An activity several days ago had the narrow angle camera pointing to the Pleiades (RA 56.871, dec 24.449). What's that all about? Here's the science plan entry:

BEGIN_TIME: 183256204.182416 (2005 OCT 22 12:29:00 UTC)

END_TIME: 183289624.182419 (2005 OCT 22 21:46:00 UTC)

PRIMARY_POINTING: ISS_NAC to 56.871/24.449

REQUEST_ID: VIMS_017OT_PLEIADES001_PRIME

REQUEST_TITLE: Pleiades

REQ_DESCRIPTION: Turn to Pleiades; Time: 30 min

Pleiades: multiple cubes w/64x64 FOV IR @ 640 ms, VIS @ 40000 ms; Time: 7.0 hr 47 min; Volume: 86.5 Mbits; CompRatio: 2.0

Turn to waypoint: 30 min

Primary: -y to Pleiades; Secondary: +z to TBD

Not time critical.

RWA REQUIRED.

SECONDARY_POINTING: POS_X to NSP

SEQUENCE: S15

SUBSYSTEM: VIMS

Instrument calibration purposes? Acquiring of stellar spectra for VIMS and PSF, flatfields for the cameras? AFAIK there are supposed to be regular calibration measurements made at least once per year to better understand the instrument performance.

This is understandable, aiming the instruments at known targets to compare their output. The Pleiades were selected perhaps because they offer a great number of stars of varied sizes ( ideal to plot the luminance response curve of the cameras) into only one frame, and also they are very close together (separation power of the cameras).

The Pleiades seem to be a popular target for in-flight calibration purposes. I recall at least one other mission that used them for camera testing, OTOH it might have been Deep Impact or MESSENGER.

Star clusters are great targets for calibration because their dense...clustering of stars at varied brightnesses allow for montioring of the PSF and the instrument response. HiRISE looked at Omega Centauri, for example, last month.

Could Cassini conduct any significant stellar astronomy? It is about a billion miles closer to the rest of the galaxy than we are on Earth.

That's a bit like climbing up a step ladder to get a better look at the moon.

I think this is one of the Pleiades images, but can't really tell. The FOV of the narrow angle camera is much less than the width of the Pleiades, so maybe only one bright star is in the field (Alcyone?):



http://saturn.jpl.nasa.gov/multimedia/imag...5/N00042058.jpg

Well, technically, one would be closer to the Moon.

Repeating smiley here:

Seriously, I read a proposal for an astrometry mission which would be sent on an Interstellar Probe type trajectory. The purpose would be to get a longer baseline for parallax. However, the resolution of Cassini's cameras is not good enough to give it an advantage at Saturn.

And also quite necessary if one's view of the moon is blocked by one's neighbor's privacy fence

nope, this is from ISS_017ST_MONITOR001_PRIME, the bright star is Vega

Should have known you'd track it down.

gr-qc/0510105 [abs, ps, pdf, other] :

Title: A comment on the possibility of testing the Dvali-Gabadadze-Porrati gravity model with the outer planets of the Solar System

Authors: Lorenzo Iorio

Comments: LaTex, 7 pages, 2 tables, 0 figures, 18 references

Subj-class: General Relativity and Quantum Cosmology; Space Physics

The multidimensional braneworld gravity model by Dvali, Gabadadze and Porrati was primarily put forth to explain the observed acceleration of the expansion of the Universe without resorting to dark energy. One of the most intriguing features of such a model is that it also predicts small effects on the orbital motion of test particles which could be tested in such a way that local measurements at Solar System scales would allow to get information on the global properties of the Universe. Lue and Starkman derived a secular extra-perihelion \omega precession of 5\times 10^-4 arcseconds per century, while Iorio showed that the mean longitude \lambda is affected by a secular precession of about 10^-3 arcseconds per century. Such effects depend only on the eccentricities e of the orbits via second-order terms: they are, instead, independent of their semimajor axes a. Up to now, the observational efforts focused on the dynamics of the inner planets of the Solar System whose orbits are the best known via radar ranging. Since the competing Newtonian and Einsteinian effects like the precessions due to the solar quadrupole mass moment J2, the gravitoelectric and gravitomagnetic part of the equations of motion reduce with increasing distances, it would be possible to argue that an analysis of the orbital dynamics of the outer planets of the Solar System, with particular emphasis on Saturn because of the ongoing Cassini mission with its precision ranging instrumentation, could be helpful in evidencing the predicted new features of motion. In this note we investigate this possibility in view of the latest results in the planetary ephemeris field. Unfortunately, the current level of accuracy rules out this appealing possibility and it appears unlikely that Cassini and GAIA will ameliorate the situation.

http://arxiv.org/abs/gr-qc/0510105



gr-qc/0508012 [abs, ps, pdf, other] :

Title: On the impossibility of using certain existing spacecraft for the measurement of the Lense-Thirring effect in the terrestrial gravitational field

Authors: Lorenzo Iorio

Comments: LaTex2e, 13 pages, no figures, no tables, 31 references. Reference added, Section 2.4 added

Subj-class: General Relativity and Quantum Cosmology; Space Physics

In the context of the currently ongoing efforts to improve the accuracy and reliability of the measurement of the Lense-Thirring effect in the gravitational field of the Earth it has recently been proposed to use the data from the existing spacecraft endowed with some active mechanisms of compensation of the non-gravitational accelerations like GRACE. In this paper we critically discuss this interesting possibility. Unfortunately, it turns out to be unpracticable because of the impact of the uncancelled even zonal harmonic coefficients of the multipolar expansion of the terrestrial gravitational potential and of some time-dependent tidal perturbations which would resemble as superimposed linear trends over the necessarily limited observational time span of the analysis.

http://arxiv.org/abs/gr-qc/0508012

There is another Deep Sky image online.

Gasgiant Saturn and supergiant Rigel:


- PIA08187: Help from Orion