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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