About Nov. 17.5 UT, STEREO spacecraft visible from e. Asia, Australia, N.Z.

From: by way of Tony Beresford (dunham@starpower.net)
Date: Thu Nov 16 2006 - 04:28:30 EST

  • Next message: astro: "obs 8305 16nov 06320"

    On November 17 around 12h UT, less than 30h from now, the two STEREO
    spacecraft will pass through perigee, and will be visible for an
    hour or two before they will pass through the Earth's shadow.  The
    best areas from which the satellites can be observed are around the
    locations under where they enter or exit the shadow.  Observers in
    southern and eastern Asia, and New Zealand and Australia have a
    chance to observe them with telescopes; they are unlikely to be
    bright enough to be seen with binoculars.  The table below gives the
    shadow entrance and exit times and locations:
    
    Spacecraft             Ahead             Behind
    Shadow Event    Entrance    Exit    Entrance    Exit
    Universal Time  13:35:31 13:56:54   11:10:33 11:31:52
    Altitude, km       2506     3506       2180     3274
    longitude, deg.  103.2 E  164.8 E    136.9 E  154.7 W
    latitude, deg.    27.8 N   16.8 S     26.0 N   15.8 S
    Best areas       India    New        Taiwan     New
    for observation s.e.Asia  Zealand    Japan      Zealand
                      China   Australia   Philippine (low in
                                          Islands     east)
                                          China
    
    Delta-V Maneuver  Start     End      Start      End
    Universal Time  12:56:00 12:59.0    10:00:00 10:05.0
    
    In addition, observers in the areas under the shadow entrance might
    see the spacecraft brighten a little as they fire their thrusters
    for velocity-change (Delta-V) maneuvers.  The times of these are
    also given above.  The start times are exact, but the end times are
    uncertain by several seconds since the maneuvers are terminated by
    on-board accelerometers that sense when the Delta-V target has been
    achieved.  About 20 minutes before the maneuvers, and a few minutes
    after them, the spacecraft change their orientation, so a change in
    apparent brightness might be seen then as well.
    
    The satellites are thousands of kilometers apart near perigee.  They
    spend most of their orbit near apogee, near the Moon's distance; the
    direction of the apogees is near the direction to the Sun, so they
    can not be observed then, and can only be seen within a few hours of
    perigee.  They will probably be about 9th magnitude, but could be as
    faint as 10th or even 11th.  When Greg Roberts observed them shortly
    after launch, they were about 8th mag., with some brightenings to
    perhaps 6th mag., but the two spacecraft were too close to resolve
    then (so twice as bright) and they were also lower than they will be
    at this perigee.  Observers towards the Sun from the shadow points
    (that is, west of the entrance points and east of the exit points)
    will have better (smaller) phase angles so the satellites will
    appear brighter for them, but they will have less time in a
    reasonably dark sky to see them.
    
    You can generate detailed path predictions (RA, Dec, alt., az.,
    distance) in the sky for your location using the The JPL Horizons
    ephemeris generator Web site at
    http://ssd.jpl.nasa.gov/horizons.cgi .  To get predictions at
    less than a minute interval, specify "equal intervals" for the
    "units" of the step size, and then if you specify a time range
    of 20 minutes, specifying an "equal intervals" value of 600 will
    give you data at 2-second intervals.  The JPL Horizons site uses
    orbits for the satellites determined before we performed large
    maneuvers Tuesday morning, Nov. 14.  Those maneuvers were included
    in the orbital data supplied to JPL so their effects are included,
    but although the maneuvers were performed well, they did have errors
    of about 1% which cause them to deviate a little from the JPL
    ephemeris, which can not be updated before this perigee passage.  We
    have compared it with a new orbit determined after the Nov. 14th
    maneuvers, and find the following offsets that you will probably see
    relative to the Horizons prediction:
    
    Ahead:  Path 0.3  deg. south and 48 seconds late.
    Behind: Path 0.07 deg. south and 12 seconds late.
    
    These were calculated for locations right under the shadow entrance
    and exit points; for other locations where the spacecraft will be
    lower in your sky, the path offset will be a little smaller.  The
    maneuvers mentioned above are smaller than the Nov. 14th maneuvers
    and have been factored into the predictions.  The Nov. 17th
    maneuvers could have errors as well, but there will not be
    sufficient time from when they occur to the perigee passage to
    notice any significant differnce that might be caused by that.
    
    This is the 2nd perigee passage of these satellites.  The only
    inhabited area from which the 1st perigee passage was visible on
    November 6 was from part of Latin America; I have heard of no
    successful observations.
    
    The purpose of the two maneuvers this week is to target each
    spacecraft to the first lunar swingby on December 15th, and the
    Behind spacecraft to the right conditions at the first swingby so
    that it will have a second lunar swingby on January 21st.  The first
    maneuvers were successful for this purpose; if the maneuvers on
    November 17th are as successful, the targeting of the lunar swingbys
    should largely be complete, with only one or two additional very
    small maneuvers needed later on to refine the trajectories.  Much
    more about STEREO is on the main STEREO Web site at
    http://stereo.jhuapl.edu and on my STEREO Web site at
    http://highorbits.jhuapl.edu/stereo.htm .
    
    David Dunham, IOTA and STEREO mission design
    with much help from Jose Guzman, Peter Sharer, and many others on
    the STEREO team
    
    
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