Rosetta Lander Philae

ESA's Rosetta mission was launched on 2 March 2004, and is destined to reach its target comet, 67P/Churyumov-Gerasimenko, in 2014. The lander of the Rosetta mission, named Philae, is expected to be deployed around November 2014, to make the first ever controlled landing on a comet nucleus. En route, the mission's interplanetary trajectory takes in four gravity assists, three at Earth and one at Mars, and two asteroid flybys. Having matched the comet's orbit, Rosetta will close in to perform a comprehensive remote sensing survey of the nucleus and its environment prior to final selection of the landing site and deployment of the lander.

The finally launched mission had evolved a great deal over several iterations since the initial conception of a 'mission to the primitive bodies of the Solar System' around 1985 as a cornerstone of ESA's new Horizon 2000 science programme (this was almost a year before ESA's Giotto spacecraft had encountered comet Halley). The mission plan has always incorporated a surface element, though initially this was to obtain a sample for return to Earth. Known briefly as the Comet Nucleus Sample Return (CNSR) mission, it had by 1987 been renamed Rosetta. By the end of 1985 a joint ESA/NASA Science Definition Team had been formed to define in detail the mission's scientific objectives; NASA being envisaged as a partner for ESA on the mission. Planning began in earnest after the Giotto spacecraft's pioneering encounter with comet Halley in March 1986, which provided an important 'first look' at the type of body Rosetta was due to visit.

An ESA workshop was held in July 1986 to bring together the cometary community to look forward to the next European cometary space mission. The proceedings were published as ESA SP-249 (1986).

The report of the Science Definition Team was published in 1987 (ESA SCI (87)3). Work on the sample return mission scenario continued (see Atzei et al. (1989) for an overview), producing a Mission and System Definition Document (ESA SP-1125) in June 1991. This outlined the type of spacecraft and mission architecture that would be required. A large commitment from NASA was envisaged in the form of a carrier spacecraft derived from the Mariner Mark-II bus (used for Cassini). This would carry the landing stage to the comet, lifting off from the surface after about 15 days of sampling operations, to bring about 10 kg of cometary material back to Earth in an Earth Return Capsule.

Early in 1992, however, financial and programmatic difficulties within NASA (related to its own ill-fated CRAF (Comet Rendezvous and Asteroid Flyby) mission) prompted a re-examination of the original sample return concept, with a need to show that the mission could be achieved by European technology alone. As a result, Rosetta was re-oriented as a comet rendezvous and in situ analysis mission. A new System Definition Study (December 1993) was carried out to define the new mission. An ESA Study Report (ESA SCI(93)7) was produced. This re-examined the scientific objectives and model payload as well as outlining the new mission architecture.

The Rosetta 'comet rendezvous' concept now involved a main orbiter spacecraft, carrying both a payload for remote sensing of the nucleus and in situ measurements of the dust, gas and plasma environment, and a —75 kg lander to be deployed towards the surface. Rendezvous with the target comet would occur at just over 3 AU heliocentric distance and the primary mission would last until perihelion, some two years later. The major differences between this revised design and the CNSR concept were the use of solar arrays rather than RTGs, and that the orbiter would not descend to the surface with the lander (as was the case for the CNSR scenario). Rather, it would stay in orbit around the nucleus and perform a much more extensive remote sensing investigation from rendezvous until perihelion. Rosetta proceeded along these lines, although further changes to the surface element were to occur.

Following a call for PI-led lander proposals, the initially selected configuration incorporated two —45 kg landers, RoLand (from a German-led consortium) and Champollion (from a French/US consortium - see Neugebauer and Bibring, 1998). In 1996 the US withdrew from Champollion (although it survived, until cancellation in July 1999, as a New Millennium mission, DS-4 (later ST-4) Champollion),13 eventually leaving the French to team up with the RoLand consortium to provide a single, larger lander of —85-100 kg mass. This was called Rosetta Lander until it was given the name Philae in 2004, shortly before launch.

13 The payload of the (DS-4 / ST-4) Champollion comet lander was as follows: CIRCLE (Champollion Infrared and Camera Lander Experiment: near-field camera, microscope, IR spectrometer) (Yelle), ISIS (stereo panoramic camera) (Bibring), CHAMPAGNE (gamma-ray spectrometer) (d'Uston), CHARGE (Chemical Analysis of Released Gas Experiment - GCMS) (Mahaffy), CPPP (Comet Physical Properties Package) (Ahrens), CONSERT (Kofman) (while still part of Rosetta), Sample return (studied briefly). The Project Scientist was Paul Weissman.

The scientific objectives of Rosetta as a whole are as follows (Schwehm and Hechler, 1994).

• Global characterisation of the nucleus, determination of dynamic properties, surface morphology and composition.

• Chemical, mineralogical and isotopic compositions of volatiles and refractories in a cometary nucleus.

• Physical properties and interrelation of volatiles and refractories in a cometary nucleus.

• Study of the development of cometary activity and the processes in the surface layer of the nucleus and in the inner coma (dust-gas interaction).

• Origin of comets; relationship between cometary and interstellar material; and implications for the origin of the Solar System.

Philae's aim is to address those objectives of Rosetta that cannot be achieved from the orbiter, including: geochemical analyses requiring sampling or close contact, surface and sub-surface physical properties, ground truth measurements for the orbiter, and the high resolution study of a single site. Philae, together with the orbiter, also provides a baseline for radio transmission tomography of the nucleus. Specifically, Philae's scientific objectives are as follows

• The determination of the composition of cometary surface matter: bulk elemental abundances, isotopes, minerals, ices, carbonaceous compounds, organic volatiles - as a function of time and insolation.

• The investigation of the structure, physical, chemical and mineralogical properties of the cometary surface: topography, texture, roughness, mechanical, electrical, optical and thermal properties.

• The investigation of the local depth structure (stratigraphy), and the global internal structure.

• Investigation of the plasma environment.

The selected payload, totalling 27.6 kg, can be seen in Section 20.3. The ten instruments include a sampling drill and two evolved gas analysers, imaging and microscopy, an alpha-X-ray spectrometer, and various sensors for studying the thermal, mechanical and electromagnetic properties of the nucleus and its near-surface environment. A mass breakdown is given in Table 26.1.

The main particular challenges of Philae's design and mission arise from the low surface gravity on the comet nucleus, the uncertain nature of the terrain (topography and mechanical properties), and the wide variation in solar flux and thermal conditions to be experienced as the comet's elliptical orbit nears the Sun. Landing will occur at about 3 AU, while perihelion is at 1.29 AU.

Power is provided initially by a primary kWh LiSOCl2 battery. This ensures operation for the first science sequence of 120 h. Thereafter a secondary ~100 Wh Li-ion battery with body-mounted solar array is intended to provide

Table 26.1. Mass Breakdown of Philae, the comet lander of the Rosetta Mission

Item

Mass (kg)

APX

1.32

CIVA

3.39

COSAC

4.95

CONSERT

1.79

MUPUS

2.16

Ptolemy

4.53

ROMAP

0.74

ROLIS

1.36

SESAME

1.76

SD2

4.77

Payload total

26.82

ADS (Active Descent System)

3.69

Anchors

0.89

Landing gear

9.36

Separation structure

1.30

Non-payload common electronics

5.80

Thermal subsystem

7.42

Flywheel

2.90

Solar generator

1.72

Battery

8.50

Power Hardware

0.74

Communications subsystem

2.34

System harness

6.06

Structure

18.02

Balance mass

2.32

Lander total

97.89

Lander support equipment on Rosetta

13.09

Total

110.98

power for the long-term mission of about 3 months, until the comet reaches 2 AU heliocentric distance. After that, the extended mission will sooner or later end when the lander overheats.

The basic configuration of the lander is based on a jointed tripod carrying a baseplate, on top of which sits the lander's main body, much of the external surface of which carries solar cells. The thermal design for the lander involves a central, thermally controlled 'warm' compartment within the main body, housing the main electronic equipment and other critical subsystems, the evolved gas analysers and several of the cameras. Two solar absorbers on the lander's top panel are used to absorb heat from the Sun during the early part of surface operations.

Many of the experiments require access to the external environment, however. Most of these are mounted on the 'cold balcony', an area sharing the same baseplate as the main body of the lander. Other sensors are mounted on the landing gear. Several mechanisms are required by the payload, in addition to those serving mainly the landing and anchoring aspects. The sampling drill is due to obtain samples from a depth of up to ~20 cm and feed them to the microscope and evolved gas analysers via a carousel-based sample handling and distribution system. The APXS instrument needs close contact with the surface material and is thus lowered down through the lander baseplate. The magnetometer is deployed by a hinged boom, and the MUPUS thermal and mechanical properties experiment includes a thermal probe deployed by an arm comprising two parallel booms. An electromechanical hammering mechanism then drives the probe into the surface, to a depth of around 35 cm.

The lander will be ejected from the orbiter with a speed adjustable from 5 to 52 cms-1, by means of a screw-mechanism. This speed will cancel out enough of the lander's orbital speed for it to fall towards the surface, its attitude stabilised by an internal momentum wheel.

On landing, rebound must be avoided since this could lead to overturn of the lander. As the comet approaches closer to the Sun, outgassing of water and other volatiles from the nucleus may increase to such an extent that the lander could be blown off the surface. For these reasons the lander is equipped with a redundant system of cold-gas hold-down thruster, anchoring harpoons (2) and 'foot screws' on each of the three feet.

For most of the development and construction of the Rosetta mission, the target comet had been 46P/Wirtanen, with a launch scheduled for early 2003. However, problems with the Ariane 5 launch vehicle led to a change in target and launch date. The new comet, 67P/Churyumov-Gerasimenko, is larger (~4km diameter versus 1.2 km) and so Philae is expected to have a higher landing speed: up to 1.2 m s-1 as opposed to 0.5 m s-1 for Wirtanen (the landing gear's original design limit being 1ms-1). The deployment manoeuvre can mitigate this to some degree, and a 'tilt-limiter' structure was added to the landing gear to prevent the main body of the lander tilting too far (>5°) on landing.

Two-way relay communications are achieved via the Rosetta orbiter. Two S-band antennas are located on the lander's upper surface. The first few hours of operation are planned to take the form of a pre-programmed sequence, after which ground controllers can modify the operations plan based on the data received.

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