How to View the ISS

Look! Up in the sky! It's a bird! It's a plane! It's a steady, non-blinking, non-winking, non-flashing white dot that moves across the San Antonio sky in a maximum of six minutes! It's the International Space Station - or ISS for short. The ISS is easily visible to the naked eye on a clear pre-dawn morning or post-sunset evening when its orbit takes it over San Antonio. --But, its arrival needs to be within a window when the Sun is just over the horizon (i.e. no earlier than about 90 minutes before sunrise and no later than about 90 minutes after sunset). At ground level during those windows it's dark or dusk, but the orbital altitude of the ISS is high enough for it to continue to be directly illuminated by the Sun while over San Antonio. The ISS's modules are painted white and its solar panels are tinted gold. It reflects sunlight quite well.

If you surf the Internet to the following NASA webpage, you can find near term ISS viewing opportunities (usually within a week or two): ISS Flyover Sightings

Sometimes there will be no viewing opportunities in the near future. When there are viewing opportunities, they are generally clustered within a handful of consecutive days.

The webpage informs you at what San Antonio time and at what compass direction the ISS should appear. You don't literally need a compass to view the ISS. As long as you generally know north from south and east from west you're good to go. The appearance is a stated elevation above the horizon. It lists the duration of the flyover (anywhere less than 1 minute up to a maximum of about 6 minutes). It tells you the compass direction and at what elevation it should depart your field-of-view. Finally, it reports the maximum elevation angle from your vantage point that the ISS should attain during the flyover. An elevation of 0 degrees is the horizon and 90 degrees is directly overhead. Why isn't the viewing data always the same? Because the ISS doesn't always trace the same path on the ground. Its orbit is tilted relative to the Earth's equator (i.e. 51 degree inclination relative to the equator), and the Earth continues to rotate on its axis underneath the orbit of the ISS.

Aside: If an object orbits directly over the equator it is said to have an orbital inclination of 0 degrees. A 90 degree orbital inclination describes an object in a polar orbit (i.e. orbiting from pole to pole).

If the sky is clear, the first time you try to observe an ISS flyover you'll be wondering if every little speck is the ISS as its appearance time approaches. Don't worry, when it enters your field-of-view from its direction of approach it is obvious that it isn't a star or planet or plane or jet (or a locomotive or tall building :-). Stars and planets appear stationary in the sky compared to the ISS. Planes and jets must have colored blinking lights. The ISS doesn't blink and it moves across the sky much faster than any high altitude jet.

Occasionally, other objects in orbit are also visible. A space shuttle is much smaller than the ISS. Its upper body is painted white, whereas its thermal tiles are black. It is less reflective than the ISS, but still easily visible to the naked eye if its orbit takes it over San Antonio during a viewing window. When a space shuttle is docked with the ISS, the combined reflectivity of a space shuttle and ISS makes it especially bright. When a space shuttle is within about 1 day prior to docking or 1 day after undocking you might view a special treat if they pass over San Antonio during a clear viewing window. The ISS appears first and then within 1 minute or so the less bright space shuttle follows along the same - or nearly the same - vector. The space shuttle looks as if it's chasing the ISS. With the planned retirement schedule of the shuttle fleet, you might have as little as one more opportunity to observe a space shuttle chase the ISS.

You can't see the ISS or a space shuttle in orbit with the naked eye during the day because the Sun above the horizon is too bright. Why can't you see the ISS or a shuttle each time its orbit takes it over San Antonio at night? An ISS or space shuttle is only visible to the naked eye before sunrise or after sunset if sunlight is reflecting off its exterior. They each generate too little light of their own to be visible from the ground to the naked eye. Once the ISS enters the Earth's shadow it quickly disappears from view. During most of the night when you look up at the sky, you're looking into Earth's shadow. Because the Earth is roughly spherical, it casts a shadow in the shape of a cone - narrowing the farther you move away from the Earth in the opposite direction from the Sun. Any object in low earth orbit is within the bounds of that cone of darkness when you look up at night. However, the Moon is far enough away that even when it's behind the Earth at least part of it remains out of the Earth's shadow most of the time and reflects the Sun's light back to Earth. The only time the Moon is completely obscured by the shadow of the Earth is during a total lunar eclipse.

Is the ISS faster than a speeding bullet? The ISS travels at approximately 4.8 miles/sec. That's over 25,000 ft/sec which is why it only requires a little over 90 minutes to complete an orbit of the Earth. A quick search of the Net indicates that modern rifles with high performance cartridges achieve a muzzle velocity of up to 4,000 ft/sec with a tank's main gun at 5,900 ft/sec. A bullet doesn't stand much chance of catching up to the ISS - or to Superman.


Much thanks to a NCTONA neighbor for providing this article.