February 22, 2001
Landing Site Steering Committee
From:M. Golombek and J.Grant
Re: Update on Thermal Constraints and ROTO Targeting
The following is an update on two issues that have surfaced related to landing site selection that we would like to make you aware of and receive any comments you might have. In order to ensure the opportunity to incorporate your input and discuss any other issues that may arise, we have scheduled a Steering Committee Meeting during the upcoming LPSC in Houston. Plans are to convene over lunch on Tuesday, 3/13, at a venue to be determined (but that will be close to the meeting).
Cold Temperature Sites:
As was noted and discussed during the Ames landing site workshop, thermal modeling of the spacecraft has shown extreme difficulty in testing the spacecraft in the extremely low temperatures likely at low thermal inertia, high albedo sites.Simple atmospheric thermal modeling shows that sites with thermal inertias <250 SI units with albedos >0.26 and <200 SI units with albedos >0.18, yield atmospheric temperatures colder than -95C. The current thermal design is based on a minimum atmospheric temperature of -95C, which thermal modeling shows that certain external hardware is about 10C colder (105C).For the required 15C test margin, this requires thermal qualification testing to -120C, which is the limit of present test facilities. Testing at temperatures colder than this would likely impact the cost, schedule and success of the test program.
Even if a test program could be devised, there would be a significant impact to the science that could be accomplished at cold temperature locations. Thermal modeling shows that about 56 W-hr is required to keep the rover interior warm at these locations, which translates into about 4 hours less instrument usage or 0.5 hours less data downlink per Sol. In addition, for external equipment, heaters would have to be added that would require an additional 20 W-hr (as well as mass and complexity), for these colder sites. Sites that have low thermal inertia and high albedo are generally considered to be dusty.Excess dust could further reduce solar power and coat rocks and surface materials. If surfaces at the site are coated with dust, it will be much harder to distinguish different materials with the Mini-TES, thereby reducing the variety of materials that can be measured in situ with the APXS and Mossbauer spectrometer and reducing the potential science return at these sites. As a result, the project is unanimous in requesting and endorsing this change.
The impact on the landing sites under consideration is as follows. Of the highest priority sites identified at the landing site workshop, only the Elysium Marte Vallis outflow site (EP49B) would be eliminated. Additional nadir only sites of opportunity that would be eliminated are Apollinaris at roughly 9.5S, 190.2W, Durius Valles (EP56A), and the other Elysium site (EP74A). None of the other high priority or targets of opportunity sites would be affected.
ROTO Imaging:
Images released by MOC of the landing sites in the past month are considerable (about 60).Many of the landing site ellipses are already covered by multiple MOC image swaths and we can look forward to ongoing excellent MOC coverage of the landing sites. These images have been released to the community and are available at the USGS and Ames web sites.
Preliminary review of these images permits identification of potential landing hazards and interesting science targets and has warranted a slight shift in two of the high priority landing ellipses. In Gusev Crater, the ellipse has been shifted slightly to the east, whereas in Melas Chasma, the ellipse has been moved slightly to the west. The new ellipse centers are located at approximately 184.15W, 14.85S and 77.78W, 8.80S for the Gusev and Melas sites, resepectively (see attached images).
There are two high priority landing regions, however, where many ellipses are bunched together (Hematite and Isidis), and decisions must be made regarding which ellipse to target with MOC in any given orbit as only one can be imaged during any given ROTO. Hence, it is necessary to prioritize targets within these two areas.
For the Hematite Region there are 5 ellipses (see attached image). If one of the landers is sent to a high priority site in Valles Marineris or a crater lake, for example, then MER B would most likely be sent to Hematite due to the limited range of latitudes accessible by the two landers. As a result, MER B sites in the Hematite Region probably should be prioritized higher than MER A sites. Given this and the preference expressed at the Ames landing site workshop, the following prioritized list (from highest to lowest) is suggested for the Hematite sites.
TM20B, TM21B, TM19B, TM10A, TM9A
Based on similar arguments for Isidis Planitia (see attached image), the following is a suggested prioritized list (from highest to lowest):
IP85A, IP98B
Summary Request:
Because of the potential impact to the MER Project (due to the thermal issue) and MOC imaging of the landing sites (due to the need to target during ROTOs), we request your review and comment on these issues ASAP. Please review these requests and attached materials at your earliest convenience and provide any feedback via e-mail to both John and Matt. We will discuss these items further during the lunch meeting at LPSC.