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2.
The International Rosetta Mission is set for a rendezvous with Comet 67 P/Churyumov-Gerasimenko in 2014. On its 10 year journey
to the comet, the spacecraft will also perform a fly-by of the two asteroids Stein and Lutetia in 2008 and 2010, respectively.
The mission goal is to study the origin of comets, the relationship between cometary and interstellar material and its implications
with regard to the origin of the Solar System. Measurements will be performed that shed light into the development of cometary
activity and the processes in the surface layer of the nucleus and the inner coma.
The Micro-Imaging Dust Analysis System (MIDAS) instrument is an essential element of Rosetta’s scientific payload. It will
provide 3D images and statistical parameters of pristine cometary particles in the nm-μm range from Comet 67P/Churyumov-Gerasimenko.
According to cometary dust models and experience gained from the Giotto and Vega missions to 1P/Halley, there appears to be
an abundance of particles in this size range, which also covers the building blocks of pristine interplanetary dust particles.
The dust collector of MIDAS will point at the comet and collect particles drifting outwards from the nucleus surface. MIDAS
is based on an Atomic Force Microscope (AFM), a type of scanning microprobe able to image small structures in 3D. AFM images
provide morphological and statistical information on the dust population, including texture, shape, size and flux. Although
the AFM uses proven laboratory technology, MIDAS is its first such application in space. This paper describes the scientific
objectives and background, the technical implementation and the capabilities of MIDAS as they stand after the commissioning
of the flight instrument, and the implications for cometary measurements. 相似文献
3.
The gas flux from a volatile icy-dust mixture is computed using a comet nucleus thermal model in order to study the evolution
of CO outgassing during several apparitions from long-period Comet Hale-Bopp and short-period Comet Wirtanen. The comet model
assumes a spherical, porous body containing a dust component, one major ice component (H 2O), and one minor ice component of higher volatility (CO). The initial chemical composition is assumed to be homogeneous.
The following processes are taken into account: heat and gas diffusion inside the rotating nucleus; release of outward diffusing
gas from the comet nucleus; chemical differentiation by sublimation of volatile ices in the surface layers and recondensation
of gas in deeper, cooler layers. A 2-D time dependent solution is obtained through the dependence of the boundary conditions
on the local solar illumination as the nucleus rotates. The model for Comet Hale-Bopp was compared with observational measurements
(Biver et al., 1999). The best agreement was obtained for a model with amorphous water ice and CO, assuming that a part of the latter is
trapped by the water ice, another part is condensed as an independent ice phase. The model confirms that sublimation of CO
ice at large heliocentric distance produces a gradual increase in the comet's activity as it approaches the Sun. Crystallization
of amorphous water ice begins at 7 AU from the Sun, but no outbursts were found. Seasonal effects and thermal inertia of the
nucleus material lead to larger CO outgassing rates as the comet recedes from the Sun. In the second part of this work the
model was run with the orbital parameters of Comet Wirtanen. Unlike Comet Hale-Bopp, the predicted CO outgassing from Comet
Wirtanen is almost constant throughout its orbit. Such behavior can be explained by a thermally evolved and chemically differentiated
comet nucleus.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
4.
The investigation of the volatile material in the coma of comets is a key to understanding the origin of cometary material,
the physical and chemical conditions in the early solar system, the process of comet formation, and the changes that comets
have undergone during the last 4.6 billion years. So far, in situ investigations of the volatile constituents have been confined
to a single comet, namely P/Halley in 1986. Although, the Giotto mission gave only a few hours of data from the coma, it has
yielded a surprising amount of new data and has advanced cometary science by a large step. In the present article the most
important results of the measurements of the volatile material of Halley's comet are summarized and an overview of the identified
molecules is given. Furthermore, a list of identified radicals and unstable molecules is presented for the first time. At
least one of the radicals, namely CH 2, seems to be present as such in the cometary ice.
As an outlook to the future we present a list of open questions concerning cometary volatiles and a short preview on the next
generation of mass spectrometers that are being built for the International Rosetta Mission to explore the coma of Comet Wirtanen.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
5.
The ROSETTA Mission, the Planetary Cornerstone Mission in the European Space Agency’s long-term programme Horizon 2000, will
rendezvous in 2014 with comet 67P/Churyumov-Gerasimenko close to its aphelion and will study the physical and chemical properties
of the nucleus, the evolution of the coma during the comet’s approach to the Sun, and the development of the interaction region
of the solar wind and the comet, for more than one year until it reaches perihelion. In addition to the investigations performed
by the scientific instruments on board the orbiter, the ROSETTA lander PHILAE will be deployed onto the surface of the nucleus.
On its way to comet 67P/Churyumov-Gerasimenko, ROSETTA will fly by and study the two asteroids 2867 Steins and 21 Lutetia. 相似文献
6.
In 2003, comet 67P/Churyumov–Gerasimenko was selected as the new target of the Rosetta mission as the most suitable alternative
to the original target, comet 46P/Wirtanen, on the basis of orbital considerations even though very little was known about
the physical properties of its nucleus. In a matter of a few years and based on highly focused observational campaigns as
well as thorough theoretical investigations, a detailed portrait of this nucleus has been established that will serve as a
baseline for planning the Rosetta operations and observations. In this review article, we present a novel method to determine
the size and shape of a cometary nucleus: several visible light curves were inverted to produce a size–scale free three–dimensional
shape, the size scaling being imposed by a thermal light curve. The procedure converges to two solutions which are only marginally
different. The nucleus of comet 67P/Churyumov–Gerasimenko emerges as an irregular body with an effective radius (that of the
sphere having the same volume) = 1.72 km and moderate axial ratios a/b = 1.26 and a/c = 1.5 to 1.6. The overall dimensions
measured along the principal axis for the two solutions are 4.49–4.75 km, 3.54–3.77 km and 2.94–2.92 km. The nucleus is found
to be in principal axis rotation with a period = 12.4–12.7 h. Merging all observational constraints allow us to specify two
regions for the direction of the rotational axis of the nucleus: RA = 220° +50°
−30° and Dec = −70 ° ± 10 ° (retrograde rotation) or RA = 40° +50°
-30° and Dec = +70 °± 10 ° (prograde), the better convergence of the various determinations presently favoring the first solution. The phase function,
although constrained by only two data points, exhibits a strong opposition effect rather similar to that of comet 9P/Tempel
1. The definition of the disk–integrated albedo of an irregular body having a strong opposition effect raises problems, and
the various alternatives led to a R-band geometric albedo in the range 0.045–0.060, consistent with our present knowledge of cometary nuclei. The active fraction
is low, not exceeding ~ 7% at perihelion, and is probably limited to one or two active regions subjected to a strong seasonal
effect, a picture coherent with the asymmetric behaviour of the coma. Our slightly downward revision of the size of the nucleus
of comet 67P/Churyumov-Gerasimenko resulting from the present analysis (with the correlative increase of the albedo compared
to the originally assumed value of 0.04), and our best estimate of the bulk density of 370 kg m −3, lead to a mass of ~ 8 × 10 12 kg which should ease the landing of Philae and insure the overall success of the Rosetta mission. 相似文献
7.
ESA’s Rosetta mission was launched in March 2004 and is on its way to comet 67P/Churyumov-Gerasimenko, where it is scheduled
to arrive in summer 2014. It comprises a payload of 12 scientific instruments and a Lander. All instruments are provided by
Principal Investigators, which are responsible for their operations.
As for most ESA science missions, the ground segment of the mission consists of a Mission Operations Centre (MOC) and a Science
Operations Centre (SOC). While the MOC is responsible for all spacecraft-related aspects and the final uplink of all command
timelines to the spacecraft, the scientific operations of the instruments and the collection of the data and ingestion into
the Planetary Science Archive are coordinated by the SOC. This paper focuses on the tasks of the SOC and in particular on
the methodology and constraints to convert the scientific goals of the Rosetta mission to operational timelines. 相似文献
8.
The European Space Agency (ESA) Rosetta Spacecraft, launched on March 2, 2004 toward Comet 67P/Churyumov-Gerasimenko (C-G), carries a complementary set of instruments on both the orbiter and lander (Philae) portions of the spacecraft, to measure the composition of the Comet C-G. The primary composition measuring instruments on the Orbiter are Alice, COSIMA, ICA, MIRO, OSIRIS, ROSINA and VIRTIS. These instruments collectively are capable of providing compositional information, including temporal and spatial distributions of important atomic, molecular, and ionic species, minerals, and ices in the coma and nucleus. The instruments utilize a variety of techniques and wavelength ranges to accomplish their objectives. This paper provides an overview of composition measurements that will be possible using the suite of orbiter composition measuring instruments. A table is provided that lists important species detectable (depending on abundances) with each instrument. 相似文献
9.
Surface temperature and the available effective energy strongly influence the mass flux of H 2O and minor volatiles from the nucleus. We perform computer simulations to model the gas flux from volatile, icy components
in porous ice-dust surfaces, in order to better understand results from observations of comets. Our model assumes a porous
body containing dust, one major ice component (H 2O) and up to eight minor components of higher volatility (e.g. CO, CH 4, CH 3OH, HCN, C 2H 2, H 2S), The body's porous structure is modeled as a bundle of tubes with a given tortuosity and an initially constant pore diameter.
Heat is conducted by the matrix and carried by the vapors. The model includes radially inward and outward flowing vapor within
the body, escape of outward flowing gas from the body, complete depletion of less volatile ices in outer layers, and recondensation
of vapor in deeper, cooler layers. From the calculations we obtain temperature profiles and changes in relative chemical abundances,
porosity and pore size distribution as a function of depth, and the gas flux into the interior and into the atmosphere for
each of the volatiles at various positions of the body in its orbit.
In this paper we relate the observed relative molecular abundances in the coma of Comet C/1995 O1 (Hale-Bopp) and of Comet
46P/Wirtanen to molecular fluxes at the surface calculated from our model.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
10.
The paper describes the Rosetta Lander named Philae and introduces its complement of scientific instruments. Philae was launched
aboard the European Space Agency Rosetta spacecraft on 02 March 2004 and is expected to land and operate on the nucleus of
67P/Churyumov-Gerasimenko at a distance of about 3 AU from the Sun. Its overall mass is ~98 kg (plus the support systems remaining
on the Orbiter), including its scientific payload of ~27 kg. It will operate autonomously, using the Rosetta Orbiter as a
communication relay to Earth. The scientific goals of its experiments focus on elemental, isotopic, molecular and mineralogical
composition of the cometary material, the characterization of physical properties of the surface and subsurface material,
the large-scale structure and the magnetic and plasma environment of the nucleus. In particular, surface and sub-surface samples
will be acquired and sequentially analyzed by a suite of instruments. Measurements will be performed primarily during descent
and along the first five days following touch-down. Philae is designed to also operate on a long time-scale, to monitor the
evolution of the nucleus properties. Philae is a very integrated project at system, science and management levels, provided
by an international consortium. The Philae experiments have the potential of providing unique scientific outcomes, complementing
by in situ ground truth the Rosetta Orbiter investigations.
Philae team members are listed in the acknowledgements 相似文献
11.
The main objective of the Mutual Impedance Probe (MIP), part of the Rosetta Plasma Consortium (RPC), is to measure the electron
density and temperature of Comet 67P/Churyumov-Gerasimenko’s coma, in particular inside the contact surface. Furthermore,
MIP will determine the bulk velocity of the ionised outflowing atmosphere, define the spectral distribution of natural plasma
waves, and monitor dust and gas activities around the nucleus. The MIP instrumentation consists of an electronics board for
signal processing in the 7 kHz to 3.5 MHz range and a sensor unit of two receiving and two transmitting electrodes mounted
on a 1-m long bar. In addition, the Langmuir probe of the RPC/LAP instrument that is at about 4 m from the MIP sensor can
be used as a transmitter (in place of the MIP ones) and MIP as a receiver in order to have access to the density and temperature
of plasmas at higher Debye lengths than those for which the MIP is originally designed. 相似文献
12.
The European Space Agency Rosetta Spacecraft, launched on March 2, 2004 toward Comet 67P/Churyumov-Gerasimenko, carries a
relatively small and lightweight millimeter-submillimeter spectrometer instrument, the first of its kind launched into deep
space. The instrument will be used to study the evolution of outgassing water and other molecules from the target comet as
a function of heliocentric distance. During flybys of the asteroids (2867) Steins and (21) Lutetia in 2008 and 2010 respectively,
the instrument will measure thermal emission and search for water vapor in the vicinity of these asteroids.
The instrument, named MIRO (Microwave Instrument for the Rosetta Orbiter), consists of a 30-cm diameter, offset parabolic
reflector telescope followed by two heterodyne receivers. Center-band operating frequencies of the receivers are near 190
GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband continuum channels are implemented in both frequency bands for the measurement
of near surface temperatures and temperature gradients in Comet 67P/Churyumov-Gerasimenko and the asteroids (2867) Steins
and (21) Lutetia. A 4096 channel CTS (Chirp Transform Spectrometer) spectrometer having 180 MHz total bandwidth and 44 kHz
resolution is, in addition to the continuum channel, connected to the submillimeter receiver. The submillimeter radiometer/spectrometer
is fixed tuned to measure four volatile species – CO, CH 3OH, NH 3 and three, oxygen-related isotopologues of water, H 2
16O, H 2
17O and H 2
18O. The basic quantities measured with the MIRO instrument are surface temperature, gas production rates and relative abundances,
and velocity and excitation temperature of each species, along with their spatial and temporal variability. This paper provides
a short discussion of the scientific objectives of the investigation, and a detailed discussion of the MIRO instrument system. 相似文献
13.
The Deep Impact observations of low thermal inertia for comet 9P/Tempel 1 are of profound importance for the observations to be made by the Rosetta spacecraft at comet 67P/Churyumov-Gerasimenko. While sub-surface sublimation is necessary to explain the observations, the depth at which this occurs is no more than 2–3 cm and possibly less. The low thermal conductivity when combined with local surface roughness (also observed with Deep Impact) implies that local variations in outgassing rates can be substantial. These variations are likely to be on scales smaller than the resolution limits of all experiments on the Rosetta orbiter. The observed physico-chemical inhomogeneity further suggests that the Rosetta lander will only provide a local snapshot of conditions in the nucleus layer. 相似文献
14.
The Optical, Spectroscopic, and Infrared Remote Imaging System OSIRIS is the scientific camera system onboard the Rosetta
spacecraft (Figure 1). The advanced high performance imaging system will be pivotal for the success of the Rosetta mission.
OSIRIS will detect 67P/Churyumov-Gerasimenko from a distance of more than 10 6 km, characterise the comet shape and volume, its rotational state and find a suitable landing spot for Philae, the Rosetta
lander. OSIRIS will observe the nucleus, its activity and surroundings down to a scale of ~2 cm px −1. The observations will begin well before the onset of cometary activity and will extend over months until the comet reaches
perihelion. During the rendezvous episode of the Rosetta mission, OSIRIS will provide key information about the nature of
cometary nuclei and reveal the physics of cometary activity that leads to the gas and dust coma.
OSIRIS comprises a high resolution Narrow Angle Camera (NAC) unit and a Wide Angle Camera (WAC) unit accompanied by three
electronics boxes. The NAC is designed to obtain high resolution images of the surface of comet 67P/Churyumov-Gerasimenko
through 12 discrete filters over the wavelength range 250–1000 nm at an angular resolution of 18.6 μrad px −1. The WAC is optimised to provide images of the near-nucleus environment in 14 discrete filters at an angular resolution of
101 μrad px −1. The two units use identical shutter, filter wheel, front door, and detector systems. They are operated by a common Data
Processing Unit. The OSIRIS instrument has a total mass of 35 kg and is provided by institutes from six European countries. 相似文献
15.
The Rosetta spacecraft has been successfully launched on 2nd March 2004 to its new target comet 67 P/Churyumov-Gerasimenko. The science objectives of the Rosetta Radio Science Investigations (RSI) experiment address fundamental aspects of cometary physics such as the mass and bulk density of the nucleus, its gravity field, its interplanetary orbit perturbed by nongravitational forces, its size and shape, its internal structure, the composition and roughness of the nucleus surface, the abundance of large dust grains, the plasma content in the coma and the combined dust and gas mass flux. The masses of two asteroids, Steins and Lutetia, shall be determined during flybys in 2008 and 2010, respectively. Secondary objectives are the radio sounding of the solar corona during the superior conjunctions of the spacecraft with the Sun during the cruise phase. The radio carrier links of the spacecraft Telemetry, Tracking and Command (TT&C) subsystem between the orbiter and the Earth will be used for these investigations. An Ultrastable oscillator (USO) connected to both transponders of the radio subsystem serves as a stable frequency reference source for both radio downlinks at X-band (8.4 GHz) and S-band (2.3 GHz) in the one-way mode. The simultaneous and coherent dual-frequency downlinks via the High Gain Antenna (HGA) permit separation of contributions from the classical Doppler shift and the dispersive media effects caused by the motion of the spacecraft with respect to the Earth and the propagation of the signals through the dispersive media, respectively. The investigation relies on the observation of the phase, amplitude, polarization and propagation times of radio signals transmitted from the spacecraft and received with ground station antennas on Earth. The radio signals are affected by the medium through which the signals propagate (atmospheres, ionospheres, interplanetary medium, solar corona), by the gravitational influence of the planet on the spacecraft and finally by the performance of the various systems involved both on the spacecraft and on ground. 相似文献
16.
In 1998, Comet 9P/Tempel 1 was chosen as the target of the Deep Impact mission (A’Hearn, M. F., Belton, M. J. S., and Delamere, A., Space Sci. Rev., 2005) even though very little was known about its physical properties. Efforts were immediately begun to improve this situation
by the Deep Impact Science Team leading to the founding of a worldwide observing campaign (Meech et al., Space Sci. Rev., 2005a). This campaign has already produced a great deal of information on the global properties of the comet’s nucleus
(summarized in Table I) that is vital to the planning and the assessment of the chances of success at the impact and encounter.
Since the mission was begun the successful encounters of the Deep Space 1 spacecraft at Comet 19P/Borrelly and the Stardust spacecraft at Comet 81P/Wild 2 have occurred yielding new information on the state of the nuclei of these two comets. This
information, together with earlier results on the nucleus of comet 1P/Halley from the European Space Agency’s Giotto, the Soviet Vega mission, and various ground-based observational and theoretical studies, is used as a basis for conjectures on the morphological,
geological, mechanical, and compositional properties of the surface and subsurface that Deep Impact may find at 9P/Tempel 1. We adopt the following working values ( circa December 2004) for the nucleus parameters of prime importance to Deep Impact as follows: mean effective radius = 3.25± 0.2 km, shape – irregular triaxial ellipsoid with a/ b = 3.2± 0.4 and overall dimensions of ∼14.4 × 4.4 × 4.4 km, principal axis rotation with period = 41.85± 0.1 hr, pole directions
(RA, Dec, J2000) = 46± 10, 73± 10 deg (Pole 1) or 287± 14, 16.5± 10 deg (Pole 2) (the two poles are photometrically, but not
geometrically, equivalent), Kron-Cousins (V-R) color = 0.56± 0.02, V-band geometric albedo = 0.04± 0.01, R-band geometric
albedo = 0.05± 0.01, R-band H(1,1,0) = 14.441± 0.067, and mass ∼7×10 13 kg assuming a bulk density of 500 kg m −3. As these are working values, {i.e.}, based on preliminary analyses, it is expected that adjustments to their values may be made before encounter
as improved estimates become available through further analysis of the large database being made available by the Deep Impact observing campaign. Given the parameters listed above the impact will occur in an environment where the local gravity is
estimated at 0.027–0.04 cm s −2 and the escape velocity between 1.4 and 2 m s −1. For both of the rotation poles found here, the Deep Impact spacecraft on approach to encounter will find the rotation axis close to the plane of the sky (aspect angles 82.2 and 69.7
deg. for pole 1 and 2, respectively). However, until the rotation period estimate is substantially improved, it will remain
uncertain whether the impactor will collide with the broadside or the ends of the nucleus. 相似文献
17.
The scientific objectives, design and capabilities of the Rosetta Lander’s ROMAP instrument are presented. ROMAP’s main scientific
goals are longterm magnetic field and plasma measurements of the surface of Comet 67P/Churyumov-Gerasimenko in order to study
cometary activity as a function of heliocentric distance, and measurements during the Lander’s descent to investigate the
structure of the comet’s remanent magnetisation. The ROMAP fluxgate magnetometer, electrostatic analyser and Faraday cup measure
the magnetic field from 0 to 32 Hz, ions of up to 8000 keV and electrons of up to 4200 keV. Additional two types of pressure
sensors – Penning and Minipirani – cover a pressure range from 10 −8 to 10 1 mbar. ROMAP’s sensors and electronics are highly integrated, as required by a combined field/plasma instrument with less
than 1 W power consumption and 1 kg mass. 相似文献
18.
The ion and electron sensor (IES) is part of the Rosetta Plasma Consortium (RPC). The IES consists of two electrostatic plasma
analyzers, one each for ions and electrons, which share a common entrance aperture. Each analyzer covers an energy/charge
range from 1 eV/e to 22 keV/e with a resolution of 4%. Electrostatic deflection is used at the entrance aperture to achieve
a field of view of 90°× 360° (2.8π sr). Angular resolution is 5°× 22.5° for electrons and 5°× 45° for ions with the sector
containing the solar wind being further segmented to 5°× 5°. The three-dimensional plasma distributions obtained by IES will
be used to investigate the interaction of the solar wind with asteroids Steins and Lutetia and the coma and nucleus of comet
67P/Churyumov–Gerasimenko (CG). In addition, photoelectron spectra obtained at these bodies will help determine their composition. 相似文献
19.
ROLIS (Rosetta Lander Imaging System) is one of the two imaging systems carried by Rosetta’s Lander Philae, successfully launched
to comet 67P/ Churyumov-Gerasimenko in March 2004. Consisting of a highly-miniaturized CCD camera, ROLIS will operate as a descent imager, acquiring imagery
of the landing site with increasing spatial resolution. After touchdown ROLIS will focus at an object distance of 30 cm, taking
pictures of the comet’s surface below the Lander. Multispectral imaging is achieved through an illumination device consisting
of four arrays of monochromatic light emitting diodes working in the 470, 530, 640 and 870 nm spectral bands. The drill sample
sites, as well as the Alpha X-Ray Spectrometer (APXS) target locations will be imaged to provide context for the measurements
performed by the in situ analyzers. After the drilling operation, the borehole will be inspected to study its morphology and to search for stratification.
Taking advantage of the Lander’s rotation capability, stereo image pairs will be acquired, which will facilitate the mapping
and identification of surface structures. 相似文献
20.
The Rosetta observations have greatly advanced our knowledge of the cometary nucleus and its immediate environment. However, constraints on the mission (both planned and unplanned), the only partially successful Philae lander, and other instrumental issues have inevitably resulted in open questions. Surprising results from the many successful Rosetta observations have also opened new questions, unimagined when Rosetta was first planned. We discuss these and introduce several mission concepts that might address these issues. It is apparent that a sample return mission as originally conceived in the 1980s during the genesis of Rosetta would provide many answers but it is arguable whether it is technically feasible even with today’s technology and knowledge. Less ambitious mission concepts are described to address the suggested main outstanding scientific goals. 相似文献
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