Iridium 30 and Iridium 31 exchange

Iridium 30 and 31 exchanged places in the constellation on September 19-22, 2002.

I would like to thank Walter Nissen for initially drawing my attention to this - see Status and Identity of Iridium 30 and Iridium 31 on SeeSat-L at http://www.satobs.org/seesat/Jul-2003/0151.html

Up to and including September 18, 2002, Iridium 30 was followed in the regular pattern by Iridium 31 :

Iridium 30
1 24949U 97051F   02261.14806094 -.00000231  00000-0 -89653-4 0  8872
2 24949  86.3995  17.9087 0002209  56.9878 303.1529 14.34213466262410
Iridium 31
1 24950U 97051G   02260.87532995 -.00000360  00000-0 -13554-3 0  8889
2 24950  86.3993  18.0069 0002271  58.9272 301.2147 14.34212368262382

IRIDIUM 30
1 24949U 97051F   02261.84574731 -.00000193 +00000-0 -76017-4 0 08944
2 24949 086.3998 017.6177 0002235 060.2566 299.8858 14.34213526262515
IRIDIUM 31
1 24950U 97051G   02261.85209058 -.00000211 +00000-0 -82404-4 0 08986
2 24950 086.3990 017.5985 0002292 063.4936 296.6506 14.34213018262526

A day later, Iridium 30 had been raised by 10km and Iridium 31 lowered by 10km. These changes appear to have taken place at around 03:00 UT on September 19, 2002:

IRIDIUM 30
1 24949U 97051F   02262.68297198  .00066488  00000-0  24949-1 0  8880
2 24949  86.4041  17.2727 0002372  77.9375 276.0562 14.31204631262633
IRIDIUM 31
1 24950U 97051G   02262.68931529  .00036298  00000-0  12095-1 0  8891
2 24950  86.4053  17.2485 0002117  83.7475 282.6644 14.37378306262644

IRIDIUM 30
1 24949U 97051F   02262.82399796  .00037339  00000-0  14069-1 0  8899
2 24949  86.4033  17.2127 0002324  77.5571 282.5854 14.31202862262651
IRIDIUM 31
1 24950U 97051G   02262.82733350  .00016341  00000-0  54583-2 0  8907
2 24950  86.4042  17.1891 0002128  86.9874 273.1520 14.37374441262668

IRIDIUM 30
1 24949U 97051F   02263.24349255  .00002886  00000-0  10863-2 0  8901
2 24949  86.4031  17.0390 0002388  81.1700 278.9537 14.31186517262716
IRIDIUM 31
1 24950U 97051G   02263.24502930 -.00000147  00000-0 -55923-4 0  8913
2 24950  86.4039  17.0140 0002177  89.5276 270.6120 14.37364776262724

On this basis, Iridium 31 would have overtaken Iridium 30 at around 14:30 UT on September 20, 2002.

IRIDIUM 30
1 24949U 97051F   02264.36216666 -.00003261  00000-0 -12451-2 0  8921
2 24949  86.4020  16.5744 0002307  79.9658 280.1786 14.31170241262874
IRIDIUM 31
1 24950U 97051G   02264.42850689 -.00002106  00000-0 -71311-3 0  8927
2 24950  86.4037  16.5201 0002285  89.2377 270.9082 14.37355916262897

Subsequent elsets show Iridium 31 preceding Iridium 30 in the regular pattern, the exchange having taken around 3 days:

IRIDIUM 30
1 24949U 97051F   02266.59952037 -.00107328  00000-0 -38984-1 0  8935
2 24949  86.4056  15.6428 0002675  71.8218 304.7160 14.34168198263199
IRIDIUM 31
1 24950U 97051G   02266.58661706 -.00070580  00000-0 -25474-1 0  8935
2 24950  86.4059  15.6161 0003051  92.4409 250.2627 14.34220528263209

Over the next few days, minor adjustments appear to have been made to get the satellites into precise new positions.

I have no idea of the reason for this changeover, and no official information has been forthcoming on the matter! Possibilities include a collision avoidance manouver or matching up failed cross-link antennae in adjacent orbital planes.

[Rod Sladen's Iridium Page] [Rod Sladen's Astronomy Page] [Rod Sladen's Home Page]