RATIONALE FOR OBSERVING MAN-MADE SPACECRAFT
by Paul D. Maley
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.
Photo of a Russian rocket launch causing fear in Russia. Part of the problem is lack of disclosure when such launches occur. This is another reason for why one should observe and understand night sky phenomena to help distinguish fact from fiction.
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).