Emily Lakdawalla
Page 13
0000MD0000000000100344E01 to 0000MD0000000000100358E01, taken between 5:16:45
and 5:16:49. NASA/JPL-Caltech/MSSS.
After dropping the heat shield, MSL waited three seconds, ready to use its thrusters to
cancel any rocking motion caused by the separation, but the spacecraft was steady and
2.3 EDL: Entry, Descent, and Landing 93
needed no correction. After another two seconds, it activated the Terminal Descent Sensor
radar system. The five-second delay after heat shield separation was necessary to prevent
the radar system from confusing the nearby heat shield with the ground. The Terminal
Descent Sensor achieved radar lock about 20 seconds after heat shield jettison. One radar
beam showed a spurious measurement of the “ground” at a range of 1003.66 meters and a
velocity of –47.76 meters per second about 30 seconds after dropping the heat shield. This
was probably a detection of the heat shield falling toward the ground!44
Although the Terminal Descent Sensor provided information on the distance to the
ground, the instantaneous altitude of the spacecraft is not necessarily the same as its altitude relative to its final landing site some distance away; it was mainly for this reason that the landing site needed to be flat throughout the landing ellipse. 45
Shortly after dropping the heat shield, direct-to-Earth transmission of the X-band signal
ceased. The MSL team was now entirely dependent upon Mars Odyssey for real-time
information on the status of the landing. Odyssey performed well throughout the landing,
delivering continuous updates on MSL’s health.
As MSL descended, Mars Reconnaissance Orbiter sped toward it from the south (see
Figure 2.14 and look for 5:14 UTC on all three ground tracks, then follow the time forward). Less than a second after the heat shield dropped, Mars Reconnaissance Orbiter’s
HiRISE camera began acquiring an image of the landing site. Like most Mars orbiting
cameras, HiRISE is a “pushbroom” instrument that sweeps a long, skinny detector across
the surface, taking advantage of spacecraft motion to build up an image swath about
10,000 pixels wide by as many as 126,000 pixels long. It can take as many as 100 seconds
to capture a single image. Ordinarily, not much changes on the Martian surface during
such a short period of time, but things were happening fast as MSL descended. HiRISE’s
beam swept across the heat shield at 353 seconds after entry; it caught the backshell and
then the parachute about 3 seconds later.46 So the amazing HiRISE image actually captures different moments in time for the two pieces of hardware (Figure 2.26).
At an altitude of 3000 meters, the rover prepared its descent stage for powered descent.
When the data from the Terminal Descent Sensor indicated that the spacecraft had reached
a speed of 79 meters per second and an altitude of 1671 meters, 117 seconds and 10.4
kilometers’ altitude after deploying the parachute, it primed the descent stage engines,
flowing fuel to them at 1% throttle, and abruptly cut the connection to the backshell and
parachute.47 For two seconds, the spacecraft plummeted, making room between it and the backshell. The parachute remained attached to the backshell, and both fell together.
Lacking rockets to slow their descent further, they landed before the rover did, to the west-southwest of the rover’s landing site.
44 Chen and Pollard (2014)
45 Steltzner et al (2010)
46 Christian Schaller, personal communication, email dated February 17, 2016
47 Karlgaard et al (2014)
94 Getting to Mars
Figure 2.26. HiRISE’s amazing image of MSL under parachute, its heat shield significantly below it. HiRISE image ESP_028256_9022. NASA/JPL-Caltech/UA.
2.3 EDL: Entry, Descent, and Landing 95
2.3.8 Powered descent: 378 to 412 seconds
The Mars Lander Engines throttled up, beginning the “powered approach” phase, at
5:17:04. The rockets worked to smoothly zero out the spacecraft’s horizontal motion while
bringing the vertical descent rate to 32 meters per second. Figure 2.27 summarizes the
work of the landing engines. At the beginning of powered approach, the descent stage also
performed a divert maneuver, shifting the spacecraft’s position 300 meters to the left of the entry trajectory, a distance sufficient to ensure that the rover’s eventual landing site would
not be directly on top of the already-landed backshell and parachute.48
Following powered approach, the spacecraft was finally directly above its eventual
landing site. For the first time, the terminal descent sensor’s altitude readings directly
measured the remaining distance to the surface: 247.9 meters. A brief descent phase called
the “constant velocity accordion” saw the rover continuing to descend at 32 meters per
second. The constant velocity accordion was intended to accommodate any mismatch
between the terminal descent sensor’s measured altitude at the beginning of powered
approach and the altitude of the actual landing site now beneath the rover. The constant
velocity accordion could have accommodated as many as 100 meters of altitude differ-
ence; in fact, there were only 5.5 meters of altitude difference between the estimated and
Figure 2.27. Mars Lander Engine (MLE) thruster operation during the spacecraft’s final descent. The position of a propellant injection device, called a pintle, in the throat of the rocket controlled the amount of thrust. Emily Lakdawalla after Baker et al ( 2014 ).
48 Steltzner et al (2010)
96 Getting to Mars
actual altitude.49 With that out of the way, at an altitude of 142 meters, MSL entered the constant deceleration phase, smoothly slowing the spacecraft from a descent rate of 32
meters per second to 0.75 meters per second. 50 It was time to deploy the landing gear.
2.3.9 The lander
Before the MSL mission could rove Mars, its rover had to perform the functions of a
lander, deploying landing legs and coming to a stable halt. Once on Mars, the landing gear
had to transform into the rover’s mobility system. The rocker-bogie suspension system
uses a number of passive pivots to balance out rough terrain and keep the rover body as
level as possible. But during landing, with the wheels not yet touching ground, the inter-
connected levers of the mobility system needed to be carefully restrained until the last
possible moment, to keep the six wheels as close to flat as possible upon touchdown. 51 The
mobility system was restrained at five points: at the four corners of the rover, connecting the rockers and bogies to the rover body, and also in the center of the rover’s back, holding the differential arm still. During flight, the long rocker arm connecting the front wheel to the rear bogie was folded nearly at a right angle in order to fit the mobility system within the cramped space of the aeroshell (Figure 2.28 and Figure 1.7).
Many of the devices now visible on the top deck of the rover are related to cruise, entry,
descent, and landing, and several are not used in the surface mission (Figure 2.29).
2.3.10 Sky crane and landing: 412 to 432 seconds
The descent stage switched from decelerating at a constant rate to descending at a constant rate of 0.75 meters per second, so required less rocket power. Out of concern that the
descent stage rocket exhaust could impinge on the rover, the four engines canted at only
5° were throttled down to 1%, the other four throttling up to compensate. The descent
stage wobbled a bit in response to the sudden change in the descent engines
’ activity; the
spacecraft allowed 2.5 seconds for those wobbles to settle out before proceeding, of which
it needed only 1.25 seconds.
At 5:17:38, at an altitude of about 21 meters, with the descent stage stable and descend-
ing at 0.75 meters per second, three pyros fired to separate the rover from the descent
stage. The weight of the rover pulled on three nylon/Vectran cords wrapped across a con-
fluence point pulley and then around a spool attached to the descent stage, called the bridle umbilical device (Figure 2.30). A brake within the spool controlled the rate of descent. The rover had pulled the cords to their full length of about 7.5 meters in 5 seconds (Figure 2.31).
Along with the three strings of the bridle, the bridle umbilical device also deployed an
umbilical cable that allowed commands to be passed from the rover computer to the
descent stage. (An artist’s concept of the extended bridle and umbilical can be found in
Figure 1.21.) The tapered shape of the spool made it spin at a higher angular rate as the 49 Way et al (2013)
50 Sell et al (2014)
51 Jordan (2012)
2.3 EDL: Entry, Descent, and Landing 97
Figure 2.28. Rover mobility system in stowed configuration. Photo taken at Kennedy Space Center in November 2011. NASA/JPL-Caltech release PIA15021.
rover descended, and the faster it spun, the more the brake resisted the motion; this con-
trolled the rate at which the rover descended under Mars’ gravitational acceleration. 52
As the rover descended on its cables, it also deployed its landing gear. Pyros fired to
separate the rear bogies from the rover body 0.7 seconds after the rover separated. The
bogies fell, pulling downward on the bent rockers, and locking them into their final,
straight positions. After the rover reached the end of the bridle, another pair of pyros fired to separate both rockers.53 Finally, just before touchdown, one more pyro fired to release the differential restraint; waiting until the very last moment kept the wheels as coplanar as possible for touchdown, and would allow the landing gear to passively accommodate any
surface roughness.54 One thing the landing gear could not handle however, would be the presence of a rock more than 66 centimeters tall positioned to spear the rover’s belly pan.
HiRISE images had shown few such rocks in the landing ellipse, but bad luck could win,
and MSL had no active terrain hazard avoidance capability.
52 Gallon (2012)
53 Sell et al (2014)
54 Jordan (2012)
98 Getting to Mars
Figure 2.29. Parts of the rover relevant to cruise, entry, descent, and landing. The base image is a self-portrait taken with the Mastcam on sol 1197 (19 December 2015). NASA/JPL-Caltech/MSSS/Emily Lakdawalla.
2.3 EDL: Entry, Descent, and Landing 99
Figure 2.30. The bridle umbilical device connection to the rover. Emily Lakdawalla after Gallon (2012 ).
Figure 2.31. Artist’s concept of the rover pulling to the end of its bridle as the Mars Lander Engines fire to maintain a steady rate of descent. NASA/JPL-Caltech release PIA14839.
Throughout, the descent stage should have continued to drop at a slow rate of 0.75
meters per second. It should then have taken 15.67 seconds for the rover’s wheels to touch
the ground. However, the actual time was 17.9 seconds, far longer than estimated. That is,
the rover actually descended slower than planned, at only 0.6 meters per second at the
100 Getting to Mars
moment of touchdown. Moreover, the rover was still drifting horizontally at more than 0.1
meters per second at the moment of touchdown, more than twice as fast as expected. 55 This
slower-than-expected descent, right at the moment of touchdown, was a very serious error.
Rob Manning explains:
We were to discover after MSL had landed on Mars that we had missed a crucial item.
The long list of variable parameters had not included one that should be obvious:
gravity. In the simulations, the EDL team used a fixed value for gravity that was
rather generic for that part of Mars. We failed to take into account that the shape of
the surrounding terrain and hills might affect the actual gravity, and because we didn’t
try other values, we didn’t notice just how sensitive the landing was to being slightly
off with the value the team had chosen. The value for Mars gravity used in the simula-
tion turned out to be slightly too high – very slightly, only 0.1 percent – but significant enough that MSL’s slowest-ever landing was even slower than we expected.56
Had the value for gravity been off by 0.1% in the other direction, the maximum design
touchdown velocity could have been exceeded, potentially damaging the mobility sys-
tem. 57 Fortunately, the error was in a safe direction, and the rover touched down on its wheels very gently at 05:17:57, or 431 seconds after entering the Martian atmosphere. At
that moment, the rover computer stopped the descent of the descent stage, and gave com-
mand of the descent stage to the descent stage thruster system computer.58 The rover commanded pyros within the bridle exit guides on the rover’s top deck to fire guillotine-like
blades that cut through the three bridle cables and the umbilical. Spring-loaded spools
within the bridle exit guides retracted the cut ends of the cables attached to the rover, and a tensioned cable that had unwound with the last few meters of the umbilical lifted the cut ends of the umbilical and bridle cables dangling from the descent stage. The Curiosity
rover was all by itself on the surface of Mars – but wasn’t yet out of danger.
The descent stage hovered for 0.7 seconds. To avoid dragging rocket exhaust across the
rover, it needed to depart the rover either forward or backward, not sideways. Because the rover was landing to the north of the eventual science target, the descent stage had been commanded to depart whichever of those two directions was the more northerly, taking it away from the likely drive direction.59 The rover knew it had landed facing east-southeast, so the descent stage pitched backward and then burned the four canted engines at full throttle for 6 seconds, sending the descent stage on a long parabolic arc away from the rover, to a crash landing 650 meters away about 20 seconds later. 60 Throughout powered descent, it had burned 270.4 kilograms of fuel, leaving 119 kilograms of usable hydrazine in the tanks during the crash.
Back on Earth, engineers were waiting for three distinct signals to confirm that the
landing had been successful and that the rover and descent stage were safely separated.
Jody Davis announced the first at 05:31:45 UTC, when she noticed that the Mars Lander
Engines had throttled down to half their former power, indicating that the descent stage
55 Way et al (2013)
56 Manning and Simon (2014)
57 Way et al (2013)
58 Baker et al (2014)
59 This was explained at the August 6, 2012 post-landing press briefing
60 Baker et al (2014)
2.4 Curiosity on Mars 101
was no longer supporting the weight of the rover: “Tango Delta nominal.” Several seconds
of quiet followed that comment, because the landing would not be over safely until the
descent stage had disconnected and flown safely away.
David Way announced the second positive landing signal when he noticed that the
Rover Inertial Measurement Unit was no longer reporting a changing position: “RIMU
stable.” The rover was therefore not being dragged by a connection to the descent stage,
nor was it sliding down a slope, or tumbling off a cliff. The third announcement came from
EDL communications enginee
r Brian Schratz, who was monitoring the strength of the
UHF radio signal between rover and orbiter, which would vary (or worse, disappear) if the
descent stage dragged the rover off the ground, or landed atop the rover. Eight seconds
after landing, he announced “UHF strong.” 61
The last two announcements collided with each other over the microphones. Adam
Steltzner walked over to Allen Chen while pointing to Schratz, asking him to repeat him-
self; “UHF strong,” Schratz said again. Steltzner tapped Chen on the shoulder and gave
him a thumbs up signal. “Touchdown confirmed,” Chen said. “Time to see where our
Curiosity will take us.” The room erupted.
2.4 CURIOSITY ON MARS
It had all gone precisely according to script. Curiosity’s landing had been targeted at
4.5965°S and 137.4019°E. The actual landing location was 4.5895°S, 137.4417°E. Curiosity
had arrived only 2385 meters away from its intended target, slightly downrange and to the
north of the center of the landing ellipse. In computer simulations of the landing, only 24%
of simulated landings got closer to the target. 62
Curiosity remained in contact with Mars Reconnaissance Orbiter and Odyssey for
another six minutes after landing. That was long enough for Odyssey to receive the first
data that Curiosity returned from the surface of Mars, and dutifully relay the images onward to Earth. As the dust settled, Curiosity snapped photos with its belly-mounted Hazcams,
giving it a fish-eye view of the ground immediately around the rover. Months prior, the mission had offered the science team a choice: receive the rear Hazcam image first, or the front one first? The mission had assumed that the scientists would want to see the forward view
first, because the view of Mars would be less obscured by hardware. The science team
replied that the first image is not about science; it’s about seeing wheels on the dirt. They requested that the rover’s first image show a wheel in contact with the ground. 63
So the first image to arrive on the computer monitors of the landing engineers, two