Alison, these are the vists we want, but the Visit Planner says it can't find an ephemeris -- do I recall we may need to define a 2nd target Lutetia, since the first target was "used" by visits 1-5?----- Attempting Submission 8 (Mon Dec 08 02:32:33 GMT 2008) -----The Visit Planner won't run but we know we have good windows.Hubble Investigation of Asteroid 21 Lutetia in Support of the Rosetta Mission FlybyThe Rosetta mission is gearing up for a flyby encounter with the large, main belt asteroid 21 Lutetia, with closest approach on 2010 June 10. The next opposition, on 2008 December 1, is our last opportunity to make observations in time to affect the planning of the Rosetta program. We request a total of 5 orbits of Hubble observing time (2 using ACS/SBC and 3 using WFPC2) to characterize the UV albedo of Lutetia, to search for any dust debris near the main body, and to perform a deep search for companions. Even one orbit of Hubble time would provide valuable data for planning the Rosetta-Alice ultraviolet spectrometer observations, and 2 orbits are sufficient to characterize the far-UV albedo, but a 5-orbit program provides a richer scientific investigation with potentially much broader implications, both scientifically and for Rosetta planning.We request a single orbit of observations of Lutetia using the ACS/SBC to measure the
far-UV albedo, which will allow us to optimize the Rosetta observationsmade with the R-Alice UV
spectrometer. A second orbit of observations, also using the ACS/SBC but made approximately
2–3 orbits after the first, will allow us to observe the opposite hemisphere on Lutetia and constrain
any variation in the UV albedo with asteroid rotational phase (Lutetia’s rotational period is 8.17 hr),
something that cannot be done during the Rosetta fast flyby. Two additional orbits of Hubble
observations of Lutetia using the WFPC2/PC will place the most stringent constraints possible on
any dusty debris and nearby companions. (The second orbit is mainly to verify that any additional
objects observed are physically associated with Lutetia.) Finally, we request one additional orbit,
ideally close in time to the others, to measure the near-UV albedo of Lutetia, which will provide
reflectivity data spanning the far-UV to the visible. Thus, we request a grand total of 5 orbits
of Hubble time to perform an intensive investigation of Lutetia. If the 5 orbits can be scheduled
consecutively, we suggest the following cadence: ACS/SBC, PC/F606W, PC/F606W, ACS/SBC,
PC/F218W-F255W-F300W.
We obtained the Lutetia orbital elements and ephemerides from the JPL Horizons system.
On the opposition date (2008 Dec 1), the geocentric RA and DEC (J2000) at 0h UTC are
04H:27M:07S and +20◦ 36′ 38′′, respectively, the heliocentric distance is 2.42 AU, the geocentric
distance is 1.44 AU, and the apparent geocentric rate of motion is 40′′ hr−1. The relatively high
rate of motion means that we should be able to verify that any new objects detected in the field are
physically associated with Lutetia by observing just 1 orbit after the initial detection.
We used the STScI exposure time calculators (ETCs) to determine the appropriate exposure
times for all the observations. The visible albedo (22%) and average diameter (100 km) of Lutetia
are well-known, and that information is used to calculate the V -mag at the time of our proposed
Hubble observations. For the ETC runs, we adopted V = 10.4 (this is the faintest Lutetia will
be during the observing window; it should be 0.4 mag brighter at opposition) and a solar-type
spectral distribution for Lutetia. However, since the UV albedo is expected to be significantly
lower than the visible albedo, we decreased the brightness used for normalizing the ACS/SBC
ETC calculations by 1.85 mag, which effectively means we assumed a UV albedo of 4%.
In order to obtain the best possible measurement of the UV albedo, we will take two long
( 1000 s each) exposures using the ACS/SBC: one with the F140LP filter and one with the F165LP
filter. By subtracting the F165LP image from the F140LP image, we effectively obtain an image of
Lutetia averaged over the wavelength range 1400–1600 °A, which is perfect for planning exposure
times for the R-Alice observations. In the net subtracted image, the ETC predicts that Lutetia
will produce 100 counts (i.e., S/N 10), which easily meets our objectives. (Measuring the UV
albedo to 30–50% accuracy should be sufficient for planning the R-Alice program.)
To search for dust debris and companions, we want to go as deep as possible at visible wavelengths.
Thus, we have selected theWFPC2/PC with the F606Wfilter. We plan to follow a strategy
similar to that employed during our discovery (using Hubble) of Pluto’s faint satellites Nix and Hydra
(Weaver et al. Nature, v439, 943–945, 2006), namely, take a short exposure of the central object
(Lutetia in this case) so that it doesn’t saturate and use the rest of the orbit for several much longer
exposures that can be combined to create an extremely deep image. For the short PC exposures
(we will take at least two short exposures at two different CCD locations to avoid bad pixels), the
brightness of Lutetia forces us to use the smallest available exposure time of 0.11 s. There should
be no saturation in this case, even at opposition when V = 10, because the asteroid will be slightly
resolved. Nevertheless, we expect to use the 14 e/DN gain setting to ensure that none of the pixels
are saturated. These short PC exposures will allow us to measure the visible brightness of Lutetia
itself with high S/N ( 300) and also to obtain an accurate measure of the radial brightness profile
in the immediate vicinity of Lutetia to search for dust debris. For the long PC exposures, taken in
the same orbit as the short exposures, we must trade-off the competing constraints of going deep,
not saturating near Lutetia where we want to search for dust debris and companions, and minimizing
operational overheads. We conclude that taking 10 dithered images, each having an exposure
time of 160 s, is the optimal strategy for our objectives. The effective exposure time of 1600 s for
the combined deep image will allow us to reach a limiting magnitude (S/N=5) of V 27.5 in the
region of the image not strongly affected by the wings of the Lutetia image. For an object having
the same visible albedo of Lutetia (22%), this magnitude limit corresponds to an effective diameter
of 30 m. Following the work of Steffl et al. (AJ, v132, 614–619, 2006), we estimate that
we should be able to search for objects down to a diameter of 100 m (corresponding to V =25)
at 3′′ from Lutetia. By carefully combining the data from both the short and long exposures to
create a radial brightness profile that extends right up to the surface of Lutetia, and by comparing
that profile to synthetic PSFs generated by the TinyTim software, we expect to put the best limits
possible on any dust debris near Lutetia. (Setting strict quantitative upper limits on the debris field
requires extensive modeling that is beyond the scope of this proposal. At this stage, we are simply
saying that a sensitive search for dust debris will be performed. If our Hubble data show convincing
evidence for dust debris, extensive modeling will be done to assess any potential hazard to the
Rosetta spacecraft and this, in turn, may affect the selection of the exact flyby distance.)
As mentioned earlier, the rotational period of Lutetia is well-determined at 8.17 hr. The solutions
for the rotational pole location suggest that it lies in the ecliptic plane7, which implies there
is some chance that the pole could be pointing toward Earth now, or nearly so, and that essentially
only a single hemisphere of Lutetia will be visible during the Hubble observations. But since there
are large uncertainties in the pole location, since significant ( 15–20%) brightness variations have
been detected during all the previous observations of Lutetia, since the UV variation is not necessarily
identical to the visible and IR variations, and since the UV variation with rotational phase
cannot be measured during the fast Rosetta flyby, we have concluded it would be prudent to attempt
two sets of UV observations of Lutetia separated by approximately one-half of the rotational
period, which translates to making the second set of ACS/SBC observations 2–3 orbits after the
first one.
For the fifth and final orbit, our objective is to measure the near-UV albedo of Lutetia in
a region that is inaccessible from ground-based facilities. Specifically, we plan to take images
through three near-UV filters: F218W, F255W, and F300W. (We recognize that the F218W data
may be dominated by long wavelength light because the spectrum of the asteroid is so red, but we
plan to model that effect.) For our ETC calculations, we used a G0 stellar spectrum with V = 10.8,
which is equivalent to using a near-UV albedo of 15%. The total exposure times need to achieve
S/N=100 for the F218W, F255W, and F300W filters, respectively, are 287 s, 55 s, and 2 s. For the
observations, we plan to take 5 F218W images, each with an exposure time of 160 s, 5 F255W
images, each with an exposure time of 60 s, and 5 F300W images, each with an exposure time of
2 s. We will additionally take (in the same orbit) 3 short (0.11 s with a gain of 14 e/DN) exposures
with the F606W filter to more accurately tie together the near-UV and visible results.Visits 13 and 14 were added, after the original program executed, to repeat and follow up on visits 03 and 04. We request TDRSS dumps to be scheduled as soon as possible after both of these visits.SOLAR SYSTEM0false0.00.0falsefalsefalsefalsefalseSUPPORTEDSOLAR SYSTEMASTEROIDAsteroid LutetiaTYPE=ASTEROID,A=2.436655154268493,E=0.1617304906531877,I=3.066937765356453,O=80.99203587426756,W=250.1298331884249,M=176.8215074616499,EQUINOX=J2000,EPOCH=12-DEC-1990:00:00:00EARTHACS/SBCImagingNoNoF140LPACS/SBCImagingNoNoF165LPWFPC2ImagingNoF606WWFPC2ImagingNoF606WACS/SBCImagingNoNoF140LPACS/SBCImagingNoNoF165LPWFPC2ImagingNoF218WWFPC2ImagingNoF255WWFPC2ImagingNoF300WUsing ephemeris for target 11957-01-LUTETIAephemeris data for target 11957-01-LUTETIA does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 08.304 to 09.335. ephemeris interval: 08.335 to 10.210Timing link suitability includes constraints ([Sequence (01 02 03 04 05) within 0.4 days])The scheduling data was generated by CASM 17.2.CASM was run in Phase II mode.C:\Documents and Settings\Max\Desktop\DD_lutetia\OP-cache\11957_confirm-v01.xml1228064093000353.1085533029692(Tic-Visit
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)true10Using ephemeris for target 11957-02-LUTETIAephemeris data for target 11957-02-LUTETIA does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 08.304 to 09.335. ephemeris interval: 08.335 to 10.210Timing link suitability includes constraints ([Sequence (01 02 03 04 05) within 0.4 days])The scheduling data was generated by CASM 17.2.CASM was run in Phase II mode.C:\Documents and Settings\Max\Desktop\DD_lutetia\OP-cache\11957_confirm-v02.xml122806984600019.98350432251499(Tic-Visit
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)true10Using ephemeris for target 11957-03-LUTETIAephemeris data for target 11957-03-LUTETIA does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 08.304 to 09.335. ephemeris interval: 08.335 to 10.210Timing link suitability includes constraints ([Sequence (01 02 03 04 05) within 0.4 days])The scheduling data was generated by CASM 17.2.CASM was run in Phase II mode.C:\Documents and Settings\Max\Desktop\DD_lutetia\OP-cache\11957_confirm-v03.xml122807559900021.186741262764514(Tic-Visit
:proposal-id 11957
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)true10Using ephemeris for target 11957-04-LUTETIAephemeris data for target 11957-04-LUTETIA does not cover planning interval. Physical constraints are assumed to be unschedulable outside of the ephemeris data. Planning interval: 08.304 to 09.335. ephemeris interval: 08.335 to 10.210Timing link suitability includes constraints ([Sequence (01 02 03 04 05) within 0.4 days])The scheduling data was generated by CASM 17.2.CASM was run in Phase II mode.C:\Documents and Settings\Max\Desktop\DD_lutetia\OP-cache\11957_confirm-v04.xml122808135300018.76728379783441(Tic-Visit
:proposal-id 11957
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