In a Saturday Night Fever mood? Break out the medallions and your best polyester – we’re heading outdoors tonight to spot the 7-foot diameter orbiting disco ball called Hydrangea Flower (Ajisai in Japanese). Known better among satellite watcher by its more formal name – Experimental Geodetic Payload or EGP – this twinking “star” will amaze your eyes as you track it during a typical 18-minute long passage across the sky.
18 minutes is a long time compared to the approximately five minutes it takes for the space station to pass from horizon to horizon. That’s because EGP orbits Earth at an altitude of 900 miles versus the station’s approximately 230 mile orbit. Being farther from Earth, it moves more slowly through the sky.
EGP is a Japanese satellite launched in 1986 with a dual mission: to test a new launch vehicle and determine the exact positions of isolated and remote Japanese islands. And it really does look like a disco ball. The hollow sphere is covered in 318 mirrors and 1436 “cube corners” or retro-reflectors. Retro-reflectors are optical devices that reflect any light falling on them directly back in the direction from which it came. They’re used in everyday things like the insides of red tail-light covers on cars and bicycle reflectors. Even the paint on road signs contains reflectors in the form of tiny glass beads.
Scientists on the ground bounce a laser beam off one of the many mirrors or retro-reflectors on EGP and detect the return beam reflected back. Since the satellite’s orbit is exactly known, the round-trip return time for the beam will yield a location for the observer’s position on the ground accurate to millimeters. Satellite geodesy, as it’s known, is so accurate, it can detect the molasses-like movement of Earth’s crustal plates over short time scales.
Two nights ago I watched a pass of EGP. There’s nothing quite like it. With all those mirrors, the satellite flashes rapidly and irregularly like a string of firecrackers going off. The flashes were too faint to see with the naked eye – except from a dark sky site – but extremely easy in any pair of binoculars. The key to finding it is to use the simple maps available on the Heavens-Above satellite observing website.
Once you log on and select your city, look under the Satellites heading and click on Select another satellite. In the U.S. Space Command ID box, type in EGP’s ID number 16908 and click Submit. Clicking on the Ajisai (EGP) link takes you to a info page. At upper right, click on Passes (visible). Now just pick a convenient viewing time. There are many, since EGP is so high up it’s visible late into the night and early morning. I scanned the list of passes and selected the 22:09 (10:09 p.m.) pass tonight for Duluth, Minn. (see below).
When you click on the date link for the pass you’d like to see, you’ll be shown two maps. The top one is a wide view of the whole sky showing the entire length of the pass. The bottom one shows the brightest segment of the pass along with tick marks showing where EGP will be at particular times along its path. Pick a pass or a part of EGP’s path that takes it near a bright, easy-to-find star. Note the time when it’s near that star, then go outside five or 10 minutes before EGP’s arrival to let your eyes to get used to the dark. A minute before it shows up, point your binoculars at your “guide star” and just wait. Pretty soon you’ll see it popping and flashing along.
While watching it Saturday, I counted about two strobe-like flashes per second. Twinkle, twinkle little star, indeed!