Subject: Highway In The Sky (Speed Limit: 7.7 km/s)
From: Daniel Adamo <[email-masked]>
Date: Sat, 08 Jun 2013 11:36:36 -0700

ATIG Members-

The attached photo was taken from my home looking north toward the city of
Salem, Oregon with the state capitol about 10 km away, so please forgive a
little light pollution.  It shows an early stage of the European Space
Agency's (ESA's) Automated Transfer Vehicle-4 (ATV-4, christened Albert
Einstein, see http://esamultimedia.esa.int/HSO/Publications/ATV4-EN/)
rendezvous with the International Space Station (ISS).

The photo's exposure began on June 7 at 10:28:59 PM PDT when only ATV-4
(dimmer, lower trail) was in the camera's field of view, and exposure was
terminated 55 sec later when only ISS (brighter, higher trail) was in the
field of view.  Because the two spacecraft are orbiting in nearly the same
plane with ATV-4 at a slightly lower altitude and higher speed, their trails
are parallel but displaced during the exposure.  Orbit motion among the
stars is down and to the right in the photo, which shows a maple and a fir
tree in silhouette against Salem's sky-glow at bottom.  There's also a very
faint trail at lower left running nearly perpendicular to that of ISS, but I
have no idea what Earth satellite caused it.

For those unfamiliar with constellations, I've "connected the dots" for Ursa
Minor, alias The Little Dipper, using faint gray lines.  The star at the end
of the Dipper's handle, and closest to the end of the ISS trail, is Polaris.

Launched on June 5 at 2:52 PM CDT, ATV-4's docking with ISS is planned for
June 15 at 6:46 AM PDT.  The attached June 7 PDT photo catches ATV-4 just
after it had "lapped" ISS and moved ahead of it in orbit.  On the previous
PDT evening, ATV-4 was on the other side of Earth from ISS, making a single
exposure capturing both spacecraft impossible for Earthbound photographers.
The following table shows daily differences in the times ISS and ATV-4
appear in Salem's sky after sunset.  This Dt is reckoned by subtracting the
ISS time from the ATV-4 time for a particular PDT evening, so a negative
value implies ISS trails ATV-4.  The concept of trailing or leading
degenerates near +45 min or -45 min Dt when ISS and ATV-4 are on opposite
sides of the Earth.

PDT Date   Dt (min)
June 5     +10
June 6     -44
June 7     0
June 8     +44
June 9     -11
June 10    +40
June 11    -3
June 12    -43
June 13    +16
June 14    0

From data in the foregoing table, ATV-4 can be inferred to pass beneath ISS
on multiple occasions at roughly 2-day intervals prior to about a day before
docking.  If you're interested in sighting opportunities for ISS, ATV-4, or
various other satellites from your location, point your web browser at
http://spaceflight.nasa.gov/realdata/sightings/index.html to obtain
predictions.

Wishing you clear skies!

-Dan