Since 2000, the NASA Mars Exploration Rover mission (MER) has
been the main objective of hundreds of scientists and engineers;
their life being completely dedicated to the mission for years.
In January 2004, two twin rovers —
Spirit and Opportunity — landed on
different sites on Mars. Newspapers
and TVs showed beautiful close-up
images of the Martian surface taken
from the rovers (see Figure 1).
Biologists around the Earth have
been amazed by the discovery of geological evidence of water in Mars’ past, thus
confirming the intuition that the Red
Planet can sustain future life. As a matter
of fact, MER has been the most successful mission ever: Spirit and Opportunity
operated for three Earth years, each one
exploring over eight miles of Martian
terrain. This means an unprecedented
mobile surface exploration.
Nonetheless, Spirit and Opportunity
were not the first to navigate on Mars:
Pathfinder and Sojourner were there in
1996, even if they were only an
engineering test-bed mission to validate technology for surface mobility.
However, both Spirit and Opportunity
have greatly exceeded most of their
initial requirements.
Among the major MER objectives,
we can identify: (1) looking for past life
through the study of the planet soil
and the discovery of the presence of
water; ( 2) understanding the climate of
the planet which is supposed to have
been a “green heaven” in the past; and
( 3) performing experiments both in the
field of geology and in studies of
the atmosphere in order to prepare
for future human exploration and
settlements (see Figure 2). In other
words, planetary astrobiology.
These objectives require that tens
of kilometers must be traversed in
order to measure and study biodiversi-ty. From a rover’s perspective, this turns
out to be a daunting task: long
distance mobility and autonomy are the
two major issues in mobile robotics.
Rovers cannot be tele-operated.
From Earth to Mars, there are over 300
million miles and the response times
would cause rover ungovernability
caused by the extremely limited bandwidth. Despite that, the MER’s mission
assessed a number of key points and
techniques which paved the way for all
future planetary exploration research
agendas: remote planning, command
sequencing and validation on Earth,
data gathering, and reduced independence on Mars.
This article focuses on the entire
control loop which is used to allow a
rover to safely navigate on Mars
using information processed on Earth,
ranging from data acquisition to low
level command actuation.
A Typical Control Loop
A mission on Mars is limited by the
fact that rover work cycles are tied to the
FIGURE 1
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