Re: [guide-user] Re: Phobos transit in Guide

Luc Desamore Sep 5, 2013

Hi Bill,
There is well a three days error in the Nasa News when they said that the transit took place on Aug 17.
It is proved by the caption of their own raw image.
This image was taken by Mastcam: Right (MAST_RIGHT) onboard NASA's Mars rover Curiosity on Sol 369 (2013-08-20 07:12:27 UTC).
Without Guide, I would not have realize that.
The comparison looks pretty good, there is indeed a 1m15s différence in time.
And a slight rotation of the image (from Phobos or the camera ?).
Look at the updated jpeg file "eclipse_phases.jpg" in the Yahoo group files.
This latter includes an accurate position of Curiosity measured on :
Note that the JPL horizons does not include an updated position of Curiosity.
The JPL position is 137°24' E  4°30' S
Actual position on aug 21th  is 137°25'58.5" E 4°35'59.5"
From: Bill Gray
Sent: Thursday, 05 September, 2013 03:37
Subject: [guide-user] Re: Phobos transit in Guide

Hi Luc,

   In this case,  I'm afraid it's Guide that is in error.  The orbits Guide
has for Phobos and Deimos are more than good enough for visual observations,
and put the moons in almost exactly the right places as seen from the
Martian surface.  But they aren't good enough to show transits,  at least
not exactly.

   This is going to be a bit tricky to fix.  It used to be that I could
get precessing-ellipse elements for Phobos and Deimos (essentially,
orbital elements with added terms to have the ascending node and
argument of perihelion increase linearly),  but those are no longer
being produced at JPL;  they're integrating the orbits numerically now.
The ephemerides for Phobos and Deimos have been updated over the years,
using a combination of transits observed by Spirit and Opportunity
and from Mars Express images.  (And I expect they'll use the Curiosity
observation to improve the orbit;  the observation was a kilometer or
two off prediction.)

   I may try creating my own ephemerides.  As it happens,  I used the
recent Curiosity observation to test out the solution I got with Find_Orb:

   This is a somewhat rough solution,  as described in the text at the
top of the page.  The data is still not quite good enough to get the
transit to show up;  all you get is a close approach.  Still some
work to be done here... of course,  one thing I can do is to include
the Curiosity observation itself in my orbit solution!

-- Bill

On 09/04/2013 05:56 AM, Luc Desamore wrote:
> Hi all,

> Thanks to Guide, I found an error in the Nasa News concerning the Phobos transit.
> The Nasa (and the whole internet) says that the transit took place on aug 17.


> With Guide there is no transit seen from Curiosity at this date but there is a good candidate on aug 20 at 07:11:16 UT.
> Try on Mars with the latest coordinates of Curiosity : 4°36'06" S and 137°25'48" E and compare with the Nasa picture.

> Finally I found the original picture of the transit/eclipse at :

> The picture was taken at 07:12:27 UT nearly the same instant found by Guide.

> Congratulations to Bill and Guide.

> Luc Desamore



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