Not-So-Soft Landings on Other Worlds
26 Nov 2001
(Source: European Space Agency)
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."
[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)