From the September 2015 issue

Catch the fleeting ISS

Celestial sketcher Erika Rix works against the clock to catch the International Space Station zipping across the Moon or sky.
By | Published: September 28, 2015
Chances are you’ve watched the International Space Station (ISS) zipping across the sky. But did you realize that it is possible to track this fast-moving satellite telescopically and then sketch a surprising amount of its structure? If the timing is right, you may even capture an arriving or departing spacecraft.

You can see the basic shape of the ISS through any telescope, although steady tracking and higher magnifications will reveal increased detail. The golden color of the solar arrays is more noticeable when the luminosity is less intense. If sunlight reflects directly from the panels, the space station will glow brilliant white and reach magnitude –6 or brighter before fading once again.

Sketch of the ISS transiting the Moon
The author sketched the ISS transit of the Moon on September 11, 2014, from Leander, Texas. She used a 4-inch f/9.8 refractor on an equatorial mount with a 16mm eyepiece and a 13 percent transmission Moon filter for a magnification of 62.5x. She completed the sketch on black Strathmore Artagain paper using white pastels and black and white charcoal. She drew the ISS and transit line with a white gel pen and a black artist pen, respectively, and then scanned the sketch. North is to the top, and west is to the left.
Erika Rix
The first step to capturing these details in a sketch is to use satellite prediction services like to determine where and when visible passes will occur for your location. I prefer to track the ISS manually with a Dobsonian telescope that is fitted with a wide-field eyepiece. I achieve focus on a star field prior to the pass.

Rather than writing notes, I extend eyepiece time by using a digital voice recorder to describe the space station’s appearance and how it evolves during a pass. Focus on its aspect, especially the angles of the solar arrays in comparison to the modular components. Other notable changes are color, size, and brightness. I keep a sketch pad, pencil, and observing light on hand so that once the ISS is no longer visible, I can render a series of sketches fresh from memory. To add the color and luminance, use your sketches and voice recordings to create digital illustrations of the observation, as shown in the tracking sequence by Jeremy Perez at right.

ISS transits of the Moon and Sun are a real treat and require a different approach. The Web-based celestial calendar at can predict the exact time and place to observe the central line of a transit and will email you with alerts for upcoming events. A tracking mount is useful so that the Sun or Moon remains centered in the eyepiece.

For my observation, I started the lunar sketch well in advance and used the app GoSatWatch on my smartphone to set off two alarms — five minutes prior to the pass and at the expected time of transit. When the first alarm sounded, I stopped the sketch and continuously observed the Moon’s limb edge where I expected the ISS to appear.

International Space Station
Jeremy Perez created this digital illustration based on his visual observation and sketch of the ISS on August 29, 2012, at 2:25 UT using an 8-inch f/5.9 Newtonian with a 10mm wide-field eyepiece for a magnification of 120x.
Jeremy Perez
The space station crosses the disk in under a second. In that brief moment, I made a mental image of the points of its entry and exit, as well as its brightness, aspect, and size compared to nearby craters. Sunlight illuminated the ISS the morning of my observation. It would have appeared as a silhouette had it been in Earth’s shadow. Immediately following the event, I added the ISS and its path before completing the sketch.

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