April 1982:the Kepler Mars orbiter

Kepler, named after the 17th century astronomer Johannes Kepler, is an ESA proposal to place an unmanned geophysical research satellite in orbit around Mars, with launch opportunities in 1988 and 1990. Kepler will be a logical step in the first stage of unmanned Martian exploration, filling the gap left by the American Mariner and Viking and the Soviet "Mars” probes. Photographic reconnaissance and a search for evidence of life were the main scientific objectives of the NASA probes, but they gathered relatively little geophysical data. This is Kepler’s role. (The Soviet probes did return some geophysical data but it was patchy and inconclusive - much like the "Mars” series of probes overall).

The limitations of Mariner and Viking in this respect were recognised by the Mars Science Working Croup charged by NASA to study post-Viking missions. This Group suggested the inclusion in a very large “Mars 1984” mission (not approved) of a “Mars polar orbiter that would conduct a comprehensive global mapping of the geochemical provinces of the planet, its atmosphere, its gravity field, its magnetic field and the charged particles in its vicinity.” Today we know that, after the NASA Pioneer Venus Orbiter mission (a mission comparable to Kepler, but around Venus) a similar geophysical orbiter of Mars could cover most of the areas of interest not studied by Mariner 9 and Vikings 1 and 2.

Kepler’s scientific objectives are distributed over three broad areas: atmospheric science, interaction with the solar wind, and planetary surface and interior science.

Atmospheric Science

Fascinating glimpses of the climatology and meteorology of Mars have been obtained by previous space missions, but the subjects have never been systematically studied. To do that requires dedicated remote sounding instrumentation on a probe in an orbit inclined enough to the Martian equator to eventually cover all of the surface. Surveys by earlier craft have revealed the basic properties of the atmosphere: composition mainly carbon dioxide, surface pressure less than 1% of Earth’s, varying strongly with season; vigorous circulation, strongly modified by surface topography. The latter, in turn, evolves under the influence of wind erosion and large-scale transport of dust; wind-blown dust is an important mechanism on mars for heating and cooling the atmosphere.

Kepler's objectives are: (1) to determine the structure, composition and dynamics of the upper atmosphere, and how all of these vary with time, location and solar activity; (2) to monitor the global temperature pattern to permit an understanding of atmospheric circulation and its driving forces (measurements of the vertical temperature profile by the descending Viking landers were naturally very limited in coverage, in both time and area); (3) to study the distribution of airborne dust and the onset and evolution of major dust storms; (4) to study seasonal pressure variations (discovered by Viking); (5) to investigate ozone amounts (found by Mariner 9 to vary by a factor of 100 between the winter and summer hemispheres).

Interaction with the Solar Wind

A primary objective of Kepler is to determine the nature of the interaction between the solar wind and Mars. Two types of interaction have been proposed: (1) an ionospheric Venus- like interaction in which the pressure of the atmospheric charged particles holds off the solar wind; (2) a magnetospheric Mercury-like interaction in which the pressure of the planetary magnetic field holds off the solar wind. Recent studies tend to support the idea that Mars’ solar wind interaction represents an intermediate case between these two.

None of the spacecraft flown around Mars to date has had good instrumentation for measuring the magnetic field or the solar wind (Mariners 6, 7 and 9, and the two Vikings had no such instruments). Mars 2. 3 and 5 did not approach closer than 1,100 km and the information they did pick up suffered from alow 'bit rate’ of return and gaps in the data (two to ten minutes long). The Soviet probes did detect a local magnetic field, and also - together with Mariner 4 - what appeared to be a bow shock in the solar wind flow close to Mars. But none of these craft directly sampled the region where the solar wind actually interacts with the upper atmosphere, ionosphere, or magnetosphere. Kepler will do so, using a plasma analyser, a plasma wave detector, and a three-axis magnetometer.

Surface and Interior Science

The study of Mars’ internal structure and the dynamics of the interior has not been even a secondary objective of previous missions. These topics will be studied by Kepler through a better knowledge of the gravity field, a determination of Mars’ topography, and a study of its magnetic field.

Two-way Doppler tracking data from the Mariner 9 and Viking missions has been used to compute a model of the Martian gravity field, and gravity anomalies directly associated with visible topographic features such as Olympus Mons and Valles Ma rineris have been discovered. The accuracy of these results depend strongly on the periapsis altitude (the lowest point of the orbit: Mariner 9, 1,500 km; Viking I & 2. 300 km) and also the location of the periapsis on the surface of the planet (Mariner 9’s periapsis was within a few degrees of 23 degrees latitude; Viking orbiters had a periapsis covering from 45°S to 60°N latitude). Kepler will be a major step forward as it will have a periapsis as low as 150 km and will cover the entire surface. This gravity field experiment requires only good tracking capabilities.

So far, the topography of Mars has been inferred from pressure maps, occultation measurements, very rough ground-based radar observations and photogrammetric reduction of a few Viking pictures. These data suffer greatly from lack of coverage in the polar regions and overall poor accuracy. Kepler’s contribution to topographic study will come through a radar altimeter; this will be the only instrument so far not flown by European scientists but it does have the characteristics needed by Kepler. However, enough experience exists such that its inclusion should not give rise to any worries. A 10 kg radar altimeter was flown on the Pioneer Venus Orbiter.

With Kepler, the opportunity of studying the magnetic properties of Mars is given-high priority, and a magnetometer will be carried for this purpose.

Spacecraft Design

Kepler, with a maximum diameter of 2.8 m, will be spin-stabilised (5 rpm) with a despun antenna. Tracking and data gathering in one revolution out of five (for which one Earth ground station, Weilheim in Germany, is sufficient) is considered scientifically satisfactory.

Useful spacecraft mass in Mars orbit (i.e. excluding motor casing and remaining fuel) is estimated at 302 kg. The craft carries a bipropellant liquid motor for mid course corrections, injection into Mars orbit, orbital modifications and spin control. The motor chosen is derived from an engine developed by MBB and successfully used in Symphonie and currently adapted as the Galileo retro-propulsion module. Kepler will also have ten small rockets for attitude control.

Flight Plan

The mission can be broken down into four phases:

Launch and Trans-Mars Injection

Launch will be from ESA’s Kourou Launch Centre in French Guiana, by one of two developments of the Ariane launcher - either Ariane 3, or Ariane 2 with a fourth stage. The latter allows a better weight margin, the fourth stage allowing a useful spacecraft mass in final Mars orbit of 450 kg, as against 350 kg for Ariane 3 on its own (302 kg is the actual estimated mass under the present proposal). Launch opportunities to Mars occur approximately every two years and the mid-1988 launch window (centred around 17 July 1988 and lasting for about one month) is considered the most realistic, though a backup date in mid- 1990 is possible, with very limited adjustment in the spacecraft configuration for the same scientific mission.

Cruise Phase

Kepler will be monitored for about four hours each day with the payload switched off during this period. Using the 1988 launch window means that the cruise will last about six months. No cruise science is proposed as yet, though the mission could be expanded to provide a baseline in the ecliptic plane for solar wind and solar activity studies being made at the same time by the International Solar Polar Mission. A solar cosmic rays experiment could be added to the payload for this purpose.

Mars Orbit Insertion

The aim will be to establish an apoapsis of 6300 km. periapsis being subsequently lowered to give an orbit of 6,300 km/200 km with an inclination of 80° and period of 4.8 hours.

Mars Mission Operations

Kepler will make five revolutions each day. The periapsis will move around the planet covering the entire surface twice in one Mars year. Kepler will be tracked for at least one Mars year (687 days), giving a planned mission time of 2 years from launch. As during the other three phases, operations will be controlled from the Kepler-Mars Payload Operations Control Centre at ESOC (European Space Operations Centre) at Darmstadt, West Germany.