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| Titel |
Ptolemy operations as part of the Rosetta mission during the targeted flyby of asteroid 21 Lutetia. |
| VerfasserIn |
Dan Andrews, Andrew Morse, Simeon Barber, Mark Leese, Geraint Morgan, Simon Sheridan, Ian Wright, Colin Pillinger |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056371
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| Zusammenfassung |
| Rosetta is a European Space Agency ‘Planetary Cornerstone’ mission intended to solve many
of the unanswered questions surrounding the formation of the Solar System. The Rosetta
cruise has included: three gravitational assist manoeuvres using Earth, one such manoeuvre
past Mars, an 800 km flyby of the 5 km diameter E-type asteroid 2867 Šteins, and a 3,100 km
flyby of the 100 km diameter asteroid 21 Lutetia which took place on July 10th 2010. In
2014, Rosetta will rendezvous with, map, and then deposit a lander upon the surface of comet
67P/Churyumov-Gerasimenko.
Ptolemy is a miniature chemical analysis laboratory aboard the Philae lander intended to
determine the chemical and isotopic composition of material sourced from beneath, on and
above the surface of the target comet. The primary science will be returned from solid
cometary samples processed in the ovens of the Sampler, Drill and Distribution system
(SD2). The resulting sample gas mixture is processed in a chemical preparation suite
before delivery to a three channel gas chromatograph (GC) for separation. Elution
products are passed to a quadrupole ion trap mass spectrometer for identification and
quantification.
Ptolemy can also passively adsorb coma material onto CarbosphereTM molecular sieve
within one of the SD2 sample ovens for later release and analysis. Also possible are simple
‘sniff’ detections of the current spacecraft environment bypassing the sample inlet and GC
system and instead directly analysing the inside of the mass spectrometer, itself connected to
space via a vent pipe. During post launch testing the detection limit was measured to be in the
region of 10-11 mbar, using an on-board supply of argon doped at 100 ppm in the helium
carrier gas.
The once clear-cut differentiation between volatile rich comets and volatile depleted
asteroids has been somewhat eroded by recent ground based observations of the main belt
asteroids 24 Themis and 65 Cybele, showing that these bodies harbour organic-rich surfaces
with exposed water ice. In addition, at least four comets reside within the main asteroid belt,
the prototype being 133P/Elst-Pizarro and there are likely to be many more such bodies as yet
undiscovered.
It was known prior to the encounter that 21 Lutetia had carbonaceous material and
hydrated minerals on its surface, both were potential sources of outgassing. However, the
volatile composition and outgassing nature of main belt asteroids are only loosely
constrained; it was therefore decided to attempt to detect the plausible exosphere surrounding
Lutetia during the 2010 Rosetta flyby.
Not being among the primary mission objectives of the Ptolemy instrument, the asteroid
observations were constrained by spacecraft considerations and instrument resource
limitations. Only a small number of observations were possible and not at the closest
approach distance. As such it was decided to target Ptolemy’s exosphere search by taking six
mass spectra around the period of zero phase angle – 15 minutes prior to closest approach
above the area on the surface of Lutetia receiving the greatest level of insolation. Ptolemy
also made four further sets of six measurements several hours either side of ‘close approach’
to provide background data.
These spectra covered the mass range 10-140 Da, detecting water and a variety of other
species with some enhancement around the sub-Solar point, indicating the possibility of
an extant exosphere. The question remains open as to whether this was indeed an
extant exosphere or if the increased signal recorded was the result of spacecraft
outgassing.
The following presents Ptolemy’s operations to date, science results from the flyby of 21
Lutetia including possible implications for outgassing of main belt asteroids, and shows the
road ahead for the Ptolemy instrument with plans for pre- and on-comet operations. |
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