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.