by The Design
the south, while Mars Odyssey approached from the north (Figure 2.14). Mars
Reconnaissance Orbiter’s path took it across the westernmost rim of Gale crater, carrying it nearly overhead during landing, while Odyssey passed considerably to the east. That geometry would allow Odyssey to have a second communications pass with MSL later on land-
ing day, passing to the west about two hours after landing.25 Mars Reconnaissance Orbiter began its “open-loop” recording of the MSL signal at 8 minutes 7 seconds before entry. 26
2.3.4 Entry: 0 to 259 seconds
MSL entered the Martian atmosphere at 05:10:46 at an altitude of 125 kilometers.
Traveling at a relative speed of 5.8 kilometers per second, it shed all of that velocity within the next 7 minutes. Within one minute, it had plunged to only 40 kilometers’ altitude,
broadcasting tones to keep Earth updated. Watching the X-band tones arrive on Earth,
Allen Chen had a moment of sheer terror: a tone had arrived that indicated that the vehicle orientation was out of control, suggesting that the loss of the spacecraft could be immi-nent. Fortunately, it turned out to be a calibration issue with the MEADS sensors, not an
actual anomaly, and the rest of the landing events happened as expected. 27
At 46 seconds after entry, the descent stage inertial measurement unit had begun to
sense the atmosphere as a drag force of 0.2 gees, beginning the range control phase of
guided entry. This was earlier than expected, because the navigation team’s atmospheric
model had overpredicted the temperatures there, underpredicting the pressures, although
the pressures and temperatures that MSL measured were consistent with those reported by
Mars Climate Sounder.28 The mismatch between prediction and reality had little effect on the landing process.
25 Abilleira and Shidner (2012)
26 Way et al (2013)
27 Allen Chen, personal communication, email dated February 24, 2016
28 Martin-Mur et al (2014)
2.3 EDL: Entry, Descent, and Landing 79
Figure 2.14. Geometry of MSL and orbiter ground tracks during entry, descent, and landing.
Base image is from Viking Orbiter; spacecraft positions retrieved from JPL Horizons. ODY =
2001 Mars Odyssey; MRO = Mars Reconnaissance Orbiter; LOS = loss of signal.
During the range control phase, the rover computer predicted the downrange distance it
would fly and adjusted lift as necessary in order to shoot for the correct range. Unlike an airplane, MSL had no flaps or elevators to change its angle of attack, so the way that the
spacecraft adjusted its range was to perform a series of banking turns, rotating its center of gravity around the axis of its blunt nose. Its initial entry point was biased to the left (north) of the intended landing site, so it began with a banking turn to the right. The computer
monitored the spacecraft’s cross-range drift, and commanded a bank reversal when the
drift passed a threshold. It reversed its bank angle to the left, then right, then left again.
Figure 2.15 shows how the velocity, altitude, and bank angle varied with time. The first, commanded bank was at very nearly 90° (resulting in no lift being generated), so the
80 Getting to Mars
Figure 2.15. Best estimate of entry trajectory, based on spacecraft telemetry. Modified from Mendeck and Craig McGrew (2014 ).
2.3 EDL: Entry, Descent, and Landing 81
spacecraft descended on an almost ballistic path. By the time of the first bank reversal, it had slowed dramatically and the spacecraft commanded less bank angle. The capsule truly
began to fly in the Martian atmosphere.29
All this time, the heat shield was doing its job. Initially, the spacecraft continued to lose altitude at a rate of a kilometer per second. The hypersonic entry pressurized the air in
front of the capsule, creating a shock wave with temperatures as high as 4000 kelvins
(Figure 2.16). At 65 seconds after atmospheric entry, the atmosphere had become thick enough that the flow of air across the heat shield abruptly transitioned from laminar
(smooth) to turbulent. 30 The heat shield’s temperature increased rapidly. At 85 seconds after atmospheric entry, the surface of the heat shield reached its peak temperature, of
around 1300 kelvins (Figure 2.17). MEDLI data showed that peak heating happened at a different location and lower temperature than had been predicted during heat shield development, possibly because the flow of air over the heat shield became turbulent earlier than predicted. The PICA heat shield material withstood these forces easily, with little of it
receding away: every single MEDLI thermocouple survived entry, even though some were
installed just 2.54 millimeters underneath the surface. 31
Figure 2.16. Artist’s concept of the MSL aeroshell creating a shock wave during entry. NASA/
JPL-Caltech release PIA14835.
29 Mendeck and Craig McGrew (2014)
30 Bose et al (2013)
31 Little et al (2013)
82 Getting to Mars
Figure 2.17. MEDLI MISP temperature flight data (solid lines) compared to preflight predictions (dashed lines), from Bose et al ( 2014 ). Each MISP sensor has four thermocouples at different depths. The colorful pattern on the heat shield shows the preflight predictions. Peak heating actually occurred closer to sensor T7 near the center of the heat shield, not at the most leeward sensor T3 as predicted.
2.3 EDL: Entry, Descent, and Landing 83
With every second of entry, the spacecraft flew into denser air. It reached peak decelera-
tion 80 seconds after entry, the pressure of the air decelerating it at 12.5 gees. As the
spacecraft began its first bank reversal, dropping below 20 kilometers altitude, those forces began to wane, and the flying saucer entered a period of nearly level flight for two full
minutes. It flew with a tailwind of about 20 meters per second, but the spacecraft’s reckoning of its downrange target depended on an inertial measurement unit that wasn’t affected
by the wind, and the spacecraft stayed on course. The final bank reversal left it with about 1 kilometer of downrange error, well within tolerances.32
At an altitude of 14 kilometers and speed of 1.1 kilometers per second, the spacecraft
transitioned into the “heading alignment” phase of guided entry. 33 The spacecraft banked left to correct its cross-range heading, probably to compensate for a 10-meter-per-second
crosswind. 34 It flew downrange for 100 seconds at a near-constant altitude, steering lightly to arrive at the optimal location for parachute deployment. It was during heading alignment, at 222 seconds after entry, when Mars Odyssey achieved lock on MSL’s UHF signal
and began relaying telemetry directly to Earth at a rate of 8 kilobits per second through the Deep Space Network station in Canberra, Australia. 35 Back on Earth, engineers applauded
the news; the landing would occur just the same with or without Odyssey communica-
tions, but only Odyssey could give Earth real-time telemetry. “Real” time being 13.8 min-
utes after the events on Mars, thanks to the distance separating Mars and Earth.
MSL waited until its inertial measurement unit registered a speed of only about 400
meters per second and then changed its configuration again to prepare to deploy its para-
chute. MSL prepared for parachute deployment with the “straighten up and fly right”
maneuver. The falling spacecraft threw away six 25-kilogram entry balance masses in
pairs at two-second intervals. (You can see the entry balance masses on the backshell in
Figure 2.10.) The release of the entry balance masses counteracted the off-center weight distribution that had been imparted by the release of the cruise balance masses.
The aeroshell tipped up, aligning its angle of attack to wi
thin 5° of its descent trajec-
tory. At the same time, the reaction control system rolled the spacecraft 180° (a maneuver
referred to as the “victory roll”) to the desired bank angle for later radar operation pur-
poses. Straighten up and fly right took a total of 14 seconds. 36 The work of the descent stage reaction control thruster system was complete. Throughout entry and descent, the
reaction control thrusters had performed a total of 2256 thrust pulses, operating for a total of 110.725 seconds (Figure 2.18). 37 The aeroshell had dissipated 99.6% of the vehicle’s kinetic energy through friction with the atmosphere. 38 The spacecraft was now ready to deploy its parachute. The balance masses continued along their ballistic trajectories, later impacting the ground at the northern edge of Mount Sharp, beyond the landing site to the
east (Figure 2.7).
32 Mendeck and Craig McGrew (2014)
33 Mendeck and Craig McGrew (2014)
34 Martin-Mur et al (2014)
35 Way et al (2013)
36 Cruz et al (2014)
37 Baker et al (2014)
38 Way et al (2013)
84 Getting to Mars
Figure 2.18. Descent reaction control system thruster use. Odd-numbered thrusters were used for all pulses; even-numbered thrusters were secondary, used only when more thrust was needed. SUFR = Straighten Up and Fly Right. Modified from Baker et al ( 2014 ).
2.3.5 The parachute
MSL’s parachute had the same shape as the Vikings’, but with a diameter of 21.35 meters
it was 33% larger (Figure 2.19). Another crucial difference was the distance between the backshell and parachute: Viking’s parachute trailed by 8.5 times the parachute diameter,
but MSL’s lines were longer, to separate it by 10.32 times the diameter. This increased
separation was designed to reduce “area oscillations” of the parachute. The parachute was
composed of orange and white ripstop nylon, except for the crown, which was made of a
heavier ripstop polyester. The suspension lines were made of Technora and Kevlar, both
synthetic fibers with high strength and heat resistance.39
39 Cruz et al (2014)
2.3 EDL: Entry, Descent, and Landing 85
Figure 2.19. The MSL parachute. Top: dimensions, from Cruz et al ( 2014 ). The bottom two images were taken during April 2009 testing of parachute deployment in a wind tunnel at NASA Ames Research Center. NASA/JPL-Caltech releases PIA11992 and PIA11993, annotated by Emily Lakdawalla.
86 Getting to Mars
2.3.6 The descent stage
The descent stage was a complicated spacecraft all on its own, with a mind-boggling num-
ber of systems crammed into its open structure (Figure 2.20 and Figure 2.21). At its heart was the most sophisticated propulsion system JPL had ever built. It was actually two distinct propulsion systems that had to share components to conserve mass and volume. The
descent stage also served as the structural link between all other spacecraft components,
with six separation nuts each connecting the top hexagon of the descent stage to the cruise stage and backshell, and three connecting the bottom of the descent stage structure to the
top deck of the rover. It weighed 1068 kilograms, of which 397 was fuel.40
Although the rover’s main computer ultimately commanded the descent stage, the
descent stage contained numerous avionics of its own, including a computer to control the
thruster systems; the descent inertial measurement unit, with gyroscopes that facilitated
the precision flying of the guided-entry phase; an X-band radio system that was used
throughout cruise, entry, descent, and landing; the Terminal Descent Sensor radar system
used to measure altitude and velocity; and the bridle umbilical device used to lower the
“rover-on-a-rope”.
The Descent Reaction Control System (DRCS) that steered the aeroshell throughout
entry and descent consisted of eight 250-newton thrusters in four pairs, one primary and
one secondary. Holes cut into the backshell allowed these thrusters to protrude. MSL used
the primary (odd-numbered) thrusters for small pulses; the secondary (even-numbered)
thrusters came into play for larger pulses. These eight thrusters drew fuel from only one of the descent stage’s three fuel tanks, the one mounted toward the rover’s front. The rover’s computer updated commands to the thrusters every 125 milliseconds, commanding thrusts
in increments of 15.625 milliseconds.
The eight downward-pointing Mars Lander Engines (MLE) were much larger than the
upward-pointing Reaction Control System thrusters, at 3300 newtons as compared to 250.
The landing used only about two-thirds of the descent engines’ thrust capability because
the low altitude of the landing site gave MSL ample time to decelerate. They drew on three
propellant tanks using a flow regulator that had been launched into Earth orbit multiple
times as part of the Space Shuttle Discovery before being rebuilt for MSL. 41 Four of the engines were canted at 5° outboard from the rover, and four were canted at 22.5°. When
the descent stage was connected to the rover, the nozzles of the engines projected beyond
the rover’s belly pan, keeping exhaust clear of the rover (Figure 2.22).
The Terminal Descent Sensor sensed the ground with six radar beams. One beam
pointed directly downward; three pointed at an elevation of 20° in different directions (one toward the rear and one each to left and right); and two, called the “headlight” beams,
pointed forward and slightly left and right at elevations of 50° (Figure 2.23). Unlike other landing radar systems, MSL’s was “memoryless” – measurements of range and velocity
were essentially instantaneous, not relying on previous measurements or even on sharing
of information between beams. It was computationally intensive, but a “bad lock” didn’t
propagate error forward in time, allowing the system to be robust to spurious signals.
40 Hoffman et al (2007)
41 Pearlman (2017)
2.3 EDL: Entry, Descent, and Landing 87
Figure 2.20. Descent stage parts (part 1). Top photo taken at JPL in early October 2008, bottom photo around November 2008. NASA/JPL-Caltech release PIA11425, annotated by
Emily Lakdawalla.
88 Getting to Mars
Figure 2.21. Descent stage parts (part 2). Top photo taken at JPL around November 2008.
Bottom photo taken at Kennedy Space Center around November 2011. NASA/JPL-Caltech
releases PIA11808 and PIA15020, annotated by Emily Lakdawalla.
2.3 EDL: Entry, Descent, and Landing 89
Figure 2.22. Descent stage mated to the rover and inside the backshell. The red caps on descent stage rockets, sliver wrap on rover wheels, and yellow covers on MARDI camera
(square) and terminal descent sensors (round) were removed before flight. Photo taken at Kennedy Space Center in in October 2011. NASA/JPL-Caltech release PIA14756.
2.3.7 Descent under parachute: 259 to 375 seconds
Still traveling at Mach 1.7, MSL fired the explosive sabot that deployed the parachute 259
seconds after hitting the entry interface, at 5:15:05 Spacecraft Event Time. The parachute
filled with air, stretching its suspension lines in 1.135 seconds and fully inflating in under two seconds. In those two seconds, Mars’ gravity was still accelerating the spacecraft; it
sped up by 0.743 meters per second. The parachute was qualified to survive deployment at
up to Mach 2.3 and able to withstand an opening force of 289 kilonewtons. In the event, it
experienced only 153.8 kilonewtons. The reaction control system thrusters remained ready
to work to cancel out any spinning or rocking motions, but MSL’s desc
ent was stable
90 Getting to Mars
Figure 2.23. Terminal Descent Sensor beam pattern. Emily Lakdawalla after Pollard ( 2012 ).
enough for them not to be needed. 42 With the parachute inflated, the MEDLI instrument suite shut down. Only 20 seconds after the parachute deployed, MSL had slowed to sub-sonic speeds, so it dropped the heat shield, exposing the rover and descent stage to the
Martian air at 5:15:24.43
About 6 seconds before the heat shield separated, the Mars Descent Imager (MARDI)
had switched on and begun taking images at an average 3.88 frames per second. The first
26 MARDI photos were black; the next 622 documented the final 2.5 minutes of landing.
As the heat shield fell away, a white-balance target on the inside of the heat shield helped MARDI’s autoexposure algorithm to adjust quickly from the pitch-black interior of the
capsule to the brightly lit Martian day (Figure 2.24).
Angled to the east along the descent path and with a field of view of 70-by-55°,
MARDI’s first images encompassed much of the eastern half of the landing ellipse. The
heat shield can be clearly tracked through the first 250 of the images, and MARDI even
documented the moment of its impact onto the Martian surface in image number 345,
taken at 05:16:48 on the spacecraft’s clock (Figure 2.25). (Note: Time stamps in MARDI image files appear to be 3 seconds later than the spacecraft clock times in the same files.
The given spacecraft clock times correctly correspond to the landing timeline in Table 2.2
within a fraction of a second.)
42 Cruz et al (2014)
43 Karlgaard et al (2014)
2.3 EDL: Entry, Descent, and Landing 91
Figure 2.24. MARDI image of the heat shield taken at a spacecraft clock time of 397501995, one second after the release of the heat shield. MARDI image 0000MD0000000000100033E01.
NASA/JPL-Caltech/MSSS.
92 Getting to Mars
Figure 2.25. Series of MARDI images documenting the impact of the heat shield onto the Martian surface. A plume of material spread for several seconds after the impact before the MARDI field of view no longer encompassed the impact site. MARDI images
