Dear Colleagues:
The MER Project is currently evaluating several scenarios related to the
number of trajectory correction maneuvers (TCM's) and mode of tracking that
may be implemented on one or both of the MER spacecraft. While some of
these scenarios result in landing ellipses considerably larger than those
presently being considered, others lead to smaller landing ellipse sizes.
The feasibility of implementing a 5th TCM together with DeltaDOR tracking
(using two DSN stations simultaneously) during cruise would improve the
ability to precisely target desired landing sites and may lead to
significantly smaller ellipses than considered thus far in the MER landing
site evaluation process. The project will not make a decision on whether
to employ a 5th TCM and/or DeltaDOR tracking until September. If the
project does decide in favor of both a 5th TCM and DeltaDOR tracking, then
the science community may want to be prepared with any additional, new
viable landing sites for the MER spacecraft. Because it is unlikely that
any such newly identified sites will have been evaluated before, this memo
describes how the process will be accomplished.
Ellipses considered thus far in the landing site identification and
analysis process have varied in length (3 sigma) from about 340 km to 75 km
(dependant on latitude) with a cross track width of 30 km. These ellipses
result from 5 TCMs and regular Doppler and range tracking. Preliminary
evaluation of employing DeltaDOR tracking along with 5 TCMs results in
ellipses that are roughly a factor of 2 smaller in length (still dependant
on latitude) and a factor of 3 narrower in width. For the top 6 sites that
are presently being evaluated for MER, the northernmost Isidis sites would
be about 100 km long and the southernmost Gusev site would be about 40 km
long using this technique. Ellipses of this size were never evaluated for
the MER site election effort. Given that the number of potential landing
ellipses scales with size and almost 200 ellipses were identified with
dimensions 2-3 times larger than this, there are probably numerous ellipses
of this smaller size that fit within the elevation and latitude constraints
for MER. It is possible that some high-priority scientific ellipses of
this size exist that have never been identified or evaluated.
Atmospheric dust has precluded MOC imaging of landing sites during this
summer while the data rate is high and before Odyssey arrives at Mars.
Nevertheless, project constraints on site evaluation requires that the
number of sites being considered are significantly reduced at the close of
the September landing site workshop. Hence, about 4 prime sites with a
small number of possible backups will remain on the list of potential MER
landing sites following the landing site workshop in Ithaca. These four
sites will undergo focused evaluation for landing safety and increased
targeting for acquisition of MOC images. The program for the Ithaca
workshop has been fashioned to accomplish this down selection.
It is not yet known whether smaller (or larger) ellipses will be possible
and the ellipse size will not be known until just before the September
meeting in Ithaca. Furthermore, it is important to note that the present
group of sites under consideration was selected for because they appear to
best enable the main scientific objectives of the MER mission to be
addressed. These science objectives include determining the aqueous,
climatic, and geologic history of a site on Mars where conditions may have
been favorable to the preservation of evidence of possible pre-biotic or
biotic processes. Given these points, it is uncertain whether higher
priority science sites may be identified even with smaller ellipses. To
provide an answer to this "what if" question, we are asking the science
community to evaluate and propose (where warranted) possible new ellipses
of smaller size before the September meeting in Ithaca.
The rules for accomplishing this are as follows. Any proposed new site
must meet all the engineering constraints (below -1.3 km MOLA elevation,
between 10N and 15S, thermal inertia's >250 SI units with albedos <0.26 and
>200 SI units with albedos <0.18, appear smooth and flat over the entire
ellipse (ellipse length 100 km at 5N to 40 km at 15S; ellipse width about
10 km) in available Viking images and MOLA topography, less than 20% rock
abundance, and contain MOC images to assess the surface texture at small
scale (and that appear safe). Additional information regarding the mission
engineering constraints is located at
http://marsoweb.nas.nasa.gov/landingsites/ and http://webgis.wr.usgs.gov/.
In addition, the site must be at least as compelling scientifically
(relative to the mission science objectives) as the existing sites under
consideration. If sites are identified that meet these criteria, please
notify Matt Golombek, John Grant, and Tim Parker as soon as possible and
send us the supporting information. Please submit this information no
later than COB on Friday, August 17th, 2001.
A critical requirement of any new site that may be proposed is that it be
covered by sufficient MOC imagery to characterize major surface units
within the ellipse and to permit evaluation of landing hazards. We will
review the sites with key project personnel and the landing site steering
committee to decide if the site should be considered at the landing site
workshop in September. If so, the meeting agenda will be revised
accordingly.
If the project decides to implement DeltaDOR tracking with 5 TCMs, we will
be able to notify workshop attendees about a week prior to the meeting, so
that these locations can be discussed at the workshop. Note that there is
no need to send new ellipse locations at the existing sites currently under
consideration.
For your information, Matt Golombek, John Grant, and Tim Parker will review
all sites proposed during the '01 lander site selection effort as well as
all sites proposed for '03 as a guide for identifying possible new sites
with smaller ellipses. Recall that the '01 site selection effort
identified high priority sites in the same latitude band as MER, but with a
higher minimum elevation. These as well as sites originally proposed for
'03 were selected for their science potential. In addition, we will look
at areas similar to the high-priority sites that have been proposed for MER
that have MOC images to identify new potential sites with smaller ellipses.
We welcome your comments, suggestions and any new sites identified.
Thank you for your continued interest and participation in the Mars landing
site selection process.
Regards,
John Grant and Matt Golombek
Co-Chairs, Landing Site Steering Committee
John Grant
Center for Earth and Planetary Studies
National Air and Space Museum
Smithsonian Institution
4th and Independence SW
Washington, DC 20560-0315
202-357-1494 (Voice)