by The Design
stage weighed 466.5 kilograms when dry, and carried 73.8 kilograms of propellant. 3
To aid navigation, the cruise stage carried one star scanner and two sun-sensor assem-
blies, each of which had four sensors pointed in different directions. One sun-sensor
assembly was connected to each of the rover’s two computers. The cruise stage had no
independent brain, relying instead on the rover’s computers.
Figure 2.4. Cruise stage parts as seen at JPL in late 2008. The thruster clusters are enclosed in protective cages (red) that were removed before launch. NASA/JPL-Caltech release
PIA11440, annotated by Emily Lakdawalla.
3 Allen Chen, personal communication, email dated July 1, 2016, correcting numbers published before launch
62 Getting to Mars
MSL’s power system was incredibly complex, due in part to its being controlled by the
rover avionics. Although the cruise stage derived power from solar arrays, the power had
to be passed through the descent stage to the rover avionics for maintenance of voltage
stability and power levels. To generate power, the cruise stage had 6 arrays of solar cells (visible in Figure 2.3), capable of producing as many as 2500 watts of power at Earth’s distance from the Sun. As the spacecraft approached Mars, the power output would diminish to about 1000 watts, both because of the increasing distance and because the solar
panels would no longer be pointed directly at the Sun.
Keeping the interior of the interplanetary spacecraft cool was a major challenge during
cruise, just as it was during the final launch preparations. Encased within the aeroshell, the MMRTG sat in close proximity to pressurized fuel tanks for the descent stage whose temperature should not rise above 30°C. The 2000 watts of waste heat that the MMRTG gener-
ated needed to be radiated to space. Using fluid running inside metal tubing, the cruise stage gathered heat from within the aeroshell. The cruise stage had a redundant pair of pumps
(only one in use at a time) that moved Freon through 70 meters of tubing in the Cruise Heat Rejection System (CHRS). The flowing Freon gathered heat from the cold plates of the
rover’s heat exchanger and the roots of the MMRTG fins, then carried it behind the 10
cruise stage radiators (read section 4.4 for more about rover thermal control). After passing the last radiator, the tubing carried cool fluid past heat exchanger plates on electronic components of the cruise and descent stages before returning to the rover.4 The connections between CHRS and the rover were severed with pyros shortly before landing; the now-disused tubing is still being carried around on Mars by the rover (see Figure 2.29).
For propulsion, the cruise stage had two propellant tanks, each 48 centimeters in diam-
eter. The tanks fed two thruster clusters, each of which consisted of four 5-newton thrust-
ers pointed in different directions. Two of the thrusters in each cluster were “axial”, and were tilted 40° away from the spacecraft’s axis of rotation (one each toward the positive
and negative ends of the axis). Continuously firing the axial thrusters decreased or
increased the speed of the spacecraft in the direction of its rotation axis. The other two
thrusters in each cluster were oriented perpendicular to the spin axis. They could be used
to change the lateral speed of the spacecraft by being pulsed for 5 seconds at a time, twice per two-minute revolution. 5
2.2.2 Cruise phase
Figure 2.5 illustrates MSL’s cruise trajectory. Just three days after launch, on November 29, the spacecraft went into safe mode while attempting to use its star scanner to determine its orientation. It took weeks to track down the root cause of the problem: “a previously unknown design idiosyncrasy in the memory management unit of the MSL computer processor. In
rare sets of circumstances unique to how this mission uses the processor, cache access errors could occur, resulting in instructions not being executed properly. ”6 Because the problem
originated in hardware, not software, it could not be repaired, only worked around.
4 Bhandari et al (2011)
5 Abilleira (2013)
6 JPL (2012a)
2.2 Cruise 63
Figure 2.5. Schematic diagram of MSL cruise trajectory between Earth and Mars. Time ticks on MSL trajectory are placed every 20 days. Modified from NASA/JPL-Caltech landing press kit.
Without use of the star scanner, the spacecraft could not turn to keep its solar panels and radio antenna precisely pointed at Earth. A planned December 11 trajectory correction
maneuver couldn’t be performed without turning the spacecraft. Without the star scanner
they couldn’t determine the spacecraft’s orientation and spin rate precisely, as required to time the position and duration of the multiple jet firings for the maneuver. With every passing day, the spacecraft’s orientation drifted farther away from the optimal direction, so
engineers rushed to develop a solution to the problem.
The mission formed a Tiger Team to try to understand the reboots triggered by the use
of the star tracker. Fortunately, the initial trajectory toward Mars was so close to predictions that the mission was able to delay the necessary maneuver by a month. Still, they
were unable to solve the problem before the need for the maneuver pressed. To get the
orientation and spin rate information that they needed, navigators employed a trick that
had been developed during a similar circumstance on Pathfinder. They measured the min-
ute Doppler shift of the spacecraft’s radio signal, caused by the spacecraft’s spin; the
Doppler showed up as a sine wave in the radio frequency. From the magnitude of the
Doppler signal, they determined the orientation of the spacecraft. With that knowledge,
they were able to command the maneuver with sufficient precision.
64 Getting to Mars
By far the largest of all of the deep-space course changes, the January 11, 2012 maneu-
ver changed the spacecraft’s speed by 5.635 meters per second, at a cost of about 18 kilo-
grams of fuel. It wasn’t perfectly aligned, but it was close enough for later maneuvers to
clean up any errors. 7 In early February, engineers applied a software update to avoid use of the memory functions that triggered the safe mode.
The mission used the otherwise quiet time of cruise to turn on and test all the science instruments. One of them, RAD, actually began doing mission science in December 2011, studying
what human astronauts might experience on their own future cruise to Mars.8 The mission checked out the other nine science instruments in the middle of March; all passed. 9 Afterward, on March 26, the spacecraft performed a second trajectory correction maneuver. The maneuver finally cleaned up the residual trajectory error and aimed MSL directly at Mars. In fact, the second maneuver was so precise that the mission was able to delay the third maneuver to
accommodate a flight software update and some additional instrument checkouts.10
Throughout cruise, the navigation team tested their ability to determine the spacecraft’s
orientation in space with a series of commanded turns.11 When test results were fed back into their landing simulations, they were able to predict the landing site more accurately: the landing ellipse could be shrunk from 20-by-25 kilometers, to only 7-by-21 kilometers.
The navigators presented the opportunity to the science team, who eventually decided to
shift the target closer to the mountain in the middle of Gale crater, 6.5 kilometers south and 1.3 kilometers west of the original target. On June 26, the third trajectory correction
maneuver targeted MSL to the new landing ellipse. The fourth maneuver, on July 28,
cleaned up residual errors.12 During cruise, the spacecraft consumed fewer than 30 kilograms of fuel, less than 40
% of the total amount available (Table 2.1). 13
2.2.3 Approaching Mars
On June 22, six weeks before landing, NASA and JPL released a video to YouTube titled
“Seven Minutes of Terror: The Challenges of Getting to Mars.” Like a summer blockbuster
movie trailer, with strident music accompanying disconcertingly lit appearances by lead
landing engineer Adam Steltzner and others, the video presented the impending landing as
seven minutes of terrifying and helpless uncertainty for the MSL team. The video struck a
chord with the public. The New York Times reported it had already been viewed half a million times by July 10. 14 By early September, the views had hit 3 million.15
7 The story about how the navigators pulled off the first Trajectory Correction Maneuver was shared with me in an email by Rob Manning on January 8, 2015, and corrects timeline errors he made in his book, Mars Rover Curiosity
8 NASA (2011c)
9 JPL (2012b)
10 Martin-Mur et al (2012)
11 Martin-Mur et al (2014)
12 Martin-Mur et al (2014)
13 Table data are from Abilleira, 2013. For a detailed accounting of the nature and reasons of all the cruise turns and calibrations, read Martin-Mur et al (2014)
14 Chang (2012)
15 Guy Webster, personal communication, email dated May 17, 2017
2.2 Cruise 65
elocity
Actual v
change (m/s)
–
0.27
–
5.6350
–
0.7119
–
0.0418
–
0.0104
–
elocity
Planned v
change (m/s)
–
0.27
–
5.5071
–
0.7116
–
0.0414
–
0.0111
–
Propellant mass
available (kg)
73.8
72.0
70.4
52.3
49.0
46.8
44.9
44.7
44.6
44.5
44.5
Propellant
mass used (kg)
–
1.78
1.63
18.03
3.33
2.23
1.90
0.14
0.19
0.03
0.07
Total space-
craft mass (kg)
3840.5
3838.7
3837.1
3819.0
3815.7
3813.5
3811.6
3811.4
3811.2
3811.2
3811.2
er 1
er 2
er 3
er 4
al (2013).
a et
Abilleir
wn, turns, calibrations
ent
. Based on
rajectory correction maneuv
urns, calibrations
rajectory correction maneuv
urns, calibrations
rajectory correction maneuv
urns
rajectory correction maneuv
urn
Ev
Launch
Separation
Spindo
T
T
T
T
T
T
T
T
06
07
18
18
Cruise performance
Table 2.1.
Date
2011-11-26
2011-11-26
2011-11-28 to 2012-01-
2012-01-11
2012-01-25 to 2012-03-
2012-03-26
2012-03-26 to 2012-06-
2012-06-26
2012-06-26 to 2012-07-
2012-07-29
2012-07-29
66 Getting to Mars
The approach phase began 45 days before arrival, as the Deep Space Network collected
nearly continuous Doppler and range data by monitoring MSL’s radio carrier signal. The
hoped-for precision landing depended upon how well navigators could determine MSL’s posi-
tion at entry, and how well they could communicate that information to the rover computer.
Four weeks before landing, navigators began twice-daily radio tracking sessions, using widely separated ground stations as a giant interferometer to measure the spacecraft's position with incredible precision. At the same time, scientists on the Mars Climate Sounder and Mars Color Imager (MARCI) instruments on Mars Reconnaissance Orbiter delivered daily updates on
atmospheric conditions over the landing site to the navigation team.16 Mars Climate Sounder couldn’t see all the way to the surface at the landing site because of seasonal water-ice clouds, indicating cool weather. MARCI images showed no dust storm activity near the landing site.
By any measure, the navigational guidance of MSL to Mars was a feat of accuracy,
“possibly at the limit of what is possible with current calibration and tracking measure-
ment errors.” 17 The spacecraft was aimed at a target at the top of Mars’ atmosphere 2.5
kilometers wide by 11.5 kilometers long and 3522.2 kilometers from the center of Mars.
Following the fourth trajectory correction maneuver on July 29, 2012, navigators found
the spacecraft to be aimed at a spot only 200 meters and 0.11 meters per second off of its
target position and velocity. This was good enough not to risk any further maneuvering.
Both of the final two trajectory correction maneuver opportunities were canceled, and the
spacecraft was on its final course from six days prior to entry. When the landing was over
and the navigators determined the spacecraft’s actual path into the atmosphere, they found
it had hit within 700 meters of its entry interface target. 18
2.3 EDL: ENTRY, DESCENT, AND LANDING
On landing day, August 6, 2012 UTC (August 5, local California time), NASA aired a live
television broadcast of the landing from JPL beginning about 53 minutes before atmo-
spheric entry. The cameras were trained on a glass-walled room at one side of the main
Spaceflight Operations Facility floor. The engineers seated behind monitors in that room
comprised only about half of the workers monitoring the landing; the rest were holed up
in an “EDL War Room” closer to the mission operations area in a separate building. On
TV, EDL Operations Lead Allen Chen served as the play-by-play announcer of landing
events, interpreting the X-band tones and the tersely worded, acronym-filled communica-
tions chatter for the watching world.
Forty minutes before atmospheric entry, a command shut down the rover’s autonomous
system fault protection. About 18 minutes before entry, the Odyssey mission reported
acquisition of signal from their spacecraft. Fifteen minutes before entry, Chen reported
that the flight team had run simulations of MSL’s course based upon the last navigational
data received, and that it looked like they were “right in the middle of the ellipse.” Thirteen and a half minutes before entry, the cruise stage vented the Freon refrigerant that had piped heat from inside the capsule to the cruise stage radiators. The engineers monitoring the
X-band radio signal from MSL were able to detect the minute effect that the venting of the
16 Chen et al (2014)
17 Martin-Mur et al (2014)
18 Abilleira (2013)
&nb
sp; 2.3 EDL: Entry, Descent, and Landing 67
Freon had on MSL’s velocity. Twelve minutes before entry, the Odyssey team reported that
they were “go” to serve as the communications relay for MSL’s descent.
The remaining events of entry, descent, and landing unfolded very rapidly. Figure 2.6
and Table 2.2 summarize them. Figure 2.7 shows the spacecraft trajectory across Mars’
surface.
Figure 2.6. Diagram of approach, entry, descent, and landing process. Emily Lakdawalla after Kornfeld et al. 2014 .
68 Getting to Mars
Source*
N2016
K2014
K2014
K2014
W2013
K2014
K2014
MC2014
K2014
K2014
K2014
K2014
K2014
W2013
K2014
K2014
K2014
K2014
K2014
K2014
Sc2014
esponds to the
s consider the entry
” that corr 0
ime (MSL SCLK)T 397500889.00 397501115.00 397501175.00 397501225.64 397501228.00 397501715.00 397501760.88 397501778.00 397501787.88 397501795.33 397501808.88 397501838.38 397501850.63 397501937.00 397501954.87 397501968.87 397501974.12 397501983.86 397501993.87 397502012.12 397502014.00
efer to a “t
0
ime (rel. to entry)
45.88
63
72.88
80.33
93.88
T
–826
–600
–540
–489.36
–487
123.38
135.63
222
239.87
253.87
259.12
268.86
278.87
297.12
299
. Some publications r
) 0
ead 397501174.997338 seconds; other
k r
0
aft cloc
ime (rel. to t
50.64
53
T
–286
–60
540
585.88
603
612.88
620.33
633.88
663.38
675.63
762
779.87
793.87
799.12
808.86
818.87
837.12
839
, SCET)