Emily Lakdawalla
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and +70°C. 18 But some instruments, particularly ChemCam’s body unit, don’t like running hot. Other instruments, like SAM, have components that generate a lot of heat. These
instruments have integrated coolers to keep their electronics at safe temperatures.
4.4.1 Rover avionics mounting panel
All the temperature-sensitive electronics and systems are bolted to a rover avionics mount-
ing panel (RAMP). The panel is, in turn, attached to the top deck of the rover with titanium structures designed not to conduct heat from the interior to the exterior of the rover.
Martian atmosphere occupies the small amount of space inside the rover that is not filled
with electronics. The mostly carbon dioxide gas inhibits the transfer of heat between the
internal hardware and the rover’s sides and belly panel.
4.4.2 Sensors and survival heaters
A total of 221 temperature sensors monitor conditions all over the rover, though only
about half are in use at any given time, since there are redundant sensors for the rover’s
A- and B- side electronics. A few critical components (such as the batteries) have their
own survival and warm-up heaters controlled by a total of 8 mechanical thermostats.
Curiosity’s 17 cameras and 32 motors can survive all expected ambient temperatures
outside the rover’s body, but have minimum operating temperatures between –55°C
and –40°C. When they are too cold (as they always are overnight and early in the morning,
and can be during the day, depending on the season), they have to be warmed before use.
The rover’s main computer switches these warm-up heaters on and off as commanded.
4.4.3 Rover heat rejection system
The rover heat rejection system (RHRS) pumps Freon (trichlorofluoromethane, CFC-11)
through tubing that loops near the MMRTG to pick up waste heat and then into the rover
body to warm the electronics (Figure 4.6). The heat rejection system contains a total of 60
meters of aluminum and stainless-steel tubing. The rover integrated pump assembly (also
visible in Figure 4.3) acts like the rover’s heart, pumping Freon near all the parts of the rover that need to be warmed and cooled. It contains a large accumulator, or tank, that
gives the Freon room to expand when it warms. Peak temperatures in the system have
never risen above 72°C; the system can handle Freon temperatures as high as 90°C.19
17 Keith Novak, personal communication, email dated February 28, 2017
18 The description of the heat rejection system in this section is based on Novak et al (2013)
19 Keith Novak, personal communication, email dated February 28, 2017
150 How the Rover Works
Figure 4.6. Rover heat rejection system. (a) Layout of the tubing. (b) Schematic diagram of the system, color coded to match (a). (c) Pumps, valves, filters, and manifolds that make up the interior of the pump assembly. Emily Lakdawalla after Novak et al ( 2013 ).
4.4 Thermal Control 151
There are two heat exchangers on the back of the rover, one on each side of the MMRTG
(Figure 4.7). The heat exchangers have tubing bonded to both sides. There is a hot plate on the inward-facing side of each heat exchanger, where the fluid picks up waste heat from
the MMRTG and returns to the pump. On the outward-facing side of each heat exchanger
is a cold plate, where fluid flowing through the tubing radiates heat away. Aerogel fills the honeycomb core of the heat exchangers, thermally separating the hot inner face from the
cold outer face. If the interior of the rover needs to be heated, the pump sends fluid warmed by the MMRTG through the tubing connected to the rover avionics mounting plate. When
the rover runs hot, the pump can send fluid from the rover avionics mounting plate to the
cold plates on the outside of the heat exchangers and just underneath the rover’s top deck.
The MMRTG is exposed to the Martian elements, including wind. During the coldest
winter months, high winds could rob the rover of heat necessary to survive. The heat
exchangers and body of the rover shield the MMRTG from winds blowing from the front
or sides of the rover, but the back is unprotected. A fabric windbreaker (Figure 4.7) bridges
the cold plates on the back of the rover, dramatically reducing the wind’s chilling effect.
Because the heat rejection system is absolutely essential to rover health, there are two
redundant pumps and two redundant mixer valves and splitter valves (Figure 4.6c). The
mixer and splitter valves allow the heat rejection system to selectively heat or cool the
Freon as needed. For rover safety, they work independently of any computer, operating
passively in response to the temperature of the fluid flowing through them.
Figure 4.7. External parts of the rover’s heat rejection system. Cruise and ground heat rejection system tubing in direct contact with the MMRTG is not used on Mars; it was for cooling the MMRTG on Earth and during cruise (see section 2.2.1). The base image is the Mastcam self-portrait taken on sol 1197. NASA/JPL-Caltech/MSSS/Emily Lakdawalla.
152 How the Rover Works
The mixer valve controls the amount of flow across the rover’s hot plates. If the mixer
valve falls below a temperature of –10°C, it opens all the way, sending 97% of the fluid
through the hot plates. If the mixer valve measures a temperature of 10°C, it closes to its minimum setting of 55%, which runs just enough fluid in the hot plates to keep the fluid
temperatures below 90°C. The splitter valve controls the flow to the cold plates and top
deck. When its temperature rises above 35°C, it opens all the way to 96%; it closes to its
minimum setting of 4% at 15°C.20
4.4.4 Heater tables
All of Curiosity’s components have minimum allowable operating temperatures. Some
components would spend too much time at those temperatures without assistance, so have
built-in heaters. The most heat-demanding components are the motors. Curiosity’s motors
do not operate well at temperatures below –55°C because the wet lubricant inside the
gearbox is highly viscous at that temperature. All the motors have heaters to permit their
operation when ambient temperatures are lower than that. Heating requires both power
and time, two limited resources, so during tactical planning it is imperative to predict how long and how much power it will take to prepare motors for use. Time of day, season, wind
speeds, and rover orientation (potentially causing shadowing) all have strong effects on the start temperatures of rover hardware. With so many variables involved, engineers can’t
predict exactly how much heating will be necessary for a given motor on a given sol to be
operated at a given time. Instead, they budget enough power to heat the motors as much as
necessary for a predicted worst-case scenario.
Prior to landing, thermal engineers prepared two “heater tables” that laid out the energy
requirements for motor heating for the worst-case environments for every motor for each
hour of the day for two representative days in the Martian year: landing day, which was on
Ls 151 (approaching the southern hemisphere vernal equinox), and the coldest day, winter
solstice, Ls 90. The landing-day table would allow conservative budgeting of energy
expenditures for heating throughout the rest of spring and most of summer until fall started to bring cooler temperatures, when they would have to switch to the winter table.
The heater tables stipulate, for each heater, for each hour of the day:
• Warmup period. This varies with the mass of the motor. The largest motors that
&
nbsp; drive and steer the wheels weigh 6 kilograms and can take up to 2 hours to preheat
when they are coldest.
• Target temperature of the temperature sensor. The sensors and heaters are on the outside, not inside, the motors. But it’s the deep interior of the gearbox that has to
be brought to –55°C. This unfortunate arrangement results from the late switch to
wet-lubricated motors (see section 1.5.2). To drive in the morning, the outside of the motor and its temperature sensor may be heated as high as –5°C in order to
generate a big enough pulse of heat to bring the interior to temperature within a
reasonable amount of time.
20 Novak et al (2013)
4.4 Thermal Control 153
• High and low set points for temperature cycling during the maintenance heating
period. Once the motor preheats, the exterior temperature is allowed to fall to a lower set point before the heater turns on again and continues to oscillate within a small
temperature range. For some motors, after preheating, the heater turns on when
needed to maintain the exterior temperature of the motor between –49°C and –44°C.
• Duty cycle prediction. The amount of time during the maintenance heating period that the heater will be turned on.
• Timeout period. How long to wait for the target temperature to be reached before giving up, shutting down the heater, and aborting the day’s plan. If this happens, the
rover will continue performing remote sensing activities, but will not proceed with
any drives or arm work.
• Energy (in watt-hours) associated with warmup and maintenance heating of the
motor. This is taken out of the power budget in the day’s plan.
Fortunately, it isn’t always necessary to preheat motors. Even on the coldest winter
days, Gale crater heats up to about –25°C, well above the motors’ minimum operating
temperature. The biggest motors take the longest to heat, and are the ones that enable
Curiosity to drive. During the winter, there is a 3-hour period when the rover can drive
without spending time or energy preheating, from about 14:40 to 17:50 local true solar
time each day. (Heater tables are a case where it is necessary to employ true rather than
mean solar time; see section 3.2.2.) During the spring and summer this is a 6-hour period, but still in the afternoon, from about 12:30 to 18:30 each day.
Therefore, waiting for the motors to preheat naturally requires waiting until the after-
noon to drive. However, the mission would often prefer to move the rover in the morning
in order to allow sufficient time for driving and post-drive imaging to complete before the afternoon orbiter relay. Rover planners compromise by usually starting drives between
11:00 and 12:30, which means motors usually need to be preheated, but for a relatively
short time.
4.4.5 Performance on Mars
Curiosity’s thermal control systems have operated flawlessly.21 The temperature of the interior of the electronics box has varied within acceptable limits, ranging from lows near 5°C to highs of 17°C in winter and 37°C in summer. The rover’s temperature profile has
been reliably the same, day after day, making it easy for rover planners to decide when to
operate the instruments that need cool ambient temperatures. The battery survival heaters
have never been turned on and likely never will be.
There were several surprises after the spacecraft landed on Mars. While REMS mea-
sured ground temperatures that were in good agreement with predictions, it found air
temperatures to be much higher than predicted: 25°C warmer than predicted during the
day, and 10°C warmer at night. The team now uses current REMS data on atmosphere
temperatures to help them predict rover temperatures (see section 8.4.3).
21 Cucullu et al (2014)
154 How the Rover Works
The rover operated using the Ls 151 or spring-summer heater table until sol 434, when
some of the wheel motors got cold enough that the thermal team switched to the Ls 90
(winter) heater table. The sudden switch to preheating for winter solstice temperatures
dramatically reduced available power and drive time, and was, of course, overly conserva-
tive. It also added complexity, because sometimes heating had to happen in one sol’s plan
for the subsequent sol’s activities. The worst impact was on drive time, because increased
preheat time imposed a limitation on drive time at a point in the mission when they were
attempting to extend drives to cover more distance.
The team briefly tried a hybrid approach (using the spring-summer heater table for
some systems and winter table for others), but this was operationally complex, and couldn’t last long in any case because of rapidly cooling temperatures. They switched all of the
mobility system completely to the winter table on sol 456, and all remaining subsystems
to the winter table on sol 463. Because the stepwise switch to winter heating requirements
dramatically affected rover activities, the thermal team began the process of developing an intermediate set of tables, optimized for Ls 130, covering early spring and late fall
seasons.
4.5 TELECOMMUNICATION
Curiosity receives commands directly from Earth, but returns more than 99% of its data
through an orbital relay. Telecommunications bandwidth is one of the primary limitations
on the science return from Curiosity (or any other deep-space mission). Curiosity typically returns about 500 megabits per sol. Actual volumes in any given transmission depend on
many factors, especially the geometry of an orbiter’s communications pass (range, eleva-
tion, and duration of the pass).
Curiosity’s operational schedule is dictated by communication opportunities, sched-
uled months in advance. The sol begins at about 10:00 a.m. local time, when Curiosity
usually receives the day’s command sequence directly from Earth via an X-band transmis-
sion between a Deep Space Network dish and Curiosity’s high-gain antenna. Curiosity
warms up and performs the commands – driving, arm operations, and/or remote sensing –
and typically wraps up work in time for the afternoon communications passes. Overnight,
Curiosity usually rouses from sleep to return more data. Whichever communications pass
is the last one before the next sol’s tactical planning shift begins is called the “decisional”
data pass (see section 3.4 for more about tactical planning).
4.5.1 The Deep Space Network
For more than 50 years, Earth has listened to faint signals from distant spacecraft with the giant radio antennas of NASA’s Deep Space Network (DSN). The DSN consists of three
ground stations positioned approximately 120° of longitude apart from each other, so that
at least one station can “see” a spacecraft at all times. The three stations are Goldstone, located near Barstow, in California; Madrid, in Spain; and Canberra, Australia. Each station has multiple dishes, including one 70-meter dish and several 34-meter dishes
(Figure 4.8). The 34-meter dishes can be arrayed to create a single aperture comparable to
4.5 Telecommunication 155
Figure 4.8. Dishes of the Canberra Deep Space Network pointed at Mars. In this photo, taken on November 18, 2013, two 34-meter dishes (DSS-34 at center and DSS-45 at right) were
listening to signals from the MAVEN orbiter as it arrived at Mars. At the same time, at left, the 70-meter DSS-43 simultaneously received data from Mars Odyssey and Mars
Reconnaissance Orbiter. Photo courtesy Glen Nagle, Canberra Deep Space Communi
cation
Complex.
a single 70-meter dish. For Mars, which is nearby, this usually isn’t necessary. In fact, a single antenna can receive signals from multiple Mars spacecraft simultaneously. The
DSN provides support for European and Indian Mars missions as well as NASA ones.
4.5.2 Curiosity hardware
Figure 4.9 details external parts of Curiosity’s telecommunications hardware; refer to Figure 4.3 for locations of internal parts. 22 Curiosity has three antennas. Two X-band antennas can communicate directly with Earth. A UHF antenna links Curiosity with orbiters.
There is one high-gain and one low-gain X-band antenna. X-band communications happen
through one of two redundant Rover Small Deep Space Transponders (RSDST). The tran-
sponders are an improved version of the design used for the Mars Exploration Rovers.
4.5.2.1 High-gain antenna
The high-gain antenna is hexagon-shaped, 28 centimeters in diameter, and is steerable in
both azimuth and elevation. It can both receive commands from and transmit telemetry to
Earth, but it has to be aimed properly. It has a 5° pointing accuracy, limited in part by the accuracy of the rover’s knowledge of its own orientation. It can provide a downlink of 160
22 Curiosity’s telecommunications hardware is described in Makovsky et al (2009)
156 How the Rover Works
Figure 4.9. External parts of Curiosity’s telecommunications hardware as seen in the sol 1197 Mastcam self-portrait. NASA/JPL-Caltech/MSSS/Emily Lakdawalla.
bits per second to a 34-meter Deep Space Network radio antenna, or 800 bits per second
to a 70-meter antenna. Running in the other direction, the high-gain antenna can receive
uplinked commands at a rate of 1 or 2 kilobits per second. A typical command load is
about 225 kilobits. Planners schedule 15 minutes for communication sessions to allow
sufficient margin.
Curiosity’s daily commanding is scheduled for approximately 10:00 a.m. local time
because Earth is always above the horizon at that time. The high-gain antenna sits above
the rover’s deck, but its view of Earth can be blocked by the rover mast or the hardware