Discovering New WordsDiscovering New Words

Angioletta CoradiniAngioletta Coradini

Outline

Comets: where they come from?Their Travel Comets: what is their structure?The Rosetta mission: remote sensing experiments

OsirisVirtisMiroUVIS

Conclusions

The Oort Cloud

The Oort Cloud was first proposed by Jan Oort in 1950. This "land of comets" contains comet-forming nuclei left over from the formation of the solar system. It is currently thought that this is the location where all comets originate. The way they enter the inner solar system is by gravitational pushes, usually by a passing star.The Oort Cloud is believed by most scientists to exist, but it is still only theoretical. There have been no direct sightings of any Oort Cloud members, unlike the Kuiper Belt.

The Oort cloud

Kuiper Belt

Recently, theory postulates that there exists a very large belt of asteroids and comets beyond Pluto that extends outwards for several A.U., that is also suspected of being the source of many short period comets.KBO's are very difficult to detect due to their vast distances from the sun and very low surface reflectivity. Once identified, it is also very difficult to determine anything about them, even their size. The sizes in the table below are estimates that are based upon guesses as to how much light they reflect.

1992 QB1 : the first KBO

The Travel

The source regions envisaged for the two main components of the population of short-period comets, i.e. the Halley-type (HT) ones, and those belonging to the Jupiter-family (JF) is the so-called Edgeworth-Kuiper belt, a disc-like reservoir immediately outside the orbit of Neptune.In order to discriminate between them it is crucial to understand how multi-stage capture, i.e. the process by which potential short-period comets are passed from the dynamical control of one planet to that of the next inner one, takes place; these transfers are essentially due to close encounters with the controlling planet, unless the approaches are prevented by resonances or by peculiar orbital arrangements (Manara and Valsecchi, 1997).

Our Comet

The Objects: what to expect?

What is a comet?

Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases.

They have highly elliptical orbits that bring them very close to the Sun and swing them deeply into space, often beyond the orbit of Pluto.

Halley: the nucleus

Halley Nucleus

Churiomov-Gerasimenko

The NASA/ESA Hubble Space Telescope was used to make precise measurements of the size, shape and rotational period of the comet. Observations made by Hubble in 2003 revealed that Comet 67P/Churyumov-Gerasimenko is approximately five kilometres by three kilometres in size and shaped like a rugby ball.

Churiomov-GerasimenkoA total of 16 nights of observations were obtained at Lowell Observatory during the 1982/3 and 1995/6 apparitions. Production rates were determined for OH, NH, CN, C3, and C2, along with A(\theta)f\rho, a measure of the dust production. All species exhibit larger production rates following perihelion, with water having a ~2\times pre/post-perihelion asymmetry, while minor species and dust have a larger asymmetry. Peak water production (which occured about 1 month after perihelion) was ~1.0\times1028 mol s-1 and, when combined with a standard water vaporization model, implies an effective active area on the surface of the nucleus of ~2~km2 The peak dust production (as measured by A(\theta)f\rho) was ~450 cm, while the color of the dust is slightly reddened. In comparison to original ROSETTA target Comet 46P/Wirtanen, Comet Churyumov-Gerasimenko has essentially the same peak water production and a peak dust production about 3\times greater than does Wirtanen (assuming that the properties of the dust grains are similar)

Rosetta Asteroids

Rosetta's planners have already carried out an extensive hunt for any asteroids it could fly past during this revised series of loops around the Sun, and have indeed found two. The first is 437 Rhodia, which it would fly past in Sept. 2008 at a speed of about 41,000 km/hour.The second asteroid target would be 21 Lutetia, a big asteroid about 100 km wide which Rosetta would fly past at about 55,000 km per hour in July 2010

Remote Sensing Experiments

OSIRISOSIRIS

OSIRIS (Optical, Spectroscopic, and Infrared Remote Imaging System) is a wide-angle camera and a narrow-angle camera to obtain high-resolution images of the comet’s nucleus and the asteroids that Rosetta passes on its voyage to Comet 67P/Churyumov-Gerasimenko. It will help in identifying the best landing sites. Principal Investigator: H.U. Keller, MPAe, Katlenburg-Lindau, Germany.  

OSIRISOSIRIS

Wide and Narrow Angle CameraWide and Narrow Angle Camera NAC is a high resolution camera able to image the comet nucleus.

NAC is a high resolution camera able to image the comet nucleus.

NAC WAC are together OSIRIS Orbiter Rosetta. Cameras have the same CCD and share the Main Electronics.

The OSIRIS weight is 23 Kg

NAC WAC are together OSIRIS Orbiter Rosetta. Cameras have the same CCD and share the Main Electronics.

The OSIRIS weight is 23 Kg

WAC

25

19P/Borrelly

Wild

Wild

Halley

Osiris: some expected results

To determine the volume and bulk density of the nucleus to better than 1% and to derive surface gravity fields and moments of inertia.To search for residual evidence of formation (accretion) mechanisms and scale lengths.To investigate topographical features and identify the physical processes which have influenced their creation.To map surface ices.To investigate the structure, development, and inhomogeneity and erosion processes of active regions and compare their properties to the inactive surface.To observe the cratering of inactive surfaces.To characterise the landing sites, to relate these sites to the rest of the surface, and to search for effects of the SSP impacts.

VIRTIS-H VIRTIS-M

Radiator

Baseplate (S/C Interface)

Cold Box

VIRTIS Consortium

3 Partner: Italy France e Germany

3 Subsystems:

VIRTIS M

VIRTIS H

DPU

VIRTIS-M is a Visual and Infrared Imaging Spectrometer in a single optical head. A Shafer telescope is mated to an Offner slit spectrometer. Spectral images are formed on two matrix detectors to cover the 0.25-1 m and 1-5 m ranges. One single grating uses three different groove densities and grooves shapes to cover the full spectral range.

VIRTIS-H is a high-resolution infrared spectrometer. An Off axis parabolic mirror telescope is mated to an echelle spectrometer to achieve a spectral resolution R > 1300. The 2-5 m spectrum is dispersed in 9 orders on a matrix detector identical to the VIRTIS-M one.

VIRTIS-H and M IR detectors are cooled to 70K by means of Stirling Cycle Cryocoolers.

The Main Electronic Box main features are the DPU, the Coolers Control Unit, the Power Supply and the High Speed S/C Interface.

VIRTIS Expected results: Ices & Organic VIRTIS Expected results: Ices & Organic CompoundsCompounds

The most abundant species, besides H2O that is probably more than 80%, are CO, CH3OH, and CO2, H2CO (NH3 being less abundant).

Radiation processes may have altered the primary mineral phases, as well as the ices (H2O, CH3OH, etc.). Simple hydrocarbons could have been polymerised.

The spectral structure of this kind of materials should be studied by means of dedicated experiments (Moroz et al. 1998).

VIrtis expected results:silicatesVIrtis expected results:silicates

Gas-solid processes could have altered primary silicates present on comets.

Secondary phyllosilicates Fe-oxides (magnetite) and some salts may have been formed. All these elements should be searched for.

The presence of altered material has been found by Galileo NIMS on Galilean satellites: spectra showing different kind of distorted water bands indicating the presence of hydrated minerals (Europa), as well as the signature of clay minerals and salts (McCord et al 1998).

ASTEROIDSASTEROIDS

Asteroids surface mineralogy and petrology can be studied by means of visible and infrared spectroscopy.

Spectrophotometric phase curves will give information on surface structures, microroughness and porosity of the surface layers of the asteroid.

Several molecular species produce also important visible and near infrared absorption features such as, water, OH bearing minerals, carbonates and hydrocarbons are possibly found also on asteroid surfaces.

What is MIRO?

MIRO (Microwave Instrument for the Rosetta Orbiter) is a scientific instrument on the ROSETTA Spacecraft. MIRO will measure the near surface temperatures of the asteroids and the comet C-G thereby allowing scientists to estimate the thermal and electrical properties of these surfaces. In addition, the spectrometer portion of MIRO will allow measurements of water, carbon monoxide, ammonia, and methanol in the gaseous coma of comet C-G. These measurements will allow scientists to study how the comet material sublimates (changes from its frozen state, ice, to a gas) in time and distance from the sun.The data from MIRO, along with data from other instruments on the orbiter and the comet lander, will give scientists a better idea of how comets formed, what they are made of and how they change with time.

MIROThe MIRO instrument consists of two heterodyne radiometers, one operating at millimeter wavelengths (190 GHz, ~1.6 mm) and one operating at submillimeter wavelengths (562 GHz, ~0.5 mm). The millimeter and submillimeter radiometers are both configured with a broadband continuum detector for the determination of the brightness temperature of the comet nucleus and the target asteroids. The submillimeter receiver is also configured as a very high resolution spectrometer.

AliceALICE: a lightweight (2.2 kg), low-power (2.9 W), and low-cost UV imaging spectrometer for the ESA Rosetta Orbiter. Ultraviolet spectroscopy is a powerful tool for studying astrophysical objects, and has been applied with great success to the study of comets. ALICE is designed to obtain far-UV (FUV) spectra of the Rosetta comet nucleus and coma in the 700-2050 Å bandpass; it will achieve spectral resolutions between 9.8 and 12.5 Å across the bandpass for extended sources that fill its 0.1 x 6.0 deg.2 field-of-view. It employs an off-axis telescope feeding a 0.15-m normal incidence Rowland circle spectrograph with a concave holographic reflection grating. The imaging microchannel plate detector utilizes dual solar-blind opaque photocathodes (KBr and CsI) and a 2-D wedge-and-strip readout array.

Alice Expected results

ALICE will deepen the Rosetta Orbiter remote sensing investigation through its ability to detect and measure (1) noble gases; (2) atomic abundances in the coma; (3) major ion abundances in the tail; and (4) production rates, variability, and structure of H2O and CO/CO2 molecules that generate cometary activity. In addition, ALICE will allow an investigation of the FUV properties of the nucleus and its solid grains, and can provide unique information during asteroid flybys and at en-route planetary encounters, most notably, Mars.

Alice

Developed at SwRI facilities in San Antonio, ALICE is one of the first instruments to be delivered for installation on Rosetta. The ALICE science team includes prominent comet scientists from France, the University of Maryland, and Johns Hopkins University.

Conclusions

Comets are observed since a long time, but the knowledge of the nucleus structure is very poor.

The comet chemical composition is dominated by H2O water, but is the presence of high volatility ices thet dominates the gas emission processes at large distance from the Sun.

The composition of refracrory materials can be only guessed from ground-based observations.

The isotopic composition is only known for a limited number of (D, H).

ROSETTA clarify many of these open points!