1.0 Executive Summary
1.1Brief overall description of Lander/Probe
In Greek mythology, Europa is a mortal concubine of Zeus. Zeus carried Europa off to a remote island and had several demigod children with her. Our probe takes the name of Zeus in the spirit of life which is embodied in this story. Life forming on a distant deserted island is exactly what may have occurred on Jupiter’s moon Europa. Along the same lines as in mythology, it is hoped that the union of the moon Europa with our Zeus spacecraft will present to the world the fruits of nature in the form of life on another world.
According to the 1997/98 AIAA Undergraduate Team Space Design Competition RFP, “The objective of this project is to produce a complete system design for a spacecraft that can land on the surface of Jupiter’s moon Europa and examine the ice and water of which it is made.” To accomplish this objective, the lander/orbiter and the ice/water probe need to possess certain instruments and experiments. The spacecraft needs to re-map certain areas of Europa that have been selected from data from the previous Europa Orbiter mission. This mapping will be done from a 20km orbit and will include photography and surface topography. Once the spacecraft lands, surface tests must be conducted to determine the physical and chemical properties of Europa. Photography and seismic testing are also essential portions of the lander/orbiter’s mission. Once the spacecraft lands, the ice/water probe can be released. This probe has the duty of providing chemical analysis, taking photographs, and determining the pressure and temperature distribution beneath the surface of Europa, as well as conducting life detection experiments.
The Zeus spacecraft is furnished with the RIEGL Laser Altimeter LD90-31K to aid in mapping the surface of Europa. The lander is equipped with 1024x1024x24-bit CCD cameras, which are also used for surface mapping from orbit, as well as providing photographs of the surface once the spacecraft has landed. The lander carries a robotic arm, which is modeled after the Mars Surveyor 2001 lander. An optical spectrometer is attached to this arm to perform chemical analysis of the Europa’s surface. To observe the ice shifting and “Moonquakes,” a JPL Microseismometer is imbedded in one of the feet of the landing gear to perform these measurements. Before any of the scientific operations can occur, the spacecraft must be able to maneuver to an orbit around Europa. Zeus’s three main engines provide a maximum thrust of 488 N and use monomethelhydrazine and nitrogen tetraoxide propellants. The ten monopropellant hydrazine thrusters on this spacecraft provide a maximum thrust of 6.15 N each. The power is generated by a Radioactive Power Source (RPS), which converts thermal energy to electrical power. The communications system consists of a parabolic high gain antenna constructed out of an aluminum honeycomb. A low gain antenna is used for low data rate communications and in case of high gain antenna malfunction. The spacecraft guidance and navigation is performed by autonomous star trackers, Fine Sun Sensors from the Swedish Space Corporation, an Analog Devices/ ADXL05 accelerometer, and a Litton LN-200 Fiber Optic Inertial Measurement Unit. To organize all of these instruments, a redundant computer is used to conduct the operations of the spacecraft.
The ice/water probe, like the lander/orbiter, is also required to perform scientific operations. The chemical analysis is executed by capillary electrophoresis system. This system separates molecules based on their movement through a fluid under the influence of an applied electric field (Weinberger, 1993). A set of cameras is used to take pictures near the ice/water boundary. The cameras are equipped with halogen lamps to provide the light within the underdwellings of Europa. These cameras view the inside of the moon through ports provided for the transducers used to measure the pressure and temperature distribution of Europa. The ice/water probe contains instruments vital to the success of this portion of the mission. Like the lander/orbiter, the ice/water probe houses an RTG for power. In addition to the RTG, the probe contains radioactive heating units (RHU) which aid in melting the ice for its journey to the ice/water boundary. Once the probe has melted its way down to about 30 m from the ice/water boundary, the Probe Arresting Sub-System (PASS) is activated. This system consists of titanium blades that, when released, hold the upper portion of the probe in the ice while the lower portion continues to fall into the hopefully present water.
The purpose for the PASS is to ensure contact with the ice for the communications between the ice/water probe and the lander. The communications system between the probe and the lander uses sonar through the ice. This system consists of hydrophones and an acoustic modem. Sound, instead of an electrical impulse, is sent through the ice. This means that a cable between the probe and the lander is not necessary. The sonar system can also be used to determine the depth of the ice and the distance from the probe to the lander.
1.2 Mission Events Sequence
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1.3 Mission Time-Line
The following is a time-line explaining the order and date of key aspects of the mission.
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2.0 Introduction
The exploration of the world and universe has always been a preoccupation for humans. Since the dawn of our species, as we traveled across continents to settle into different territories, to the modern space age, we have spent tremendous resources on expanding our horizons. In the last few years of the 20th century, we are on the cusp of discovering life outside of our planet. New telescopes are able to see planets forming around distant stars, and planets and moons in our own solar system show promise of having what it takes to form life as we know it—liquid water. With the discovery of oceans covered by a thick ice crust on Europa, the possibility of finding life is more imminent than at any other time.
As part of this study of our solar system, and our quest for life outside of our planet, NASA has proposed a possible mission to Europa. This mission is going to do a detailed analysis of the surface and subsurface of Europa. No probe prior to this one will have probed these features in as much detail. This will be the first time a probe will be landed on a Jovian moon. The choice of Europa is based on the interest surrounding the liquid water, which is theorized to be present.
As a moon of Jupiter, Europa is brought under the gravitational forces of the second largest body in the solar system. Since Jupiter comprises as much mass as all other bodies in orbit around the sun, any object which comes in close proximity to this planet experiences huge tidal forces. These huge tidal forces continually oscillate through Europa’s orbit, due to the oblateness of Jupiter. Further tidal forces are encountered as nearby moons are passed. This motion continually generates heat in the center of Europa. It is theorized that this heating is what causes the liquid ocean under the crust to exist.
The scientific community has only recently accepted the concept of a floating ice shell. The first real evidence of this situation has come from high resolution, <1km, imaging from the Galileo probe. The fracture level and local plate shifting shown in these images quickly lead scientists to conclude that the ice was indeed floating. The nearly complete absence of craters also points to floating ice, since large objects drawn into the Jovian gravity will bombard any object orbiting Jupiter.
The surface of Europa is a thick and relatively smooth coating of ice. There are no mountains or canyons. There is practically no appreciable atmosphere either, since Europa doesn’t generate enough gravity to hold one. Current gravity analysis and plate motions lead scientists to conclude that Europa has a 1-10km ice crust followed by approximately 100km of ocean. The brine content of the ice and the water is not known at this time. At the core of the moon is a rocky center—probably iron.
To determine the accuracy of these predictions our probe will employ several experiments meant to study these features. From orbit a detailed photographic and topographical study will be carried out over a few pre-selected regions. This will augment a previous study done by an orbiting NASA probe before our arrival. Besides using this information for scientific purposes, NASA will use this data for final selection of the landing site. Upon making a soft landing, a probe designed to penetrate through the ice will be released. This probe will melt through the ice and study the subterranean oceans. Pictures will also be taken of this undersea environment and transmitted to earth.
Chemical analysis will also be carried out to better determine the composition of Europa. The lander will perform one set of chemical analysis, and the probe will perform another. The lander experiment is modeled after the spectrometry experiment, which is planned for the Mars Lander 2001. Along with studying the elemental composition, the probe will also conduct experiments to determine if life has ever existed on Europa.
With the current array of science payloads, the Zeus probe will be able to provide new insights into Europa, and by extension our own planet.
2.1 General Performance Parameters of the Configuration
Table 2.1: Weights and power requirements of major components
UNIT
/WEIGHT (kg)
/POWER (Watts)
PROBE
Power Generation / 20.72 / 44-62.5 (generated)Capillary Electrophoresis / 5.5 / 10.75
Probe Shell / 14
PASS / 8.571 / 20
Photographic Equipment / 0.6 / 0.6 to 10.1
Acoustic Modem / 1.1 / 1.66
Computer / 1 / 5
Pressure Transducers / 0.1 / 0.1
Thermocouple amp
/ 0.25 / 0.1PROBE TOTAL
/ 51.84LANDER
Laser Altimeter
/ 1.5 / 1Arm/Spectrometer
/ 2.5 / 0.25Fixed Camera(3)
/ 0.25 / 1.5Zoom Cameras(4)
/ 2 / 2Seismometer
/ 0.1 / 0.1Main Propellant Tanks
/ 48 / -Main Engine Propellant
/ 956 / -RCS Tank
/ 3.63 / -RCS Propellant
/ 30.25 / -RCS Thrusters
/ 3.6 / -Helium Tank
/ 31.3 / -Helium
/ 5.89 / -Engines
/ 13.5 / 5Valves
/ 5 / 2Radioisotope Power Source
/ 7.96 / Generates 163WCommunications System
/ 35 / 53(High Gain), 20 (Low Gain)Star Tracker (2)
/ 2.8 / 7Sun Sensors(4)
/ 1 / 1.4Accelerometer
/ 5x10-3 / 0.35IMU
/ 0.7 / 10Computers
/ 2 / 20Radiator Fins
/ 7.172 / -Heat Pipes
/ 1.42 / -Jacket
/ 3.34 / -Acoustic Modem
/ 1.1 / -Radiation Shielding
/ 10 / -LANDER TOTAL
/ 1210.6DRY MASS / 306.5
TOTAL(inc. propellant) / 1262.5
3.0 Design Evolution
The design of the spacecraft has gone through several stages over the course of the project. The arrangement of all of the components and the components themselves were selected to minimize size and complexity. To keep launch costs to a minimum, we decided to limit ourselves to the payload capability of the Atlas IIA launcher. As the design progressed, the launch vehicle became the Delta III, which has even less stringent mass and size constraints. Several major iterations were done to perfect the design, and each is presented below.
From the beginning the structure has been a truss made of tubular graphite epoxy members connected by titanium joints. Similarly all designs have had a set of three engines located centrally. The ice water probe has also remained at the center of the configuration to make it easier to mount and to protect from radiation. The landing pad design has also been held constant. To increase traction on the ice surface, a series of small spikes line the landing pads. This will reduce any skidding which may occur if the spacecraft lands on a slight incline.
Figure 3.1: Initial Design Concept
Figure 3.1 shows the initial design of the spacecraft. Obscured from view is the probe. This crude drawing also lacks the instrumentation section, which would have been mounted on a palette directly on top of the tanks. At this point two primary factors drove us towards this design. First of all, we assumed that we wanted a full 360 degrees of motion for the main antenna. This explains why it is so high above the main lander body. At this point the antenna was also 3m in diameter to accommodate a data throughput rate which proved too extravagant. The tanks are mounted directly to the structure at two hard points on each tank. Each leg of the landing gear is a single member which connects into the center of the main structure.
There were several problems with this design. First of all, the landing gear base was too small. The structure lacked inherent stability. The tank mounting was also a problem since local stresses at the hard points would have been too large at launch. The antenna in such a position added too much mass to the structure and further contributed to stability problems. After some analysis it became apparent that it would be possible to land with an orientation that would allow for effective communication with only 180 degrees of rotation. The mounting of the instrumentation on a palette above the tanks proved unwise also. First of all, the instrumentation was not large enough to need their own separate section of the spacecraft; it is adequate for them to be stored throughout the spacecraft. Second of all, in this configuration very heavy radiation shielding would be necessary to protect the instrumentation. At this stage we had a radio echo sounder (RES) for ice thickness measuring and a far range spectrometer. Both of these were abandoned when NASA released news that a separate mission would carry out these tasks. At this stage there was no robotic arm or seismometer. The sonar transducer was also mounted above the ice/water probe.
Figure 3.2: Second Iteration of Design Evolution
After more careful analysis a second major design was arrived at. This design incorporated the lessons learned from the first iteration, and some basic design changes that occurred between the two iterations. The second iteration can be seen in Figure 3.2. By the time the second iteration was started the RES system and the far range spectrometer were removed from the design. The sonar transducer was mounted in one of the landing pads. A seismometer was also added to the design, on one of the landing pads. Both of these instruments need a firm contact with the surface. This is provided by the lander’s weight.
The tank arrangement was altered slightly, and the formerly symmetric shape was stretched to accompany the side-mounted antenna. The antenna, which is now only two meters in diameter, was moved from its top position to the side to minimize the height of the probe and to reduce the amount of additional structural support needed for the dish. The computer and electronics are mounted internally, surrounded by tanks. This provides a shield from the radiation and reduces the amount of radiation shielding that is necessary. The tanks are now mounted on a set of equatorial support straps. These straps are attached to the structure via hard points. The landing gear has been spread out to extend further away from the center of gravity.
While this design is more stable, there were still some problems that had to be overcome. First of all, the landing gear legs were only supported in one plane. This caused their thickness to be too large. Extra bracing was decided to be the solution for this problem. The tanks still had mounting problems. While the mounting stresses were distributed throughout the tank, the hard points were on the same side of the tank. This means that excessive moments would be generated under accelerations. To reduce these moments, the hard points were moved to be on opposite sides of the tank. This design also did not explicitly place the RCS fuel tanks or the helium storage tank. At this stage the thermal analysis was not completed, so thermal control systems were not mounted either.
All the design lessons from the second iteration were applied to the final iteration, see Figures 3.3 through 3.6. Furthermore any equipment which was not included explicitly in the second design, such as thermal management equipment or storage tanks, are now present. All equipment on the lander can be seen in these two views. This system has an initial total weight of 1262.5 kg, including propellant. The length from dish reflector to the far tank is 4.1m and the height of the system is 2.2m. These parameters are well within proposed Delta III launch limits.
Figure 3.3: Isometric view of final spacecraft configuration