Mars Lander Timekeeping and the Sol Counting System
When NASA sends a spacecraft to the Red Planet, timekeeping shifts from Earth-based hours to the sol—the term used to describe a single solar day on Mars. Because a Martian day differs slightly in length from an Earth day, mission controllers use a numerical sol count to track the duration of lander operations and schedule scientific activities.
The Logic of Sol 0 vs. Sol 1
Across various NASA missions, there has been no single universal standard for when the sol count begins. Instead, the starting point—whether the day of touchdown is designated as "Sol 0" or "Sol 1"—is typically determined by the timing of the landing relative to the Martian day.
The primary goal is to ensure that Sol 1 represents the first day of meaningful or useful lander operations. If a spacecraft touches down late in the Martian day, the landing date is recorded as Sol 0. Conversely, if the touchdown occurs early in the day, it is designated as Sol 1.
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Missions Starting at Sol 0
The following projects adopted the Sol 0 convention for their touchdown date:
- The two Viking landers
- Mars Phoenix
- Mars Science Laboratory (Curiosity rover)
- InSight
- Perseverance rover
Missions Starting at Sol 1
The following missions designated their touchdown date as Sol 1:
- Mars Pathfinder
- The two Mars Exploration Rovers (Spirit and Opportunity)
Synchronization Challenges in Paired Missions
Even when NASA has deployed landers in pairs to operate simultaneously, there has been no effort to synchronize their sol counts. Because each lander begins its count based on its own specific touchdown time, their calendars can diverge significantly.
A prime example of this occurred with the Mars Exploration Rovers, Spirit and Opportunity. Although both were designed to work in tandem, each independently counted its own landing date as Sol 1. This resulted in their respective calendars being approximately 21 sols out of sync.
Key Facts
- A sol is the scientific term for one solar day on Mars.
- The choice between starting at Sol 0 or Sol 1 depends on whether the landing occurred early or late in the Martian day.
- Sol 1 is intended to be the first day of "meaningful" operations.
- Paired missions, such as Spirit and Opportunity, do not synchronize their sol counts.
- The Spirit and Opportunity rovers operated with calendars roughly 21 sols apart.
| Touchdown Designation | Associated NASA Missions |
|---|---|
| Sol 0 | Viking landers, Mars Phoenix, Curiosity, InSight, Perseverance |
| Sol 1 | Mars Pathfinder, Spirit, Opportunity |
Frequently Asked Questions
What is a sol?
A sol is a solar day on Mars, used by spacecraft and scientists to track time on the Martian surface.
Why do some missions start at Sol 0 and others at Sol 1?
The designation depends on the time of day the lander touched down; the count is set so that Sol 1 marks the first day of useful operations.
Do paired Mars rovers use the same calendar?
No, NASA does not synchronize sol counts between paired landers, meaning they may have different calendars despite operating at the same time.
How far apart were the calendars of Spirit and Opportunity?
The calendars of the Spirit and Opportunity rovers were approximately 21 sols out of sync.
Which recent rovers used the Sol 0 starting point?
The Curiosity and Perseverance rovers, along with the InSight lander, all counted their touchdown as Sol 0.