Emily Lakdawalla
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Earth time schedule. 1 Curiosity followed in their tracks, but the complexity of its instrument package required some changes to the Spirit and Opportunity way of doing things.
3.2 MARS’ CALENDAR
3.2.1 Mars sols and seasons
Mars missions are bound to the same kinds of celestial cycles that dictate Earth’s 24-hour
days, 365-day years, and four seasons. When it comes to days and seasons, Mars has some
coincidental similarities to Earth. Mars’ solar days are just 3% longer than Earth days,
being 24 hours 39 minutes 35.244 seconds long, on average. To differentiate Earth days
from Mars days, one Mars solar day is called a “sol”, a term coined during the Viking
1 Bass et al (2005)
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https://doi.org/10.1007/978-3-319-68146-7_3
3.2 Mars’ Calendar 111
missions. 2 Because of the similarity in day length, Mars landed mission time can be run on a 24-hour clock, with hours split into 60 minutes of 60 seconds each, just like on Earth.
Mars seconds are 1.0275 Earth seconds long.
Mars’ axial tilt is also similar to Earth’s at 25.2°, resulting in Earth-like seasons of
spring, summer, fall, and winter. Curiosity landed 4.6° south of the equator, so the south-
ern hemisphere seasons are relevant. Scientists measure the seasons on Mars using solar
longitude, abbreviated Ls (pronounced “ell sub ess”). Ls is 0° at the southern autumnal
equinox (beginning of northern spring), 90° at southern winter solstice, 180° at southern
vernal equinox, and 270° at summer solstice.
Mars’ years last 687.9726 Earth days or 668.5921 Mars sols. Because Mars’ orbit is
eccentric, Mars’ distance to the Sun varies over the course of a year: it is 206.62 million kilometers from the Sun at perihelion, but 249.23 million kilometers away at aphelion.
The difference in distance means the Sun is 45% brighter at perihelion than at aphelion.
Aphelion happens at Ls=70°, just before southern winter solstice (Figure 3.1).
Figure 3.1. Geometry of Mars’ orbit. See text for discussion.
2 Reichardt (2015)
112 Mars Operations
Table 3.1. Mars seasons relevant to the Curiosity mission. Data from Cantor et al (2010 ).
Northern spring/
Northern summer /
Northern autumn /
Northern winter/
Mars
southern autumn
southern winter
southern spring
southern summer
year
equinox (Ls = 0°)
solstice (Ls = 90°)
equinox (Ls = 180°)
solstice (Ls = 270°)
31
Sep 13 2011
Mar 30 2012
Sep 29 2012 / sol 53
Feb 23 2013 / sol 196
32
Jul 31 2013 / sol 350
Feb 15 2014 / sol 543
Aug 17 2014 / sol 722
Jan 11 2015 / sol 865
33
Jun 18 2015 / sol 1019
Jan 03 2016 / sol 1212
Jul 04 2016 / sol 1390
Nov 28 2016 / sol 1533
34
May 05 2017 / sol 1687
Nov 20 2017 / sol 2059
May 22 2018
Oct 16 2018
35
Mar 23 2019
Oct 08 2019
Apr 08 2020
Sep 02 2020
36
Feb 07 2021
Aug 25 2021
Feb 24 2022
Jul 21 2022
37
Dec 26 2022
Jul 12 2023
Jan 12 2024
Jun 07 2024
38
Nov 12 2024
May 29 2025
Nov 29 2025
Apr 25 2026
39
Sep 30 2026
Apr 16 2027
Oct 17 2027
Mar 12 2028
40
Aug 17 2028
Mar 03 2029
Sep 03 2029
Jan 28 2030
Planets move faster when close to perihelion than they do when near to aphelion, and
the difference is stark for Mars. Winters in Mars’ southern hemisphere, which begin near
aphelion, are long and cold under a fainter, distant Sun. Summers are short and hot, with a big Sun overhead. Autumn is the longest season in the southern hemisphere, at 194 sols;
winter has 178 sols; spring has 142 sols; and summer has 154 sols.
To discuss the passage of years, Mars atmospheric scientists have settled upon a con-
vention first defined by Bruce Cantor, Philip James, and Wendy Calvin in a 2010 paper.
Mars years begin at Ls=0°, and the beginning of Mars year 1 on April 11, 1955. The choice
of year 1 is, of course, arbitrary, but it is about one Mars year before the Space Age began
with the launch of Sputnik in 1957.3 Curiosity landed at Ls=151° of Mars year 31, after the coldest part of winter had passed, headed into spring. Years and seasons relevant to the
Curiosity mission are listed in Table 3.1.
3.2.2 Mars solar time
Curiosity may be nuclear-powered but needs sunlight to take photos and warm its motors,
so solar time rules its activities. Keeping track of sunrise and sunset times has to take into account Mars’ orbital eccentricity, which makes the solar day length vary slightly over the course of the year. To simplify timekeeping, there is a defined convention of local mean
solar time (LMST), counting time in evenly advancing increments. It approximates local
true solar time (LTST). Mean and true solar time are identical close to aphelion and peri-
helion, at Ls=57.7° and 258.0°. They are most dissimilar at Ls=187.9°, when mean solar
time runs behind true solar time by 39.9 minutes, and at Ls=329.1°, when mean solar time
runs ahead of true solar time by 51.1 minutes (Figure 3.2).4 Curiosity (like all landed Mars missions) operates according to a mean solar time clock so that every sol is of the same
3 Allison (1997)
4 Allison (1997)
3.2 Mars’ Calendar 113
Figure 3.2. Difference between mean and true solar time as a function of solar longitude.
Emily Lakdawalla after Allison ( 1997 )
length, but tracking local true solar time is very important for thermal control and camera systems. 5 In this book I’ll generally refer to the two interchangeably as “local time,” differentiating between local mean and local true solar time only if the situation requires it.
3.2.3 Rover timekeeping
One way the mission measures time is with the spacecraft clock, which counts time in
seconds. Landing happened at a spacecraft clock time of 397502503. The mission calen-
dar is reckoned in sols, counting up from landing day on sol 0.
The rover motion counter (RMC) is another way of ordering rover activities, including
science, in time. The rover motion counter comprises 10 integer indices, keeping track of
when various rover motors have operated. In order, the 10 indices are site, drive, pose, arm, CHIMRA, drill, mast, high-gain antenna, brush, and inlet covers. The site index
5 Local Mean Solar Time is defined for a fixed longitude on the surface. For Curiosity, that longitude was defined before landing to be 137.42°E. Curiosity’s mission time does not shift with Curiosity’s changing longitude. For every 246 meters that Curiosity drives west of the initial landing position, the Sun rises 1 second later than it does at the longitude of the landing site. Curiosity actually landed at 137.441635°E, which meant that
there was about a 4-second difference between Curiosity mission time and the Local Mean Solar Time – not enough of a difference to make it worth it to adjust the software, especially because this tiny difference is swamped by the variations in sun rise/set times caused by the difference between True and Mean Solar Time.
114 Mars Operations
increments every time the rover updates its knowledge of its geographic location, so has
been counting up since the beginning of the mission. Curiosity increments the site index
when it updates its knowledge of its orientation by sighting the Sun with the Navcam (see
section 6.5.1); incrementing the site index sets all other indices to 0. The drive index increments every time the rover rolls or steers its wheels. Incrementing the drive index sets all other indices except site to 0. All science data with the same site/drive index is from the same geographic location. The rest of the indices increment whenever the relevant motor
is operated, and help to place activities in order at a specific site/drive location.
3.3 STRATEGIC, SUPRATACTICAL, AND TACTICAL PLANNING
Curiosity is far more capable than its predecessors, especially when it comes to drilling
and sampling, but there are no more hours in the day for Curiosity operations to be planned than there were for Spirit and Opportunity. There are 10 science instruments, and more
than 400 scientists, and planning has to be mindful of the needs of multi-sol campaigns.
Performing daily operations with a complex rover while keeping eyes set on long-term
goals is difficult for such an unwieldy team.
Many different factors limit what the rover can accomplish in a given sol. Power is one:
most activities draw power from the batteries at a faster rate than the MMRTG can replen-
ish it, but for safety reasons the tactical team is usually required to leave the rover batteries nearly fully charged at the end of each sol’s plan. Communications are another serious
bottleneck: the rover can capture far more data than there is capacity to transmit it to Earth.
But at the beginning of the mission, the most stringent limit on Curiosity’s activities was imposed by the sheer complexity of the machine.
To make it all work, Curiosity mission operations are planned on four different times-
cales, with all four working in parallel:
• The Project Science Group (PSG), a committee consisting of the project scientist
(John Grotzinger at landing, and later Ashwin Vasavada), NASA Mars Program
Scientist Michael Meyer, and the principal investigators of the science instruments,
establishes the overarching scientific questions motivating Curiosity’s mission, and
determines the very long-term driving destinations. The goals, questions, and des-
tinations were established in the mission proposal and subsequent extended- mission
proposals.
• The strategic or long-term planning process addresses development and testing of first-time activities, planning science and sampling campaigns, and long-term management of rover resources. Long-term planners map out a “sol path” covering
several sols, a high-level list of activities that directs the mission toward accom-
plishing the Project Science Group’s goals. Strategic planning works on week- to
months-long time scales, and is mostly conducted on Earth time.
• The plans needed to implement the strategic plans are developed first in a “supratac-
tical” planning process. One to several sols ahead of time, the supratactical team
sequences the “look-ahead plan”, beginning to sketch out the actual list of com-
mands to be sent to the rover. At the beginning of the mission, the supratactical
process was conducted on Mars time.
3.4 Tactical Planning Process 115
• Finally, the tactical team produces each sol’s plan. The supratactical team hands the tactical team an outline of the activity plan for that sol, called a skeleton plan,
along with guidelines for what the rover needs to accomplish to stay on the look-
ahead plan. Each skeleton plan contains science blocks, periods of time during
which the tactical team can add in science observations, resource limitations per-
mitting. The tactical team responds to data downlinked from the rover each sol,
fleshes out the plan handed to them by the supratactical team, and generates com-
mands to uplink to the rover for the next sol. Tactical planning was conducted on
Mars time when the mission began, with the tactical planning timeline taking 1 sol,
operating every day of the Earth week, around the clock.6
Curiosity differs from its predecessors Spirit and Opportunity in having the supratacti-
cal planning process, which is necessary because of Curiosity’s complexity and the inten-
sive resource demands of its analytical instruments, CheMin and SAM. In parallel with
strategic planning, the supratactical process takes care of the negotiations among different instruments for rover resources, assigning activities to different sols to balance out
demands.
3.4 TACTICAL PLANNING PROCESS
3.4.1 Mars time operations
Since the rover needs a plan for each sol that responds to what happened the previous sol,
the ideal way to operate Curiosity is to begin planning the next sol at the end of each active sol on Mars. When the mission operated on Mars time, the planners worked over the rover’s night to deliver a new tactical plan at around 10:00 local time each rover morning.
Late in the afternoon (between 3:00 and 6:00 p.m. local time), both Mars Odyssey and
Mars Reconnaissance Orbiter fly over the landing site. (Read more about rover telecom-
munications in section 4.5.) Colloquially, a communications session is referred to as a
“pass”, because the orbiter is passing across the rover’s sky as the rover uplinks its data.
The orbiters relay the data onward to Deep Space Network radio dishes on Earth. The last
orbiter communications session before Earth planning begins is called the “decisional data
pass” because it is the last pass containing data that Earth planners can use to make deci-
sions about rover activities. Decisional data includes telemetry on the health and safety of the rover, and Hazcam and Navcam images that can be used to build a terrain mesh, a 3D
map of the terrain around the rover.
The rest of the data comes down in priority order. The tactical team carefully assigns
priority to every data product that they command the rover to produce. They assign highest
priority to science data that is beneficial for planning – such as Mastcam images of the area that the arm instruments can reach, or ChemCam can zap; these usually arrive quickly.
Other data may come down days later. The Mastcams, in particular, generate huge vol-
umes of data and can store it for months inside large flash memory drives within each
instrument’s electronics. Low-priority Mastcam data can easily sit on the rover for a year
6 Chattopadhyay et al (2014)
116 Mars Operations
before downlink. The mission always tries to keep some volume of data in memory so that
if an onboard anomaly prevents new activities, all available downlink sessions can be used
to downlink science data. The team makes all these priority assignments during tactical
planning, but can also reprioritize data still on the rover to force it to return to Earth sooner or later, as appropriate.
On Earth, a downlink team of instrument scientists and rover engineers studies the
downlinked data to assess rover health and suggest a set of activities. Responsibility shifts to an uplink team, w
hich includes representatives of every science instrument as well as
rover planners (also known as rover drivers). The uplink team generates a command
sequence and sends it to the rover. Usually the Deep Space Network transmits the sequence
directly to the rover’s steerable high-gain antenna around 10:00 a.m. local solar time, after the Sun has warmed the rover slightly, in time for it to begin its next sol of operations on Mars.
Operating on Mars time presents two main challenges. One: the full cycle from data
downlink to sequence uplink has to be completed within about 16 hours, over the rover’s
night; if the sequence doesn’t get prepared by the end of that time, they miss their uplink window and lose a whole sol of activity on Mars. Two: Mars time and Earth time are not
the same. To work on Mars time is to begin the planning day 39 minutes later each day. In
38 Earth days, there are 37 Mars sols. If the Earth and Mars schedules are perfectly aligned one day, then, 19 days later, the two schedules are perfectly out of sync, and operating on Mars time requires working through the Earth night.
For the first 90 sols after landing, the mission operated on Mars time, through nights
and weekends, with the whole science team co-located with the engineers at the Jet
Propulsion Laboratory (JPL) in Pasadena, California. Mars time helped the engineers
maintain a tight connection with the rover as they commissioned all its instruments and
tools, and permitted them to use all of the 16-hour rover night to prepare each sol’s worth of activities. But Mars time is grueling for humans, whose circadian rhythms and private
lives still run on Earth time.
3.4.2 Slide sols, restricted sols, and solidays
After sol 90, the science team members returned to their home institutions and the mission
transitioned to Earth time. They had increased planning efficiency to the point that one
sol’s worth of activities could be developed in a 9-hour planning day. Earth time opera-
tions are permitted to take place between the hours of approximately 6:00 a.m. to 10:00
p.m. California time. So in a day where uplink needed to take place no later than 4 p.m.
California time, they could begin the planning day at 7 a.m., an “early slide sol.” In this way the team could operate as though they were on Mars time for about half of the sols, as