A relatively small communications satellite has been providing spectacular visible reflective flares/glints to observers on the ground (*refer to respectfully submitted definition for flares/glints).
With only a normal brightness of +6 magnitude (binoculars are useful to spot it), occasionally some of the Iridium satellites provide reflective flares/glints of magnitude (-)8. For comparison, Venus can be as bright as magnitude (-)4.9, thus reflections can be up to 30 times brighter than Venus. The flares/glints can last anywhere from 5 to 20 seconds before the satellite once again becomes almost invisible to the naked eye. Some flares have been observed during the daylight hours which is very unusual for reflective glares from satellites. Knowing where to look to observe these flares during the daylight hours is essential.
Initially, the Iridium satellite is placed in a circular orbit at an altitude of approximately 500 km. Normally, over a two week period the satellites are individually raised to a circular orbit of approximately 780 km. With its job done, the satellite dispenser at the 500 km altitude is placed in a lower eccentric orbit to accelerate decay and re-entry. During the first launch provided by Boeing's Delta II, the dispenser failed to achieve a lower eccentric orbit and went into a higher eccentric orbit.
The mechanism providing the flare/glint are the Main Mission Antenna (MMA) on each of the satellites. These antennas (of which there are three- 120 degrees apart, 188 cm wide x 86 cm long x 4 cm thick each) are highly reflective aluminum flat plates (treated with silver-coated Teflon for thermal control) that are angled 40 degrees away from the axis of the body of the satellite. Here are detailed views of the satellite taken at the NASM. Daniel Deak also provides photos taken at the NASM.
The axis of the satellite body is maintained vertical to the Earth's surface. It is the maintenance of the operational satellite's axial and longitudinal position that allows the flares to be predicted.
On November 15, 2000 it was announced that Iridium Satellite LLC will take over the assets of Iridium LLC and maintain the constellation in orbit. Details of the purchase can be found at SpaceFlightNow.Com .
On December 12, 2000, Iridium Satellite LLC announced additional satellite launches for 7 more satellites. Tentatively planned was a launch in late 2001 by a Delta II with five satellites. The launch date was later moved to early 2002. There is also a Russian launch tentatively planned in June 2002 with two satellites. The goal is to maintain 6 orbital planes with 11 operable satellites per plane along with two or more spares per plane.
The plate or MMA can provide a direct (specular) reflection of the sun's disk. This specular reflection is only tens of kilometers wide at the Earth's surface. In order to see a very bright reflection, the observer must be within this relatively small area. Prediction programs are available to determine this area.
The three sided (similar to an equilateral triangle) satellites themselves are not very large, approximately 4 meters long and less than one meter in width.
Alan Rohwer, a SeeSat-L subscriber, provided the initial construction details and provides additional information on the construction of the satellite which may account for unexplained reflections not predicted by the flaring prediction programs.
The flares/glints can now be predicted. A fully operational Iridium satellite maintains its axial and longitudinal position to within very close tolerances. Knowing the satellite's position, its orientation, the relative Sun angle to the satellite, the reflective properties of a mirror and the observers position on Earth, a program can be developed to calculate the projected specular reflection of the Sun from the Main Mission Antennas to an observer. Rob Matson provides additional information in a response on SeeSat-L.
Rob Matson and Randy John have developed software aimed at predicting these events. You can obtain more details on Randy John's SKYSAT program (version 0.64) , while you can obtain Rob Matson's newly revised DOS based program iridflar (version 2.21) . In addition, Rob Matson's windows program SkyMap 6.3 & 6.4 can now find Iridium flares.
*You will need to provide Iridium satellite elsets for these programs and links to them can be found immediately below.*
The following web site provides the Iridium Group element set listings:
Here's the break down of a typical orbital elset .
Observers should not assume that all Iridiums will flare per Rob/Randy's programs. Since several Iridium satellites listed below are either tumbling or otherwise not operational, those can NOT be predicted. The mean motion that signals a fully operational satellite should be 14.3422 revolutions/day.
Heavens-Above in Munich, Germany now provides flare predictions. Because the flare effect is fairly local in nature, the user should provide their best known coordinates in order to observe the flare successfully. This site has a large coordinate database to allow the user to determine their coordinates.
Observations and comments posted by SeeSat-L subscribers can be found either in the archives or by joining the SeeSat-List.
Below is a listing of all the Iridium satellites assigned to their respective orbital planes. Motorola had provided an official list of Iridium Satellite Nomenclature on or before December 17, 1998. Motorola is no longer involved in the maintainance of the satellite orbits. The Boeing Company is now providing this service.
There are now 6 separate orbital planes of
operational satellites spaced approximately 30 degrees apart. In the list below
the different groupings or planes are identified by their Right Ascension of
Ascending Node (RAAN) .
There are three categories of satellite identification. First is the satellite catalog number, the international designation, and the common name number. "Slot" refers to Motorola's positional assignment of the designated Iridium satellite in each of the six planes.
Iridium 79 (inoperable) was the first satellite in the constellation to reenter. Iridium 85 (inoperable) was the second satellite to reenter on Dec 31, 2000. Iridium 48 (inoperable) was the third satellite to reenter on May 5, 2001. Iridium 27 (inoperable) was the fourth satellite to reenter on Feb 1, 2002. Predictions for satellite decays are provided by Alan Pickup.
Note: The visual characteristics of all the satellites are not included below but are for the satellites that have been previously reported inoperable. You can find links with more up-to-date status here.
NORAD COSPAR Common Name Slot ----- ------ ---------- ---- (PLANE 1) 25285 98021A Iridium 62 05 25286 98021B Iridium 63 06 25287 98021C Iridium 64 07 25288 98021D Iridium 65 08 25289 98021E Iridium 66 09 25290 98021F Iridium 67 10 25291 98021G Iridium 68 11 25342 98032A Iridium 70 04 25343 98032B Iridium 72 02 25344 98032C Iridium 73 -- Inoperable 25345 98032D Iridium 74 01 25346 98032E Iridium 75 03 25777 99032A Iridium 14 -- Spare; previously called Iridium 14A 25778 99032B Iridium 21 -- Spare; previously called Iridium 21A (PLANE 2) 24903 97043A Iridium 26 10 24904 97043B Iridium 25 04 24905 97043C Iridium 46 06 24906 97043D Iridium 23 02 24907 97043E Iridium 22 01 25104 97082A Iridium 45 05 25105 97082B Iridium 24 -- Inoperable 25106 97082C Iridium 47 07 25108 97082E Iridium 49 09 25319 98026A Iridium 69 -- Inoperable 25320 98026B Iridium 71 -- Inoperable-tumbling 25431 98048A Iridium 03 11 25432 98048B Iridium 76 03 25577 98074A Iridium 11 08 previously called Iridium 11A 25578 98074B Iridium 20 -- Spare; previously called Iridium 20A (PLANE 3) 24944 97051A Iridium 29 03 24945 97051B Iridium 32 06 24946 97051C Iridium 33 07 24948 97051E Iridium 28 02 24949 97051F Iridium 30 04 24950 97051G Iridium 31 05 25272 98019A Iridium 55 01 25273 98019B Iridium 57 08 25274 98019C Iridium 58 09 25275 98019D Iridium 59 10 25276 98019E Iridium 60 11 27372 02005A Iridium 90 -- Spare 27373 02005B Iridium 91 -- Spare 27374 02005C Iridium 94 -- Spare 27375 02005D Iridium 95 -- Spare 27376 02005E Iridium 96 -- Spare (PLANE 4) 24792 97020A Iridium 08 08 24793 97020B Iridium 07 07 24794 97020C Iridium 06 06 24795 97020D Iridium 05 05 24796 97020E Iridium 04 09 24965 97056A Iridium 19 01 24966 97056B Iridium 35 03 24967 97056C Iridium 36 04 24968 97056D Iridium 37 10 24969 97056E Iridium 34 02 25262 98018A Iridium 51 -- Spare 25263 98018B Iridium 61 11 (PLANE 5) 24836 97030A Iridium 914 -- tumbling; previously called Iridium 14 24837 97030B Iridium 12 08 24838 97030C Iridium 09 -- not operational-tumbling 24839 97030D Iridium 10 06 24840 97030E Iridium 13 09 24841 97030F Iridium 16 11 24842 97030G Iridium 911 -- not operational-tumbling; previously Iridium 11 25169 98010A Iridium 52 03 25170 98010B Iridium 56 02 25171 98010C Iridium 54 07 25172 98010D Iridium 50 01 25173 98010E Iridium 53 04 25527 98066A Iridium 02 ? (no longer in plane 5) 25528 98066B Iridium 86 -- Spare 25530 98066D Iridium 84 05 25531 98066E Iridium 83 10 (PLANE 6) 24869 97034A Iridium 15 07 24870 97034B Iridium 17 06 24871 97034C Iridium 920 -- not operational-tumbling; previously Iridium 20 24872 97034D Iridium 18 01 24873 97034E Iridium 921 -- not operational-tumbling; previously Iridium 21 25039 97069A Iridium 43 11 25040 97069B Iridium 41 10 25041 97069C Iridium 40 03 25042 97069D Iridium 39 04 25043 97069E Iridium 38 09 25077 97077A Iridium 42 02 25078 97077B Iridium 44 -- not operational-tumbling 25467 98051A Iridium 82 -- Spare 25468 98051B Iridium 81 08 25469 98051C Iridium 80 05 25471 98051E Iridium 77 -- Spare
An unoffical status page is maintained by Rod Sladen.
A total of 88 satellites were launched between May 5, 1997 and June 11, 1999. Five additional more were launched on February 11, 2002.
The expected lifetime of a satellite is 5-8 years.
Three satellite carriers were used to launch the original satellite constellation.
In the first week of September 1997, China launched two dummy Iridium satellites (cat. no. 24925 and 24926) to test the launch dispenser. The launch appeared successful. These Motion Flight Simulators (MFS) satellites are not normally included in the Iridium Group elsets.
Paul Maley, who is an experienced satellite observer, has produced an excellent web site on satellite observing.
Denis Denissenko from Russia has some pictures of Iridium flares observed near Jupiter taken from Moscow.
Anthony Ayiomamitis from Athens, Greece has a nice collection of various Iridium satellites flaring.
Jim Nix maintains a web site that has a lot of Iridium flare photos.
Matt Lowe provides photos of Iridium and other satellites.
Visit Kevin S. Forsyth's web site for further information on the Boeing Delta family of launchers.
Bjorn Gimle provides analysis of Paul Maley's observation (see image above) plus a picture of a *daytme* iridium flare and additional links.
Further details on the Iridium constellation, together with a multitude of links, can be found on Lloyd Wood's satellite constellations pages.
Visit Russell Sipe's Iridium Flares page
SpaceRef.Com reviews the history of the Iridium communications satellite.
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