| MISSION_DESCRIPTION |
Mission Overview
================
Mars Global Surveyor (MGS) was the successor to the failed
Mars Observer (MO) spacecraft, which was lost as it approached
orbit insertion in August 1993. MGS carried five of the
original seven MO investigations; the Gamma Ray Spectrometer
(GRS) was reflown on the 2001 Mars Odyssey, and the Pressure
Modulated Infrared Radiometer (PMIRR) eventually reached Mars
in 2006 aboard Mars Reconnaissance Orbiter (MRO) under the
name Mars Climate Sounder (MCS). When the reports on the
first year of MGS operations were published in 2001, MGS had
already returned more information about Mars than all previous
missions to that planet combined [ALBEE2001]. The mission to
that point was described in an overview paper [ALBEEETAL2001]
and a series of papers submitted by members of the science
teams which will be cited below.
The Mars Global Surveyor (MGS) spacecraft was launched from
the Cape Canaveral Air Station in Florida on 7 November 1996
aboard a Delta-2/7925 rocket. The 1062-kilogram spacecraft,
built by Lockheed Martin Astronautics, traveled nearly 750
million kilometers over the course of a 300-day cruise to
reach Mars on 12 September 1997 [JPLD-12088].
Upon reaching Mars, MGS fired its main rocket engine for a
25-minute Mars orbit insertion (MOI) burn. This maneuver
slowed the spacecraft and allowed the planet's gravity to
capture it into orbit. The initial MGS orbit was highly
elliptical and had a period of 45 hours.
After orbit insertion, MGS performed a series of orbit changes
to drop the low point of its orbit into the upper fringes of
the Martian atmosphere at an altitude of about 110 kilometers.
During every atmospheric pass, the spacecraft slowed by a small
amount because of air resistance. This slowing caused the
spacecraft to lose altitude on its next pass through the
orbit's high point. MGS was to use this aerobraking technique
over a period of four months to lower the high point of its
orbit from 56,000 km to near 400 km in altitude, resulting in
a nearly circular orbit for mapping. Aerobraking was
complicated by discovery of a broken damper arm on one of the
solar panels about three weeks into the procedure. After study,
spacecraft engineers concluded that aerobraking could resume,
but with less stress on the panel. As a result, MGS reached
its mapping orbit about a year later than planned; but science
observations were interleaved with orbit adjustments.
The spacecraft began its primary mission (the Mapping Phase)
on March 9, 1999; at the same time its orbit counter was reset
to 1, meaning that the first 1683 orbit numbers were repeated.
During mapping operations, the spacecraft orbited Mars with
a period of 117.65 minutes at an 'index' altitude of 378 km.
The orbit parameters resulted in an 88 revolution near-repeat
cycle of approximately 7 martian days. With the true altitude
ranging between 368 and 438 km and an inclination of 92.96
degrees, navigators could use interactions between the orbit
and the gravity field to maintain equator crossings at
approximately 2 AM and 2 PM local time without human
intervention or expenditure of fuel. The mapping phase of
the mission lasted for approximately one Mars year (687 days),
ending January 31, 2001, on orbit 8505 [ALBEEETAL2001].
A series of four extended mission phases began on February 1,
2001, (orbit 8506), and continued until communication with the
spacecraft was lost on November 2, 2006 (orbit 34202). During
that time operations continued in much the same way as during
the primary mission, with ongoing data collection by the
science instruments. Failure of an oscillator in the laser
altimeter reduced its capabilities, but it continued to
function in a radiometry mode. Performance of some other
instruments was degraded over time; but the MGS camera system
continued to provide high-quality imaging of potential
landing sites, the radio science experiment was measuring
atmospheric profiles, and the spacecraft was providing
backup relay (communications) functions until the end.
MGS was built of lightweight composite materials and divided
into four sub-assemblies: the equipment module, the
propulsion module, the solar array support structure, and
the high-gain antenna support structure. On board power
was provided by the solar arrays; attitude was controlled
by gyroscopes and small thrusters working in conjunction with
celestial and sun sensors.
Mars Global Surveyor carried four on-board science instruments.
The Mars Orbiter Camera (MOC) had both a wide-angle mode for
global coverage and a narrow-angle mode with resolution of 1.4
meters [MALINETAL1992]. Results from the first Mars year of
operation have been summarized by [MALIN&EDGETT2001]. The
Thermal Emission Spectrometer (TES) measured infrared radiation
[CHRISTENSENETAL1992]. TES was used to determine the general
mineral composition of patches of ground as small as 9.0
square kilometers; in addition, TES also scanned the Martian
atmosphere to provide data for the study of the clouds and
weather [CHRISTENSENETAL2001]. The Magnetometer and
Electron Reflectometer (MAG/ER) were used to measure the global
magnetic properties of Mars, which provided insight on internal
structure [ACUNAETAL1992] [ACUNAETAL2001] [MITCHELLETAL2001].
The Mars Orbiting Laser Altimeter (MOLA) gathered data that
allowed calculation of surface feature heights to accuracies
of 30 meters [ZUBERETAL1992] [SMITHETAL2001B].
An ultra-stable oscillator (USO) in conjunction with
the on-board telecommunications equipment and ground equipment
at stations of the NASA Deep Space Network (DSN) made up the
Radio Science Subsystem (RSS) [TYLERETAL1992]. RSS measurements
included radio tracking of the spacecraft to improve the
gravity field model of Mars, radio occultation observations to
study the structure of the atmosphere and ionosphere, and
surface scattering measurements to characterize potential
landing sites [TYLERETAL2001].
A sixth 'instrument' was the Mars Relay, a cylindrically
shaped antenna that was used to collect data transmitted
to MGS from landers on the Martian surface. These landers
were carried to Mars by later spacecraft and operated after
completion of the MGS primary mission.
A seventh instrument was the Accelerometer, which measured
the deceleration of the spacecraft as it passed through
periapsis during the aerobraking orbits. The deceleration
could be used to infer atmospheric drag and, thereby, density.
From the atmospheric density and altitude, it was then
possible to infer pressures and temperatures above 100 km
altitude, a region not accessible to other instruments
[BOUGHER&KEATING1999].
The MGS Horizon Sensor, originally included for attitude
monitoring and control, was also used for study of the
martian atmosphere [MARTIN&MURPHY2001].
Mission Phases
==============
Six mission phases were originally defined for significant
spacecraft activity periods. These were the Pre-Launch, Launch,
Cruise, Orbit Insertion, Mapping, and Relay Phases. The Cruise
Phase included both Inner and Outer Cruise components. Once
every seven Martian days during the Mapping Phase, the
spacecraft approximately retraced its ground track; these
88-orbit intervals are known as 'repeat cycles.'
The final mission phase, Relay, was intended to support the 1998
Mars Polar Lander and possibly the Mars 2001 Lander. It was
planned to run from February 1, 2001, through January 1, 2003.
Since the Mars Polar Lander was lost and the 2001 mission was
reconfigured without a lander, MGS no longer needed a Relay
phase. Instead, a series of Extended Mission phases replaced
Relay. Relay support, such as for the Mars Exploration Rovers
starting in 2004, was woven into the Extended Mission planning.
PRELAUNCH
---------
The Prelaunch Phase extended from beginning of the MGS
mission until the start of the launch countdown at the
Kennedy Space Center.
Mission Phase Start Time : 1994-10-12
Mission Phase Stop Time : 1996-11-06
LAUNCH
------
The Launch Phase extended from the start of launch countdown
until completion of the injection into the Earth-Mars
trajectory.
Mission Phase Start Time : 1996-11-06
Mission Phase Stop Time : 1996-11-07
CRUISE
------
The Cruise Phase extended from injection into the Earth-Mars
trajectory until Mars orbit insertion. During the Inner
Cruise sub-phase, MGS aimed its solar panels toward the Sun
and communicated through its low-gain antenna; during
the Outer Cruise sub-phase, the high-gain antenna could be
used while the solar panels generated acceptable levels of
power. The transition occurred on 1997-01-09.
Mission Phase Start Time : 1996-11-07
Mission Phase Stop Time : 1997-09-12
ORBIT INSERTION
---------------
After orbit insertion, MGS performed a series of orbit changes
to drop the low point of its orbit into the upper fringes of
the Martian atmosphere at an altitude of about 110 kilometers.
During every atmospheric pass, the spacecraft slowed by a
small amount because of air resistance. This slowing caused
the spacecraft to lose altitude on its next pass through the
orbit's high point. MGS was to use this aerobraking technique
over a period of four months to lower the high point of its
orbit from 56,000 km to near 400 km in altitude, resulting in
a nearly circular orbit for mapping.
At the low point of orbit 15, on October 8, 1997, MGS
experienced difficulties, later diagnosed as due to excess
vibrations of one of the solar panels. The problem was
associated with the fracture of a panel damper arm
[ALBEEETAL1998]. While an evaluation of the solar array
problem was underway, periapsis was raised to about 172 km on
October 13, 1997, and remained near that altitude until
November 7, 1997 (orbits 19 through 36). During this 26-day
period the spacecraft instrument panel was pointed toward
Mars during close approaches (i.e., near periapsis) and the
first extensive set of science observations was conducted.
Orbits 19-36 are known as the Assessment Orbits or the
Aerobraking Hiatus. The science observations were acquired
during the descending leg of each orbit -- that is, as the
spacecraft moved from north to south.
Aerobraking resumed on November 8, 1997 (orbit 37), but with
a periapsis approximately 10 km higher. Aerobraking was
then conducted at about one-third the rate originally planned,
placing the spacecraft in a 2 AM Sun-synchronous mapping
orbit by March 1999 rather than the planned 2 PM mapping
orbit in March 1998. The 2 PM orbit meant that the
spacecraft would have crossed the equator in the descending
leg of the orbit -- north to south -- at 2 PM, a desirable
observing time for some instruments. This orbit could not be
achieved given the new aerobraking constraints. However, a
2 AM orbit was possible because, although the descending leg
of the orbit crossed the equator at 2 AM, the ascending leg
(south to north) crossed the equator at the desired time of
2 PM.
Aerobraking was halted again on March 27, 1998, and resumed
on September 24, 1998. The period from March 27 through
April 28 was known as Science Phasing Orbit 1 (SPO-1, orbits
202 through 268). There was a break for solar conjunction
(May 12) between April 29 and May 27. Then Science Phasing
Orbit 2 (SPO-2, orbits 329-573) followed from May 28 through
September 23. The two science phasing orbits were needed to
synchronize the two-hour circular orbit period with the
equatorial crossing time of 2 AM. A final period of
aerobraking began September 24, 1998, and ended February 4,
1999. Another month was then used for the Gravity
Calibration Orbit, other calibration activities, and final
trajectory adjustments to put the spacecraft into its mapping
orbit. The period between arrival at Mars and completion of
the orbit adjustment activities is known collectively as the
Orbit Insertion Phase. It ended on March 9, 1999, with
orbit 1683.
Mission Phase Start Time : 1997-09-12
Mission Phase Stop Time : 1999-03-09
Subphases Dates Orbits
--------- ----- ---------
Aerobraking Phase 1A 1997-09-12 to 1997-10-12 0001-0018
Aerobraking Hiatus 1997-10-13 to 1997-11-07 0019-0036
Aerobraking Phase 1B 1997-11-08 to 1998-03-27 0037-0201
Science Phasing Orbit 1 1998-03-27 to 1998-04-28 0202-0268
(SPO-1)
Solar conjunction 1998-04-29 to 1998-05-27 0269-0328
Science Phasing Orbit 2 1998-05-28 to 1998-09-23 0329-0573
(SPO-2)
Aerobraking Phase 2 1998-09-24 to 1999-02-04 0574-1284
Transition to Mapping 1999-02-04 to 1999-03-09 1285-1683
MAPPING
-------
The Mapping Phase was the period of concentrated science data
acquisition. At the beginning of this phase, orbit numbering
was restarted at 1. The Mapping Phase lasted for 687 days,
approximately one Martian year. Mars was at opposition with
the Sun on April 24, 1999, and in conjunction with the Sun on
July 1, 2000.
As a risk reduction measure against possible problems with the
deployment of the High-Gain Antenna, the first 20 days of the
Mapping Phase were operated in so-called 'Fixed High-Gain
Antenna' or FHGA mode. In this mode, the undeployed HGA was
pointed at Earth for four to five orbits out of every twelve
to transmit data. During data transmission, the science
instruments were not pointed at Mars. The HGA was deployed on
March 29, 1999, and the first day of full mapping was
April 3, 1999.
Soon after the antenna was deployed (April 16, 1999) its
azimuth gimbal jammed, causing an entry into contingency mode
and interruption in the acquisition of science data. This
interruption lasted until April 29, 1999, and then data were
acquired in a modified FHGA mode (HGA deployed, but boresight
fixed in the spacecraft +x direction) until May 6, 1999, when
normal mapping resumed. It was determined that the restricted
range of travel on the azimuth gimbal would allow normal
mapping operations until early 2000.
So-called 'beta supplement mode' operations, in which the
antenna was reoriented to allow Earth tracking during data
acquisition, were begun on February 7, 2000. But beta
supplement mode required that the antenna be 'rewound' while
the spacecraft was being tracked and precluded collection of
egress (exit) radio occultations.
Approximately three days of FHGA operation were inserted into
the schedule (March 5, 2000, to March 7, 2000) to mitigate
impact on radio science. Unexpected heating of MOLA resulted,
and further FHGA operation was suspended pending resolution
of the thermal problems. Radio science 'egress campaigns' were
eventually resumed at a rate of approximately 24 hours every
month.
Mission Phase Start Time : 1999-03-09 (orbit 0001)
Mission Phase Stop Time : 2001-01-31 (orbit 8505)
EXTENDED MISSION E1
-------------------
The first Extended Mission phase (E1) began at the end of the
Mapping Phase, 1 February 2001, and continued through 22 April
2002. Mars was at opposition with the Sun on June 13, 2001.
Operations were much the same as during Mapping. MGS
supported orbit insertion and aerobraking for the 2001 Mars
Odyssey spacecraft with rapid release of MOC and TES data in
late 2001 and early 2002.
A new type of spacecraft maneuver was designed for MGS
targeted science observations during the extended mission:
the Roll Only Targeted Observation (ROTO). The maneuver was
constrained to occur primarily in the roll axis and could not
exceed +/- 30 degrees off nadir. The spacecraft was
rolled during selected orbits to acquire off-nadir contiguous
MOC, MOLA, and TES data, which could be used to support
landing site certification for future missions.
Starting on August 16, 2001, the spacecraft was put into
the 'Relay 16' or R16 attitude, in which it was pitched
back along the velocity vector by 16 degrees. This reduced
gravity-gradient torques, slowing momentum buildup in the
spacecraft reaction wheels and reducing fuel consumption.
Mission Phase Start Time : 2001-02-01 (orbit 8506)
Mission Phase Stop Time : 2002-04-22 (orbit 13960)
EXTENDED-EXTENDED (E2) MISSION
------------------------------
The E2 Mission phase began at the end of the E1 Extended
Mission Phase, April 22, 2002, and continued until September
26, 2004. Mars was in conjunction with the Sun on August
10, 2002, and on September 15, 2004, and was in opposition
with the Sun on August 28, 2003. There was a major maneuver
in May 2004 to ensure aphelion power within local sun angle
constraints.
Operations were similar to those in the E1 phase. ROTOs,
the R16 attitude, and radio science 'egress
campaigns' were continued. MGS life expectancy was increased
to 10 years based on the rate of fuel consumption. Support
of the Mars Exploration Rover (MER) missions was increased
with both imaging of potential landing sites and testing of
the UHF communication relay system. The MGS orbit was
synchronized to support descent and landing of both MER
rovers; over 6 terabits of surface science data were
returned via the MGS Relay.
Mission Phase Start Time : 2002-04-22 (orbit 13961)
Mission Phase Stop Time : 2004-09-26 (orbit 24836)
EXTENDED MISSION (E3)
---------------------
The E3 Mission phase began at the end of the E2 phase,
September 27, 2004, and continued until September 26, 2006.
Mars was at opposition with the Sun on November 7, 2005.
Operations were similar to those in the E2 phase except that
a safe mode event in August 2006 showed that the high-gain
antenna azimuth gimbal obstruction was no longer present and
that Beta Supplement operation was no longer required. This
saved gimbal life, eased sequencing, reduced fuel consumption,
and allowed collection of more radio occultation data. A new
estimate of attitude control fuel added 10 kg to the
inventory, effectively removing fuel as a limiting factor in
extending the mission lifetime. MGS supported MRO orbit
insertion and aerobraking after its arrival in March 2006.
Mission Phase Start Time : 2004-09-27 (orbit 24837)
Mission Phase Stop Time : 2006-09-26 (orbit 33815)
EXTENDED MISSION (E4)
---------------------
The E4 Mission phase began at the end of the E3 Extended
Mission phase, September 27,2006, and continued until contact
with the spacecraft was unexpectedly lost in early November
2006. Normal operations were suspended between October 17
and November 2, 2006, while Mars was in conjunction with the
Sun (October 23).
Operations were similar to those in the previous
Extended Mission phases. During this phase the 250000-th
image was transmitted to Earth, of which 1750 were collected
using ROTOs and 250 were collected using motion compensated
ROTOs (CPROTOs) with surface resolution as small as 50 cm.
Spacecraft loss may have resulted from a solar panel hitting
a hard stop during an eclipse rewind, leading to a cascade
of other events which culminated in catastrophic battery
failure within 10 hours.
Mission Phase Start Time : 2006-09-27 (orbit 33816)
Mission Phase Stop Time : 2006-11-02 (orbit 34202)
|
| MISSION_OBJECTIVES_SUMMARY |
One of the most intriguing, unanswered scientific questions is
why do Earth and Mars appear different today? At the time of
their formation several billion years ago, Mars and Earth shared
similar conditions. Both planets harbored vast quantities of
surface water, thick atmospheres, and climates warmer than at
present. Today, Earth is a lush world filled with a countless
number of animal and plant species. In contrast, data gathered
from Mars prior to MGS showed that the planet was trapped in
conditions reminiscent of a global ice age. The dry and
seemingly lifeless Martian surface makes the Sahara look like
an ocean in comparison, and average daily temperatures make
Antarctica seen balmy. Comparing the history and evolution of
the two planets yields clues into Earth's past and possibly
its future.
Science objectives for the failed Mars Observer Mission
[ALBEEETAL1992] were essentially identical to those for Mars
Global Surveyor [ALBEEETAL2001].
Basic Measurements and Data Collection
======================================
Although several spacecraft preceded MGS to Mars, fundamental
measurements remained to be made. No topographic model of
the planet existed at the 100 meter level (and many areas were
uncertain by kilometers); MOLA provided one with typical
accuracies of 30 m. Preliminary measurements on the magnetic
field were carried out by early spacecraft; but MGS MAG/ER
was the first instrument to carry out a systematic mapping
effort. Gravity models had been compiled from Mariner 9 and
Viking data, but MGS RSS provided an order of magnitude
improvement in these -- leading to improved understanding of
the planet's interior.
Atmospheric Processes
=====================
Despite its forbidding climate, surface temperatures on Mars
resemble the Earth's more than any other planet. These
similarities in temperature result in part from the fact that
Mars orbits the Sun only slightly farther out than the Earth as
compared to other planets. For example, the ground at some
locations near Mars' equator may warm up to as high as 25C
at noontime. However, daytime temperatures still average well
below freezing, and night temperatures dip much lower.
Martian temperatures may seem almost inviting to the seasoned
outdoors explorer, but the composition of the atmosphere leaves
much to be desired from a human perspective. Most of the
martian air consists of carbon dioxide (CO2), similar to
conditions on Venus. If breathing carbon dioxide seems
uninviting, the density of the air will appear worse. Average
barometric pressures on Mars are lower than those found at
Earth's sea level by a factor of more than 125. In other
words, the air at the surface of Mars is thinner than that
found on Earth at an altitude 19 times higher than Denver,
Colorado.
The extremely thin Martian air directly impacts the mystery of
potential life on Mars, either in the past or present. The
reason is that almost all of the water lies trapped in the
Martian polar ice caps or frozen beneath the surface. Liquid
water cannot exist on the surface because the thin atmosphere
will cause melting ice to evaporate directly into water vapor.
Despite the hostile composition, density, and temperature by
today's standards on Earth, the atmosphere of Mars is both
interesting and dynamic. MGS objectives in this area included
recording global daily images of the planet so that cloud
patterns could be followed and the growth of dust storms could
be monitored over a full martian year. TES and RSS were both
able to measure vertical structure within the atmosphere,
another key to understanding transport of material within the
atmosphere -- including precipitation of CO2 itself on the
winter polar cap.
Surface Processes
=================
Geologically, Mars is one of the most interesting planets in
the Solar System. Although only half the diameter of Earth,
Mars maintains large water and CO2 ice caps at the poles, a
canyon much deeper than the Grand Canyon and longer than the
contiguous 48 United States are wide, crater valleys as
large as the western United States, and a handful of monstrous
volcanoes that make Mount Everest appear tiny in comparison.
A study of Martian geology is crucial to deciphering clues
about the history of the Earth. Mars is the only planet in the
solar system that both has an atmosphere and contains surface
features that cover almost the entire range of history. On
Earth, pristine rocks and other surface features from
the first billion years of our planet's existence do not exist
because geological events, weather, and life have caused
drastic alterations. Because Earth and Mars shared similar
conditions near the time of their formation, the MGS
exploration of Mars allows us to take a peek into Earth's
past in a way not possible by studying the Earth by itself.
Although liquid water on Mars will quickly evaporate,
photographs transmitted back to Earth by NASA missions prior
to MGS revealed giant flood channels, dry river beds, and
flood plains on the surface. This evidence of past water on
Mars led some scientists to consider Mars as the prime
location in the Solar System to search for extraterrestrial
life. The speculation was that because Mars once possessed
a thicker atmosphere and vast quantities of surface water
billions of years ago, then the planet may have harbored
conditions favorable to the formation of life despite its
present forbidding climate.
Viking and Mars Pathfinder returned information on elemental
composition of some Mars surface materials at specific
landing sites. But regional and global information was needed
to understand both the current state and history of rocky
surfaces. MOC provided high-resolution image data; and TES
acquired spectral signatures of rock units so that thermal
inertia, surface rock distributions, and composition could
be inferred.
MOC also revealed contemporary activity on the surface during
the instrument's own lifetime including 20 new impact craters,
numerous boulder trails, secular enlargement of south polar
pits, and fresh channel outflows likely to be water-related.
Search for Life
===============
Sensors aboard various NASA spacecraft launched to Mars over
the 30 years prior to MGS showed that advanced life forms
almost certainly do not exist on the planet today. However,
many felt that the planet might hide bacterial forms of life
or their fossil remains. Although Mars Global Surveyor did
not conduct a search for life on Mars, it gathered detailed
data that will help in understanding the mystery of the
missing water. This type of study provides important
background data to help scientists in their search for
Martian life on future missions.
Other Studies
=============
In addition to studying Mars, the spacecraft was also
used for experiments of opportunity, such as searching for
gravitational waves during cruise [ESTABROOKETAL1995] and
probing the Sun's corona during solar conjunction [WOO1993].
|
.png)
.png)
