Astrophotography by Anthony Ayiomamitis

The ISS Transitting the Sun and Moon

Interested in finding the proverbial needle in a haystack with ultimate precision? If you are up to such a challenge, I suggest you consider trying to observe and/or image the International Space Station transitting the sun and/or moon.

This is a little project that occurred to me in 2001 when I accidently observed a satellite pass through my field of view while observing the moon and which has kept me on an incessant hunt ever since then with four predicted (solar) transits during this interval. Two other predictions did not materialize due to a shuttle launch which occurred two days earlier and led to an orbit change of the ISS following docking (standard procedure).

Following my capture of the ISS with the shuttle Discovery (STS-114) transitting the sun in late July/2005, I received many requests for further information from people around the world after seeing the photo to the right on NASA's APOD (AstroPhoto of the Day, July 29, 2005). Rather than regurgitate the same information to each interested individual with back-and-forth emails, I decided to prepare this reference page for everyone's benefit.

There are three resources that one may use to identify such transit opportunities. The first two such methods include the Internet (what else?) and, more specifically, the CalSKY website and Thomas Fly's (free) subscription service. The third method involves downloading two-line orbital elements for the ISS (I prefer doing it for all satellites) and then running an analysis using Rob Matson's DOS-based SkyMap application. Without getting into specifics, suffice it to say that I prefer the last method (Matson's application) for a variety of reasons including stunning graphics of the transit path itself across the sky and a super-accurate transit depiction against the solar or lunar disk.

One key assumption to generating such predictions is that you have a very good estimate of your geographical coordinates. This is a perfect application for a GPS unit which can get you a reading (supposedly) accurate to one or two meters. These coordinates must be as accurate as possible by virtue of the fact that a transit footprint is usually five kilometers or so in width and effectively represents a very tight fence for you to sit on for all of this to be realized.

With the geographical coordinates identified, we enter this information when visiting the CalSKY website or specify them when subscribing to Thomas Fly's service. For the former, we then click on the "Satellites" hyperlink to the far right of the home page and proceed to click on "Sun/Moon crossovers" on the new page which pops up. By default, the website will generate requests for transits and passes involving any satellite during the following 24 hours but a pull-down menu can be used for predictions up to a week.

For the latter, namely Thomas Fly's subscription service, bulletins are sent usually on a weekly basis as electronic emails specifying all such ISS transits within a radius of 160 km of your home location (as identified by your geographical coordinates). The advantage of these reports is that they identify close misses which gives you the opportunity to travel a nominal distance (ex. 10.3 km) so that you will now find yourself in the middle of the transit path and thereby observe (or image) what would otherwise have been a close but missed opportunity.

When using Matson's SkyMap application, we specify a default text file containing the downloaded orbital elements and then proceed to execute a search for solar or lunar transits for a user-specified interval into the future (I do so for the next fourteen days). I download fresh elements EVERY two to three days and execute the same search for the next fourteen days.

Once a POTENTIAL transit has been identified using any one of the three methods above, it is then that all of the fun and excitement begins, for there are a number of factors which can invariably lead to a false alarm. Perhaps the most vital factor is the epoch of the orbital elements since the ISS is characterized with minor orbit corrections and which can make or break a predicted transit. Therefore, it is crucial that one has the freshest possible orbital elements which hopefully will account for such an orbital change. Equally important at times is a launch of one of the shuttles destined to dock with the International Space Station; these launches inevitably lead to a docking 42 hours later with ensuing orbital boosts to higher orbits thereafter. With the latter, I have on two occasions lost a predicted transit due to these orbital boosts. For the transit image above, I was aware of the predicted solar transit on the Monday of the same week but had the problem of a shuttle launch the following day (Tuesday) with my original prediction indicating a Thursday late afternoon transit. Although the docking did occur four hours prior to my proposed transit, I was most fortunate that the orbit boosts occurred after the transit itself and, hence, this particular transit opportunity was spared.

During the four years preceeding the transit depicted in the photo above, I had four opportunities to observe and/or image the transitting ISS. The first two opportunities were lost due to the weather (misplaced dark clouds on one occasion and very heavy winds on the other) whereas a miscalibration of my cellular phone did me in for the third opportunity.

Depending on how close to the center line one is in relation to the transit path, the transits themselves can be grazing events or bisect the solar/lunar disk in half. For the latter, the transit will last a "whopping" one second (or just slightly over it) whereas grazing events last for fractions of a second. Even though a one-second transit may seem like eternity, you will be impressed at how quickly the ISS will zip bye in the field of view!

Happy ISS hunting!

Note: The above text is a very brief synopsis of an article being submitted for publication.