Satellite observation is an underground facet of observational astronomy.
If you have an interest in space and what is up there, you will have
at your disposal thousands of possible moving (and some not moving)
targets for your eyes, binoculars, or telescopes. This is a past-time
or sport that is available to persons of all ages, genders and transcends
country and region. You can spot satellites if you live in large urban
areas, sail the ocean or travel by train, and with some difficulty, on
aircraft. When I briefly lived in the center of Houston, Texas (population
over 2 million), I was able to spot satellites down to +6 magnitude on
a clear night from the roof of an apartment building. I have observed
satellites from many large cities and countries around the world, primarily
from roofs of hotels (e.g. Amsterdam, Seoul, Beijing, Antwerp, London,
Mexico City, to name just a few). I videotaped the Russian Mir space
station from a moving train between Sofia, Bulgaria and Bucharest,
Romania some years ago, and have observed a number of satellites with
the aid of binoculars while on cruise ships in the Caribbean Sea. The
advent of the Internet has made it possible to download freeware and
have access to orbital elements as well as complete predictions and
star charts.
When I sat down to try to think of why satellite observing
provides immense satisfaction, I found a varied number of
reasons that have struck me over the years since I observed my
first satellite, Echo I, in 1960 from San Antonio, Texas. The catalyst
for my interest began that night when I watched this spacecraft for
several minutes and then saw it completely vanish without a trace.
Since that moment I have observed satellites at home, while on vacation,
and during business trips. Hobbyists can develop levels of viewing
purposes from the lowest level (armchair analyst) to the most
sophisticated observer with research applications goals in mind:
1. For some spacecraft such as the Space Shuttle and International
Space Station there are real people inhabiting these vehicles.
That in itself is a reason to try to watch flyovers of these satellites.
2. There is a tremendous challenge in determining whether you can
actually see an object for which a prediction has been made. Will
it be visible or not? Will clouds interfere? Have I made an error
in running a satellite prediction program or using an online database?
Am I sure that I am looking in the right direction?
3. Many amateur astronomers are familiar with the Messier Marathon
in which the challenge is to observe all of the more than 100
Messier objects in the space of one night. Every summer, it is
possible to observe high inclination objects such as the International
Space Station on a number of consecutive orbits between sunset and
sunrise during a short period when the orbit is fully sunlit. Usually
a satellite can be observed once or maybe twice in one night. This
can be adventurous in itself.
4. Special objects, such as those that have been 'lost' by ground-
based sensors, pose a challenge in that their orbits are no longer or
are poorly known. It takes special talents and initiative to track
down an elusive object missing in space, e.g. Saturn and Apollo era
pieces.
5. A satellite may be involved in active maneuvering (e.g. Soyuz,
Progress, Shenzou, Space Shuttle) that can add to the uncertainty of
whether you can spot it.
6. Some satellites are old and have been in orbit more than 30 years.
Observing some of the oldest manmade objects in orbit (e.g.
Vanguard 2) is like opening a window in the page of aerospace
history.
7. Satellites known as 'Iridium' can be visible in broad daylight
unlike any of the thousands of others in orbit.
8. Certain satellites give off spectacular flashes that can be seen
from far away (e.g. Telstar 401, Gorizont 14, and TDF amongst others
in geostationary orbit can sometimes be seen flashing to naked eye
brightness from 22,000 miles distant). Such 'one power flashers' are
a remarkable sight to see.
9. Amateur radio operators may be interested in attempting to observe
amateur radio satellites (e.g. OSCAR) actually built by amateurs.
These satellites are normally very small and difficult to spot.
10. Ejection of water (water dumps or waste dumps) from manned vehicles
like the Space Shuttle will form comet-like tails. They can be easily
visible to the naked eye and are beautiful and impressive sights on
a dark night when they appear!
11. Rendezvous of the Space Shuttle with a payload that it is about
to retrieve (or when it is deploying a payload) can be fun to watch.
The two will station keep for a time and will appear as two moving
stars in close proximity to one another.
12. Reentries are a real challenge in that the odds of actually
spotting one is nearly impossible. Yet when one is seen, it can prove
a wonderful light show to photograph or videotape (e.g. Space
Shuttle).
13. Amateur satellite observer groups have their own web sites and chat
rooms online. There are also sites for downloading freeware and also
for providing viewing angles, times and star charts at the click of a
mouse. Compare your own experiences with those of others and get
helpful advice from experts.
14. There are specialty satellites like the Global Positioning
System spacecraft that can be interesting to try to see since they
normally are around +10th magnitude. They are responsible for
providing accurate locations for some automobile navigation systems
and for hand-held GPS units used by backpackers, surveyors, boaters,
etc.
15. Military satellites for which orbital information is not published
(e.g. Lacrosse) is another specialty field in which a few hardy
observers attempt to find and track and maintain their orbits. This
is an interesting specialized subordinate field of satellite watching.
16. The Space Shuttle crew can be seen on cable TV (NASA select)
and, at times, their activities can be watched on TV simultaneously as
they may fly overhead.
17. A few satellites have special shapes (e.g.TiPS), which make them
look like flying lines moving across the night sky. They are sometimes
viewed with the unaided eye or by telescope.
18. On occasion, flying formations of 3 satellites can be viewed
(e.g. NOSS) as they ply the night sky. These form a highly unusual
flying triangle and can sometimes be viewed with the naked eye.
Satellites are normally only seen individually or possibly moving
in an orbit with a nearby carrier rocket body.
19. Decaying objects nearing reentry are fun to attempt. As they
reach their end of life, the atmospheric drag increases radically
and it is an enormous challenge to spot them. Hence there is a
challenge in trying to use the latest decaying orbital elements
to predict where the reentry might occur. An interesting exercise
for computing buffs and observers working in tandem.
20. Low perigees are the trademarks of some decaying satellites
especially from geostationary transfer orbit. On occasion the perigee
can dip below 90 miles (some time in the months before reentry). It
is conceivable if you track the perigee and are in the right place,
you could see a glow as the atmosphere interacts with the satellite
before it swings back out.
21. Atmospheric drag can severely affect the orbits of some spacecraft,
especially those that might have a 'light' construction and hence
are affected significantly by solar radiation pressure. Tracking
these objects is a real challenge as they are whipped around from
day to day. The drag term of the orbital elements varies considerably,
and one usually has to define an observing window for a number of
minutes on either side of the predicted time.
22. Reconnaissance satellites operate at low altitudes and make
infrequent, speedy passes over an area. These are fun to try to spot.
23. Serious observers can plot the light curves of tumbling rockets
in order to obtain information on their rotations and rotational
changes over time. These are dynamic opportunities for which study
can be done for many years. Accelerations (or decelerations) are
often obvious as unvented fuel all of a sudden impacts the rotation
rate.
24. Failures of some satellites mean that they can change their
orientation, rotation rate or can even be maneuvered out of orbit.
These newsworthy events are interesting to attempt in order to
help determine if the impact of the failure has optical characteristic
indicators (e.g. WIRE, ETS VII, SUNSAT, ABRIXAS).
25. Trash can be accidentally or intentionally jettisoned by EVA
astronauts working on a manned space station. Trash can orbit in close
proximity to the parent vehicle for the short term. In rare instances
(e.g. Mir) many objects have been seen 'leading' the station after
having been jettisoned, providing a 'parade' effect.
26. Some satellites create clouds of chemicals, which can be viewed
by the naked eye at high altitudes as black and white or colorful
displays.
27. Launches of rockets can often be viewed in the night sky from
U.S. east or west coast locations, and they can create cloud-like
formations that are really unique to observe and photograph. Other
launches have had unexpected displays from fuel venting or motor
firings that have caused reports of unidentified flying objects in
e.g. Russia and South America.
28. Geostationary satellites form a belt around the celestial
equator. Observing these objects can be fun during the equinoxes
when active solar arrays tend to be favorably oriented causing the
normally 12-14th magnitude objects to brighten to +9 or even brighter
during a one week period in the vicinity of the months of March and
September.
29. Some satellites (e.g. Ofeq) are launched into retrograde orbits
and seem to defy the rotation of the earth as they orbit in the
reverse sense.
30. A number of spacecraft orbits are affected by gravitational
harmonics. Their positions are studied over time to ascertain
information about the earth's gravitational field. Observers can
make measurements on the positions of these objects to assist in
such endeavors. Similar contribution to other professional research
projects can occur, e.g. LAGEOS and Ariane IV amateur payloads.
31. Interplanetary spacecraft, which conduct maneuvers to bring
them close to earth, can be a major challenge to observe at great
distances (e.g. Galileo, NEAR, STARDUST) extending millions of
miles from earth. Amateur satellite watchers have been able to
observe these flybys using telescopes or CCD cameras. In at least
one case NEAR was reportedly visible to the unaided eye.
32. Distant earth-orbiting satellites can sometimes be mistaken
from asteroids. Certain objects have perigees thousands of miles
from the earth's surface and apogees even farther out. Again,
these are extremely challenging objects to find and observe. Assisting
in the investigation of such mystery objects can help to determine
the truth about their source orbits.
33. Iridium satellites can flare up at night or in twilight to
where they are brighter than any other object (except the moon)
and possibly even create a shadow under the right conditions. This
creates interesting video/photo opportunities.
34. Observing transits of satellites across the moon or sun are
another challenge for satellite observers.
35. Space debris includes a wide range of objects size-wise. The
smallest debris, such as results of explosions, can also present
a major challenge to observers. They can give clues as to the
reflectivity of objects in comparison to their radar cross section.
36. Other space debris resulting from rare collisions between two
man-made objects (e.g. Cerise) are another fascinating challenge.
37. Imaging the shape of a satellite using a large telescope and
auto tracking is a mostly unexplored field that offers lots of new
horizons. The International Space Station offers a sizable target
whose shape can often be discerned in moderate sized telescopes.
38. Observing and monitoring light curves of moving satellites offers
a good training ground for persons wanting to learn magnitudes and
develop acuity skills that can be applied elsewhere in astronomy
such as to variable star measurements.
39. Use of a telescope and star charts in support of satellite
observing provides one an expanded skill base for being able to
track down star fields quickly and use different instruments for
detecting faint objects.
40. Sometimes amateur observers are called upon by institutions
to attempt to observe certain satellites that have a news or science
value (e.g. launch of first North Korean satellite).