Eos, Vol. 95, No. 35, 2 September 2014
VOLUME 95
NUMBER 35
2 September 2014
EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION
PAGES 313–324
Future Mars Rovers: The Next
Places to Direct Our Curiosity
from 10°N to 15°S latitude due to insolation
and concomitant power constraints [Golombek
et al., 2003]. Although the MSL rover is powered by a radioisotope thermoelectric generator, it too had a range restricted to ±30°
latitude for thermal considerations [Golombek
et al., 2012].
Yet not all engineering constraints are hard
limits; some, such as latitude, may be somewhat flexible. For example, the initial MSL
latitude constraints were ±60° north or south
from the equator of Mars [Golombek et al.,
2006]; later, these were reduced to ±30°. Although the nuclear power source of MSL
does not depend on sunlight to operate, the
PAGES 313–314
When the $2.5 billion dollar Mars Science
Laboratory (MSL) rover Curiosity was designed
and built, its landing site was not known in
advance. Rather, a series of open-invitation
workshops was convened to solicit input from
the scientific community to help evaluate potential sites.
Think of the selection process as the science fair to end all science fairs. Many elements of the site selection process for MSL
trace their heritage to procedures developed
during the Apollo era. Here we trace some of
the connections to past site selection activity
and envision the future through similar series
of workshops planned for the European
Space Agency’s (ESA) ExoMars rover (slated
for launch in 2018) as well as NASA’s Mars
2020 rover (see Figure 1).
The primary incentive for participating in
the process is information: Scientists who
volunteer their time are rewarded by new
data on their favored locales on Mars. This, in
turn, spurs more research and fuels more curiosity about Mars.
plus orbital constraints of the command
module limited the initial site selection to an
“Apollo Operational Zone” that encompassed
±45° longitude and ±5° latitude (i.e., the
equatorial nearside of the Moon) [Beattie and
El-Baz, 1970]. In an analogous fashion on Mars,
potential landing sites for the solar-powered
Mars Exploration Rovers (MERs) were similarly confined to an equatorial zone extending
Engineering Constraints Based
on Environmental Factors
The most basic dictates as to where and
when a spacecraft can land are termed “engineering constraints,” and they are primarily
determined by mission safety. Safety trumps
science and for good reason. If a mission
cannot land safely, it will be over before it has
begun.
Selection of landing sites is therefore governed by a set of mission- specific engineering
constraints that are unique to a given mission’s
particular architecture. These constraints
provide the broad framework of potentially
acceptable places to land, and scientists must
abide by these engineering constraints in
selecting the most scientifically relevant landing sites.
For the Apollo missions, the need to maintain constant Earth-Moon communications
BY B. J. THOMSON AND F. EL-BAZ
Fig. 1. Mars global topography overlain with proposed landing sites for the Mars 2020 rover
(white diamonds), ExoMars rover (pink squares), and past landing sites (solid black circles). The
insets depict type examples of landing sites (see online supplement for further details). (a) Perspective view of Mawrth Vallis region (20°N−28°N, 17°W−22°W) using Mars Orbiter Laser
Altimeter (MOLA) topography overlain with Observatoire pour la Minéralogie, l’Eau, les Glaces
et l’Activité (OMEGA) spectral detections of clay [from Michalski et al., 2010]. (b) Nili Fossae
(21°N, 74°E), with proposed landing ellipse located on the graben floor adjacent to a small,
theater- headed valley. MOLA color elevation values overlain on Thermal Emission Imaging System ( THEMIS) daytime infrared data. (c) East Margaritifer Terra (5.6°S, 6.2°W), with proposed
landing ellipse next to outcrop of chloride- and clay- bearing material. THEMIS nighttime infrared
data. (d) Eberswalde delta (24.3°S, 33.5°W), with proposed landing ellipse on flat terrain to the
east. Mars Orbiter Camera (MOC) image data with THEMIS visible color. (e) Holden crater (26°S,
34°W), with proposed ellipse on alluvial fan along southwest crater wall just north of channel
inlet breech. MOLA color elevation values overlain on THEMIS daytime infrared. Data credits:
MOC: NASA/JPL/MSSS; MOLA: NASA/JPL/GSFC; OMEGA: ESA/IAS; THEMIS: NASA/JPL/ASU.
© 2014. American Geophysical Union. All Rights Reserved.
Eos, Vol. 95, No. 35, 2 September 2014
spacecraft has a preferred operational temperature range. Colder temperatures mean
more power would need to be diverted to the
heaters to keep various sensitive electronic
components warm, and hence, less power
would be available for science and mobility
(meaning smaller traverse distances, slower
movement rates, and fewer samples analyzed
by the instruments).
Landing System Constraints
In addition to power and thermal constraints, many of the key parameters that determine if a given site is accessible for landing
stem from the incoming spacecraft trajectory
and landing system. For example, for the Mars
Pathfinder, MER, and MSL missions, a landing
site had to be at or below an elevation of 0.0,
−1.3, and −1.0 kilometers, respectively (as
Mars has no sea level, elevations are reported
relative to an average reference elevation).
Sites above these elevation limits would have
insufficient column density of atmosphere to
ensure full deployment of the parachutes used
to slow the spacecraft during entry, descent,
and landing (EDL) processes.
Following the precedents of the Apollo
landings on the Moon, the totality of trajectory parameters during the EDL process is used
to define the three- sigma landing dispersion
ellipse (or landing ellipse for short), which is
the probability envelope for a given region
within which the spacecraft has a 99.7%
chance of landing. For MSL, the landing ellipse was initially about 20 by 25 kilometers.
The area of MSL’s landing ellipse shows that
as EDL technology improved, the size of the
landing ellipse shrank. The long axes of the
landing ellipses for the Viking, Pathfinder, and
MER spacecraft, in contrast to MSL, were 300,
200, and 82 kilometers, respectively [Soffen,
1977; Golombek et al., 1997; Golombek et al.,
2003].
A smaller landing ellipse permits landing in
a larger variety of candidate sites. Gale crater,
the eventual landing site for MSL, had been
considered as a potential MER site but was
dropped from consideration because it could
not accommodate the larger MER landing
ellipse. The nominal landing ellipse size for
Mars 2020 is identical to MSL, while the
ExoMars ellipse is 100 by 15 kilometers [Vago
et al., 2013], which is comparable in size to
MER and Mars Phoenix landing ellipses.
A final element of consideration is the
roughness within the landing ellipse and
approach pathway. A rough or rocky landing
site presents a significant hazard to any landing system.
Although the presence of a manned crew
significantly increases the degree of complexity of a mission, in the case of Apollo 11, the
ability of its commander to make last-minute
course corrections during descent turned a
potential failed landing into a successful one.
Such a feat would be difficult to replicate with
an automated landing system—mission planners could only mitigate this risk for unmanned spacecraft sent to Mars by avoiding
placing landing ellipses in rocky areas. Hence,
there is a growing demand for high-resolution
photographs of potential landing sites to certify that they are nearly rock free.
Site Selection Workshops
During the Apollo program, NASA established an advisory body known as the Group
for Lunar Exploration Planning [Beattie and
El-Baz, 1970]. It had expert astronomers,
geologists, geochemists, geophysicists, and
engineers who were charged with selecting
scientific instruments for successive missions.
Within this group, a landing site selection
committee took all of the scientific objectives
into account in the selection process and
incorporated all relevant engineering constraints. Site selection during the Apollo years
was not a static activity but rather a continuous process, with later mission selections
planned with flexibility to incorporate data
returned by previous missions.
Starting with the Mariner photographic
missions in the late 1960s, the exploration of
Mars began during the preparation for the
Apollo landings. Because of this temporal overlap, the site selection for the first spacecraft
to land on Mars, Viking 1 in 1976, benefited
greatly from the procedures of site selection
developed during the Apollo lunar landings.
By default, the process was limited to a small
group of scientists who were heavily involved
with NASA’s efforts.
A larger group of scientists and engineers
participated in the open site selection process
for the Mars Pathfinder mission [Golombek
et al., 1997], and a still larger group participated in the process for the MER missions
[Golombek et al., 2003; Grant et al., 2004]. The
growing inclusiveness of the site selection
process has increased with the advent of the
Internet, that is, the facilitation of instant,
worldwide communications with all those
interested in planetary exploration. The broad
community input into the selection process is
a testament to this exceptional development.
For the MSL mission, five open public
workshops were convened between 2006 and
2011 by its landing site steering committee to
solicit and evaluate potential landing sites
[Grant et al., 2011; Golombek et al., 2012]. Of
the sites proposed, several dozen were later
designated for special targeting by existing
spacecraft orbiting Mars, including the Mars
Reconnaissance Orbiter (MRO), giving workshop participants access to high-resolution
data on specific locations on Mars.
These targeted observations were key to
getting scientists involved with the workshops.
High-resolution cameras and spectrometers
on MRO and other orbiters are enormously
powerful, but their detailed views of the surface come with a trade-off: limited spatial
coverage. For example, it is estimated that the
High Resolution Imaging Science Experiment
(HiRISE) will only be able to view less than
2% of the Martian surface at its maximum
resolution, even after spending 8 years in orbit
with near- continual operations.
© 2014. American Geophysical Union. All Rights Reserved.
Thus, targeting orbital cameras to potential
landing sites on Mars allows scientists to get
a detailed picture of a favorite location that
otherwise would not be scrutinized—this in
itself can be a real boon to participating scientists. Indeed, this is one of the main incentives
used to get the participants to donate time and
effort to a thoughtful site selection process.
Picking Future Mars Landing Sites
The first landing site workshop for the ESA’s
ExoMars 2018 rover was held 26−28 March
2014 in Madrid, Spain. Sites under consideration for NASA’s Mars 2020 rover received a
similar first airing at a workshop that was held
14−16 May 2014 in Washington, D. C.
What kind of site do we want to visit next
on Mars? First and foremost, the chosen locale must align with the scientific objectives
of the mission. ExoMars and Mars 2020 have
related but distinct science objectives, and
each will be targeted at a site to maximize its
scientific return. The ExoMars rover will have
the capability to drill up to 2 meters into the
subsurface and will search for evidence of
organics from an environment that is shielded
from surface radiation. The Mars 2020 rover
will characterize an astrobiologically relevant
ancient environment and search for potential
biosignatures, and it is tasked with assembling a diverse cache of Mars samples to be
returned to Earth by a future mission.
One way to narrow the potential slate of
candidate sites is by inferred mode of deposition or formation. Among the highest-priority
sites are those with subaqueous sediments or
hydrothermal deposits [Mustard et al., 2013].
These formative processes are inferred
from both morphological and mineralogical
evidence.
For example, some of the clearest morphological indicators of past aqueous activity are
channel deposits indicative of past fluvial
activity or the terminal fan or delta deposits
present within basins (such as in Holden crater in Mars’ southern highlands or at nearby
Eberswalde delta). While there is some spectroscopic evidence for limited alteration
phases present at these sites, the strength of
these spectral features is dwarfed by the spectral signatures of the massive clay deposits in
the Mawrth Vallis region (possibly formed by
volcanic ash or the weathering of rocks to
form soil). Other sites of interest include claychloride sites such as East Margaritifer Terra
and sites with potential hydrothermal alteration near Nili Fossae. See Figure 1 for these
sites’ locations on Mars.
An attractive element of the list of candidate reference sites identified by McLennan
et al. [2012] is that they contain both aqueous
or sedimentary rocks and igneous rocks.
Access to igneous material from a widespread
volcanic unit would provide an important
calibration point for understanding the Martian timescale. A more complete preview
of some of the potential sites is provided in
the additional supporting information in the
online version of this article.
Eos, Vol. 95, No. 35, 2 September 2014
Some caution is warranted, however, when
attempting to select a landing site on the basis
of its perceived mode of formation. The selection of Gusev crater as a MER landing site
was motivated by evidence for past lacustrine activity, but evidence for paleolacustrine
processes turned out to be buried beneath
subsequent lava flows. Fortuitous discoveries
in the nearby Columbia Hills instead indicate
that Gusev is a prime example of a site with
past hydrothermal activity, but such a connection could not be established from orbital
data alone.
A more pragmatic division of potential
landing sites is between those for which the
primary scientific objectives lie entirely within
the landing ellipse (dubbed “land-on” sites)
versus sites where the science objectives that
motivated its selection are located outside the
landing ellipse (called “go-to” sites). Such a
distinction factors in to considerations about
potential EDL technology improvements.
Lessons Learned From MSL
Just as the later Apollo missions benefited
from the experiences gained in the early mission, so too has the Mars exploration program
built upon past experience. With MSL, despite
favorable placement of the landing ellipse
within Gale crater, it appears unlikely that the
rover will achieve its primary science objectives in the central mound [e.g., Thomson
et al., 2011] before the end of its primary mission. This is due in part to its extended commissioning phase and the rover’s relatively
slow rate of movement (i.e., including pauses
for stationary surface science operations).
MSL will still likely attain this goal during an
extended mission. However, this drawn-out
pace of exploration should give the scientific
community pause in its conception of long
rover traverses at future landing sites.
Relevant to the characterization of potential sites as land-on versus go-to, a critical
element for the Mars 2020 landing site selection process will be the potential addition of
new EDL technologies. For example, triggering
the parachute to open using a measure of the
range to the ground rather than the current
velocity-based trigger could reduce the size of
the landing ellipse from 25 by 20 kilometers to
18 by 13 kilometers, thus enabling placement
of the ellipse closer to targets of interest that
are too rough for a direct landing and reducing necessary traverse distances.
In addition to this “range trigger” option,
there are some additional considerations of
enabling maneuverability of the spacecraft
during the descent phase [Mustard et al., 2013].
Landings conducted with these enhanced
capabilities could potentially be more tolerant
of hazards (e.g., isolated areas with steep
slopes or rocks), thus opening up more sites
for potential consideration.
Being involved in site selection is a tremendous opportunity for all students of science
and engineering; anyone with an interest in
planetary exploration is welcome to pitch
in and help choose a future landing site on
Mars. Whether the next missions to Mars are
sent to previously proposed sites or some
as yet unidentified locales remains to be determined, but it is never too early to start planning ahead.
Acknowledgments
B. J. T. was supported in part for this work
by NASA MDAP grant NNX11AN01G. We
thank John Grant for helpful discussions and
Bethany Ehlmann, Joseph Michalski, and an
anonymous reviewer for insightful reviews
that improved the manuscript.
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Author Information
BRADLEY J. THOMSON and FAROUK EL-BAZ, Center
for Remote Sensing, Boston University, Boston,
Mass.; email: [email protected]