Emily Lakdawalla
Page 11
ent ways of measuring timefer
4:57:00
5:00:46
5:01:46
5:02:37
5:02:39
5:10:46
5:11:31
5:11:49
5:11:58
5:12:06
5:12:19
5:12:49
5:13:01
5:14:28
5:14:45
5:14:59
5:15:05
5:15:14
5:15:24
5:15:43
5:15:45
s, when the spacecr
ime (UTT
ding to dif
OS
vated
A
yment
ence point to Mar
efer
OS
yment
OS = acquisition of signal. LOS = loss of signal.
y A
A
ace
versal 1
versal 2
versal 3
, descent, and landing accor
Odysse
ent
ent Cruise Heat Rejection System
ait for parachute deplo
Ev
V
Cruise stage separation
EDL guidance & control acti
Cruise balance mass jettison
Mars Reconnaissance Orbiter
Entry interf
Guidance start
Peak heating
Bank re
Peak deceleration
Bank re
Bank re
Heading alignment
Mars
Entry balance mass jettison
W
Parachute deplo
Last MEDLI measurement
Heat shield separation
Radar lock
End direct-to-Earth transmission
vents in entry
Timeline of e
ait for Guidance Start
Table 2.2.
time that the computer updated its navigational r
interface time to be 540 seconds after that.
Phase
Approach
Approach
Approach
Approach
Approach
W
Range Control
Range Control
Range Control
Range Control
Range Control
Range Control
Heading alignment
Heading alignment
Straighten Up and Fly Right
Straighten Up and Fly Right
Parachute descent
Parachute descent
Parachute descent
Parachute descent
Parachute descent
2.3 EDL: Entry, Descent, and Landing 69
S2016
S2016
S2016
L2016
K2014
K2014
K2014
K2014
S2014
S2014
S2014
K2014
S2016
K2014
K2014
L2016
L2016
W2013
W2013
397502048.48
397502068.35
397502071.18
397502074.00
397502074.63
397502090.92
397502093.38
397502127.89
397502128.59
397502132.89
397502133.89
397502136.86
397502145.90
397502146.52
397502147.31
397502189.07
397502306.50
397502502.00
397502503.00
333.482
353.353
356.178
359
359.63
375.92
378.38
412.89
413.59
417.89
418.89
421.86
430.9
431.52
432.31
474.07
591.50
787
788
, personal communication, email dated February 17, 2016; Sc2014:
873.482
893.353
896.178
899
899.63
915.92
918.38
952.89
953.59
957.89
958.89
961.86
970.9
971.52
972.31
1014.068662
1131.501662
1327
1328
5:16:19
5:16:39
5:16:42
5:16:45
5:16:45
5:17:01
5:17:04
5:17:38
5:17:39
5:17:43
5:17:44
5:17:47
5:17:57
5:17:57
5:17:58
5:18:40
5:20:37
5:23:53
5:23:54
); L2016: calculated by me from MARDI image time stamps; MC2014: Mendeck and Craig
2014
); S2016: Christian Schaller
2014
utty (
ets
al (
wn
aard and K
y
y LOS
).
); S2014: Sell et
wn sensed
2013
er deplo
ayw
2016
al (
wered approach
y crane start
ver reaches end of bridle
al (
HiRISE image start
HiRISE image heat shield
HiRISE image lander
Heat shield impact (approx.)
Prime descent engine rock
Backshell separation
Po
Sk
Rock
Ro
Bogie release
Ready for touchdo
HiRISE image end
Touchdo
Flya
First rear Hazcam image
First front Hazcam image
Mars Odysse
Mars Reconnaissance Orbiter LOS
ay et
vak et
W
W2013: );
); N2016: No
2014
2014
al (
w (
wered descent
wered descent
wered descent
wered descent
wered descent
wered descent
wered descent
Parachute descent
Parachute descent
Parachute descent
Parachute descent
Parachute descent
Parachute descent
Po
Po
Po
Po
Po
Po
Po
Landing
Landing
Landing
Landing
Landing
Landing
*Sources for the data in the table are: K2014: Karlg
McGre
Schratz et
70 Getting to Mars
Figure 2.7. Entry, descent, and landing trajectory. Base image is from Viking global mosaic, trajectories from JPL Horizons.
2.3.1 Telecommunications during landing
Ever since the loss of Mars Polar Lander, NASA has required Mars landers to be in con-
stant communication with Earth during the dramatic and risky events of entry, descent, and
landing. On the day of MSL’s landing, Mars and Earth were separated by about 248.2
million kilometers, or 828.0 light-seconds. The entire entry, descent, and
landing took
only 432 seconds from start to finish. There is nothing that anyone on Earth could do to
rescue a mission should something go wrong; instead, any telemetry received would serve
to help engineers determine the cause of a landing failure, with the goal of preventing a
future one. MSL was required to transmit all highest-priority data within 3 seconds of the
event it recorded. That way, some information could be salvaged from a catastrophic acci-
dent to improve future missions.19
Ideally, the spacecraft would use a single radio configuration for communications
throughout entry, descent, and landing. But the MSL landing sequence had the spacecraft
reconfiguring itself multiple times, throwing away hardware on which antennas were
mounted. To handle communications, MSL had to switch among multiple radio systems
and antennas.
There were two primary X-band radio systems for communicating directly with Earth,
one within the rover (still used now for surface operations) and one attached to the descent stage. During cruise, the descent stage X-band system handled communications through a
19 The details of EDL telecommunications in this section are based on Schratz et al (2014)
2.3 EDL: Entry, Descent, and Landing 71
medium-gain antenna mounted to the cruise stage. During entry and descent, the descent
stage communicated with Earth through two low-gain antennas mounted on the back-
shell’s parachute cone, beginning with the parachute low-gain antenna and later switching
to the tilted low-gain antenna (Figure 2.10). The signal from such small antennas, 112
million kilometers from Earth, was weak, so they transmitted no telemetry. Rather, they
broadcast 11-second-long “tones,” signals with frequencies slightly offset from the main
carrier frequency, with different frequencies signifying different events. Events during the cruise, approach, and guided-entry phases were separated far enough in time that
11- second-long signals were good enough to communicate the spacecraft’s status, but
after that, MSL needed higher-rate communications. When events did overlap in time,
complicated logic governed which tone would play first:
If additional tone events occurred while one tone was playing, the new event was
queued until the currently playing tone had completed. Then, the queued tone
played. Each tone had a defined priority. Nominal tones were generally prioritized
lower than tones indicating faults or specific critical events during EDL. Of the
available tone events, a small subset were labeled as “stomping tones,” which inter-
rupted a currently playing tone, causing the interrupted tone to be re-queued by
flight software to replay when time permits. In the event of multiple queued tones,
the highest priority tones were played first. In the event of tones with the same prior-
ity, the most recent tones were played first, because the newest information during
EDL was generally favored over stale information. Because of this software logic,
the time between a tone event occurring and when it actually was radiated varied by
several seconds, and some tones appeared to play out of order. Although this made
real-time operations more complicated, it was the preferred strategy to enhance the
probability of receiving indications of off-nominal behavior in the event of a fault…
Parachute deploy and touchdown tones [were] carrier-only tone, where no subcar-
rier modulation is used. 20
For transmitting telemetry during descent and landing, MSL used the Electra-Lite UHF
radio within the rover, broadcasting to receivers on Mars orbiters. It transmitted through
three different low-gain UHF antennas at different times: one on the parachute cone of the
backshell, one mounted to the top of the descent stage, and finally the rover’s helix antenna, the one that it still uses for surface operations (see section 4.5). Amazingly, during entry, descent, and landing, the Parkes radio telescope back on Earth, in Australia, was able to
pick up the signal from MSL’s UHF antennas. While the signal was too weak for Parkes to
decode any telemetry, it did provide Doppler information as the spacecraft decelerated
toward landing.
Telemetry arrived on Earth with the assistance of orbital relays. Odyssey provided the
primary conduit for real-time communication. It served as a bent-pipe relay: it received
MSL’s signals, demodulated them, and immediately sent the decoded telemetry to Earth.
The Odyssey relay arrived only seconds slower than the direct-to-Earth tones. Mars
Reconnaissance Orbiter recorded MSL’s UHF signals in an “open-loop” mode, without
demodulating them. This recording wasn’t available on Earth until hours after landing, but
20 Schratz et al (2014)
72 Getting to Mars
would have provided more data if an anomaly happened that caused Odyssey to lose lock
on the signal. The European Space Agency’s Mars Express also listened for MSL’s signal.
It operated in an open-loop carrier-only detection mode, which didn’t record telemetry but
provided an alternate angle for Doppler tracking relative to that recorded from Earth.
2.3.2 The aeroshell and MEDLI
MSL’s was the most challenging Mars atmospheric entry in history, for two main reasons.
MSL was dramatically larger than any previous landed Mars spacecraft, and its goal was
a much more precise landing than previously attempted (Figure 2.8). The aeroshell was 4.5 meters in diameter, the heat shield a 70° cone (Figure 2.9). The heat shield’s shape was the same as for all previous Mars landers, but Curiosity’s aeroshell was a meter larger and more than three times heavier than any previous one. 21 In a throwback to Viking, the aeroshell was able to generate lift. Parts of the backshell and heat shield are labeled in
Figure 2.10 and Figure 2.11. The backshell with parachute and balance masses weighed 576.6 kilograms. 22 The heat shield weighed 440.7 kilograms.
Figure 2.8. Comparison of NASA Mars aeroshells. Emily Lakdawalla after Edquist et al (2009) and Wallace (2012 ).
21 Edquist K et al (2009)
22 Allen Chen, personal communication, email dated July 1, 2016, correcting numbers published before the launch
2.3 EDL: Entry, Descent, and Landing 73
Figure 2.9. Top: Aeroshell dimensions. The aeroshell consists of the backshell and heat shield. Bottom: geometry of the aeroshell during guided flight. Images: NASA/JPL-Caltech.
Top diagram based on Karlgaard et al ( 2014 ). Bottom based on Steltzner et al (2010 ).
The heat shield gathered an unprecedented amount of information about the descent
through the Martian atmosphere, thanks to the MSL Entry, Descent, and Landing
Instrumentation (MEDLI) sensors embedded within it. MEDLI had two kinds of sensors.
74 Getting to Mars
Figure 2.10. Parts of the MSL backshell. NASA/KSC image releases KSC-2011-4526 and KSC-2011-7183, annotated by Emily Lakdawalla.
Seven transducers of the Mars Entry Atmospheric Data System (MEADS) measured
atmospheric pressure by tiny 2.5-millimeter through-holes in the shield. Seven MEDLI
Integrated Sensor Plugs (MISP) were embedded in the heatshield within 33-millimeter-
diameter plugs. They consisted of thermocouples to measure temperature and recession
sensors to document how the PICA material weathered entry. Locations of the MEDLI
sensors are shown in Figure 2.11 and Figure 2.12. 23
23 Little et al (2013)
2.3 EDL: Entry, Descent, and Landing 75
Figure 2.11. Parts of the MSL heat shield. Top: exterior surface of the heat shield, with locations of the MEDLI Integrated Sensor Plugs marked. Bottom: interior surface of the heat shield. NASA/JPL-Caltech/Lockheed Martin image release PIA14128, taken at Lockheed
Martin Space Systems, Denver, in April 2011, annotated by Emily Lakdawalla.
76 Getting to Mars
Figure 2.12. Locations of the MEDLI MEADS (orange) and MISP (white) sensors on the MSL heat shield. Flow lines show the direction of expected air flow. Based on Little et al (2013 ) and Bec k et al ( 2010 ).
2.3.3 Final approach
Ten minutes before entry, at 5:00:46 Spacecraft Event Time on August 6, 2012, the cruise
stage separated, its work complete. Later images from Mars Reconnaissance Orbiter
HiRISE and CTX instruments show numerous impact craters from the cruise stage scat-
tered over a strewnfield 12 kilometers long, indicating that the cruise stage – unprotected by an aeroshell – broke up in the atmosphere (Figure 2.13).24 The cruise stage took with it MSL’s star trackers. From that point on, the rover computer would maintain its sense of its own orientation by dead reckoning. MEDLI began to acquire data from the heat shield.
24 McEwen A (2012)
2.3 EDL: Entry, Descent, and Landing 77
Figure 2.13. Impact sites of the cruise balance masses and fragments of the cruise stage. At about 4 meters in diameter, the two largest craters are probably the cruise balance mass impact sites. All the other, smaller impacts are likely from fragments of the cruise stage.
HiRISE images ESP_029245_1755 and ESP_029601_1755. NASA/JPL-Caltech/UA.
78 Getting to Mars
Nine minutes prior to entry, the guidance, navigation, and control system activated, and
the rover computer fed it the navigators’ best estimate of the spacecraft’s position and
velocity. (Many publications about the landing refer to this moment, 540 seconds before
entry, or 397501174.997338 seconds on the spacecraft clock, as “t0” for the landing phase, while others use the moment of entry as the zero point.) The spacecraft stilled its rotation and oriented to the correct angle for hitting the top of the atmosphere. It ejected two 75-kilogram blocks of tungsten, the cruise balance masses, which went on to impact the surface
close to the cruise stage (Figure 2.13). The sudden loss of 150 kilograms of mass offset the capsule’s center of mass away from its centerline. Once the capsule was in the atmosphere,
this offset gave it a 16° angle of attack. The capsule was ready to fly in the Martian air.
MSL switched X-band antennas, now broadcasting tones from the tilted low-gain
antenna, which was pointed 17.5° away from the aeroshell’s axis of symmetry (Figure 2.10).
The switch of antennas caused only a very brief loss of communication with the
spacecraft.
As MSL approached Mars, Mars Reconnaissance Orbiter approached the equator from