Subject: Rendezvous Endgame
From: Daniel Adamo <[email-masked]>
Date: Sat, 15 Jun 2013 10:36:07 -0700

ATIG Members-

About 8 hours before ATV-4 docked with ISS at 7:07 AM PDT on June 15, the
attached image was obtained from my home south of Salem, Oregon.  Both
spacecraft are recorded moving southeastward past the star Deneb in the
constellation Cygnus.  The 30-second exposure began at 11:14:42 PM PDT on
June 14 with ISS leading ATV-4 at a point near its closest approach to
Deneb.  For those unfamiliar with constellations, I've "connected the dots"
for Cygnus, alias The Northern Cross, using faint gray lines.

Note how much closer the two trails are to overlaying each other in this
late phase of rendezvous than they were in the image obtained a week
earlier.  Because ISS elevation above the horizon is similar in both images,
about 40 degrees, the ATV-4 trail has shifted closer to that of ISS in the
later image due to its increased orbit altitude.  This is a typical
signature in "rendezvous from below and behind" operations.  In the endgame,
the spacecraft about to dock with ISS progressively reduces its catch-up
rate by incrementing its orbit altitude to more closely match that of ISS.
By carefully timing these increments, ATV-4 achieved an altitude match just
as it caught up with ISS, allowing docking operations to proceed safely at
low relative speeds.

Yet another faint unidentified satellite trail is visible in the attached
image.  For scale, assign angular distance d to the apparent separation
between Deneb and the star Gamma Cygni at which the two Northern Cross
members intersect.  From Gamma Cygni, proceed in the 4 o'clock direction
about 0.7 d to the trail, which is about 0.2 d in length.  Because the trail
is so short compared to those of ISS and ATV-4, it's undoubtedly from a
satellite at much higher altitude.

Congratulations to ESA for a successful (and picturesque) rendezvous and
docking with ISS!

-Dan

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

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