A month from now, the
Mars Science Laboratory
(Curiosity) rover is set to touch down on the surface of the Red Planet
and begin its mission to learn more about the possible existence of
life - past or present. Curiosity will attempt to touch down using a
complex and unusual landing sequence unlike any other used for previous
Mars rovers ... here's how the plan will unfold.
The challenge
In the past, NASA's preferred modus operandi for landing Mars rovers
has been to wrap them into a spheric "airbag" that breaks the fall and
absorbs the impact with the terrain. This time around NASA is going for a
much more complicated, multi-stage approach that seems to have come out
of a science fiction movie.
Among the stages are a sophisticated rocket-guided entry system, a
huge supersonic parachute that will be traveling almost parallel to the
Martian surface, and a skycrane that will tether the rover directly onto
the Martian surface while hovering just a few feet above. The entire
process will be executed completely autonomously, managed not by human
intervention, but by a computer algorithm made of some 500,000 lines of
code. The success of this ambitious US$2.5 billion mission lays in the
balance.
"Most people look at this system - particularly the skycrane at the
end - and they say, 'What are you guys thinking, are you out of your
mind?,'" says Pete Theisinger, project manager of the Mars Science
Laboratory. "But the vehicle is too big and heavy for airbags."
Curiosity weighs 2,000 lbs (making it five times as heavy as the
Spirit and Opportunity rovers launched in 2003) and carries an
impressive 180 lbs of science payload. Theisinger says that, for its
size, this is the safest, simplest landing sequence that NASA could
muster.
During the landing phase, the main challenge is that Mars's
atmosphere is 100 times thinner than Earth's - thick enough that
engineers need to worry about a heat shield, but not quite thick enough
to slow the spacecraft fast enough to prevent it from crashing to the
ground at high speed. Altitude on Mars ranges from minus 4 to plus 12
miles (minus 6 to plus 20 km) and the whole of the southern hemisphere
has positive altitude. Until now, no attempt has been made to explore
this region because engineers need the extra space to slow down the
rovers.
This is going to be a very risky landing. Only 40 percent of missions
to Mars have been successful, either because of engineering problems or
because of the hostile Mars environment. But at the very least, should
the landing falter, the data collected on it by the three current Mars
orbiters - Mars Express, Odyssey and Mars Reconnaissance Orbiter - will
help scientists learn from their mistakes and increase the probability
of success in future missions.
The testing phase
The technology behind the landing is an interplay of hardware and
software. On the software side, the computer algorithms that guide each
part of the craft can be tested from Earth, simulations can be run, and
new software updates can be installed - the final stable version was
uploaded in the last few days of May.
Testing the hardware was not nearly as easy, since the right
conditions can't be recreated on Earth. "One of the problems you have
with entry and descent landing with any Martian vehicle is, how do you
test it on Earth? We have the wrong atmosphere, the wrong gravity, and
we would need to start at 13,000 mph outside the atmosphere," says
Theisinger.
NASA's answer was to construct a long series of compartmentalized
tests, and to then stitch them together using computer simulations. The
individual tests were quite elaborate, and the scientists often had to
go to great lengths to simulate the conditions they would be facing on
Mars.
To test the radars that will help direct the thrusters toward the
landing site, the devices were flown on a helicopter over a desert
landscape (representative of the Martian terrain). To characterize the
high-velocity, high altitude portion of the landing sequence, the
equipment was put on a F-18 accelerating toward the ground (each dive
only gathered about six seconds worth of data).
Landing on Mars, with style
In the context of Mars exploration, the landing ellipse describes the
area inside of which a rover has a 99 percent chance of landing.
Previous Mars landers (Spirit, Opportunity, Pathfinder and Phoenix) have
operated with a ballistic landing system that meant a very elongated
landing ellipse: Pathfinder, for instance had a 185 by 9 miles (300 by
15 km) ellipse, and the limited mobility of the rovers meant that
scientist have had very little control over exactly which terrain the
rovers would find themselves in.
By contrast, Curiosity will use guided entry, including thrusters
during the supersonic phase of the mission, to achieve a much smaller
landing ellipse of only 4 by 12 miles (6 by 18 km). This allows
scientists to select landing sites that would have otherwise been
inaccessible, with the potential of a much greater scientific payoff.
Built to operate for at least two Earth years, Curiosity will be the
first mission in which the rover will be able to venture outside its own
landing ellipse.
The landing sequence will start at 13,200 miles (21,000 km) above the
planetary surface, and will last only seven minutes. At the date of the
scheduled landing, Earth and Mars will be separated by 14
light-minutes. The process will therefore be performed completely
autonomously by the spacecraft, and it will be a grueling few minutes at
NASA and around the world before news on the result, whether good or
bad, reaches Earth.
Ten minutes before hitting the atmosphere, the "cruise stage" of the
craft will separate and the final preparations for entry begin. Hitting
the atmosphere at 13,000 mph, the spacecraft will start to slow down
while using thrusters, guided by radars and data from the Mars orbiters,
to help steer toward the landing target.
A supersonic parachute will be deployed to slow the craft down to the
speed of sound and enable the rover to descend on an angle almost
parallel to the Martian ground, gaining more time to slow the craft
down. Meanwhile, the heat shield will separate to clear the view for
MARDI, the rover's camera system, which will hopefully provide us with a
spectacular, hi-def video of the descent at eight frames per second.
At an altitude of about a mile and speeds of 200 mph (320 km/h), the
craft will then fire up its six landing engines, bringing the rover down
very gently to only a few yards of altitude. The rover will then deploy
its wheels and - this is the fancy part - a skycrane will slowly start
lowering the vehicle to the ground.
After detecting touchdown, the skycrane will remove its tethers and
fly away to a controlled crash far from the landing site, leaving the
rover on the surface of Mars. Or, at least, that is the plan.
The NASA video below illustrates the different phases of the landing.
Source:
NASA
