Rock Out Under The Space Disco Ball Tonight

Japan's high-tech disco ball, EGP, revolves around Earth every 116 minutes at an altitude of 900 miles. It's used to pinpoint remote Japanese islands and to study Earth's gravity field and motions of crustal plates. Credit: JAXA

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.

A 35-second time exposure with a telephoto lens shows a series of flashes (some in groups of three) from EGP this past Saturday night as the satellite traveled through Draco the Dragon. As sunlight strikes the satellite's many mirrors, we see flashes of light. Details: 100mm lens, f/2.8, 35-seconds at ISO 1600. Photo: Bob King

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).

For the Duluth, Minn. region tonight, the space disco ball will pass very close to Vega, one of the easiest-to-find bright stars in the sky. At the times shown, Vega will lie almost directly overhead. Created with Stellarium

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!

6 Responses

  1. Mike Thiele

    Thanks Bob for the recent post about the ISS flyby dates and times. As I watched the 10:34 flyby the ISS appeared in the west and passed near the big dipper as you had stated. As I was watching it approach Cassiopeia it “winked out” just prior to crossing into the constellation! Is this because it entered the shadow of earth? If so I am amazed because it is so high.

  2. Roxane Hebert

    I really enjoyed being able to see this satellite back when you posted this article. It was my first time trying to spot one, and it was very rewarding to find it and see it go sparkling by until I lost it in the clouds. Tonight, I thought I’d look it up again on the heavens-above site and I’m having trouble finding it. Do you know if the satellite is no longer in orbit? I’m wondering if it has been taken off of their list of satellites?

    1. astrobob

      It’s still in the database. Click the “Select another satellite” link and then type in these two words: Ajisai (EGP). Then click the Passes link. If it doesn’t work, let me know.

  3. Troy

    I saw it through binoculars, very cool! Flashes like an airplane. Thanks, I never heard about it until I saw your post.

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