Iridium Flares and the ISS

Tuesday, January 19 2010 @ 03:55 pm EST

Contributed by: dgrosvold

By Dave Grosvold

The real astronomical treat this week will not be planets, stars, or even comets, but man-made objects in the heavens. Iridium Flares are the result of sunlight reflecting off the surface of Iridium Satellites as they pass overhead.

Flare from Iridium 84, November 20, 2006 at 7:06 PM CST. Andrew Echols Photo.
Iridium Satellites are telecommunication satellites launched in 1997 and 1998. There are about 70 Iridium Satellites currently in orbit. What makes them interesting is that their particular shape and antenna configuration contribute to the occurrence of very bright flares – sometimes so bright they can be seen in daylight. The flares last only a few seconds, but are most impressive at night. You can view a flare simulation on Wikipedia.

These flares can reach a brightness nearly twice that of Venus at maximum, or nearly −8.0 in apparent magnitude, a measure of the apparent brightness of an object in the night sky when viewed from Earth. Although there will be several flares over the next week, there are two that will be visible in the evening. On Wednesday evening, January 20th, Iridium 50 will pass overhead to your NE at approximately 6:55 PM. Look for it at about 52° above the horizon. Remember – straight up (the zenith) is 90° above the horizon, so 52° is more than halfway from the horizon to the zenith. To make sure you see it, start observing at about 6:50 PM CST, and continue until at least 7:00 PM. It’s even better if you have several people looking in different parts of the sky.

On Thursday, January 21st, Iridium 53 will take nearly the same path across the sky, but just a bit earlier, at 6:49 PM CST. On Thursday, start looking at 6:45 and continue until about 6:55 PM. Look to the NE at an altitude of about 52° If you’re adventurous, Iridium 54 and Iridium 13 will pass at a low altitude of 22° in the SSE just after sunrise at 6:25 AM CST on Thursday, January 21st and at 6:10 AM CST Monday, January 25 respectively. These may be difficult to spot, but they are bright enough to see for sharp-eyed observers.

Saturn as viewed in a small telescope, 1:00 AM CST Thursday, January 21, 2010.
The International Space Station (ISS) will also make several bright passes in the evening this week, each brighter than Sirius, the brightest star in the night sky. The ISS is visible over a much larger path in the sky than Iridium Flares, and consequently stays visible for a longer period of time.

On the evening of Thursday, January 21st, the ISS will pass low in the N starting at 6:09 PM CST. It will swing higher to a maximum altitude of just 16° above the horizon in the NE, then pass low to the ENE over a period of about 6 minutes. You can estimate 15° of altitude on the sky by viewing the width of your fist held at arms’ length.

Observers get another chance at ISS on Friday, January 22nd. This time, the ISS will rise above the horizon in the NW at about 6:30 PM CST, swing high overhead to about 45° altitude in the NE, and then sink back to the horizon in the ESE, again over about a six minute period.

On Saturday, January 23rd, the ISS makes another pass from the NW at about 6:51 PM CST over to the SW at a maximum altitude of 40°, and then back toward the horizon in the SSW at about 6:58 PM. Amateurs have reported seeing the shuttle docked at the station in small telescopes as it passes overhead in the past. However, no shuttle missions are in progress at this time.

Also, Mars and Saturn dominate the eastern sky after sunset once again this week. Mars rises at about 6:30 PM CST, and is high enough in the East by about three hours after sunset to see clearly in small telescopes. Saturn rises much later - after 10:30 PM. It will be 1:00 AM or so before Saturn is high enough above the thick eastern atmosphere for observing. You may spot several of Saturn’s largest moons, including Titan in a small telescope.

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