Mission Overview
================
The Viking mission to Mars consisted of four spacecraft: the
two orbiters VO1 and VO2, and the landers VL1 and VL2
[SOFFEN1977]. During cruise to Mars the landers were attached
to the orbiters; the combined spacecraft were then known as
Viking 1 and 2. The role of the orbiters was to transport the
landers to Mars, to carry reconnaissance instruments for
certifying the landing sites, to act as relay stations for
lander data, and to perform their own scientific
investigations. The initial orbit periapses were placed over
the candidate landing sites to allow for maximum viewing
resolution and relay of the lander data. After the primary
lander missions were completed, the orbiters' orbits were
allowed to drift so that the entire planetary surface could be
systematically mapped by the three remote sensing experiments.

The Viking 1 spacecraft was launched August 20, 1975, and
arrived at Mars on June 19, 1976. Lander 1 was deployed to the
Mars surface on July 20, 1976. The VO1 orbital inclination of
38-39 degrees was chosen to optimize communication with VL1.
Viking 2 was launched September 9, 1975 and arrived at Mars
August 7, 1976. VL2 landed on September 3, 1976, at a more
northerly site than VL1. The VO2 orbit was correspondingly
more inclined than VO1; initially 55 degrees, it was later
adjusted to 80 degrees, providing particularly good coverage
of polar regions. The areocentric locations of VL1 and VL2
have since been determined to be (22.270N, 48.264W) and
(47.669N, 226.032W), respectively [YODER&STANDISH1997].


Mission Phases
==============
The timeline for the Viking Mission is divided into a number of
mission phases in terms of the types of observations and level
of activity. The references [SNYDER1977], [SNYDER1979],
[MOOREETAL1987], and [SNYDER&MOROZ1992] provide detailed
descriptions of these mission phases. A summary of the mission
phases and the relevant dates are described below. Before Mars
encounter and orbit insertion the orbiter and lander spacecraft
are considered as one spacecraft with the same mission phases.
The primary missions for all four spacecraft (VL1, VO1, VL2,
and VO2) are listed separately because each has a different
starting date. All mission phases after the primary mission
are listed only once because all four spacecraft operated
together.


VIKING 1 MARS LAUNCH
--------------------
The Viking 1 spacecraft was launched on August 20, 1975 on a
Titan Centaur 3 booster from Cape Canaveral, Florida.

Spacecraft Id: : VO1, VL1
Target Name : MARS
Mission Phase Start Time : 1975-08-20
Mission Phase Stop Time : 1975-08-20
Spacecraft Operations Type : ORBITER, LANDER


VIKING 1 MARS CRUISE
--------------------
The Viking 1 spacecraft, consisting of the VO1 orbiter and
VL1 lander, cruised to Mars for about 10 months, during which
time the spacecraft was checked periodically.

Spacecraft Id: : VO1, VL1
Target Name : MARS
Mission Phase Start Time : 1975-08-20
Mission Phase Stop Time : 1976-06-19
Spacecraft Operations Type : ORBITER, LANDER


VIKING 2 MARS LAUNCH
--------------------
The Viking 2 spacecraft was launched on September 9, 1975 on
a Titan Centaur 3 booster from Cape Canaveral, Florida.

Spacecraft Id: : VO2, VL2
Target Name : MARS
Mission Phase Start Time : 1975-09-09
Mission Phase Stop Time : 1975-09-09
Spacecraft Operations Type : ORBITER, LANDER


VIKING 2 MARS CRUISE
--------------------
The Viking 2 spacecraft, consisting of the VO2 orbiter and
VL2 lander, cruised to Mars for about 11 months, during which
time the spacecraft was checked periodically.

Spacecraft Id: : VO2, VL2
Target Name : MARS
Mission Phase Start Time : 1975-09-09
Mission Phase Stop Time : 1976-08-07
Spacecraft Operations Type : ORBITER, LANDER


VIKING ORBITER 1 PRIMARY MISSION
--------------------------------
The Viking Orbiter 1 spacecraft entered Mars orbit on June
19, 1976. Operations commenced by supporting the selection
of a landing site for VL1. Throughout the Primary Mission,
the VO1 spacecraft supported communications with the landers
and made observations of the Martian surface and atmosphere.
The Primary Mission ended at the start of the solar
conjunction in November, 1976.

Spacecraft Id: : VO1
Target Name : MARS
Mission Phase Start Time : 1976-06-19
Mission Phase Stop Time : 1976-11-15
Spacecraft Operations Type : ORBITER


VIKING LANDER 1 PRIMARY MISSION
-------------------------------
The Viking Lander 1 spacecraft separated from the VO1 orbiter
and descended to the Martian surface on July 20, 1976. The
Primary Mission focused on the collection and analysis of
soil samples and the characterization of the landing site and
atmosphere. The Primary Mission ended at the start of the
solar conjunction in November, 1976.

Spacecraft Id: : VL1
Target Name : MARS
Mission Phase Start Time : 1976-07-20
Mission Phase Stop Time : 1976-11-15
Spacecraft Operations Type : LANDER


VIKING ORBITER 2 PRIMARY MISSION
--------------------------------
The Viking Orbiter 2 spacecraft entered Mars orbit on August
7, 1976. Operations commenced by supporting the selection of
a landing site for VL2. Throughout the Primary Mission, the
VO2 spacecraft supported communications with the landers and
made observations of the Martian surface and atmosphere. The
Primary Mission ended at the start of the solar conjunction
in November, 1976.

Spacecraft Id: : VO2
Target Name : MARS
Mission Phase Start Time : 1976-08-07
Mission Phase Stop Time : 1976-11-15
Spacecraft Operations Type : ORBITER


VIKING LANDER 2 PRIMARY MISSION
-------------------------------
The Viking Lander 2 spacecraft separated from the VO2 orbiter
and descended to the Martian surface on September 3, 1976.
The Primary Mission focused on the collection and analysis of
soil samples and the characterization of the landing site and
atmosphere. The Primary Mission ended at the start of the
solar conjunction in November, 1976.

Spacecraft Id: : VL2
Target Name : MARS
Mission Phase Start Time : 1976-09-03
Mission Phase Stop Time : 1976-11-15
Spacecraft Operations Type : LANDER


VIKING EXTENDED MISSION
-----------------------
The Viking Extended Mission began after solar conjunction.
The two orbiters continued to observe the surface and
atmosphere of Mars. The two lander spacecraft analyzed
additional soil samples and dug three deep holes in the
surface. All four spacecraft monitored the planet through
the cycle of seasons. During the winter season, the landers
operated in an automatic manner designed to allow the
spacecraft to survive the cold temperatures and still return
some data.

Spacecraft Id: : VO1, VL1, VO2, VL2
Target Name : MARS
Mission Phase Start Time : 1976-11-15
Mission Phase Stop Time : 1978-05-31
Spacecraft Operations Type : ORBITER, LANDER


VIKING CONTINUATION MISSION
---------------------------
Primary objectives of the Continuation Mission were to make
orbital observations at times of the Mars year that were
missed due to landing site selection and solar conjunction
and to collect high resolution surface images when the
atmosphere was clear. A radio science solar conjunction
relativity experiment was also done during the Continuation
Mission. Lander activities consisted of measurements by the
imaging, meteorology, and XRFS instruments operating in a
fully automated manner. Viking Orbiter 2 developed a leak in
its propulsion system and lost its attitude control gas. VO2
was turned off on July 25, 1978 after 706 orbits around Mars.

Spacecraft Id: : VO1, VL1, VO2, VL2
Target Name : MARS
Mission Phase Start Time : 1978-05-25
Mission Phase Stop Time : 1979-02-26
Spacecraft Operations Type : ORBITER, LANDER


VIKING INTERIM PERIOD
---------------------
The Interim Period mission phase occurred during the time of
the Voyager 2 encounter with Jupiter. Thus, communications
to and from the Viking spacecraft were limited. The landers
continued to operate in an automated manner making imaging
and meteorology observations. A final VL2 surface sampler
sequence was conducted during this mission phase as an
engineering test in the cold temperatures of mid winter.
Orbital data stored on spacecraft tape recorders and not
returned during the Continuation Mission were downlinked
during the Interim Period.

Spacecraft Id: : VO1, VL1, VL2
Target Name : MARS
Mission Phase Start Time : 1979-02-26
Mission Phase Stop Time : 1979-07-19
Spacecraft Operations Type : ORBITER, LANDER


VIKING SURVEY MISSION
---------------------
The prime scientific objective for VO1 during the Survey
Mission was to obtain high resolution images of possible
future landing sites. The plan for the landers was to
collect image and meteorology data for as long as possible.
Because VL2 no longer had a direct downlink capability, it
meant that VL2 could return data only as long as VO1 provided
a relay link, once every seven weeks. Communications with
VL2 ended on April 11, 1980 after its batteries could no
longer hold a charge. VL2 operated on the surface of Mars
for 1281 sols. VO1 consumed the last of its attitude control
gas on August 7, 1980 and was turned off after 1485 orbits
around Mars.

Spacecraft Id: : VO1, VL1, VL2
Target Name : MARS
Mission Phase Start Time : 1979-07-19
Mission Phase Stop Time : 1980-08-07
Spacecraft Operations Type : ORBITER, LANDER


VIKING COMPLETION MISSION
-------------------------
Viking Lander 1 continued to operated in its automatic mode
during the Completion Mission. The observation sequences
were cyclic. VL1 returned via direct downlink image and
meteorology data about once a week with image sequences
repeating every 37 sols. The VL1 high-gain antenna was
programmed to track the Earth until December, 1994. However,
communications were lost in November 1982 after a command
sequence uplink.

Spacecraft Id: : VL1
Target Name : MARS
Mission Phase Start Time : 1980-08-07
Mission Phase Stop Time : 1982-11-19
Spacecraft Operations Type : LANDER