"Pseudo-MPEC" for SOLRAD 11A = 1976-023C = NORAD 08748 = WB31C9F = XC83E0D = 6N070E7 = 6N070E8

Created 2016 Dec 29 15:10:46 UT using Find_Orb

Some interesting links about this satellite:

General information on SOLRAD 11 A and B
NSSDC information about SOLRAD 11A
Updated pseudo-MPEC based on subsequent observations
Description of experiments on the SOLRAD 11 satellites
TLEs (Two-Line Elements) for this object (and other artsats on these pages)

You can click here to get customized ephemerides for this object from the Minor Planet Center's DASO Page.

Discovery story: On 2016 February 8, the Mt. Lemmon Survey (MLS) found a new, fast-moving object, which they designated XC83E0D, and tracked it for 104 minutes. Data was, as usual, posted on NEOCP. There was enough data to see that it was definitely an artificial satellite, in a near-circular orbit with a period of about five days. (As you'll see, I wasn't optimistic about tracking it further.)

Once we realized it was an artsat, the object was dropped from the NEOCP and listed as "XC83E0D was not a minor planet". But we still had the question of what it actually was. The orbit didn't resemble any of the assorted artificial satellites tracked by minor planet observers. The orbit looked as if it might be one of the SOLRAD 11A or 11B spacecraft, or possibly the kick motor used to put them in that unusual orbit. But we didn't really have all that much data. The object was faint, and I assumed we would lose it and have to hope that somebody spotted it again by pure luck. I did make a post on MPML to see if anybody had observations that might not have made it on to NEOCP, but it was a real long shot and I didn't get any takers. Dave Tholen kindly offered to give it a try from Mauna Kea, but it would have been down around magnitude 24 with large ephemeris uncertainty; we agreed it would be a waste of good telescope time.

I ran all this past Peter Birtwhistle and Jonathan McDowell, with whom I've often discussed artsat-related questions. Peter suggested a possible link to WB31C9F, an object observed briefly at the Catalina Sky Survey (CSS) more than a year earlier.

At first, I had doubts about being able to get an orbit linking the two arcs. Running the "new" arc back a year, the uncertainty amounted to three full revolutions. The best-fit orbit missed the WB31C9F observations by more than 50 degrees. This turned out to present less of a problem than I expected, though. I tried constraining the orbit (based still only on the 2016 data) to have a slightly smaller semimajor axis, which caused that "miss distance" to decrease. Eventually, it was only about a degree. I removed the constraint, turned on all observations, and attempted to fit the result. It worked, but with mean residuals of 22 arcseconds.

So I tried again, this time increasing the semimajor axis and going around 310 degrees; in other words, I assumed there had been one fewer orbit over the intervening thirteen months. Again, I got close to the 2015 observations, turned them on, removed the constraint, and did some "full step"s. This time, the mean residual was about 11 arcseconds.

This suggested I was headed in the correct direction, and indeed, when I dropped yet another revolution, I got an orbit with mean residuals of slightly over an arcsecond. (They're only that large because the object is fast-moving. The errors are almost entirely along the direction of motion.)

Jonathan was able to find orbital elements for both SOLRAD 11A and 11B, from 1977 July 1. The satellites were deliberately kept at opposite ends of the earth. Projecting our orbit back, we ended up within four degrees of SOLRAD 11A. (With more data, this later improved to within 1.5 degrees. It actually ought to be even better than that; I suspect non-gravitational forces have confused things a little over the intervening 38 years.)

By this point, I was about 90% confident that WB31C9F and XC83E0D were the same object, and that they were SOLRAD 11A. But on 2016 February 11, Peter got some more observations (shown below) within about ten arcseconds of prediction, dispelling all doubts. Armed with this, I went through the MPC's "one-night stand" file, looking for objects with suitable motion close to the predicted location. It turned out that the object had been observed in 2006, again by CSS, designated 6N070E7 and 6N070E8, about 40' from prediction (but with exactly the correct motion). You can click here for a pseudo-MPEC with all observations included.

As you'll see, the fit is a little rough. There are some non-gravitational forces at work here that are not correctly modelled. (The usual "simple" solar radiation pressure model doesn't help, nor does adding in a tangential term. It looks more as if the object is accelerating or decelerating relative to its orbit around the earth, which would be a little weird.)

Astrometry:
     6N070E7  C2006 07 09.45497 23 20 27.70 -06 54 09.5         x16.6 Vr     703
     6N070E7  C2006 07 09.45528 23 20 31.77 -06 53 55.0         x16.5 Vr     703
     6N070E7  C2006 07 09.45552 23 20 34.94 -06 53 43.5         x16.4 Vr     703
     6N070E7  C2006 07 09.45576 23 20 38.29 -06 53 32.2         x16.5 Vr     703
     6N070E8  C2006 07 09.45629 23 20 45.42 -06 53 06.2         x16.5 Vr     703
     6N070E8  C2006 07 09.45653 23 20 48.62 -06 52 54.9         x16.5 Vr     703
     6N070E8  C2006 07 09.45676 23 20 51.79 -06 52 43.5         x16.4 Vr     703
     6N070E8  C2006 07 09.45700 23 20 54.92 -06 52 32.1         x16.4 Vr     703
1976-023C     C2015 01 17.44953 09 44 01.41 +24 19 10.1          15.1 V D0541703
1976-023C     C2015 01 17.44980 09 44 05.04 +24 18 37.9          15.1 V D0541703
1976-023C     C2015 01 17.45007 09 44 08.79 +24 18 04.6          15.1 V D0541703
1976-023C     C2015 01 17.45034 09 44 12.39 +24 17 32.6          15.1 V D0541703
1976-023C     C2015 01 17.46751 09 48 08.67 +23 42 04.7          15.2 V D0541703
1976-023C     C2015 01 17.46778 09 48 12.57 +23 41 29.0          15.7 V D0541703
1976-023C     C2015 01 17.46805 09 48 16.06 +23 40 57.3          15.2 V D0541703
1976-023C     C2015 01 17.46832 09 48 19.76 +23 40 23.6          15.2 V D0541703
1976-023C     C2015 01 17.48899 09 53 04.78 +22 56 27.6          15.3 V D0541703
1976-023C     C2015 01 17.48926 09 53 08.50 +22 55 52.7          15.2 V D0541703
1976-023C     C2015 01 17.48953 09 53 11.78 +22 55 21.6          16.1 V D0541703
1976-023C     C2015 01 17.48980 09 53 15.91 +22 54 43.3          15.3 V D0541703
1976-023C     C2016 02 08.46940 12 02 46.88 -01 16 52.6          21.0 V D0541G96
1976-023C     C2016 02 08.47597 12 03 56.88 -01 33 39.5                 D0541G96
1976-023C     C2016 02 08.48911 12 06 17.76 -02 07 07.8                 D0541G96
1976-023C     C2016 02 08.51641 12 11 14.59 -03 16 18.8                 D0541G96
1976-023C     C2016 02 08.51677 12 11 18.47 -03 17 12.7          19.6 V D0541G96
1976-023C     C2016 02 08.51713 12 11 22.32 -03 18 05.4                 D0541G96
1976-023C     C2016 02 08.51749 12 11 26.38 -03 19 01.4                 D0541G96
1976-023C     C2016 02 08.54055 12 15 42.88 -04 16 51.9          21.3 V D0541G96
1976-023C     C2016 02 08.54088 12 15 46.47 -04 17 39.5                 D0541G96
1976-023C     C2016 02 08.54153 12 15 53.67 -04 19 15.3                 D0541G96
1976-023C    KC2016 02 11.75968403 12 55.10 +26 07 22.5          17.8 R D0541J95
1976-023C    KC2016 02 11.76275503 13 41.06 +26 12 43.9          17.9 R D0541J95
1976-023C    KC2016 02 11.77222903 16 03.40 +26 29 03.0          18.1 R D0541J95
1976-023C    KC2016 02 11.78175503 18 27.54 +26 45 06.3          17.4 R D0541J95
1976-023C    KC2016 02 11.78463503 19 11.30 +26 49 53.6          17.6 R D0541J95
1976-023C    tC2016 12 29.27187 09 26 26.23 +23 41 10.5          15.7 R      Y00
1976-023C    tC2016 12 29.27351 09 26 46.43 +23 37 39.8          15.9 R      Y00
1976-023C    tC2016 12 29.27520 09 27 07.27 +23 34 01.5          16.0 R      Y00

Station data:
(703) Catalina Sky Survey (N32.417003 W110.732640)  US/Arizona.   Observers E.
    J. Christensen, A. R. Gibbs, A. D. Grauer, R. E. Hill, J. A. Johnson, R. A.
    Kowalski, S. M. Larson, F. C. Shelly.  0.68 m Schmidt + CCD.
(G96) Mt. Lemmon Survey (N32.442751 W110.788720)  US/Arizona.   Observers E.
    J. Christensen, A. R. Gibbs, A. D. Grauer, R. E. Hill, J. A. Johnson, R. A.
    Kowalski, S. M. Larson, F. C. Shelly.  1.5-m reflector + CCD.
(J95) Great Shefford (N51.474967 W1.447000)  UK.   Observer P. Birtwhistle.
    0.40-m f/6.0 Schmidt-Cassegrain + CCD.
(Y00) SONEAR Observatory, Oliveira (S20.715080 W44.784960)  Brazil.
    Observers C. Jacques, E. Pimentel, J. Barros.  Measurer C. Jacques.  0.28-m
    f/2.2 astrograph + CCD.

Orbital elements:  SOLRAD 11A = 1976-023C = NORAD 08748 = WB31C9F = XC83E0D
   Perigee 2015 Jul 1.995224 +/- 0.00794 TT = 23:53:07 (JD 2457205.495224)
Epoch 2015 Jul  1.0 TT = JDT 2457204.5                        Gray
M 289.6398750 +/- 0.56              (J2000 equatorial)
n  70.6977712758 +/- 3.87e-6        Peri.  335.3020040 +/- 0.56
a125025.81401 +/- 0.00456           Node     5.7155447 +/- 0.00037
e   0.000618539 +/- 7e-6            Incl.   35.1891558 +/- 0.00008
P7332.48m/5.092d           H 30.6   G  0.15   U  4.4
q 124948.48059 +/- 0.872    Q 125103.14744 +/- 0.879
30 of 38 observations 2015 Jan. 17-2016 Dec. 29; mean residual 3".870
Residuals in arcseconds: 
060709 703( 887+  169+)   150117 703 2.31+ 1.38-    160208 G96 2.12- 2.72+
060709 703( 885+  168+)   150117 703 .826- .842+    160208 G96 5.33- 5.56+
060709 703( 884+  168+)   150117 703 1.08- 1.08+    160208 G96 7.22- 7.40+
060709 703( 886+  168+)   150117 703 2.80- 1.78+    160208 G96 9.08- 8.95+
060709 703( 886+  168+)   150117 703 2.97- 1.81+    160211 J95 2.43- .887-
060709 703( 886+  168+)   150117 703 9.22- 5.65+    160211 J95 1.92- .752-
060709 703( 887+  168+)   150117 703 3.74- 2.30+    160211 J95 .349- .734+
060709 703( 885+  168+)   160208 G96 9.01+ 6.21-    160211 J95 2.90+ 1.85+
150117 703 2.81+ 1.24-    160208 G96 6.52+ 5.00-    160211 J95 3.82+ 2.40+
150117 703 1.64+ .461-    160208 G96 3.73+ 1.61-    161229 Y00 1.67+ 3.90-
150117 703 2.11+ .772-    160208 G96 .658- 1.81+    161229 Y00 2.82+ 3.80-
150117 703 .534+ .233+    160208 G96 1.88- 2.42+    161229 Y00 4.42+ 4.31-
150117 703 .178- .405+    160208 G96 3.56- 4.22+

Ephemerides (geocentric):
Date (UTC) HH   RA              Dec         delta   r     elong  mag " sig PA
---- -- -- --  ------------   ------------  ------ ------ -----  --- ---- ---
2016 12 29 15  10 56 13.260   +08 36 05.54  125283 .98375 116.1 17.4  0.2 125
2016 12 29 16  11 05 59.272   +06 56 50.02  125284 .98372 113.3 17.5  0.1 124
2016 12 29 17  11 15 41.138   +05 16 49.95  125283 .98368 110.4 17.6  0.0 158
2016 12 29 18  11 25 19.868   +03 36 16.18  125281 .98364 107.6 17.7  0.1 128
2016 12 29 19  11 34 56.472   +01 55 19.42  125277 .98360 104.8 17.8  0.3 128
2016 12 29 20  11 44 31.959   +00 14 10.20  125272 .98355 102.0 17.9  0.4 127
2016 12 29 21  11 54 07.342   -01 27 00.96  125266 .98351  99.1 18.0  0.6 127
2016 12 29 22  12 03 43.634   -03 08 03.57  125258 .98347  96.3 18.1  0.7 127
2016 12 29 23  12 13 21.845   -04 48 47.10  125248 .98343  93.5 18.2  0.9 127
2016 12 30 00  12 23 02.992   -06 29 00.89  125238 .98339  90.6 18.3  1.0 126
2016 12 30 01  12 32 48.086   -08 08 34.12  125225 .98334  87.8 18.4  1.2 126
2016 12 30 02  12 42 38.142   -09 47 15.74  125212 .98330  85.0 18.6  1.4 125
2016 12 30 03  12 52 34.170   -11 24 54.42  125197 .98326  82.1 18.7  1.5 125
2016 12 30 04  13 02 37.178   -13 01 18.51  125180 .98322  79.3 18.8  1.7 124
2016 12 30 05  13 12 48.164   -14 36 15.94  125163 .98318  76.5 19.0  1.9 124
2016 12 30 06  13 23 08.114   -16 09 34.24  125144 .98314  73.6 19.1  2.1 123
2016 12 30 07  13 33 37.994   -17 41 00.45  125123 .98309  70.8 19.3  2.2 122
2016 12 30 08  13 44 18.742   -19 10 21.10  125102 .98306  68.0 19.5  2.4 121
2016 12 30 09  13 55 11.260   -20 37 22.20  125079 .98302  65.1 19.6  2.6 120
2016 12 30 10  14 06 16.398   -22 01 49.19  125056 .98298  62.3 19.8  2.8 119
2016 12 30 11  14 17 34.940   -23 23 26.98  125031 .98294  59.5 20??  3.0 118
2016 12 30 12  14 29 07.588   -24 41 59.95  125006 .98291  56.6 20??  3.2 117
2016 12 30 13  14 40 54.943   -25 57 12.00  124980 .98287  53.8 20??  3.3 116
2016 12 30 14  14 52 57.478   -27 08 46.59  124953 .98284  51.0 20??  3.5 114
2016 12 30 15  15 05 15.520   -28 16 26.87  124926 .98281  48.2 21??  3.7 113
2016 12 30 16  15 17 49.222   -29 19 55.80  124898 .98278  45.4 21??  3.9 111
2016 12 30 17  15 30 38.540   -30 18 56.27  124870 .98275  42.6 21??  4.0 110
2016 12 30 18  15 43 43.208   -31 13 11.35  124841 .98272  39.8 22??  4.2 108
2016 12 30 19  15 57 02.720   -32 02 24.47  124813 .98269  37.1 22??  4.4 106
2016 12 30 20  16 10 36.310   -32 46 19.71  124784 .98267  34.3 22??  4.5 104
2016 12 30 21  16 24 22.949   -33 24 42.07  124756 .98265  31.6 23??  4.7 102
2016 12 30 22  16 38 21.341   -33 57 17.77  124728 .98263  28.9 23??  4.8 100
2016 12 30 23  16 52 29.933   -34 23 54.53  124700 .98261  26.3 24??  5.0  98
2016 12 31 00  17 06 46.935   -34 44 21.89  124673 .98259  23.7 25??  5.1  96
2016 12 31 01  17 21 10.358   -34 58 31.47  124647 .98258  21.1 25??  5.2  94
2016 12 31 02  17 35 38.047   -35 06 17.18  124622 .98256  18.7 26??  5.3  92
2016 12 31 03  17 50 07.741   -35 07 35.39  124597 .98255  16.5 27??  5.5  90
2016 12 31 04  18 04 37.128   -35 02 25.05  124574 .98255  14.5 28??  5.6  88
2016 12 31 05  18 19 03.906   -34 50 47.72  124552 .98254  12.8 29??  5.7  86
2016 12 31 06  18 33 25.840   -34 32 47.51  124531 .98254  11.6 30??  5.8  84
2016 12 31 07  18 47 40.826   -34 08 30.98  124512 .98253  11.1 31??  5.8  82
2016 12 31 08  19 01 46.933   -33 38 06.96  124494 .98253  11.4 30??  5.9  80
2016 12 31 09  19 15 42.444   -33 01 46.33  124478 .98254  12.3 30??  6.0  78
2016 12 31 10  19 29 25.888   -32 19 41.71  124464 .98254  13.8 29??  6.0  76
2016 12 31 11  19 42 56.054   -31 32 07.24  124451 .98255  15.7 27??  6.1  74
2016 12 31 12  19 56 11.999   -30 39 18.22  124441 .98255  17.9 26??  6.1  72
2016 12 31 13  20 09 13.044   -29 41 30.85  124433 .98256  20.2 26??  6.1  71
2016 12 31 14  20 21 58.765   -28 39 01.95  124426 .98258  22.7 25??  6.2  69
2016 12 31 15  20 34 28.972   -27 32 08.71  124422 .98259  25.3 24??  6.2  68
2016 12 31 16  20 46 43.690   -26 21 08.47  124420 .98261  27.9 24??  6.2  66