"Towards Mars"


   

  INTERNATIONAL STRATEGY FOR THE EXPLORATION OF MARS 

  

   International Mars Exploration Working Group (IMEWG) 
 

1. INTRODUCTION

The International Mars Exploration Working Group (IMEWG) has representatives from all space agencies and major institutions participating in Mars Exploration. The IMEWG was conceived at a meeting at Wiesbaden, Germany, May 1993, and since then has met two times a year to discuss the general strategy for the exploration of Mars. The present charter of the IMEWG (approved in 1996) is as follows: 

1. Produce and maintain an international strategy for the exploration of Mars. 

2. Provide a forum for the co-ordination of  Mars exploration missions.

3. Examine the possibilities for the next steps beyond the currently defined missions.

         

The intent of IMEWG is to lay out a broad long-range strategy for Mars exploration. The strategy must be sufficiently specific that intermediate and long-range goals can be identified, and yet sufficiently flexible that the means and schedule for achieving the goals can accommodate to programmatic and fiscal realities. The strategy must also be consistent with missions already funded or planned. 

Today IMEWG is a well functioning planning forum for Mars exploration. It has passed recommendations on surface network missions, telecommunication strategy and recovery strategy for the science on Mars Observer, lost in 1993, and Mars 96, lost in 1996. Quite recently the discussion has circulated around sample return scenarios and the preparatory activities for manned missions. The recommendations issued by IMEWG have been well met in various space organisations and led to actions that improve the complementarity of the planned and approved mission scenarios. 

The strategy to be presented below first outlines the science goals of Mars exploration and then examines the effectiveness of different techniques in realising these goals. It has to be accepted that the rationale for Mars exploration is not exclusively scientific, and that political issues and other factors such as a desire to develop new technologies will affect future plans.  

2. SCIENCE GOALS 

In the broadest sense, the scientific goals of Mars exploration are to determine:

What are the evidences of present or past life on Mars.

What can we say about the origin of the solar system.

How did Mars form and evolve to its present state.

What is the history and current state of the atmosphere and climate and how are they changing.

How does the atmospheric dynamics compare to weather on Earth.

How does Solar wind interact with Mars.

2.1  Life on Mars 

A long-range objective of Mars exploration is to determine if life ever started there. The prospects for present-day life are extremely small because under present climatic conditions liquid water is unstable everywhere within a few hundred meters of the surface. However, abundant evidence for the presence of liquid water at the surface in the past suggests that climatic conditions could have been more hospitable to life at times in the past. 

There are two main lines to proceed in studying the evidence of life. The first line is to better understand the climatic history of the planet and to search for evidence of existing subsurface water ice reservoirs. Several planned missions address this question. The second line is look for direct signatures of life that may have existed. The observations reported from the ALH 84001 meteorite challenge this issue. More meteorite search campaigns are needed and, finally, samples have to be brought from Mars to be analysed in more detail; such a mission is also under consideration. Samples (sub-surface samples would be best) could also be analysed by sophisticated instruments on the surface of Mars. In particular, those instruments need to identify the following indicators of the possible existence of past life:

      (1) presence of water and evidence of exposure of water,

  1. appropriate mineralogy,
  2. relict carbonaceus material,
  3. complex structures, and
  4. isotopic fractionation between phases.

One such lander with an appropriate set of instruments, Beagle 2 of the Mars Express mission, is under consideration. 

2.2  Origin of the solar system 

Mars is a key planet to be studied to understand how did it form and what does its mode of formation imply for the formation of the Solar System and Earth. What were the materials from which Mars accreted? How quickly and in what sequence did the planet accumulate? To what extent was accretion complete when the core formed? To what extent did the planet continue to accumulate material after the main accretionary phase was over around 4.5 billion years ago? Events during accretion established the starting conditions for the planets subsequent evolution. It is, therefore, important to know the thermal and physical state of the interior at the end of accretion, and the state of the surface and atmosphere.

The above questions can only be addressed by a combination of geophysical, geochemical and geologic techniques. The size of the core is best determined by seismic methods. Once the core size is known, its density can be determined from the planet's moment of inertia, and its composition inferred. Chemical analyses of mantle-derived rocks also enable us to date and characterise differentiation events, such as core formation. The composition of the atmosphere, and the nature of any volatile reservoirs will also contribute substantially to our understanding of the accretional processes. 

The observations made on Mars can be compared to those collected on Earth, and later, on Mercury and Venus. These results would enable us to conclude some essential facts about the origin and formation of the inner Solar System planets and provide us with more evidence about the history of the entire Solar System. 

2. 3  Formation and evolution of Mars 

The evolution of the interior of Mars must have been very different from that of Earth. The Earth's core has remained partly molten, and the mantle has stirred surface materials to depths of at least 700 km and probably to the base of the mantle at 2900 km. Isotopic patterns in SNC meteorites suggest that the Mars mantle is stably layered. However, it is unlikely that the Mars mantle is laterally homogeneous in view of the extreme localisation of volcanic activity in Tharsis and Elysium.  

In order to better understand how the core has evolved it has to be determined if the planet ever had a significant magnetic field. In the light of recent Mars Global Surveyor observations remanent magnetism exists, and the next step is to date those times when a planetary magnetic field was present. 

The evolution of the mantle is best inferred from the detailed chemistry and petrology of volcanic rocks. Another method is to study the outgassing history as revealed, for example, in the isotopics of noble gases. The thicknesses of the lithosphere and crust, including lateral variations, and variations with time, will provide additional clues about the thermal history of the interior and global fractionation. 

Because most measurements are made on the surface, its understanding is crucial for understanding the planet as a whole. Most geologic events on the surface of a solid planet, such as volcanic eruptions or meteorite impacts, leave behind some record of their occurrence and partly erase the previous record. The task of the geologist is to reconstruct the history of the planet from this fragmentary record. The surface has been modified by the action of wind, water and ice. On Earth the record dates mostly from the last half billion years, whereas on Mars the record spans almost the whole of the planet's history. 

Mainly because of its importance for understanding the evolution of the interior, it is essential to determine the sequence and nature of volcanic activity, how the activity has changed with time, and how it has varied with location. Because of the climatic implications, it is essential to understand how volcanism has affected the inventory of volatiles at the surface. With respect to tectonics, of prime importance is understanding the cause of the division of the surface into predominantly cratered uplands in the south and low-lying plains in the north. Is this a tectonic feature, the result of forces originating in the interior of the planet, or is it a giant impact scar?   

Different kinds of erosional features provide the main evidence for climate change on Mars. Although Mars has been extensively modified by wind, water and ice, the rates of such activity have been far lower than on Earth. While there is abundant evidence of water erosion, we know very little about the processes involved. What caused the large floods, what has been the history of their action, and where did the water go that was involved? Were the flat lowlands due to an ocean? These questions can best be addressed with better imaging and radar probing, both from orbit and the ground, better dating, better information on the chemistry, mineralogy and lithology of water-lain deposits and a better understanding of the inventory of surface volatiles, especially H2O and CO2.  

2.4  Climate change 

One of the most intriguing aspects of the evolution of Mars is the possibility that the planet has undergone major changes in climate. The main evidence is the widespread erosion of the surface by water and ice. To better understand climate change, better insight into the processes involved is needed, and also to the climatic conditions required for the formation of a wide variety of erosional and depositional features attributed to the action of water and ice. Also understanding some external factors responsible for climate change, like, e.g., the direct interaction of the Solar wind with Martian atmosphere and the evacuation of the upper atmosphere due to various pick-up and acceleration processes, is of great importance.   

Detection of thick carbonate deposits would support the supposition that the Mars atmosphere was thicker in the past. Samples of past atmospheres trapped in surface materials would provide direct evidence of past climate. More indirect evidence is provided by the isotopics of volatile species such as nitrogen and the noble gases, and how these isotopics have changed with time. In addition to long term climate changes, Mars may experience shorter term, more modest changes as a result of obliquity changes. The best record of such changes is probably preserved in the polar layered terrains, so sampling through sections of the polar layered terrains is of considerable importance. 

2.5  Atmospheric dynamics 

We have now identified several of the major elements of the general circulation of the Martian atmosphere: the meridional flow due to seasonal condensation of CO2, seasonally dependent meridional overturning, traveling planetary waves in the northern hemisphere, regional slope winds, internal gravity waves, great dust storms, thermal tides and boundary layer dynamics. However the knowledge of the different elements is generally not adequate to allow a quantitative description of their relationship to forcing, to boundary conditions, or to each other. Moreover, our knowledge of the boundary layer is still minimal, having only the Viking and Mars Pathfinder  (late summer only) data in a fairly limited height range above the surface at three different locations.  

The observational strategy necessary to improve our understanding requires long-lived simultaneous measurements from both surface and orbit. The PFS instrument on Mars Express will be able to provide such measurements. The orbital measurements define the height-dependent components of atmospheric structure and dynamics.  The surface measurements define the height-independent local components and characterise the boundary layer. 
 
 

2.6  Solar wind interactions 

The mechanism of the interaction of Mars with the Solar wind depends of the existence of an intrinsic magnetic field. Presently a relatively strong and localised remanent magnetic field has been observed but whether an active internal dynamo still exists is an open question. The Solar wind produces ionospheric currents which in turn create magnetic fields. These fields need to be understood in order to distinguish them from the intrinsic field.   

The magnetic measurements should be made in conjunction with measurements designed to characterise the ionosphere, such as determination of the thermal ion densities and temperatures, and the composition and temperatures of neutrals. Ideally magnetic measurements should be made simultaneously from the surface and orbit, and the orbiter measurements should be made as low in the atmosphere as possible, and over a sufficiently large range in altitude that the various components of the magnetosphere and ionosphere are sampled. 

Several lines of evidence suggest that the abundance of volatiles in the martian atmosphere has changed with time. One mechanism contributing to these changes is loss to space from the upper atmosphere. H is lost by thermal escape. N, O and C can be lost by non-thermal mechanisms such as dissociative recombination of ions with electrons, pick up of ions by the Solar wind, and sputtering of atoms by the pick-up ions. In order to understand how the atmosphere has evolved we need to both measure the loss rates of H, N, O and C and the isotopic composition of the escaping species.  

3. EXPLORATION APPROACHES 

Mars exploration may be conducted by different types of missions: 

Remote sensing.

Landers (independent landers, networks, penetrators, rovers, balloons, airplanes).

Sample return.   

3.1  Orbital remote sensing 

To understand the geologic evolution of Mars images with different spatial scales varying by factors of 10 or less are needed. Low-resolution (1-5 km/pixel) imaging is also needed, primarily for atmospheric monitoring, but also to make geodetic measurements.   

Viking provided the scientific community with almost global coverage at roughly 200 m resolution, and samples of small areas with resolutions down to 10 m. For geologic purposes highly desirable higher resolution images are provided by the on-going Mars Global Surveyor mission where the regional coverage of different areas of the planet are at 10-20 m resolution and small areas, globally distributed, at 1.5 m resolution, so that all scales are examined and imaging at each scale can be placed in context of coarser scale imaging.  For monitoring the global weather, near global low-resolution coverage should be acquired daily and more frequently at times, to describe great duststorm initiation, early morning fog dissipation, etc. There are major advantages to monitoring global weather from orbit at the same time that meteorology stations are operating at the surface. 

At present we have only very little information on chemical and mineralogical variations in the surface materials. Gamma ray spectroscopy is the best technique for mapping chemical variations from orbit. Mineralogical variations may be determined by visible and near-IR reflectance spectroscopy and by thermal emission imaging spectroscopy. Only experience will show which is most suitable for Mars. Both techniques will enable us to distinguish differences from area to area and place constraints on the causes of the differences. From a strategic viewpoint it is highly desirable that the additional  high resolution imaging and the mapping of chemical and mineralogical variations be achieved prior to placing geoscience landers on the surface. Such preparatory observations are carried out by the Mars Global Surveyor mission. Also the Geochemical Mapper to be launched in 2001 will address the elemental composition and global mineralogy aspects of Mars.    

Altimetry and gravity are crucial for understanding the density variations, thermal state, and the state of stress of the crust and upper mantle. Surface elevations have also to be accurately determined before embarking on an ambitious lander program. Elevation details are also needed for atmospheric dynamics modelling purposes. The measurements can only be made globally from orbit. To acquire the required precision, an orbiter carrying an altimeter and ultra-stable oscillator (for tracking), must be placed in a low circular orbit. Accurate global altimetry and gravity observations are carried out from the Mars Global Surveyor.

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Characterisation of the magnetic field requires a variety of measurements. Mapping surface anomalies is best done from a low circular orbit. First results have been obtained with the Mars Global Surveyor during its first aerobraking orbits deep into the atmosphere. Characterisation of magnetic fields due to ionospheric currents caused by interaction of the planet with the Solar wind, and measuring the response of the solid planet to these fields is best achieved with one or more spacecraft in highly eccentric orbits. More magnetic observations in low-altitude periapsis in near equatorial orbit will carried out by the Nozomi spacecraft in 2004. 

Orbiters can be used for sounding the surface with radars operated at long wavelengths to determine variations in density, permittivity and conductivity to depths below the surface of a few kilometers. The main interest is detection of water and ice. Such an experiment is under construction for the Mars Express mission. 

In order to understand the circulation of the atmosphere, and what drives it, we need to track how temperature, pressure and aerosol profiles in the atmosphere change globally by time of day, season, and interannually. A variety of techniques can be used from orbit to obtain such profiles, including pressure modulated infra-red radiometry, thermal infra-red radiometry, radio occultation observations (for the best vertical resolution of atmospheric temperature), and radar techniques. It is imperative to have these measurements performed at the same time that meteorology measurements are made at the surface. Such a set up might appear in connection with the forthcoming Mars Express orbiter (assuming that the spacecraft survives and the operational time is extended) and the four NetLander surface stations in 2008-9.

Characterization of the interaction of the planet with the solar wind can only be done from orbit and requires the following measurements:

  1. ionospheric and planetary magnetic fields at all local times and locations,
  2. thermal ion densities, temperatures, velocities and composition,
  3. neutral compositions, winds and temperatures,
  4. visible and ultraviolet airglow,
  5. solar EUV, and
  6. plasma wake measurements, including both electric and magnetic components.

      The orbiters should be in elliptical orbits with as low a periapsis as possible, and apoapsis approximately 3 RM. Inclusion of both Solar maximum and Solar minimum in the mission lifetime is desirable. To some extent these aspects will be studied during the joint operations of the Nozomi and Mars Express spacecraft in 2004-5 orbiting Mars simultaneously in polar and near equatorial elliptical orbits.  

3.2  Landers 

Landers can be used as independent stations or in combination with other landers and/or orbiters. Some science objectives, including most of those for geology,  geochemistry and exobiology can be achieved with independent landers, each successive landers adding incrementally to our knowledge. Other objectives, including most of those for seismology and meteorology, require simultaneous measurements from an array of landers. For meteorology, simultaneous orbiter measurements are essential.   

The main function of a geoscience station would be to characterize the geology at a particular point on the surface. Characterization could include imaging, determination of the lithology, chemistry, mineralogy and isotopics of the rocks and soils, and determination of the physical properties of local materials. The station could include a small rover to image beyond the immediate lander area, and to deploy instruments such as an alpha-proton-xray spectrometer for soil and rock chemistry, or a neutron spectrometer for determination of near-surface water. Also subsurface investigation by means of drillers, moles, etc. should be included. Operation of some stations at high latitude where water ice is stable at shallow depths, is desirable.   

The main objective of a seismic network is to determine the internal structure of the planet. To do this, at least four, but preferably several, sensitive seismometers must operate together on Mars for a period long enough (years) to ensure that several marsquakes are detected by a few stations simultaneously. The date rate should preferably be at least 10 MB/day per station. The main characteristics of the seismic stations are simplicity in instrumentation, but long-life, high data rate, and simultaneity of operation with other stations. A communications orbiter is required because of the high data rates. 

Meteorology stations could, in principle, be very simple if only pressure would be measured alone; complexity increases if other meteorological parameters are recorded. The highest priority instruments, those that measure pressure, temperature, wind and humidity are all lightweight but the complexity is increasing in the order sensors are listed. Of additional interest are the optical thickness of the atmosphere and the surface-air heat flux. The data rates are low, but the stations must last for at least one Martian year. After early missions (e.g., Vikings) have completed pilot measurements, a meteorology network is desired. In order to characterize both meridional and zonal flow such a network should have several stations both at low and at high latitudes. The value of a meteorology network is enhanced by several orders of magnitude by simultaneous sounding of the atmosphere from orbit.

Penetrators have the virtue of getting instruments below thermal and radiation effects at the surface. They can also provide efficient coupling to the ground for seismometers, and enable placement of analytical instruments, both chemical and thermal, directly against local materials. However penetrators are inherently more risky than other types of landers, and thermal problems constrain lifetimes. Penetrators are thus suitable for measuring soil chemistry, heat flow, and sub-surface water, but their restricted lifetimes do not make them very suitable as seismic and weather stations. 

Balloons can be used both as instrument platforms and to deliver small landed packages to the surface. They can be designed to float at specified altitudes and to approach the ground periodically to drag instruments across the surface in a guiderope. Remote sensing instruments in a gondola could include a camera, magnetometer, radiometer, and various meteorology sensors. A guide rope could include ground penetrating radar, and chemical and mechanical sensors. One obvious advantage of balloons is their mobility, which permits high-resolution profiles across distances, possibly as large as thousands of kilometers. Possible disadvantages are small payload masses, difficulty in controlling the flight path, and short lifetimes. 

Balloons could be complemented by light-weighted aircraft. The mass could be 35-200 kg, wingspan 6-15 m and payload 3-10 kg. The operational range could be thousands of km. A relatively high speed (100m/s) is needed. There are still several technical problems to be solved. An aircraft could collect high-resolution terrain images, atmospheric samples and visit sites (canyons and poles) that are difficult to reach otherwise. A small aircraft is under consideration to fly in the near future.

Local rovers to explore and deploy instruments in the immediate vicinity of simple geoscience landers are continuously needed. Rovers including “moles” capable of burrowing below the surface can be used to collect materials either locally or over long distance for analysis at some parent vehicle or for analysis back on Earth. They can carry instruments to perform in situ analyses on a variety of rocks and soils. Rovers can emplace small geophysical stations and, while on traverses, perform geophysical measurements such as electromagnetic sounding, and active seismometry. They could place near surface atmospheric sensors away from degrading influence of spacecraft and large obstacles and, maybe, a soil temperature probe at the same time. However, the most outstanding attribute of capable, long-range rovers is that they perform exploration since they can travel tens to hundreds of kilometers through different types of terrain. 

3.3  Sample return 

Sample return from Mars has long been a goal of planetary exploration. Many critical measurements are simply too complicated and interactive to be performed remotely, at a distant planet, in the foreseeable future. Most age dating techniques, for example, stretch the capabilities of terrestrial laboratories while trace element determination is another example. In most geologic processes, trace elements are more strongly fractionated than the major rock forming elements, and so are more sensitive tracers of geologic history. Thus, many of the most crucial measurements require samples here on Earth. Moreover, it is impossible to know, a priori, which are the most critical measurements to make. Having samples here on Earth allows the measurement strategy to shift in response to previous analytical results and new techniques can be developed as suggested by the samples themselves. 

Experience with the Moon emphasizes the enormous power of returned samples when placed in the context of global data. Sample data are the basis for almost all our current ideas about the Moon. Having lunar samples in hand allowed the complete analytical and intellectual capacity of the science community to focus on the Moon's evolution. There is no reason to believe that these enormous advantages would be any less for Mars. Indeed the apparently more complex geology of Mars, and the biologic potential, enhance the importance of the kind of comprehensive examination that returned samples allow. 

A strategic issue with respect to sample return is mission complexity. Two fundamentally different approaches may be taken. One approach is to have few, relatively large and complex missions; the other to have several simple missions. The large missions would emplace a substantial capability at a few sites. Included would be a long-range rover that could travel tens or hundreds of kilometers to collect samples representative of the region. These could amount to several kilograms and would be assembled and packaged at the landing site for delivery back to Earth. The second approach would be to send a limited capability to several sites. These landers would have a very simple rover that could travel only tens of meters from the lander, to collect a few hundred grams for delivery back to Earth.  Geologic complexity would be a desirable attribute of landing sites in the first approach, geologic simplicity in the second approach.  

Sample return is considerably easier to achieve from Phobos than from Mars. Studies of chemical, isotopic, mineral composition, age of the Phobos material should be important for understanding of  its origin and possible link with the origin of Martian satellites and also of the Solar System as whole. It would be important also to understand the possibility to use the Phobos material as a resource to support human exploration. 

4. STRATEGY 

A logical strategy for exploration of a remote planet is to start with reconnaissance from fly-bys, follow this with global characterisation from orbit, then gradually shift emphasis to the surface. Early surface stations could be relatively simple, operating independently of other stations, and performing a variety of exploratory measurements. This stage could be followed by emplacement of a network of simultaneously operating stations. In effect, these early stations would be continuing the global characterisation on the ground. Then, as the planet becomes better understood, more sophisticated missions, such as sample return and capable rovers could be sent to a limited number of locations, chosen on the basis of the previous global data and surface probes. 

4.1  Global observations from orbit

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The remote sensing objectives can be grouped into three classes according to their relation to the next steps in the exploration strategy, deployment of more surface stations, and establishing a surface network: 

Group 1 objectives should be accomplished prior to any substantial lander program, because they provide knowledge that will be useful in choosing sites for the landers. Also the sample return planning requires similar type of information. Included here are:

  1. the Mars Global Surveyor objectives to systematically map the mineralogy of the surface (Thermal Emission Spectrometer), to map more precisely the altitude of the surface (laser altimeter), and to provide globally distributed high resolution imaging samples, and
  2. the Mars Express objectives to do spectral mapping in the visible and near infra-red (OMEGA), and to acquire 10-20 m (and even 1-2 m with the Super Resolution Channel) regional imaging (HRSC). Also included is mapping global variations in the chemical composition of the surface. This is planned to be achieved by flying a Gamma Ray Spectrometer on the Mars Odysseys in 2001.
 

Group 2 objectives also have high science priority but can be addressed whenever programmatically feasible because emplacement of surface stations is only marginally dependent on their accomplishment. Included here are objectives to better define global gravity (RS) and variations in the magnetic field (MAG), which are both to be achieved by Mars Global Surveyor, and the Mars Express subsurface sounding and aeronomy objectives. Included also are studies of the magnetic field and upper atmosphere and its interaction with the solar wind, such as are being currently foreseen for the Japanese  Nozomi mission in co-operation with the Mars Express mission. 

Group 3 objectives are best achieved by simultaneous surface and orbiter measurements. Included here is the desire to sound the atmosphere while operating a network of surface stations. This is currently in the plan for the Mars Express mission possible extension 2008-2009 where instruments like PFS (Planetary Fourier Spectrometer) and SPICAM (Spectroscopic Investigation of the Characteristics of the Atmosphere of Mars) with UV- and IR-channels are operated simultaneously with four NetLander stations. The same task could be carried out by the planned Italian orbiter (2007-) carrying onboard besides a telecommunication package also some scientific instruments for atmospheric and climate observations. 

4.2 Global surface sampling   

Landed science objectives can be divided into two groups: 

      - Those that do not require a network of simultaneously operating stations.

 - Those for which simultaneity is required.  

Most of the geological and geochemical objectives fall in the first group. The main meteorology and seismology objectives fall in the second group. 

The measurements that early geology/geochemistry landers will make are not only valuable in themselves, but, in addition, by providing ground truth for calibration, enhance all the previously acquired remote sensing data.  By providing information on terrain at the lander scale, the landers also provide engineering data for design of subsequent, more ambitious, lander missions. The Viking landers were the first such stations followed by Pathfinder. The number of locations at which such measurements should be made will depend on the resources available. The stations should be distributed so as to sample most of the major geologic units of the planet.

A very important meteorology objective is to establish a climate record, beginning with one or two stations, that are permanent. Locating the landers close to the Viking 1 and 2 landing sites would allow future to be related to the past. Only by these climate observations could it be possible to determine if observations at another site are due to spatial variability or "climate" differences. It would be of special interest to study the role of great dust storms on the climatic behaviour at certain sites. A climate station would need only meteorological instruments and, preferably, a wide-angle camera. 

One disadvantage of surface landers is their limited ability to make measurements below the surface. Because temperature variations damp out rapidly with depth, the stability of soil-volatiles, including water, is very dependent on depth below the surface. Ground ice, for example, is unlikely to be encountered within 1-2 meters of the surface anywhere but at the poles. Currently, a driller capable to reach a depth of 0.5-5 meters is under design. The effects of the UV radiation on soil chemistry are also likely to be less below the surface.  

Rovers are highly desirable on the geoscience landers. They allow viewing of areas away from the immediate vicinity of the landers, provide access to a variety of materials for analysis, can be used to deploy instruments, and can be used for geophysical profiling. Moreover, small or modest size rovers, including moles, on early landed missions will provide valuable engineering data and stimulate technological development needed for more distant, more ambitious missions, such as sample return. The Pathfinder mission included a small (10 kg) rover, and the U.S. will launch in 2003 two heavier rovers (150 kg class) to Mars. The Beagle 2 lander of Mars Express will carry a mole capable of travelling both across the surface and burrowing below. 

4.3 Lander network 

A network of stations is needed to bring about the next major step in understanding.  Relatively simple stations are needed with simultaneous sounding from an orbiter.  The exact number of stations needed to make a satisfactory meteorological network is controversial, but estimates range from several to around twenty. A seismic net would need four or more landers operating simultaneously for at least one Mars year. This contrasts sharply with the geology, geochemistry, exobiology and other geophysics objectives that can be pursued with independent landers, each adding incrementally to the global picture. Even without a network, one or two climate stations would be of immense value. If it is a climate network, even better.

Depending on their design, the stations of a network could carry analytical instruments similar to those on the earlier non-networked stations and so add to the global sampling of the planet. Rovers would be desirable, as with the independent stations, to deploy analytical instruments and to make observations away from the immediate vicinity of the landing site. Some deployment device is very desirable on landers that carry seismometers in order to deploy the seismometers away from landers, to reduce interference from lander-induced noise. 

The requirements for a meteorology network and a seismic network are different. A seismic network requires relatively few stations (4-10), but each station must have a high data rate (>10 MB/day), which has implications for power and for data storage and processing. In contrast, the power and data rate needs for meteorology stations are more modest and such stations could be very small. However, a robust meteorology network requires more stations (16+) than a seismic network, and simultaneous sounding and observation of the atmosphere provides a many fold increase in the scientific return.   

4.4  Communications orbiters 

Presence of an orbiter relay capability at Mars enormously simplifies implementation of a lander program. A requirement to transmit science data directly to Earth places major demands on the power and telecommunications capabilities of the landers. Without communication orbiters, the lander's solar panels, antennae, and electronics must be significantly larger, with a consequent increase in structural mass. These mass increases propagate back and multiply through the entry, descent, and lander system to the launch mass. As a result, the landers become large, complex, costly and of limited science capability. Without a communications orbiter, the small meteorology stations described in the previous section, for example, may not be possible. All these problems become more acute at higher latitudes where Solar power is less efficient and communication with Earth more difficult. With communications orbiters, these complexities are avoided and lander design can be flexible and tailored to specific science needs rather than to the demands of the direct link. 

Considerations such as these led to a design of the Mars Express lander and NetLanders that enable them to communicate with the Mars Express orbiter. Because of the enormous implications of orbital communications for any lander program, every opportunity should be taken to place orbital relay capabilities at Mars. Future remote sensing orbiters should all carry relay capabilities, and the possibility of dedicated communications orbiters, in the framework of the micromissions opportunity, should be explored. In 2007 two more telecommunication orbiters are planned to be launched towards Mars. 

4.5 Advanced missions 

There is an almost universal agreement that sample return and the kind of in situ science that capable, versatile rovers could perform have a high science priority. Their deferral in the strategy simply reflects the high cost of such missions and the likelihood that they will not be funded in the next few years. Nevertheless, one objective of the early exploratory program should be to prepare the way for these advanced missions. Global characterisation of surface chemistry, mineralogy and terrain from orbit, characterisation of multiple surface sites, determination of engineering properties of the surface materials, gaining experience in operating rovers at the surface, and developing entry, descent and landing systems will all contribute to this end. 

4.6 Mission scenario up to 2011 

This section describes the international mission scenario envisioned by the IMEWG. The U.S. has initiated a program of Mars exploration called the Mars Surveyor Program with a sequence of launches. Japan has launched to Mars its first planetary mission in 1998. The European Space Agency has joined the Mars research programme with Mars Express to be launched in 2003. The French (CNES) and Italian (ASI) space agencies together with some other European groups are planning major investments in Mars research in the 2005-2011 time frame. Below are listed the mission scenarios known by October 2000. The missions that have failed are not listed. 

Mars Pathfinder (launched in 1996) 

The spacecraft landed on Mars successfully on 4.7.1997. The main objectives of the U. S. Mars Pathfinder mission were to develop and demonstrate a low cost entry, descent and landing system that could be used for subsequent missions. It did, however, carry some instruments of scientific interest including a stereo, color camera, a meteorology experiment, a magnetic properties experiment, and an entry experiment to determine the vertical structure of the atmosphere. It also carried a small rover that was able to move several to tens of meters from the lander. The rover carried cameras and an APX to analyse local rocks and soils. A dust deposition experiment and its wheels used to carry out extensive soil property experiments at many locations.  

Mars Global Surveyor (launched in 1996) 

The U.S. Global Surveyor mission, one element of the proposed multi-mission Mars Surveyor Program, is intended to recover part of the Mars Observer objectives. However, the spacecraft, launched on a Delta, is considerably smaller than the original Mars Observer spacecraft and the full Mars Observer payload cannot be accommodated.  It carries a wide angle camera to monitor global atmospheric and surface changes, a high resolution (1.5 m) camera to obtain high resolution terrain samples, a thermal emission spectrometer to map variations in the mineralogy of the surface and to obtain temperature profiles of the atmosphere, an altimeter to determine surface topography, a magnetometer to map the magnetic field and its variations, and a multi-purpose radio science experiment, that is expected to greatly improve determination of the gravity field, and provide extensive, high resolution vertical atmospheric soundings. It carries a relay antenna that could be used to relay back to Earth. It arrived at Mars in Sep.1997. All instruments work as planned and after arrival at Mars it spent about 1,5 years using aerobraking by the atmosphere to get into the desired mapping orbit. Systematic mapping of the planet is continuing. 
 
 
 

Nozomi (Planet-B, launched in 1998) 

In l998 Japan launched to Mars their first planetary mission, Planet-B, which after successful launch was renamed as “Nozomi”. It was launched on an M-V-3. It will start orbiting Mars in early 2004. The objectives of the mission are two-fold:

  1. to study the Martian upper atmosphere and its interaction with the solar wind, and
  2. to develop technologies for future planetary missions.

      The scientific objectives are threefold. The first set of objectives concern the structure and dynamics of the upper atmosphere, and include measurements of the structure, composition and dynamics of the ionosphere, the effects of interaction of the upper atmosphere with the solar wind, and the escape of atmospheric constituents. The second set of objectives concern the magnetic field, and include measurements of the intrinsic field, the penetration of the solar wind magnetic field, and the structure of the magnetosphere. The third set of measurements concern dust, and include experiments to measure dust in the upper atmosphere and in orbit around Mars. The observations will be co-ordinated with ESA’s Mars Express mission orbiting simultaneously Mars.

Mars Odyssey (to be launched in 2001) 

The U.S. will fly  an orbiter to Mars carrying three scince instruments. The Gamma-Ray/Neutron Spectrometer (GRS) is the last of the instruments to be reflown that were on the Mars Observer spacecraft that failed at Mars orbit insertion in 1993. The instrument will globally map the chemical composition (distribution of the 20 most abundant elements) of the surface at a spatial scale of 300 km. The Spectrometer is also sensitive to the near-surface water. The two other instruments are the Thermal Emission Imaging System (THEMIS) and Mars Radiation Environment Experiment (MARIE).

The 2003 Opportunity 

The launch window is best in nearly 20 years. Hence, several missions are under preparation or consideration for this period. 

Mars Express and Beagle-2 

ESA’s Mars Express is partially a recovery mission for the ill-fated Russian Mars -96 mission where scientists from several ESA member states lost their instruments. The selection of the payload has closely followed the recommendation passed by IMEWG in December 1996. The scientific objectives of the mission include the remote and in-situ study of the surface, subsurface, atmosphere and environment of planet Mars. The payload is composed of seven experiments and a lander, Beagle-2, to study the geomorphology, geology and atmosphere-surface interaction and also to seek indicators of the possible existence of past life. The orbiter will also carry a telecommunication package to serve various landers in the 2004-2007 (or possibly later) time frame. 
 
 

Two large Mars rovers 

In 2003, NASA plans to launch two large rovers to two very different locations on Mars. Using drop, bounce, and roll technology, these rovers are expected to reach the surface of the Red Planet in January, 2004. The rovers will weigh about 150 kilograms each. These new robotic explorers will be able to trek up to100 meters across the surface each Martian day (sol). Surface operations will last for at least 90 sols, extending to late April 2004, but could continue longer, depending on the health of the rovers.The Mars rovers will carry sophisticated sets of instruments that will allow them to search for evidence of liquid water that may have been present in the planet's past, as well as study the geologic building blocks on the surface. The exact landing sites have not yet been chosen, but one site is likely to be a location such as a former lakebed or channel deposit – a place where scientists believe there was once water. The sites will be selected on the basis of intensive study of orbital data collected by the Mars Global Surveyor spacecraft, as well as the Mars 2001 orbiter, and other missions. 

The missions planned in 2005 and beyond are not fully known in October 2000. The list below has to be considered as tentative and the readers are requested to check current situation from the www pages of different space agencies.  

The 2005 Opportunity 

NASA is planning to send one orbiter (Mars reconnaisance orbiter) to recover the objectives of Mars Climate Orbiter and to perform reconnaissance of future landing sites using high resolution (up to 20 cm in accuracy) imaging and/or spectro-imaging instruments. 

The 2007 Opportunity: NASA 

NASA is planning to send one lander to Mars in order to test and demonstrate safe landing techniques. The instruments onboard will serve manned flight preparations (Human Exploration and Development of Space (HEDS) experiments) and a variety of other in situ experiments will be selected through an international announcement of opportunity process. The lander could also be a long-range, long-duration mobile  science laboratory to prepare for a future sample return mission. 

The 2007 Opportunity: In Europe 

The launch window in 2007 seems to be reserved for several European missions to be implemented. CNES will test aerocapture techniques in the Martian atmosphere with a spacecraft that will remain orbiting Mars. The orbiter will have scientific payloads to study Mars remotely and a telecommunication relay unit (X band) designed to also support some radioscience experiments. NASA will provide an UHF unit for ground communication purposes. This orbiter will be launched to Mars using an Ariane 5 launcher. 

Along with the orbiter four 66 kg NetLanders will be transported to Mars. They will be separated before arrival and then by using parachute and airbag techniques brought to Martian surface at four different locations. NetLanders will carry meteorological and seismic experiments to study the dynamics of the Martian atmosphere and to study the internal structure of the planet by seismic sounding. Also some in situ geophysical parameters, like magneto-electrical properties and morphological aspects of the environment will be studied by different techniques. 

A separate orbiter (TELEMARS) will be provided by ASI. It will serve as a second telecommunication orbiter having a polar orbit.  

The 2009 Opportunity 

Joint NASA and ASI scientific orbiter with SAR as the main instrument. 

The 2011 Opportunity 

NASA and CNES are planning to carry out the sampling return mission by sending a lander together with a Mars Ascent Vehicle (MAV, rocket) to Mars and a French orbiter dedicated to recover the samples placed in orbit using the MAV. The orbiter will then return to Earth and a Earth Entry Vehicle will be released that will bring the samples back to Earth 3 years later for laboratory analysis.  

5.  SUMMARY 

Of the different objects within the solar system, Mars has long had a special appeal, because of its resemblance to the Earth, the variety of science issues it poses, the possibility that life might have started there, and its potential for eventual human exploration.  As a result all space agencies involved in planetary exploration are interested in participating in some way in Mars exploration. Yet in this era of fiscal constraint, individual agencies and nations are finding it difficult to mount their own independent Mars exploration programs. One way of sustaining a healthy exploration program in this environment is through international co-operation. The International Mars Exploration Working Group acts as a forum for exchange of information on national and agency plans for Mars exploration, and facilitates international co-operation in planned missions. The present plan for Mars Exploration includes missions to Mars at every launch opportunity up to the year 2011 and later. The plan is tentative and will inevitably require change as some expectations are realised and others are not. 

There have been recent dramatic losses that have influenced and changed future planning. However, in several cases different elements of the plan are in different stages of implementation and will not change due the losses. The mapping program of the Mars Global Surveyor is continuing. Nozomi is on its way to Mars. In 2001, the U.S. is to launch an orbiter for mapping of the geochemical properties of Mars. In 2003, ESA is to send its first spacecraft to Mars with a lander and the U.S. will send two large rovers. In 2005 NASA plans to launch a powerful scientific orbiter to analyse the surface at totally new scales. In 2007 safe landing with a larger platform will be tested and preparation for manned flights are started by including separate HEDS-experiment packages on the lander. In 2007 a first network of landers will be placed on the surface of Mars. Also the building of a (permanent) telecommunication infrastructure with two separate orbiters will start. In 2011 the mission to fetch samples from the surface will be launched. Besides these large elements there might be separate small elements in a form of micromissions addressing scientific and technical issues that are not covered by larger missions. Such missions might be implemented in 2005, 2007, and 2009.      

Because of the vulnerability of the exploration plan to funding and implementation problems, the International Mars Exploration Working Group will continue to meet to discuss implementation plans and to modify the exploration strategy should it become necessary. Moreover, several aspects of Mars exploration, in addition to the building, instrumenting and launching of vehicles to Mars, could benefit from an international approach. Examples are communication protocols between different vehicles, establishment of a communications infrastructure at Mars, adoption of uniform standards for scientific data, sharing of deep space communications facilities, and preparation of advanced missions such as sample return. Formulation of an international strategy is, thus, only a first step toward a broad international co-operative approach to Mars exploration.