For propulsion, the cruise stage had two propellant tanks, each 48 centimeters in diameter. The tanks fed two thruster clusters, each of which consisted of four 5-newton thrusters 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.
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 minute 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.
By far the largest of all of the deep-space course changes, the January 11, 2012 maneuver changed the spacecraft’s speed by 5.635 meters per second, at a cost of about 18 kilograms 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 Table 2.1. Cruise performance. Based on Abilleira et al (2013).
Date
Event
Total space-craft mass (kg)
Propellant mass used (kg)
Propellant mass available (kg)
Planned velocity change (m/s)
Actual velocity change (m/s)
2011-11-26
Launch
3840.5
–
73.8
–
–
2011-11-26
Separation
3838.7
1.78
72.0
0.27
0.27
2011-11-28 to 2012-01-06
Spindown, turns, calibrations
3837.1
1.63
70.4
–
–
2012-01-11
Trajectory correction maneuver 1
3819.0
18.03
52.3
5.5071
5.6350
2012-01-25 to 2012-03-07
Turns, calibrations
3815.7
3.33
49.0
–
–
2012-03-26
Trajectory correction maneuver 2
3813.5
2.23
46.8
0.7116
0.7119
2012-03-26 to 2012-06-18
Turns, calibrations
3811.6
1.90
44.9
–
–
2012-06-26
Trajectory correction maneuver 3
3811.4
0.14
44.7
0.0414
0.0418
2012-06-26 to 2012-07-18
Turns
3811.2
0.19
44.6
–
–
2012-07-29
Trajectory correction maneuver 4
3811.2
0.03
44.5
0.0111
0.0104
2012-07-29
Turn
3811.2
0.07
44.5
–
–
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
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 position 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 incre
dible 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 measurement 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 atmospheric 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 communications 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 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 .
Table 2.2. Timeline of events in entry, descent, and landing according to different ways of measuring time. Some publications refer to a “t 0 ” that corresponds to the time that the computer updated its navigational reference point to Mars, when the spacecraft clock read 397501174.997338 seconds; others consider the entry interface time to be 540 seconds after that. AOS = acquisition of signal. LOS = loss of signal.
Phase
Event
Time (UT, SCET)
Time (rel. to t0)
Time (rel. to entry)
Time (MSL SCLK)
Source*
Approach
Vent Cruise Heat Rejection System
4:57:00
–286
–826
397500889.00
N2016
Approach
Cruise stage separation
5:00:46
–60
–600
397501115.00
K2014
Approach
EDL guidance & control activated
5:01:46
0
–540
397501175.00
K2014
Approach
Cruise balance mass jettison
5:02:37
50.64
–489.36
397501225.64
K2014
Approach
Mars Reconnaissance Orbiter AOS
5:02:39
53
–487
397501228.00
W2013
Wait for Guidance Start
Entry interface
5:10:46
540
0
397501715.00
K2014
Range Control
Guidance start
5:11:31
585.88
45.88
397501760.88
K2014
Range Control
Peak heating
5:11:49
603
63
397501778.00
MC2014
Range Control
Bank reversal 1
5:11:58
612.88
72.88
397501787.88
K2014
Range Control
Peak deceleration
5:12:06
620.33
80.33
397501795.33
K2014
Range Control
Bank reversal 2
5:12:19
633.88
93.88
397501808.88
K2014
Range Control
Bank reversal 3
5:12:49
663.38
123.38
397501838.38
K2014
Heading alignment
Heading alignment
5:13:01
675.63
135.63
397501850.63
K2014
Heading alignment
Mars Odyssey AOS
5:14:28
762
222
397501937.00
W2013
Straighten Up and Fly Right
Entry balance mass jettison
5:14:45
779.87
239.87
397501954.87
K2014
Straighten Up and Fly Right
Wait for parachute deployment
5:14:59
793.87
253.87
397501968.87
K2014
Parachute descent
Parachute deployment
5:15:05
799.12
259.12
397501974.12
K2014
Parachute descent
Last MEDLI measurement
5:15:14
808.86
268.86
397501983.86
K2014
Parachute descent
Heat shield separation
5:15:24
818.87
278.87
397501993.87
K2014
Parachute descent
Radar lock
5:15:43
837.12
297.12
397502012.12
K2014
Parachute descent
End direct-to-Earth transmission
5:15:45
839
299
397502014.00
Sc2014
Parachute descent
HiRISE image start
5:16:19
873.482
333.482
397502048.48
S2016
Parachute descent
HiRISE image heat shield
5:16:39
893.353
353.353
397502068.35
S2016
Parachute descent
HiRISE image lander
5:16:42
896.178
356.178
397502071.18
S2016
Parachute descent
Heat shield impact (approx.)
5:16:45
899
359
397502074.00
L2016
Parachute descent
Prime descent engine rockets
5:16:45
899.63
359.63
397502074.63
K2014
Parachute descent
Backshell separation
5:17:01
915.92
375.92
397502090.92
K2014
Powered descent
Powered approach
5:17:04
918.38
378.38
397502093.38
K2014
Powered descent
Sky crane start
5:17:38
952.89
412.89
397502127.89
K2014
Powered descent
The Design and Engineering of Curiosity Page 9
