Proceeding of the 6th International Symposium on Artificial Intelligence and Robotics & Automation in Space:
i-SAIRAS 2001, Canadian Space Agency, St-Hubert, Quebec, Canada, June 18-22, 2001.
NANOKHOD – A MICRO-ROVER TO EXPLORE
THE SURFACE OF MERCURY
R. Bertrand*1, J. Brückner*2, M. van Winnendael*3:
*1: von Hoerner & Sulger GmbH, Schwetzingen, Germany
*2: Max-Planck-Institute for Chemistry, Mainz, Germany
*3: European Space Agency, Noordwijk, The Netherlands
Keywords Rover, Nanokhod, space exploration, Mercury.
1 Introduction
With its “BepiColombo” mission, the European Space
Agency has decided a new Cornerstone Mission to
explore the planet Mercury. Being the innermost planet,
it is still a nearly blank spot in the map of our solar
system. On the other hand, an investigation of Mercury
is of fundamental importance for understanding the
formation and evolution of our solar system, since it
provides complementary information about the hot
regions that have not been possible to be gathered so
far.
According to the preliminary planning [1], two spacecraft will be launched in 2009, one of them carrying the
so-called „Mercury Surface Element“ (MSE), a lander
that will transport scientific instruments to measure
physical properties of the planetary surface and to carry
out in-situ geochemical analysis.
The scientific benefit of especially the payload instruments will largely depend on the ability to access the
relevant samples at the landing site. In this respect, a
certain mobility to transport the instruments is a must. A
robotic arm can, in principle, increase mobility as compared to a purely stationary lander. However, such a
system suffers from the same principal limitation, which
is that the interesting samples might be just out of reach.
In this respect, a micro-rover is the enabling technology
to assure the scientific value of in-situ analysis: It allows to flexibly transport miniaturized payload instruments to the relevant sample sites, and to accurately and
repeatedly operate them according to the scientist’s
needs. It also provides means for the scientific user to
control and adjust science operations interactively,
which is a key factor for exploring an a-priori unknown
environment.
This paper describes the NANOKHOD micro-rover for
scientific exploration of the surface of Mercury as part
of the BepiColombo technology development activities.
It gives an overview on mission application, the most
important design requirements and the overall system
layout. Specific design issues with respect to Mercury
environmental compatibility are addressed, showing the
design approach suitable for a micro-rover to be operated on the surface of Mercury.
2 Background
Starting in 1996, the European Space Agency has initiated the “Micro-RoSA” technology research activity
([2], [3]) in order to establish the technological basis of
a micro-rover for scientific applications with the primary application scenario of a Mars exploration mission. Within Micro-RoSA, complementary rover concepts were assessed in order to select the optimal one
with respect to payload accommodation, locomotion
capabilities, and system resource needs. The tracked
concept "NANOKHOD” was finally selected and further developed to an advanced laboratory model. Further ESA-funded development activities covered complementary technological issues of a NANOKHOD
application, namely deep drilling and sampling for Exobiology-type missions, as well as the end to end control
system needed to operate a NANOKHOD-type vehicle
on the surface of a distant. Most recently, a follow-on
activity for “Micro-RoSA” has been kicked off. Focusing again on the micro-rover hardware, this activity
starts to adapt the Nanokhod design to the specific requirements for operation on the surface of Mercury.
3 NANOKHOD Design Requirements
The NANOKHOD rover is designed along the following requirements:
· Accommodate a “Geochemistry Package” [4], a
suite of miniaturized payload instruments, consisting of an Alpha-X-Ray Spectrometer (AXS) for the
analysis of the chemical composition of surface
material, an optional Moessbauer Spectrometer to
analyze iron-bearing minerals, and a camera system
for microscopic sample imaging and rover navigation support.
·
·
ages of the surrounding area. Based on the images,
science and rover path planning can be performed by
human operators on ground. The resulting sequence of
waypoints will be uploaded to the rover, which will then
be guided to the next waypoint autonomously, using
again the lander camera and the rover camera. A basic
autonomous traversing mode will be available, too.
Transport the instruments to sites of scientific interest in the vicinity of the lander
Deploy and operate the instruments
For the BepiColombo Mission, the micro-rover has to
operate in the rather harsh Mercury environment, which
can be characterized as follows:
· High sun irradiance of up to 14490 W/m2
· Lack of a substantial atmosphere
· Solar particle flux 10 times higher as compared to 1
AU distance
· Surface texture and regolith environment comparable to lunar regolith
· Variable lighting conditions with large shadowed
areas
Furthermore, the rover has to survive a landing shock >
200 g (for some tens of ms) and it needs a high degree
of control autonomy due to the limited number of telecommunication links.
5 Specific Design and Development Issues
In general, the mechanical configuration of the rover is
well adapted to the situation expected on Mercury:
Optimum payload accommodation and instrument orientation is still the primary goal for the application of
the micro-rover, underlining the validity of the payload
cab concept. For the regolith surface topology, the
tracks have significant advantages as compared to
wheeled or legged concepts of equal vehicle size. It is
possible to reliably traverse different types of terrain,
such as for instance fine-grained soil, partly crusty /
pebbly terrain, or rock-hard terrain. At the same time,
small obstacles can be overcome by simply driving over
them, or by using the payload cab as a climbing aid.
The thermal environment is in particular challenging.
Even at the high latitudes (85°), which are envisaged for
the landing of the surface element, the surface temperatures in sunlight may reach values above 250°, if
for instance the local terrain in inclined towards the sun
for about 10°. Furthermore, surfaces facing the sun
receive the full solar flux due to the lack of an atmosphere. On the other hand, permanently shadowed areas
such as the bottom of craters may show temperatures
well below minus 100°C. Thermal design therefore
plays a preponderant role for any system to be operated
on Mercury.
However, as mentioned above, the thermal environment
becomes a major design driver. First, the rover is subjected to radiative heat loads originating from the sun
and from sunlit areas of the surrounding. Second, the
ground may impose high or low contact temperatures to
the locomotion units, depending on the local sunlight
conditions of the terrain the rover resides on. A number
of system design adaptations have to be applied to assure, that the rover can cope with this situation:
· Mechanical Configuration: Locomotion units with
reduced width to decrease the contact area to the
ground and to allow for a wider payload cab.
· Payload and Subsystem Accommodation: All payload and subsystem elements that have a higher
sensitivity to the thermal environment will be accommodated in the payload cab, which is not in direct contact to the ground and which can be better
decoupled from the soil environment by insulation.
· Locomotion Units: Being the “hotter” parts of the
rover, these items only contain motors, gears, and
driving electronics. A careful thermal design limits
the conductive heat flow through the locomotion
units to the minimum.
· Payload Cab: Insulation combined with a “cold”
outer surface finish (low absorption, high emissivity) allows to keep the inside temperatures for payloads and electonics in the range of 40 to 100°C,
depending on the operational situation. Active
cooling can be applied for single components, that
need colder temperatures such as payload instrument detectors.
4 System Overview
Reaching back to early conceptual ideas at the MaxPlanck-Institute for Chemistry in Mainz, Germany,
NANOKHOD is tailored to the needs of scientific users
who want to operate miniaturized instruments on a
planetary surface. The payloads are integrated in a central payload cab, which is suspended between two
tracked locomotion units by levers. By this it is possible
to orient all payloads very accurately to the same sample by simply rotating the payload cab. The net rover
has a mass of about 1.5 kg. Depending on the instrument-specific accommodation needs, it can accommodate up to 0.9-1.1 kg of payload. For power provision
and telecommunication, NANOKHOD relies on the
lander, to which the rover is linked via a thin tether
cable. It is unrolled as the rover moves forward. The
cable reservoir on the rover allows for a total travel
distance of up to 100 m. In total, the rover system including payloads draws not more than 3 W. In order to
control the rover on an unknown planetary surface, it is
assumed that a pair of cameras mounted on the lander as
well as the on-rover camera can be used to acquire im-
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Selection and implementation of electronic and mechanical parts is another issue that needs careful engineering. With single exceptions, active and passive
electronic parts are available for temperatures up to
150°C. Single components can even stand temperatures
as high as 300°C. However, the combined hot and cold
thermal loads as well as thermal cycling call for a careful redesign including early prototype testing under
Mercury simulated thermal conditions. The same is true
for mechanical and electromechanical components such
as motors, gears, bearings, sealings, etc.
6 Conclusions
The Nanokhod microrover is a promising part of the
BepiColombo model payload that can enable and
largely increase the efficiency for the gathering of scientific data on the surface of Mercury. The particular
environment on the surface of Mercury imposes however a new class of driving design requirements, in
particular with respect to thermal design and the control
system. Technology development for the implementation of Nanokhod within the BepiColombo mission has
started. A flight model conceptual desing as well as
concept verification tests on a component and breadboard level are planned to become available in the end
of 2002.
7 References
[1]:
Bepi-Colombo –Interdisciplinary Mission to
Planet Mercury. ESA BR-165, September 2000.
[2]:
Micro-Robots for Scientific Applications.
Summary
Report
ESTEC
Contract
No.
12052/96/NLK/JG(SC), March 1999.
[3]:
van Winnendael, M. et al.: Nanokhod Microrover heading towards Mars. Fifth Int. Symp. on Artificial
Intelligence, Robotics and Automation in Sapce, ISAIRAS 99, ESTEC, Noordwijk, The Netherlands, 1-3
June 1999.
[4]:
Assessment Study Report Mercury Surface
Element. ESA CDF-04(A), March 2000.
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