Wireless sensor networks are finding many applications in terrestrial sensing. It seems natural to propose their use for planetary exploration. A previous study (the Mars daisy) has put forward a scenario using thousands of millimeter scale wireless sensor nodes to undertake a complete survey of an area of a planet. This paper revisits that scenario, in the light of some of the discussions surrounding its presentation. The practicality of some of the ideas put forward is examined again, and an updated design sketched out. It is concluded that the updated design could be produced using currently available technology.

1. Pennies From Heaven: a retrospective on the use of
wireless sensor networks for planetary exploration
Robert Newman, Mohammad Hammoudeh
School of Computing and IT
University of Wolverhampton
Wolverhampton, UK
R.M.Newman@coventry.ac.uk
Abstract—Wireless sensor networks are finding many II. THE MARS DAISY
applications in terrestrial sensing. It seems natural to
propose their use for planetary exploration. A previous
study (the Mars daisy) has put forward a scenario using
thousands of millimeter scale wireless sensor nodes to
undertake a complete survey of an area of a planet. This
paper revisits that scenario, in the light of some of the
discussions surrounding its presentation. The practicality of
some of the ideas put forward is examined again, and an
updated design sketched out. It is concluded that the
updated design could be produced using currently available
technology.
Keywords-component; wireless sensor network, planetary
exploration, autonomous systems.
I. INTRODUCTION
Figure 1: The original mars daisy
This paper is a further visit to a concept which
received some attention two years ago. The concept was The Mars daisy (Figure 1) was a speculative nano-
a case study of the exploration of Mars using a wireless probe first presented in 2005 [1,2]. Its origins were as
sensor network, in place of more conventional probes part of the storyline for a short film designed to present
such as landers and rovers. Each probe was in itself a the potential of wireless sensor networks in a way that
‘nano-lander’, being scaled somewhat smaller than any would capture the imagination of teenagers. The film
other probes proposed before, in the scale of millimters. presented a planetary exploration mission to Mars.
It was proposed that acting together, a host of these Rather than a conventional lander, the mission used a
landers could perform many of the funstions associated number (9000 20g probes would have the same mass as
traditionally with rovers, and cover a larger area in more the Spirit and Opportunity rovers) of nano-landers., each
detail than could a rover. It should be stated that the 40mm long by 7mm in diameter. The nano-landers
proposers are computer scientists, not planetary would be deployed within atmospheric entry vehicles,
explorers, and many of the assumptions made were and released at low altitude from where they scattered
niaive. Nonetheless, the scenario did create some and became impaled in the regolith using a spike,
resonance amongst real plantary scientists. containing the major mass (the battery).
This paper represents an attempt to revisit the design The probe design was envisaged to be based around a
of the scenario, and move a step closer to practicality. It stack of chips, forming an opto-electrical-mechanical
is organised as follows. In the next section, the original system. In addition to a processor and memory,
idea is reprised. additional chips provided power regulation, optical
sensing, chemical analysis, imaging, seismographs and
Section III discusses the perceieved advantages and communication, both between probes and with a relay
disadvantages of a mission based on wireless sensor satellite.
networks. Sections IV to VIII discuss various instruments
to be expected to be deployed on such a probe in more The precise function of the chips in the stack is
detail. Section IX discusses some issues of wireless described below.
sensor network design in this context. Section X  A 7W semiconductor laser provides
proposes the sketch of a revised design, more a ‘penny’ communications functions. Optical
than a ‘daisy’. communication was selected in order to provide a

2. sufficiently narrow beam width to allow reported. However, the details of how the actuators for
communication with the satellite using the very the petals would work and how regolith samples would
small (40mm) antenna aperture dictated by the find their way from the spike to the chromatograph were
tiny size of the probe. The laser chip also provided not resolved. In particular, the petal design called for
optical energisation for the instruments on a chip, selective application of ‘unobtainium’.
as well as energy for vapourisation of soil
samples. III. SENSOR NETWORKS VERSUS CONVENTIONAL PROBES
 A ‘lab’ on a chip, containing a number of Although much of the discussion around the original
instruments, mostly operating on the Fabry-Perot Daisy centred on the design of the individual probe, the
principal. These instruments included a core concept is that of a sensor network as a planetary
chromatograph for chemical analysis and exploration instrument. Naturally, the previous work
accelerometers for seismic analysis. emphasised the advantages of the approach, the major
ones of which which were seen to be:
 The optical sensor chip included image sensors
(cameras) to capture the images formed by the  A mission with much greater robustness than one
lenses in the ‘insect eye’ at the top of the daisy. using a ‘rover’, due to the massive redundancy of
This chip also provided sensing for the Fabry- indiviual micro probes.
Perot instruments and the receivers for the optical
communication.  Coverage of a much larger area than a rover based
mission for the same payload.
 A power management chip provided would
provide power management functions as well as Discussion resulting from the publication of the
driver circuitry for the actuators, mostly steering previous papers, some with people with experience of
mirrors supporting the various optical functions. planetary exploration, brought to light a number of ways
that a sensor network would be inferior to traditional
 The other major component was formed by the instruments, namely:
‘petals’ of the daisy. This was a multipurpose
component, serving as a steerable antenna, a solar  Because of the tiny size of the probes, the function
concentrator and also a movable aerodynamic of the instruments carried was severely limited.
surface, during the descent.
 The probes were fixed, and so could not investigate
After the daisy’s first appearance in the film, certain any site other than the very immediate vicinity of
aspects of its design became detailed and a mechanism the landin point.
for packing and disseminating the daisies via a tape was
devised, which also provided for their charging and  Conventional surface probes can be equipped with
programming before delivery into the Martian tools to allow them to take deep core samples, or
atmosphere. The scenario also provided a case study for drill into rocks which have been selected as being of
the design of autonomous networked sensor systems, interest. By contrast, a nano-probe is restricted to
including protocols, information retrieval mechanisms surface sampling where it lands.
and methods for constructing maps from fields of
sensors, such as the daisies. IV. GEOGRAPHICAL MAPPING
The systems work described above differed markedly The geographic mapping function of the daisy
from most other nano-probe studies, in that it envisaged a network was in fact the place where the thought
flat, peer to peer network, as opposed to a lander which experiment started. At the time, sensor node licalisation
operated as a base station, used in combination with a was one of the main research themes of the group
swarm of nano-landers. The major reason for this concerned, and the question arose as to whether
emphasis was the direction taken in terrestrial sensor localisation information from a sensor network could be
networks. This is heading towards a vision of global, ad used to make a detailed 3-D map of the terrain on which
hoc, sensor webs, perhaps best exemplified by Nokia’s the sensors were placed. Since the original aim of the
‘Sensor Planet’ [3]. At the hardware level, this had the scenario was popular science dissemination, a highly
disadvantage of requiring every node to communicate visual application was desired, the one selected being the
with the host satellite. The advantage is a much more use of 3-D maps derived from the nodes’ localisation to
robust and adaptable system, which is not vulnerable to support a virtual presence application, along with use of
loss of a single specialized resource, such as a ground the sensed data to provide environmental feedback (in the
station. real world, the virtual presence scenario would be
precluded by the 4-20 minute transmission delay between
Since it was never intended as a serious contribution Earth and Mars).
to the canon of planetary exploration, little further work
was done on the daisy itself, although a brief feasibility The importance of sensor localisation lies in the
study was published. The conclusion was that some parts mapping of sensed data. It is obviously imperative to
were feasible, at least in operating principle, some know the location within the map of an individual
weren’t. It appeared that such a probe would have sensor’s data point. In terrestrial applications, it is not
sufficient processing and memory to undertake the task always possible to determine location easily. For most
required, the power budget balanced, and successful existing large sensor systems, sensor locations are
implementations of the instruments envisaged had been determined as a result of a site survey. This is expensive,

3. and often problematic since the area under investigation size of the daisy. Blain, Cruz and Flemming [4] have
is not always accessible. While GPS is a possible produced a useful survey om micro-miniturised mass
solution to localising individual sensors, it has its spectrometers, and conclude that the ion-trap principle of
problems. Firstly, it is sensitive to many environmental operation shows the most promise for miniaturisation.
problems, operation under tree canopies being one. Such an instrument could be at a scale of a few
Secondly, the addition of a complete system in each centimetres, physically constructed from a stack of chips
sensor node, simply for localiseation, is likely to add or wafers. There remains the problem of how to
substantially to the cost of the mission, since each extra introduce a sample into the instrument. If a probe is static
item of node cost is multiplied several thousand times. once landed, there is one sample opportunity, and the use
For this reason, a strong trend in sensor network of imact energy to excavate and move the sample seems
localisation research has been the use of the existing to be an attractive option.
radio communication resources for localisation, using a
variety of techniques including time of flight, relative Another element of the field sensor network scenario
phase and signal strength. that was proposed as an advantage is the ability to make
detailed maps of such parameters as soil chemistry. This
For extraterrestrial exploration, it is likely that is indeed an advantage in terrestrial applications, in
systems will be provided within the overall mission for which the regolith will often be obscured from overhead
surface mapping, such as the use of synthetic aperture observation by foliage or artificial structures. When this
radar (SAR) by an orbiter. This can provide terestrial is not the case, satellite imaging spectroscopy produces
mapping to a resolution of a few metres. Against this, excellent soil chemistry maps. As an example of the
trilateration localisation techniques operating in the quality obtainable, and what might be expected from a
microwave transmission region can achieve resolutions sensor network the following maps, derived from a series
of a few centimetres, but operate only where the sensor presented by Clarke and Swaze [5] are presented.
nodes are positioned. The end result is a rather sparse
map of very precisely located points. One opportunity is The first is derived directly from these images and
the use of mult-modal systems, in which the location of shows distribution of iron minerals around Cuprite,
the sensor nodes is used to add precision to a SAR map. Nevada.
Precisely localised nodes, equipped with cameras, could
also locate precisely features on the surface of a planet,
using optical triangulation. When used in combination,
these techniques do provide an opportunity for very
precise surface mapping, if that is required.
VI. REGOLITH CHEMISTRY
Regolith chemistry, in some form, is one of the
primary investigation carried out by most planetary
exploration vehicles. Since publication of the original
work, the authors have gained practical experience of
designing sensor networks for the investigation of soil
chemistry, but unfortunately these do not provide a good
guide as to what might be done with an extraterresrial
probe. Firstly, for reasons of timeliness and cost, the
devices is constrained to use off the shelf
instrumentation. The most commonly available low cost
devices depend on electrochemistry, and sense chemicals
in aqueous solution. Typically, these networks use ion
sensitive electrodes to sense particular analytes.
In the absence of water, the mechanical arrangements
for conveying the sample to the sensor become Figure 2: Fe distribution around Cuprite, NV.
considerably more complex. The scenario for the Mars
Daisy envisaged the use of the landing impact to force a The image represents an area 2 km on a side, and
small sample into a tube in the spike of the probe, provides a very detailed account of the mineral
whereby it would be vapourised and ionised using the disribution in that area. This image has been used as the
laser and analysed using a MEMS scanning Fabry-Perot basis for a simulation of the results that might be
spectrometer, integrated onto the instrumentation chip. expected from a sensor network, sensing for evidence of
All the parts of this scenario have been reported, but a the same chemicals. Using the Dingo simulator [6], a
complete instrument has not been designed, stll less network of sensors were randomly distributed over the
produced and evaluated, so it is difficult to say with any 2km square, and the value of the image in Figure 2 used
confidence that such an approach is feasible. as the output of the sensing device at that point.
Mass spectrometry is commonly used for chemical
analysis in planetary landers. The smallest such
instruments, which use MEMS technology, are still not at
a scale that could be accommodated within a probe of the

4. The simulated sensor network was programmed to Trebling the number of sensors has clearly made a
produce a map, using the Shepard interpolation method, considerable improvement to the image quality, but it is
as has been reported previously [7]. Figure 3 still considerably poorer than the
Figure 5: Isopleths added to the 1000 sensor
map.
Figure 3: A map produced using 1000 sensor
original. Finally, Figure 5 shows the image from the
nodes. 1000 node network, with an overlaid contour map.
shows the map produced by a 1000 node network. Although this is a difference of presentation only, it does
While this is recognisably the same area, the map is make the map more readable.
clearly of much poorer quality than the original, which These maps use a relatively primitive method of
could have been obtained using an orbiting image interpolation. Improved interpolation methods, together
specrometer. with multivariate and model based techniques, might
Figure 4 shows the same image, but now produced improve the interpolated maps somewhat, but the satellite
using a 3000 node network. spectroscopy is a hard target to hit.
In conclusion to this section, it can be seen that a
sensor network, unless very densely populated, cannot
compete in terms of detail with image spectrography.
However it may well be a useful adjunct to it, allowing
calibration of the satellite images, and determination of
some species with more accuracy than is possible. Sensor
networks would also come into their own in situations
where spectroscopy was impracticable, for instance
impenetrable atmospheres.
VII. IMAGING
The ‘insect eye’ on the daisy was not, in truth, a
practical solution to a requirement for visual imaging.
The objective of the design was to gain a 360º field of
vision and, given the arbitarily determined weight and
size limitations, to do it without moving parts. In
practice, it would be difficult to make lenses with the
required characteristics, and distribution of the imaging
arrays on the chip would be difficult. In a more realistic
scenario, there would be a number of requirements for
Figure 4: A map produced using 3000 sensor visual imaging. One was mentioned in Section IV,
nodes. precision mapping by sterioscopic imaging. Using widely
spaced sensor nodes could provide very accurate
localisation of objects using triangulation, provided that
the nodes could be accurately localised and angles of
incidence could be accurately determined. The ‘insect
eye’ would not perform this function well. A better
solution would be a camera which could be panned. A
larger chip would allow a very high resolution image

5. sensor, which could allow detailed inspection of objects, started with the goal of the deployment of a 10,000 node
even at some distance. sensor network and built the world's largest deployed
WSN, some 1,200 nodes, installed over an area of 1.3km
VII. SEISMOLOGY by 300m. The network was designed to detect and track
intruders using acoustic methods. ExScal remains the
The original Daisy included a accelerometer. The most thoroughly researched and documented massively
motivation was simply that seismology is of potential plural network, and thus is a reference point for future
importance in planetary investigations, and applications.
accelerometers are a well understood component of
wireless sensor networks. Subsequently, it has become Bapat et.al., in their summary of the results of the
eveident that field sensing of seismic events Ithat is, ExScal project [10] cite the problems expected to be
monitoring the same even over a large area) provides an encoutered in the design of a very large network. They
important additional capability – to undertake seismic are:
tomography [8]. This means that, provided seismic  Failure of sensor network protocols to scale:
activity was present, it would be possible to use the The main reason for this given is inability of
sensor network to make an image of the subterranian protocols, which work at a small scale, to deal with
structure. Even in cases in which there was little seismic node failure in the large scale.
activity, releasing an object to impact the planet at speed
could provide an event which could be used to probe  Complexity of integration: here the issue is
underneath the surface. the interaction of the multiple protocols which deal
with issues such as medium access, reliable
VIII. METEOROLOGY communication, sensing, and time synchronization
Another capability of sensor networks, forseen in the  Lack of sufficient fault data: given the
original scenario, is the abilty to make meteorologic susceptibility of networks to faults, it was argued
measurements and map them precisely over an area. In that there was a need for more real fault data in a
the agricultural work, the ability to understand working context.
microclimates is very important. Whether or not
microclimates are as important in extraterrestrial  Unpredictability of network behavior:
applications is a question for meteorologists. Certainly, a Essentially the consequence of the above – little is
sensor network can make accurate maps, of a comparable known about behaviour of networks on this scale,
quality to those in figures 4 and 5, of temperature, and it is not clear that scalable solutions have been
atmospheric pressure and wind speed and direction. In validated in real-life use.
terrestrial applications other factors such as CO2 and The design approach used to address these concerns
H20 content are often sensed also. Since electronic gas was one of use of a 'planned architecture', the imposition
sensing is a relatively well developed art, it would be of various design constraints to simplify the operational
possible to detect a number of gases using well complexity of the system. Nonetheless, it would seem
understood and mature technology. If the microscale when the results are studied, that ExScal was at the limit
mass spectrometer discussed in section VI were of network size feasible with such an approach.
available, it coulds be turned also to atmospheric sensing,
and would provide a flexible, general purpose To simplify the localisation of nodes and the
instrument. interpretation of the data from them, the system was laid
out on a rectangular grid, with nodes being located on
IX DISTRIBUTED SYSTEM DESIGN installation using a hand held GPS device. Nonetheless,
11.4% of the nodes were incorrectly located.
Most of the sensing modalities proposed above come
into the category of field sensing – that is the use of an To ensure reliable data transmission, a three-level
array of sensing devices to determine the value of some hierarchical architecture was adopted, with different
measurand over a surface or volume. There have been a specialised hardware at each level. The end to end
number of terrestrial examples of such networks. For reliability achieved using this architecture was 85.61%
example, in the Microclimate Sensor and Image for the best traffic type (low bandwidth) and 55.14% for
Acquisition Networks system reported by the Center for the worst type of traffic (high bandwidth)
Embedded Network Sensors (CENS) at UCLA [9], multi A conservative attitude to hardware specification was
function sensor nodes have been deployed to allow, adopted. Despite this 6% of the nodes were non-
amongst other functions, maps to be made of climate functional after the 15 day trial. Loss of a second tier
data. node caused loss of a complete section of the network. A
The promise of field sensing, with many data points, significant amount of node malfunction (7%) was
using a massive plurality of sensors, is to produce a associated with their reprogramming.
detailed map directly from the sensed data, using Without doubt, completion of the ExScal network
thousands of nodes, as depicted in the simulations in was a considerable achievement, but it has not
section VI. The scale of networks illustrated there is established 'planned architecture' as a suitable basis for
around the current state of the art. A major attempt to networks an order of magnitude larger that was achieved
demonstrate a system of similar size is the ExScal (short there.
for 'Extreme Scale Networking) project [10]. This project

6. Furthermore, the carefully planned and installed as coins are stacked. The stacks would be retained by
network of ExScal, and the attrition over a short period, wires, which would be used to cahrge and program the
would not be feasible within the planetary exploration nodes while within the entry vehicle. A possible
scenario put forward. The network must be self arrangement is shown below:
configuring and self maintaining. This was the major
reason for the adoption of a flat, peer to peer architecture. (camera ready version will contain diagram)
The problem of self-configuration of a network becomes An advantage of the disc form factor is that power
somewhat simpler when all nodes are the same. Various cells are conveniently manufactured in disc form, so a
protocols for self configuration have been published, power cell forms the lowest layer, putting the mass at the
usually designed to presever power as much as possible. bottom, to help ensure that the node lands the right way
Examples are Leach [11] and MuHMR [12]. It should be up.
noted that these protocols have been verified in
simulation only, to date no-one has produced a network The next component is the stack of wafers that form
of autonomous sensors sufficiently large or long lived to the mass specrometer. These would be topped by several
test in real life. Another point to be noted is that the vast multi chip hybrids, integrating the circuitry and
majority of protocols and simulated results assume embedded sensors (such as accelerometers). Finally
omnidirectional, radio data transmission. The directional, would be another hybrid containing the external sensors
optical transmission envisaged in the Daisy is completely (image, pressure, temperature, gas sampling valves) and
outside the parameters of these protocols. A safer choice deployment drives. During descent the top surface
would be to revert to conventional radio transmission, would be covered with a number of layers, which would
which gas been well characterised. For nodes on a very be unfolded by the deployment drives. These could be
small scale, this probably rules out direct communication one or more solar cells, for power, a loop antenna for
with the satellite, due to the lack of directivity at these radio transmission and a steerable mirror for the image
longer wavelenths. Thus, such a system would probably sensor.
use specialised ground stations to communicate back
Although larger and heavier than the original Daisy,
home. The vulnerability of these could be eliminated by
this coin shaped probe is still scaled small enough to
replicating them sufficiently. The sensor network would
allow deployment of very many for the weight of an
still operate on an ad-hoc, peer to peer basis, routing
existing orbiter. Undoubtedly if a mission is planned in
messages back to one of the ground station sinks. So long
earnest, the real design would be very different from this,
as one remained operational, and a route from all nodes
but as a concept sketch it suggests that a multi modal
to it could be found, the network would remain
planetary exploration node is feasible with current
operational.
technology.
Energy saving is also the reason for localising
processing to the nodes as much as is possible. Typically, ACKNOWLEDGMENT
in sensor networks, power consumption is dominated by
data transmission. A previous study has shown the The original Mars Daisy scenario was a team effort.
potential energy savings attainable by selection of an Contributors included Sarah Mount, Andree Woodcock,
appropriate processor, and maximising processing so as John Burns, Jim Tabor, James Shuttleworth and Elena
to minimise data transmission [13]. The interpolation Gaura.
algorithms which produced the maps in Figures 3, 4, and
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