A first for science and humanity

By James Vincent

The Philae lander has become the firstever spacecraft to land safely on a comet after traveling through space for more than 10 years and covering a distance of some 4 billion miles. “We are there and Philae is talking to us,” confirmed Philae Lander Manager Stephan Ulamec from the European Space Agency (ESA) mission control. “The landing gear has been moved inside so we are sitting on the surface – and there’s more data to come but we are there: it’s done its job, we’re on the comet!” Although details of the landing are still emerging, the ESA


hub say that it was “a fairly gentle touch down based on amount of landing gear damping,” but that the harpoons intended to secure the craft did not fire successfully as had first been thought. This means that there’s a possible danger that the lander will not be stable as the comet moves closer to the Sun and becomes more “active” (this happens as the Sun’s rays heat up the surface), but the ESA still stress that Philae is in “great shape” for the moment and that they will re-fire the harpoons shortly. “We have no reason to think it won’t work,” said Paolo Ferri, Head of Mission Operations at the ESA, “but we have no understanding of why it is doing this.” The comet itself is about as big as a mountain: 2.5 miles wide and around 2.3 miles high. If placed on Earth it would be taller than Mount Fuji, although early images of the 67P lent themselves to more unusual comparisons: apparently fused from two separate icy bodies, the comet was most often compared to a rubber duck. Rosetta and Philae have also sent back their first images to Earth – although none from the surface of the comet. Instead, they took snaps of one another as the lander detached from its parent craft. How big is 67P? Bigger than you think. Philae’s ten onboard sensors and instruments will now begin the important work of analyzing material from the comet’s surface as well as the surrounding atmosphere of gas and dust. Rosetta will stay in orbit around the comet for the next year, peeling off sometime around December 2015. The information Philae gathers from the surface of the comet will provide new insight into the mechanisms of our solar system and could even help us answer questions about the origins of life on Earth. It’s long been theorized that life was “seeded” on Earth after hitching an interstellar ride on a comet and previous fly-bys of similar bodies have detected complex organic molecules. If Philae manages to find any water ice or amino proteins (compounds that are key to life) then it could offer some evidence for the theory.

Markers of the mission

Cost: The total cost of the mission is 1.4 billion Euro, of which the total Philae costs are 220 million Euro. This includes the launch, the spacecraft, the science payload (instruments and lander) and mission and science operations. Launch: Rosetta was launched on March 2, 2004 by an Ariane-5 from Europe’s Spaceport in Kourou, French Guiana. Planned mission lifetime: Rosetta’s mission will last for almost 12 years – until December 2015. Spacecraft design: Rosetta resembles a large black box.

The scientific instruments are mounted on the top of the box (the payload support module), while the subsystems are on the “base” (bus support module). On one side of the orbiter is the steerable 2.2 m-diameter communications dish, while the lander is attached to the opposite face. Two enormous solar wings extend from the other sides. Both panels can be rotated through ±180° to catch the maximum amount of sunlight. Mass & dimensions: Approximately 3,000 kg (fully fuelled), including 1,670 kg propellant, 165 kg scientific payload for the orbiter, and the lander weighs about 100 kg. The main spacecraft is 2.8 x 2.1 x 2.0 m, on which all subsystems and payload equipment are mounted. Two 14 m-long solar panels with a total area of 64 m2 provide electrical power.

Facts about Philae lander

Design: The lander’s structure consists of a baseplate, an instrument platform and a polygonal sandwich construction, all made of carbon fibre. Some of the instruments and subsystems are beneath a hood covered by solar cells. An antenna transmits data from the surface to Earth via the orbiter. The lander carries nine experiments, with a total mass of about 21 kg. A drill will sample the subsurface material. Alpha Proton X-ray Spectrometer – APXS: Lowered to within 4 cm of the ground, APXS will detect alpha particles and X-rays to gather information on the elemental composition of the comet’s surface. Rosetta Lander Imaging SystemÇIVA/ ROLIS: It is a CCD camera that will obtain high-resolution images during descent and stereo panoramic images of areas sampled by other instruments. Six identical micro-cameras will take panoramic pictures of the surface. A spectrometer will study the composition, texture and albedo (reflectivity) of samples collected from the surface.

Comet Nucleus Sounding –

CONSERT: It will probe the internal structure of the nucleus. Radio waves from CONSERT will travel through the nucleus and will be returned by a transponder on the lander. Cometary Sampling and Composition experiment – COSAC: It is one of two ‘evolved gas analysers’. It will detect and identify complex organic molecules from their elemental and molecular composition. Evolved Gas Analyser – MODULUS PTOLEMY is another evolved gas analyser that will obtain accurate measurements of isotopic ratios of light elements. Multi-Purpose Sensor for Surface and Subsurface Science – Mupus: This will use sensors on the lander’s anchor, probe and exterior to measure the density, thermal and mechanical properties of the surface.

Rosetta Lander Magnetometer and Plasma Monitor – Romap: This is a magnetometer and plasma monitor that will study the local magnetic field and the interaction between the comet and the solar wind. Sample and Distribution Device – SD2: This device will drill more than 20 cm into the surface, collect samples and deliver them to different ovens or for microscope inspection. Surface Electrical, Seismic and Acoustic Monitoring Experiments – SESAME’s: These three instruments will measure properties of the comet’s outer layers. The Cometary Acoustic Sounding Surface Experiment will measure the way sound travels through the surface.

The Permittivity Probe will investigate its electrical characteristics, and the Dust Impact Monitor will measure dust falling back to the surface. Operations Mission Operations Centre: European Space Operations Centre (ESOC), Darmstadt, Germany. Prime Ground Station: ESA Deep Space Antenna in New Norcia, near Perth, Australia. Rosetta Science Operations Centre: European Space Astronomy Centre (ESAC), in Villafranca, Spain. The European Space Agency has released the following information regarding the mission, its parameters and


Rosetta is the first mission designed to orbit and land on a comet. It consists of an orbiter, carrying 11 science experiments, and a lander, called “Philae”, carrying 10 additional instruments, for the most detailed study of a comet ever attempted. Rosetta’s launch was originally scheduled for January 2003 on an Ariane-5 rocket. Rosetta’s target at that time was Comet 46P/Wirtanen, with the encounter planned for 2011. However, following the failure of the first Ariane ECA rocket, in December 2002, ESA and Arianespace took the joint decision not to launch Rosetta during its January 2003 launch window.

Objectives ESA’s comet-chaser will be the first to undertake a lengthy exploration of a comet at close quarters. After entering orbit around Comet 67P/Churyumov-Gerasimenko in 2014, Rosetta will release its Philae small lander onto the icy nucleus as it did two days ago. Rosetta will orbit the comet for about a year as they head towards the Sun. Once they have passed perihelion (closest distance to the Sun), Rosetta will keep orbiting the comet for another half year, while the comet moves back out towards the orbit of Jupiter. As the most primitive objects in the solar system, comets carry essential information about our origins. Their chemical compositions have not changed much since their formation, therefore reflecting that of the solar system when it was very young and still “unfinished”, more than 4,600 million years ago.

Rosetta will also help to discover whether comets contributed to the beginnings of life on Earth. Comets are carriers of complex organic molecules, delivered to Earth through impacts, and perhaps played a role in the origin of life. Moreover, volatile light elements carried by comets may also have played an important role in forming Earth’s oceans and atmosphere. During its long journey, Rosetta was scheduled to have two close encounters with asteroids of the main asteroid belt that lies between the orbits of Mars and Jupiter. The first was with (2867) Steins, a rare E-type asteroid. The flyby started on August 4, 2008 with optical navigation of the asteroid itself – a technique never before used in ESA spacecraft operations.

Rosetta orbiter

The orbiter’s scientific payload includes 11 experiments, in addition to the lander. Scientific consortia from institutes across Europe and the United States provided these state-of-the-art instruments. The Ultraviolet Imaging Spectrometer – ALICE will analyse gases in the coma and tail and measure the comet’s production rates of water and carbon monoxide and dioxide. It will provide information on the surface composition of the nucleus. Comet Nucleus Sounding Experiment – CONSERT will probe the comet’s interior by studying radio waves reflected and scattered by the nucleus. Cometary Secondary Ion Mass Analyser – COSIMA will analyse the characteristics of dust grains emitted by the comet, such as their composition and whether they are organic or inorganic. Grain Impact Analyser and Dust Accumulator – GIADA will measure the number, mass, momentum and velocity distribution of dust grains coming from the cometary nucleus and other directions (deflected by solar radiation pressure). The Micro-Imaging Dust Analysis System, MIDAS, will study the dust around the comet.

It will provide information on particle population, size, volume and shape. Microwave Instrument for the Rosetta Orbiter – MIRO will determine the abundances of major gases, the surface outgassing rate and the nucleus subsurface temperature. Optical, Spectrocopic and Infrared Remote Imaging System – OSIRIS has a wide-angle camera and narrow-angle camera that can obtain high-resolution images of the comet’s nucleus. Rosetta Orbiter Spectrometer for Ion and Neutral Analysis – ROSINA will determine the composition of the comet’s atmosphere and ionosphere, the velocities of electrified gas particles and reactions in which they take part.

Radio science investigation signals

Rosetta Plasma Consortium – RPC will measure the physical properties of the nucleus, examine the structure of the inner coma, monitor cometary activity, and study the comet’s interaction with the solar wind. Radio Science Investigation – RSI will, by using shifts in the spacecraft’s radio signals, measure the mass, density and gravity of the nucleus, define the comet’s orbit, and study the inner coma. Visible and Infrared Mapping Spectrometer – VIRTIS will map and study the nature of the solids and the temperature on the surface. It will also identify comet gases, characterise the physical conditions of the coma and help to identify the best landing sites.

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