A Proposed Discovery Mission
Photo: NASA/JPL [Larger 99K GIF]
Photo: NASA/JPL [Larger 99K GIF] | NOTE: The following information was provided to the Astrobiology Web by the Prinicpal Investigator on this proposed mission with their permission to distribute it freely as we deemed appropriate. This is not an official NASA or JPL website and, as such, the information presented here is in no way endorsed or validated by NASA or JPL. Neither the Astrobiology Web or Reston Communications has any contractual or bidding relationship with NASA, JPL, or any of the proposing parties. We just think these are exciting ideas and wanted to share what we have learned with our readers. |
D. SCIENCE
D.2. Baseline Mission:
The baseline mission that achieves the science objectives discussed above is as follows. The spacecraft, essentially a modified version of Stardust, is launched in Nov 2000 on a Delta 7925 launcher. After a two year Earth gravity assist followed by a flight as far as 6.4 AU from the sun, the Ice Clipper reaches Europa in 2008. The favorable alignment of velocity vectors allows the Ice Clipper to fly by Europa at a relative velocity of 10 km/s, 50 km above the surface.
Before the Ice Clipper moves over Europa it has released, using a gentle spring system, a small inert mass onto a collision course with the satellite. The spacecraft does a small course correction to avoid the same fate and instead of hitting Europa it flys through the ejecta plume created by the impact mass. During the flyby, the camera on board the Ice Clipper photographs the surface of Europa and the ejecta plume. Because of Europa's low escape velocity, the spacecraft's relative velocity is the most significant factor as the spacecraft moves through the plume collecting particles and gas. The images from the camera will be used to determine the location of the impact site on Europa as well as to characterize the plume dynamics. This will include: some images over time of the ejecta plume and the particle size of the ejecta. This information, in turn, will be used to constrain models of the surface properties. Thus, the impact and the resulting plume, is not only a convenient sampling device it is also an investigation into the structural properties of the ice surface. The concept of impact sampling of airless bodies was considered for the Comet Rondezvous Asteroid Flyby mission in the early '80s and was suggested as a way of sampling Mercury's polar icecaps by J.V. Post in 1993.
The low velocity of the impactor on Europa ensures that the material in the plume is primarily unmelted particles. These particles are sampled by the JEPA (Jupiter Europa Particle Analyser) time-of-flight mass spectrometer. The particles disintegrate as they impact on a silver target plate at the entrance of the instrument and the ions are measured by time-of-flight spectroscopy. This instrument is essentially a duplicate of the CIDA (Comet and Interstellar Dust Analyzer) instrument that is being flown on Stardust. The main objective of the instrument is to detect the presence of organics, volatile inorganics, and water-soluble salts present in the water ice. As discussed above these may be direct indicators of ocean water reaching to the surface.
A primary objective of the Europa Ice Clipper will be to return a sample of the refractory material in the surface of Europa to the Earth for detailed analysis. As the spacecraft moves through the ejecta plume the Aerogel Collectors (AC) --- similar in design to those used on Stardust --- will be exposed and collect particles. It is not expected that water and other volatiles will be retained in these collectors. However, the silicate materials and refractory organics will be preserved for transport back to Earth.
A second collector is also deployed during flyby, the Active Volatiles Collector (AVC). This collector is composed of substrates made of sapphire wafers onto which a low-Z metal film is deposited during encounter of the volatile plume. Encounter velocity is too low to capture volatiles by implantation, but they can be effectively trapped by co-deposition. Using this approach we will capture water vapor in sufficient quantitates to allow for the measurement of the D/H and oxygen isotopes after return to Earth. A third collector, the Particle Capture Collector (PCC) will directly collect particles that penetrate a thin aluminum membrane. By sealing the collector volume immediately following the flyby, this material, including released volatiles, is returned to Earth also for isotopic and elemental analyses.
After the flyby and collection phase the spacecraft returns to Earth. Compared to many Discovery missions, little propellent is required because the trajectory to Jupiter is a free return. The total mission Delta V is 1054 m/s. The spacecraft is solar powered with batteries for use during the encounter and begins to transmit the images and data from the particle analysis instrument on its long flight home. The vehicle reaches Earth in 2012 for a direct entry of it's Sample Return Capsule (SRG) with a Vinf of 9 km/s. The return procedures are similar to those for Stardust. The aerogel and other collectors are transported to the JSC curatorial facility for analysis and archiving.
D.3. Science Floor Mission
Only one instrument can be descoped from the Baseline Mission before the science floor is reached. This is either the PC or the AVC. These two collectors both are designed to return water samples to Earth. Although complimentary, the performance floor science objectives can be met with one or the other of these instruments. Thus, the science floor includes both the in-situ and sample return components of the science, as shown in Table 1. As discussed in section I, our strategy for dealing with cost uncertainty is not based on descoping instruments. Instead we carry a significant reserve.
D.4. Science Implementation
In this section we describe in more detail how the selected instruments and mission design address the science goals listed above. Table 2 summarizes the measurement strategy for the two principle classes of objectives for this mission; the ocean and the formation of Europa.
Table 2. Principle Measurement Strategy
| Science Objective | Measurement | Instrument | |
| Is there an Ocean? | Organics, volatile inorganics, salts in surface materials |
JEPA | |
| Hi-res. images | Camera | ||
| Formation, location and mechanism of Europa |
D/H and Oxygen Isotopes of water, trace refractories |
Sample returned to Earth by AC , AVC, and PC. |