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This is the Timeline for MER-A, Mars Rover Spirit of the Mars Exploration Rover Mission.
During the first month of operations, announcements of Mars Exploration Rover-A Spirit achievements were made at daily press briefings at the NASA Jet Propulsion Laboratory, usually held at 09:00 PST (12:00 EST, 17:00 UTC). The briefings typically discussed activities during the most recently completed sol (a Mars solar day; see below) and plans for the upcoming sol.
 First color image from MER-A Spirit By mid February 2004, as both Spirit and its partner rover MER-B Opportunity were in regular operation, daily announcements were usually made in the form of a short release, issued shortly after the rover's latest workday was complete. Press conferences were still held but at a rate of about two per week.
The following chronology uses the dates of announcements. References to "this morning" or "tonight" are often in reference to JPL local time (PST).
Sols and Local Solar Time
A day on Mars is called a sol, and lasts 24 hours 39 minutes 35.244 seconds. The sol on which Spirit landed is designated Sol 1. The local time of the event was in the early afternoon, or at 14:34 according to the LST-A timekeeping system used by the MER team. When referring to MER-B, however, Sol 1 means its landing date, which is offset 20.5 sols behind Sol 1 of MER-A.
The Local Solar Time system used by both Spirit and Opportunity is a hybrid timekeeping system created by mission planners at JPL and is specific to each lander. The LST clock ticks away at a rate appropriate to the mean length of a Mars solar day, but the clock for each lander is offset from the mean solar time at the landing site and the zonal mean solar time. Instead, the clock for each lander was initialized with the goal that at the middle of the lander's nominal mission duration, the lander's Local Solar Time would roughly align with local true solar time. For Spirit, the approximate date of that alignment is February 19, 2004. Unfortunately, because of the way this LST timekeeping system was defined and initialized, the difference between LST-A for MER-A Spirit and LST-B for MER-B Opportunity is nearly but not exactly an integral hour; it is instead 12 plus 1 minute and 10 seconds.
- June 6 - June 8 - Two launch attempts cancelled due to weather conditions at Kennedy Space Center.
- June 8 - NASA announced that Mars Exploration Rover A will henceforth be known as "Spirit". Its companion probe MER-B will be known as "Opportunity". The names were selected by means of an essay contest open to American school children; the winning essay was submitted by Sofi Collis of Arizona.
- June 10 - Spirit was successfully launched aboard a Delta II rocket.
January 3-4 (Sol 1 ends 2004-01-04 14:15 UTC) - JPL Images from 1/3/2004 (http://www.jpl.nasa.gov/mer2004/rover-images/jan-03-2004/images-1-3-04.html)
20:35 PST - MER-A landed in Gusev crater, however not exactly at its appointed landing spot within the crater (believed by scientists to once have been a crater lake). It later became apparent that the rover had landed six miles downrange from its planned landing site, but has landed in an area possibly even more favorable for the tests scientists want the rover to perform.
A few hours after landing, first photography is received via relay through NASA's Mars Odyssey orbiter. first photography from nasa.gov (http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20040104a.html)
Three descent images showed its progress as it came down. descent images from nasa.gov (http://marsrovers.jpl.nasa.gov/gallery/press/spirit/20040104b.html).
April 5 (Sol 91)
MER-A completed 90 sols of surface operations since landing day marking the completion of the last of the official success criteria for Spirit's prime mission.
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