecting waste heat
from the DIPS, and orienting the spacecraft to minimize or maximize
sunlit area. Active thermal control is necessary in this mission
because of the difference between the thermal environments near Earth
and near Jupiter. The first-order estimate of radiator size for
Asterius is 2.5 m
,
and between 5 and 10 percent of the DIPS heat
can be absorbed. During transit, when Asterius is close to Earth,
Asterius will be oriented with its antenna pointed at the sun,
minimizing the sunlit area. Near Jupiter, this requirement is
relaxed.
Besides active control, Asterius has some passive temperature control. Both the emissivity and absorptivity of the spacecraft's skin is very low (it has a shiny, polished aluminum coating). Also, the spacecraft is well insulated to keep its interior safe from the extreme temperatures of the skin.
Using cost estimating relationships based on mass, the first-order estimate of Asterius' cost is $550 million. The cost estimating relationships used were power regression curves, based on previous data. (The cost of the plutonium was estimated separately, however.)