Eos, Vol. 71, No. 39, September 25, 1990 corded data rate to the tape recorders is about one-fifth the playback rate for a 10- Mars Observer's hour link to a 34 m NASA Deep Space Net­ work tracking station. The normal sequence of collecting scientific data will be to record Global Mapping Mission continuously for 24 hours and then to play the data back in one 10-hour link. The map­ ping orbit is a 117-minute orbit with a 7-day A. L. Albee and D. F. Palluconi repeat cycle. As a result, the planet is repeat­ edly mapped in 26-day cycles with a 58.6 km California Institute of Technology, Pasadena, California nominal path separation. Orbit trim adjust­ PAGES 1099, 1107 ments make it possible to obtain uniform cov­ erage during the course of the mission with The Mars Observer mission, scheduled for face, and provide a basis for comparison with an ultimate spacing of ground tracks at the launch in September 1992, will provide an Venus and Earth. equator or 3.1 km. orbital platform at Mars from which the en­ The Mars Observer spacecraft provides a tire Martian surface and atmosphere will be Mission Description three-axis stabilized, nadir-oriented platform observed beginning in late 1993. Mars Ob­ for continuous observations of Mars by the server will extend the exploration and charac­ Mars Observer will be launched by a Titan science instruments (Figure 2). The space­ terization of Mars by providing new and sys­ III, built by Martin Marietta, with an upper craft is being built by the General Electric As­ tematic measurements of the surface and at­ Transfer Orbit Stage from Orbital Sciences tro-Space Division. The Gamma Ray Spec­ mosphere of the planet. These measurements Corporation. After the 11-month transit the trometer and Magnetometer sensor assem­ will be made from a low-altitude polar orbiter spacecraft will be injected into an elliptical or­ blies are mounted on individual booms on over a period of one Martian year (687 Earth bit around Mars with periapsis near the north the spacecraft. All other instruments are rig­ days), permitting repetitive observations of pole. The orbit is then adjusted through a se­ idly mounted to the spacecraft structure. No the surface and of the seasonal variations of ries of maneuvers to a near-circular, sun-syn­ movable scan platform is provided; the space­ the atmosphere. The mission is being de­ chronous (2:00 A.M./P.M.), low altitude, craft is continuously nadir pointed, rotating signed in a manner that will provide new and near-polar orbit. Due to the non-uniformity at the orbital rate. Those instruments that re­ valuable scientific data at a significant reduc­ of the gravity field, the altitude varies from quire scanning or multiple fields of view have tion in cost and operational complexity. 376 km near the south pole to 430 km near internal scanning mechanisms. The footprints The scientific objectives for the mission em­ the north pole. During the Martian year in for the instruments are shown in Figure 3. phasize qualitative and quantitative determi­ this mapping orbit the instruments acquire nation of the elemental and mineralogical data in a systematic program of global map­ composition of the surface; measurement of ping. At the end of the mission the spacecraft Experiments and Instruments the global surface topography, gravity field, can be boosted to a permanent quarantine or­ and magnetic field; and the development of a bit. The instruments for Mars Observer closely synoptic data base of climatological condi­ The mapping timeline, relative to the Mar­ match the scientific objectives. Collectively the tions. This mission will provide a basic global tian seasons and the likely dust storm period, instruments cover much of the electromag­ understanding of Mars as it exists today and is summarized in Figure 1. The Mars orbit netic spectrum and form a highly comple­ will provide a framework for understanding insertion (MOI) period ends just before solar mentary set. Each instrument produces well- its past. conjunction and the beginning of the dust defined sets of measurements which address The formal scientific objectives of this geo- storm period. Since it is scientifically impor­ specific major objectives, but nearly every science and climatological mission are to de­ tant to make observations for an entire global data set also contributes to a much wider va­ termine the global elemental and mineralogi­ mapping cycle (26 days) before the onset of a riety of scientific investigations. Five interdis­ cal character of the surface material; define major dust storm, it is hoped that the fuel ciplinary scientists have been selected in addi­ globally the topography and gravitational margin will permit a shorter MOI phase than tion to the instrument teams in order to ex­ field; establish the nature of the magnetic shown. ploit the strong synergism in the data sets. field; determine the time and space distribu­ The playback data rate for a 10-hour link Moreover, participating scientists, including tion, abundance, sources, and sinks of volatile varies by a factor of 4 with Earth-Mars dis­ 10 from the Soviet Union, will be added after material and dust over a seasonal cycle; and tance during the mission. The continuous re­ launch to further exploit the data returned explore the structure and aspects of the cir­ culation of the atmosphere. These first-order scientific objectives can be MOI BEGIN LGC addressed within the framework of a low-cost orbital mission. All five objectives involve MISSION MOI z MAPPING PHASE (DURATION = 687 days) global mapping. For the geoscience objec­ PHASE: PHASE tives, this mapping is mainly time-indepen­ dent and therefore two-dimensional: latitude DUST STORM PERIOD MARS 94 ACTIVITY and longitude. For many climatology objec­ I tives, the mapping is four-dimensional: lati­ SOLAR (HIGHLY VARIABLE) CONJ tude, longitude, altitude and season. As a re­ sult of this mission we should have a system­ MARS NORTHERN SOUTHERN SOUTHERN NORTHERN NORTHERN SOUTHERN SEASON: SUMMER SPRING SUMMER SPRING SUMMER SPRING atic global characterization of Mars today. This characterization will help us to under­ V V stand the geologic and climatologic history of MAXIMUM MARS MINIMUM MARS EARTH RANGE PERIHELION EARTH RANGE APHELION Mars, the evolution of its interior and sur- (2.45 AU) (1.38 AU) (0.68 AU) (1.67 AU) 16 ksps DATA RATES 3 ksps f" 8 ksps RECORD: 4ksps Cover. By the end of 1993, the Mars DATA RATES ASSUME 34-meter DSS (HEF) Observer spacecraft will begin mapping PLAYBACK: 21 ksps 43 ksps 85 ksps 43 ksps 21 ksps the surface and atmosphere of Mars, ex­ tending the exploration and characteriza­ I h H H—r H h H h- tion of the planet by providing new and JUL OCT JAN APR JUL OCT JAN APR JUL OCT JAN systematic measurements over a period of 1 1 1 1 1 1 1 1 1 1 1 one Martian year. See "Mars Observers' 1993 + Global Mapping Mission" by A. L. Albee and D. F. Palluconi, this issue. Fig. 1. Timelines for the mapping phase of the Mars Observer mission. This page may be freely copied. Eos, Vol. 71, No. 39, September 25, 1990 examine local areas at extremely high spatial resolution in order to quantify surface/atmos­ phere interactions and geological processes. Thermal Emission Spectrometer (TES). The principal investigator for TES is P. Christen- X \ ^\6-PANEL SOLAR ARRAY sen of Arizona State University. The instru­ ment is a Michelson interferometer that cov­ ers the spectral range 6.25 to 50 micrometers with 5-10 wavenumber spectral resolution. Separate solar reflectance (0.3 to 3.9 micro­ meters) and broad band radiance (0.3 to 100 micrometers) channels are included for radia­ tion balance measurements. It has six 8.3 mrad fields of view, each with 3 km spatial resolution at nadir. Objectives of this investigation are to deter­ mine and map the composition of surface +Z (NADIR) minerals, rocks and ices; study the composi­ tion, particle size, and spatial and temporal distribution of atmospheric dust and of con­ densate clouds; study the growth, retreat and total energy balance of the polar cap depos­ Ps ORBITAL its; measure the thermophysical properties of ROTATION the Martian surface (thermal inertia, albedo) that can be used to derive surface particle HIGH-GAIN +X (VELOCITY) ANTENNA size and rock abundance; and determine at­ mospheric temperature, pressure, water va­ Fig. 2. The Mars Observer spacecraft and its payload in the fully deployed config­ por, and ozone profiles, and seasonal pres­ uration of its mapping orbit at Mars. sure variations. Pressure Modulator Infrared Radiometer (PMIRR). The principal investigator for MOC W/A (140° Cross Track) PMIRR is D. McCleese of the Jet Propulsion Laboratory. The instrument is a limb, off-na­ dir and nadir scanning radiometer. Measure­ ments are made in 9 spectral bands with 5 fil­ Fig. 3. Footprints ter channels and 2 pressure modulator cells for the nadir-panel in­ (one containing carbon dioxide, the other wa­ struments: Mars Ob­ ter vapor). The detectors are cooled to 80 K server Camera (MOC, by a passive radiator. wide angle and narrow The objectives of this investigation are to angle), Thermal Emis­ map the three-dimensional and time-varying sion Spectrometer thermal structure of the atmosphere from the (TES), Mars Observer surface to 80 km altitude; map the global, Laser Altimeter vertical and temporal variation of atmospher­ (MOLA), and Pressure ic dust and condensates; map the seasonal Modulator Infrared and spatial variation of atmospheric pressure Radiometer (PMIRR). and the vertical distribution of atmospheric water vapor; and monitor the polar radiation balance. Mars Observer Laser Altimeter (MOLA).