Team S.C.A.R.
Spring 2012
Proposal
1.0 Introduction
The Surface Composition Analysis Robot, S.C.A.R., is a payload design concept for the use of
surveying and discovering the elemental composition of the surface of Mercury using Mineral Extract
Analysis Technology, or M.E.A.T. The scientific objective of the payload is to use a “Miniature Paul Ion
Trap Spectrometer” to isolate a piece of surface material from Mercury, find the elemental composition,
and relay the data back to the landing craft. S.C.A.R. will be able to collect valuable information on one
of the least known planets in the solar system with little expense to the main craft.
1.0 Scientific Objective
The science question we want to ask is “What is the elemental composition of Mercury at the
specific landing location?” Since it is impossible to scan the entire surface of Mercury with a tiny rover,
only the readings of the regolith (soil) taken at the landing site will be used for study. In order to take
reliable surface measurements, a Paul Ion Trap Spectrometer will be used to accurately determine the
surface composition.
Science Traceability Matrix
Scientific Objective
Measurement Objective
Obtain a sample of the
regolith on Mercury
and determine its
elemental composition.
Analyze and determine
the surface composition
of Mercury.
Measurement
Requirements
Accurately determine
the elemental
composition of a
sample of regolith up to
10 times.
Selected Instrument
Miniature Paul Ion
Trap Spectrometer.
2.0 Engineering Constraints
The limiting constraints for the payload concept are:
 5kg mass and 44cm x 22cm x 24cm.
 10 watts of continuous power provided by lander, other supplementary power sources may be
used.
 Method of transmitting data to the landing craft.
 Ability to survive on the surface of Mercury.
The S.C.A.R. payload easily falls below these limits. The final estimated mass of the payload is
~3.9kg, the final estimated volume is 40cm x 20cm x 22cm, and 10 watts of power will be supplied to the
S.C.A.R. payload though a 10.8 m tether, which will also transmit data back to a solid state drive on the
lander. In order to survive the extreme temperatures of Mercury, the payload will be released from the
lander after the terminator (line between day and night) crosses the lander.
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3.0 Scientific Instrument
The S.C.A.R. payload will house a “Miniature Paul Ion Trap Spectrometer”, a specially designed
mass spectrometer with low mass, volume, and power consumption, designed to work in a vacuum or
similar hostile environments. The spectrometer will use 8 constant watts of power; take up a volume of
5cm x 5cm x 5cm; transmit data at a rate of 10Mbps; and has a mass of .03kg. In order to transport
material to from the surface to the ionization chamber of the spectrometer, an aluminum scoop with a
mass of .1kg will be used. A small motor with a mass of .2kg will be used to scrape the scoop across the
surface in order to obtain a useable sample. A sample of helium is required to achieve ionization, and also
to expunge old testing material from the ionization chamber. In order to do this, a tiny helium canister
with a volume of 2cm x 4cm x 4cm will be supplied. A measurement will be taken with the mass
spectrometer 10 times. This model of mass spectrometer is currently in use on the Vehicle Cabin
Atmosphere Monitor (VCAM) aboard the International Space Station to determine air composition.
Fig. 1: Miniature Paul Ion Trap Spectrometer Display.
Instrument Required Resources
Instrument
Miniature Paul Ion
Trap Spectrometer
Mass (kg)
~0.03kg
Power (kW/h)
8 kW/h
Volume (cm^3)
125cm^3
Data Rate (bps)
10Mbps
3.0 Payload Design
-Framework
To ensure that the S.C.A.R. payload has the lowest mass possible, a frame of carbon fiber
reinforced plastic will be used as the support structure for the robot. This specific brand of material has a
density of 800 kg/m^3. The entire framework is worked in a lattice structure to conserve mass, and has
an estimated mass of ~2.1kg. In order to shield the mass spectrometer from foreign contaminants and
temperature extremes, a shielding made of aerogel will encase the spectrometer. Aerogel has nearly no
mass and is an incredibly poor thermal conductor, making it perfect as a shielding.
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-Mobility
After the lander makes contact with the surface, a field of hydrazine will cover the landing area.
In order to keep readings accurate, the payload will need to travel at least 10 meters from the initial
landing zone. The payload will travel with two treads powered by two individual motors with a mass of
.2kg and a power usage of 4 watts. The treads themselves have a mass of .3kg. Treads provide the most
efficient means of transportation due to the rough dirt associated to Mercury’s weatherless surface. In
order avoid obstacles; a World-Beam Q12 Series Photoelectric Sensor will be used to detect any obstacle.
The sensor is incredibly small, at a volume of 22mm x 8mm x 12mm, and has a mass of less than .04kg.
-Power and Data
To retain the maximum amount of power provided by the lander, and to transmit data back to the
lander, a 10.8m tether will be attached to the payload. The tether will remain coiled on a spool located in
the holding compartment of the payload. The entire mass of the tether and spool assembly on the lander
has a mass of .45kg. In order to stay in the payloads power budget, only a single operation will be
performed on the payload at a time. This means while the spectrometer is taking a measurement, the robot
will remain still, and when the robot is moving, no measurements will be taken by the spectrometer.
-Release
The location of the measurement for the S.C.A.R. is not important, but the time of day is.
Although the temperature of the surface will not affect the elemental composition of Mercury’s soil, it
puts serious constraints on the payload. The temperature Mercury reaches 400 degrees Celsius during
daytime, while the carbon fiber frame can only withstand 200. However, it can withstand -100 degrees
Celsius. Therefore, it is crucial to deploy only at nighttime, since the temperature will be within tolerable
range of the plastic. The lander for the mission will land two days before the terminator crosses, and
during that time the S.C.A.R. will remain dormant. After two days, the S.C.A.R. will deploy from the
lander and begin its mission. The height from the lander to the surface is only 1ft, and with the
acceleration due to gravity on Mercury being 4.8 m/s^2, the force due to impact is negligible. Therefore,
the payload will simply drop to the surface from the lander.
Mass and Power Budget
Component
Framework
Treads & Motors
Mass Spectrometer
Aluminum Scoop
Photoelectric Sensor
Power Cord
Total:
Quantity
1
2
1
1
1
1
1
Mass
2.1kg
1kg
.03kg
.3kg
.04kg
.45kg
3.9kg
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Power
N/A
8W
8W
3W
.1W
N/A
3.1W – 8W
Fig. 2: Final Design concept for S.C.A.R. Payload.
4.0 Concept of Operations
-Deployment
Two days after the lander hits Mercury’s surface, the S.C.A.R. payload will be released from a
bay door on the bottom lander. The payload will then begin moving the length of the tether.
-Data Collection
After the payload is 10 meters away from the lander, it will begin taking readings of the surface
material. After the first reading, the payload will travel clockwise in a circle around the lander,
maintaining a radius of 10 meters. A new reading will be taken after every meter of travel. As the scoop
picks up a new sample to be examined, the photoelectric sensor will detect any obstructions in the way. If
an obstacle is detected, the payload will simply divert its course around it. After each measurement, the
old testing material will be expunged, and data will be sent to a solid state drive on the landing craft.
-Ending
After taking 10 measurements, the motor operating the scoop will sever the tether from the robot,
leaving the payload idle on the surface of Mercury. The lander will then broadcast the information stored
on the solid state drive.
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5.0 Alternatives
Several alternative design concepts have been considered for transportation, data transmission,
and power supply. An alternative wheel design called a “twheel” can be used instead of treads. Twheels
are shock absorbing wheels with a rubber frame and low mass that can traverse the harsh surface of
Mercury with ease. However, they face problems when encountering slopes or pebbles, potentially
flipping the payload, and have a higher risk associated.
Instead of using a tether to supply the payload with power, the use of batteries was considered.
Batteries would allow the payload to explore a greater distance from the lander, but add a major deficit to
the available mass and volume. If using batteries, there is no way to send data back to the lander without
use of a wireless transmitter. Various UHF Transmitters were considered, but all had a high mass and
volume. Solar cells were considered for power, but since the mission could only be performed at night,
the amount of energy gained through light would be very low. Therefore, the idea was completely
avoided.
Various iterations of these designs were scored in a FOM (Figure of Merit) chart, with the details
of each FOM listed below:
Mass - The total mass of the payload. Having a low mass is crucial to the mission, as there is a crippling
limit of only 5kg. Mass has a weight of 3.
Power - The amount of power available to the payload. The S.C.A.R. payload uses a very high amount of
power during its mission life, so having enough available power is crucial to the success of the mission.
Power has a weight of 9.
Mobility – How efficiently a method of transportation can traverse the surface of Mercury. The only way
to garner accurate results with the spectrometer is to clear the field of hydrazine, estimated at 10 meters. If
the payload’s method of travel cannot safely traverse this distance safely, the mission will fail, making
mobility very important. Mobility has a weight of 9.
Reliability – Ability to survive on the surface and transmit data back to the lander. Surviving on the
surface of Mercury is difficult, and the ability to survive long enough to gain all 10 measurements is
important. Reliability has a weight of 3.
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Decision Analysis Chart
Figure of Merit
Weight
Concept1(Treads
+ Tether)
Concept2(Twheels Concept3(Twheels
+ Tether)
+ Batteries)
Mass
Power
Mobility
Reliability
Final
3
9
9
3
3
9
9
9
198
9
9
3
3
126
1
3
3
3
66
Concept 1 had the greatest amount of merit at a score of 198, and was used for the final design.
Creating an iteration using a battery pack and treads was impossible, since the required mass and volume
exceeded the constraints of the mission.
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