If you threw your best camera out of a fast-moving car, you might not
expect it to work very well afterwards. European space teams are
preparing to hurl cameras and other pieces of precision engineering
onto the surfaces of other worlds, and hope that they will remain
capable of making big scientific discoveries. During the next dozen
years, at least eight landers decorated with flags of Europe will find
their destinations in widely scattered parts of the Solar System.
Five will land on Mars. They will hit the planet’s surface at 70-100
km/h, protected by airbags like a car driver in a crash. Without that
protection, another lander will thump at 20 km/h onto Saturn’s very
cold moon Titan. This will be the most distant landing ever attempted,
far from the Sun. In the opposite direction, uncomfortably close to
the Sun, the planet Mercury will receive a lander, in another 100
km/h, airbag scenario.
An eighth European lander will head for the nucleus of Comet Wirtanen.
Here the problem is not a hard impact but the comet’s extremely weak
gravity. This might allow the lander to bounce off and be lost in
space, and would give no push to a drill penetrating the surface. So
the plan is to harpoon the comet and cling to it.
“These are real adventures into the unknown,” says Jean-Pierre
Lebreton of ESA’s Space Science Department. “On Titan, for example,
we can’t even say whether the probe will come down on solid ice
or splash into a sea of hydrocarbon. So we had to plan Huygens’
measurements, once on the surface, for either possibility.”
Huygens on Titan
ESA’s Titan probe, called Huygens, is the largest of the European
landers. It is named after Christiaan Huygens, the 17th-Century Dutch
astronomer who discovered Titan. Riding on NASA’s Cassini spacecraft,
launched in 1997, Huygens is already well on its way.
Around Christmas Day 2004 Huygens will part from Cassini, and on 14
January 2005 it will hit Titan’s dense atmosphere like a meteorite,
travelling at 20,000 km/h. A heat shield will absorb the atmospheric
friction and reduce the speed to 1400 km/h. Then the first of a
succession of parachutes will be deployed, to carry the descent module
of Huygens, with a mass of 200 kilograms, down to the surface in about
two hours.
During the slow descent, onboard instruments will analyse the chemical
constitution of Titan’s mysterious, hazy and very dense atmosphere.
Others will gauge the weather and send images of clouds and the
surface seen from lower and lower altitudes. Strong winds will make
the landing site unpredictable.
When Huygens eventually thuds or splashes onto the surface, it will
start an inspection with a specially designed package, and with the
other instruments that have given their best during the descent. But
the temperature is minus 180 degrees C and time is running out. Under
the distant Sun, made even fainter by Titan’s haze, solar cells would
be useless, so Huygens relies on chemical batteries. Less than half
an hour after landing — or perhaps more — either the batteries will
be exhausted or the delicate electronic equipment will get too cold.
The adventure will be over — but not before Huygens has given the
scientists new clues to conditions on the Earth long ago, when life
began.
Beagle 2 on Mars
Although launched long after Huygens, the first of all the landers
from Europe will arrive earlier at a much nearer target, Mars, in
December 2003. This British-led project is named after the ship that
carried Charles Darwin on the voyage that nurtured his ideas about
evolution. Beagle 2 will look especially for chemical traces of life
on Mars, whilst also reporting on the weather and soil at the Martian
surface.
It will travel to the Red Planet on ESA’s Mars Express, to be launched
in late May or early June 2003. Like Huygens, Beagle 2 has a heat
shield for its impact on the atmosphere, and a parachute system for
the descent. But the atmosphere of Mars is much less dense than
Titan’s. The chosen landing site, on the edge of the Martian plain
called Isidis Planitia, is low-lying, to let the parachute reduce the
impact speed as much as possible. But at 100 km/h, protective airbags
are a necessity.
The landed mass of Beagle 2 will be around 30 kilograms. It will
unfold solar panels, which will recharge the batteries every day. The
lander should operate on the surface of Mars for at least 6 months,
and perhaps for a whole Martian year (1.8 Earth years).
A robotic arm on Beagle 2, carrying most of the instruments, can
change the pointing of the cameras. It can also deploy a burrowing
‘mole’. This will gather subsurface soil samples at up to three metres
from the spacecraft, before being winched back to bring the samples
for analysis by onboard instruments.
Four NetLanders on Mars
The first-ever network of landers will arrive on Mars in August 2008,
in a project led by the French space agency, CNES, with major
contributions from Belgium, Finland and Germany. Four identical
vehicles called NetLanders will be ejected one by one, at intervals
of a few days, from a French spacecraft that will then go into orbit
around the planet. The landers will be scattered widely, across a
range of latitudes north and south of the equator.
Space scientists have dreamed for many years of having such a network
of landers, primarily to observe marsquakes that may shake the planet
many times each year. Geophysicists probe the Earth’s interior, using
the arrival times of earthquake waves at different seismic stations.
By doing the same thing on Mars, they can compare the metallic cores
and interior layers of different rocks, to understand both planets
better.
The descent will be similar to Beagle’s. Each NetLander will have a
landed mass of 25 kilograms and is expected to operate for a full
Martian year. The seismometer to register quakes will rest directly
on the ground, disconnected mechanically from the rest of the lander,
but still sheltered by it from the Martian weather.
Each lander will also explore its own surroundings with a stereoscopic
camera, and probe for subsurface water beneath it, using radar,
magnetic and seismic methods. A complete meteorological package means
that the NetLander network will also give an impression of global
weather on Mars.
A lander on Mercury
Parachutes won’t work on the planet Mercury because it is airless,
like the Moon. Early Soviet and US lunar missions, including the
manned Apollo landers, used retro-rockets fired downwards to slow
the descent. These contaminated the landing site, but in 1966 Soviet
lunar landers first showed that they could separate from their
retro-rockets and fall elsewhere, cushioned by airbags. ESA’s
BepiColombo mission will deliver a lander to Mercury in 2012 by that
method.
BepiColombo will use a solar-electric rocket to speed it on its
interplanetary journey. A faster-acting chemical rocket will then
inject two spacecraft into orbit around Mercury and become available
as a retro-rocket for the lander. After coming to a standstill at a
height of 120 metres, the rocket will separate and the 44-kilogram
lander will fall freely to the ground, hitting it with airbag
protection at 100 km/h.
Mercury is extremely hot on the sunlit side and cold on the shaded
side. The lander will go to a polar region where the temperatures
are less extreme. It is quite likely to land at a site in permanent
shadow, so it will rely on a chemical battery, capable of sustaining
it for a week.
Scientific instruments are likely to include cameras, using
flashlights if necessary, and devices for observing heat flow,
chemical elements, and magnetism. A ‘mole’ will tunnel several metres
below the surface, and a micro-rover will be able deploy instruments
at a distance from the lander. A special reward could be the
detection of volatile materials in a shaded landing site — perhaps
even water ice.
Rosetta Lander on Comet Wirtanen
The need to strap the lander to its low-gravity target with a
harpoon is not the only exceptional feature of the German-led project
to put instruments on the surface of a comet’s nucleus in 2012. Aboard
ESA’s Rosetta spacecraft, launched in 2003, the journey will last
even longer than Huygens’ voyage to Titan. And no one really knows
what the target, Comet Wirtanen, looks like — nor how big and heavy
it is.
The main Rosetta spacecraft must orbit around the comet and examine
it, before scientists decide where the 90-kilogram Rosetta Lander
should go. Pushing off the lander towards the chosen spot will
require delicate navigation. All this will happen far beyond the
orbit of Mars, in sunlight with only one-tenth of the intensity near
the Earth. The lander will nevertheless rely on solar cells to keep
its battery charged and its instruments operating.
Several cameras, including a microscope will examine grains of the
comet, while other instruments analyse its chemical composition. The
strength, density, porosity and thermal properties of the comet’s
surface and sub-surface materials will be gauged too. Radio waves
sent through the comet from the orbiter to the lander will in effect
‘X-ray’ the comet’s interior.
How long the Rosetta Lander will survive is anyone’s guess. As the
comet swoops in towards its closest approach to the Sun, in 2013,
intensifying sunlight will improve the power supply. It will also
provoke the comet to release jets of vapour and dust from its
surface, which may engulf the lander — but perhaps not before the
instruments give close-up impressions of how an eruption alters
other parts of the comet’s surface.
The future of landers
A growing community of engineers and scientists in Europe now builds
and equips planetary landers. Although for simplicity the various
projects have been described as ESA-led, or British, French or
German-led, there is multinational participation in every case, and
collaboration with US and Russian lander specialists. A Japanese
project (Lunar-A, 2003) for driving instrumented penetrators into
the Moon’s surface at 1000 km/h, widens the community still further.
Ever smaller, neater, yet more powerful instruments can be expected
in future landers, taking advantage of the techniques of
miniaturization evolving in many branches of research. NetLander
foreshadows other missions that may scatter dozens of small landers
across a planet’s surface. Greater mobility will come from long-
range surface rovers (NASA, 2007) and perhaps even small robot
aircraft.
Soviet unmanned spacecraft returned lunar soil and rock samples for
analysis in terrestrial laboratories, in the 1970s. A US-French
sample-return project aims to do the same for Mars. Whether for
samples to be returned, or for analysis with onboard instruments on
landers, better methods of drilling deep into hard surfaces are
continually under review.
“We’ve seen huge advances in lander technology and science, in just
ten years since Huygens was designed,” comments John Zarnecki of the
UK’s Open University, who is involved with Huygens and Beagle 2,
and also in studies for the Mercury lander.” Now we’re looking at
instruments from the oil industry and even from archaeology, not
tried in space so far. And we’ll pass on techniques the other way,
so that skills developed for exploring other worlds can help in
investigating difficult parts of our own planet, like Antarctica
and the ocean floor.”
Related links
* ESA’s Huygens website
http://sci.esa.int/home/huygens/
* ESA’s Mars Express website
http://sci.esa.int/home/marsexpress/
* Beagle 2 lander homepage
* Netlander
http://www-projet.cst.cnes.fr:8060/NETLANDER/
* ESA’s Bepi-Colombo website
http://sci.esa.int/home/bepicolombo/
* ESA’s Rosetta website
http://sci.esa.int/home/rosetta/
IMAGE CAPTIONS:
[Image 1:
http://www.esa.int/export/esaCP/ESAZXCZ84UC_FeatureWeek_1.html]
The Huygens Descent Module landed safely in the snows of Sweden after
successfully testing its parachute systems. The Front Shield and Back
Cover, which will protect the Descent Module when it enters Titan’s
atmosphere, separated correctly. For this test, an extra parachute
was used for the final stages to ensure a soft landing in Earth’s
gravity.
[Image 2:
http://www.esa.int/export/esaCP/ESAZXCZ84UC_FeatureWeek_1.html#subhead1]
ESA’s Huygens probe descends through Titan’s mysterious atmosphere to
unveil the hidden surface (artist’s impression)
[Image 3:
http://www.esa.int/export/esaCP/ESAZXCZ84UC_FeatureWeek_1.html#subhead2]
Beagle 2 will travel to Mars on ESA’s Mars Express, due for launch in
the summer of 2003. Beagle 2, a UK-led project, will look fro chmecial
traces of life on Mars. On the Martian surface, the lander will open
up to expose five solar panels and let the instruments on the robot
arm get to work. (Photo: all rights reserved Beagle 2)
[Image 4:
http://www.esa.int/export/esaCP/ESAZXCZ84UC_FeatureWeek_1.html#subhead3]
With observations from four widely spaced landers, relayed via an
orbiter, the French-led NetLander project will use seismic tremors to
probe the interior of Mars.
[Image 5:
http://www.esa.int/export/esaCP/ESAZXCZ84UC_FeatureWeek_1.html#subhead4]
After arriving on Mercury’s surface during ESA’s BepiColombo mission,
this lander is expected to deploy instruments with a micro-rover (left)
and to penetrate the surface with a ‘mole’ (below).
[Image 6:
http://www.esa.int/export/esaCP/ESAZXCZ84UC_FeatureWeek_1.html#subhead5]
The Rosetta Lander is a German-led project, carried out by an
international consortium of scientific institutes and institutions
in order to investigate a comet’s nucleus in situ for the first time.
In 1999, an engineering model was severely tested to ensure that the
probe will survive shaking during launch, and extreme temperature
variations during its 9-year voyage to its target comet (Image: DLR)
