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| click image to enlarge |
>> UPDATES to this story with new observational data, updated calculations and new visualizations ARE AT THE END OF THE POST, below the main story <<
Five days ago I wrote about the odd NROL-76 payload, USA 276, which was launched as NROL-76 for the NRO by SpaceX on 1 May 2017. In that post I pointed out that its orbit was peculiarly close to that of the International Space Station ISS.
I have prepared two animations to show the extend of this, and what will happen in the first week of June if USA 276 does not change its orbit before that date (this is an important caveat!).
This is what will happen on June 3, when USA 276 would make a couple of very close approaches to the ISS, perhaps to distances as close as 20 km (!) near 14:48 UT (3 June 2017):
Note how the satellite is effectively circling around the ISS, at close range.
If the DRAGON CRS-10 history is to go by, and CRS-11 is launched on-time, the latter will be close to the ISS as well (although perhaps not as close as in the animation). [UPDATE June 2: the launch of CRS-11 was postponed to June 3 due to the weather]
The next day, June 4 near 15:30 UT, the DRAGON CRS-11 supply ship will berth to the ISS if launch goes as planned. This is the situation around the time of berthing [UPDATE June 2: the launch of CRS-11 was postponed to June 3 due to the weather] :
Again, and I can't say this enough: this will be the approximate situation if USA 276 stays in the orbit we currently have for this satellite, and does not manoeuvre.
In terms of the closest approaches, these happen the day before the CRS-11 berthing.
I calculate these close approach moments, from a USA 276 orbit that is a week old by the time these events happen (the ISS orbit used is the planned orbit for that date available here). The table provides the times for approaches closer than 500 km to the ISS [edit June 2: SEE UPDATES of table in the updates at the bottom of this post):
DATE TIME (UT) DISTANCE (km)
3 JUN 2017 03:13:34 476.5
3 JUN 2017 04:01:30 443.3
3 JUN 2017 04:46:11 411.8
3 JUN 2017 05:33:53 378.8
3 JUN 2017 06:18:48 347.1
3 JUN 2017 07:06:16 314.3
3 JUN 2017 07:51:25 282.5
3 JUN 2017 08:38:39 249.9
3 JUN 2017 09:24:02 217.8
3 JUN 2017 10:11:02 185.6
3 JUN 2017 10:56:39 153.1
3 JUN 2017 11:43:25 121.5
3 JUN 2017 12:29:16 88.5
3 JUN 2017 13:15:47 58.5
3 JUN 2017 14:01:53 24.1
3 JUN 2017 14:48:10 20.3
3 JUN 2017 15:34:31 41.3
3 JUN 2017 16:20:32 75.7
3 JUN 2017 17:07:08 105.8
3 JUN 2017 17:52:55 139.2
3 JUN 2017 18:39:45 170.4
3 JUN 2017 19:25:17 203.4
3 JUN 2017 20:12:22 235.1
3 JUN 2017 20:57:39 267.7
3 JUN 2017 21:44:59 299.7
3 JUN 2017 22:30:01 332.0
3 JUN 2017 23:17:36 364.3
4 JUN 2017 00:02:23 396.4
4 JUN 2017 00:50:14 428.9
4 JUN 2017 01:34:45 460.8
4 JUN 2017 02:22:51 493.5
Note that the calculated distances in the table have quite some uncertainty, perhaps by a factor of 2 or more. Likewise, the times listed have uncertainties of at least several seconds. And then there is the possibility that USA 276 manoeuvres into another orbit between now and June 3....
The planned moment of CRS-11 berthing to the ISS, around 4 June 15:30 UT, coincides with another close approach of USA 276, although not as close as the previous day: about 1040 km:
DATE TIME (UT) DISTANCE (km)
4 JUN 2017 15:25:53 1039.5
I am still not sure what to think of this all. Is this coincidence? You would almost start to think that USA 276 is a demonstrator for technology to closely monitor third party space berthings....
While I admittedly go out on a limb here, this idea does not come out of the blue. China and Russia have been busy practising such berthings and (very) close approaches in space with dedicated satellites disguised as space debris the past 10 years, which has the US military worried. Is the technology demonstrated by USA 276 perhaps meant to test whether such events can be observed (either optically, with radar, lidar, or whatever technology) from close by, to determine in detail what is going on?
It would be incredible (and politically sensitive) to use the International Space Station as a test subject in this way, which is why I and others are hesitant to accept this idea.
On the other hand, the ISS is there and you get frequent dockings and berthings of DRAGON's, PROGRESS, SOYUZ and HTV to watch for free, objects you don't have to launch yourself (saving development and launch costs and time. Launching a bunch of satellites for this purpose also atttracts attention, as the story with the Russian satellites shows).
I still don't know what to think of this all. Are these figments of my imagination or is there really something going on here? I am at a loss. Opinions are welcome.
Postscript, 30 May 2017, 21:15 UT :
I used the following TLE for USA 276, based on amateur tracking of the satellite between May 24 and May 27:
USA 276
1 42689U 17022A 17147.01934012 0.00004742 00000-0 65889-4 0 01
2 42689 50.0000 149.4666 0015489 97.4973 262.7756 15.56150729 04
The positions of DRAGON CRS-11 in the animations are based on elsets of DRAGON CRS-10 relative to those of ISS at the time, and (for the 3 June animation) are less certain than the ISS and USA 276 orbital positions.
UPDATES (newest at the bottom):
UPDATE 1: 31 May 2017, 8:55 UT
The issue of launch windows and orbital plane shifts was rightfully raised on e.g. the NASA Spaceflight forum. It is true that the launch time needed to target the ISS orbital plane shifts by ~20 minutes each day. The crubbed launch on April 30 targetted 11:15 UT, the same time as the eventual launch a day later. Curiously enough, the Area Warning given out before the launch does open 20 minutes earlier, at 10:55 UT. Very confusing (and I initially goofed with that in a comment on the NASA Spaceflight forum).
It should be noted that USA 276 of course isn't in the exact plane of ISS (there is a 1.6 degree inclination difference anyway). A small difference in RAAN does not matter that much in this situation, it transpires.
I have looked into the effect would NROL-76 have actually been launched at 11:15 UT on April 30, when the launch was scrubbed.
The effects of a fixed launch time at 11:15 UT rather than a daily launch time shift to match the plane crossing time are actually not that large, it turns out. To investigate the effect, I adjusted the RAAN of the current orbit accordingly to match launch on 30 April, 11:15 UT..
USA 276 actually then would have made even somewhat closer passes to the ISS (to minimum distances less than 15 km on June 3 near 18:44 UT), but with the approach times some 4 hours shifted compared to those for the actual launch date.
During CRS-11 berthing on June 4, it would actually have been somewhat closer too, although with all other parameters of the orbit kept equal the time of approach would not match so neatly with berthing. These are not things that cannot be solved by a small manoeuvre however.
UPDATE 2: 1 June 2017, 10:30 UT
After updating the orbit of USA 276 with observations from last night, the time of closest approach has shifted a bit to an earlier approach instance (14:01:53 UT, June 3) and to a slightly smaller nominal distance (~18 km). The overall scenario remains the same, its details that change.
USA 276
1 82689U 17022A 17151.89933357 0.00004751 00000-0 65887-4 0 01
2 82689 50.0016 124.1750 0015094 116.7818 243.4697 15.56210183 01
Distance of USA 276 with regard to ISS in diagram form, from June 2.0 to June 5.0 (x-axis is in decimal days, e.g. 3.50 = 3 June 12:00):
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| click diagram to enlarge |
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| click diagram to enlarge |
This is the new updated list of close approach times:
DATE TIME (UT) DISTANCE (km)
3 JUN 2017 01:40:58 503.4
3 JUN 2017 02:28:57 468.5
3 JUN 2017 03:13:35 438.3
3 JUN 2017 04:01:20 403.6
3 JUN 2017 04:46:12 373.3
3 JUN 2017 05:33:43 338.7
3 JUN 2017 06:18:49 308.2
3 JUN 2017 07:06:06 273.9
3 JUN 2017 07:51:25 243.1
3 JUN 2017 08:38:29 209.1
3 JUN 2017 09:24:02 178.1
3 JUN 2017 10:10:52 144.6
3 JUN 2017 10:56:39 113.1
3 JUN 2017 11:43:15 80.6
3 JUN 2017 12:29:16 48.2
3 JUN 2017 13:15:38 22.6
3 JUN 2017 14:01:53 17.7 * closest
3 JUN 2017 14:48:00 54.4
3 JUN 2017 15:34:30 82.3
3 JUN 2017 16:20:23 117.6
3 JUN 2017 17:07:07 147.3
3 JUN 2017 17:52:45 182.0
3 JUN 2017 18:39:44 212.3
3 JUN 2017 19:25:07 246.6
3 JUN 2017 20:12:21 277.3
3 JUN 2017 20:57:29 311.3
3 JUN 2017 21:44:58 342.2
3 JUN 2017 22:29:51 376.1
3 JUN 2017 23:17:35 407.2
4 JUN 2017 00:02:13 440.8
4 JUN 2017 00:50:12 472.2
4 JUN 2017 01:34:34 505.6
4 JUN 2017 02:22:49 537.1
4 JUN 2017 03:06:56 570.3
4 JUN 2017 03:55:26 602.1
4 JUN 2017 04:39:17 635.0
4 JUN 2017 05:28:04 667.0
4 JUN 2017 06:11:38 699.7
4 JUN 2017 07:00:41 731.9
4 JUN 2017 07:43:59 764.4
4 JUN 2017 08:33:18 796.7
4 JUN 2017 09:16:20 829.1
4 JUN 2017 10:05:55 861.6
4 JUN 2017 10:48:41 893.7
4 JUN 2017 11:38:32 926.4
4 JUN 2017 12:21:01 958.3
4 JUN 2017 13:11:09 991.2
4 JUN 2017 13:53:21 1022.9
4 JUN 2017 14:43:46 1055.9
4 JUN 2017 15:25:41 1087.4 * CRS-11 berthing
4 JUN 2017 16:16:23 1120.6
Here is a photograph of last night's pass of USA 276 over my house:
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| click image to enlarge |
I also captured part of the pass on video:
UPDATE 3: 2 June 2017, 12:45 UT
Updated orbital elements based on observations from June 1:
USA 276 389 x 408 km 1 42689U 17022A 17152.86247082 0.00004757 00000-0 65966-4 0 06 2 42689 50.0043 119.1561 0014209 109.6377 250.6127 15.56228316 08
USA 276 appears to have been making small manoeuvers over the past days. The current schedule for close approaches to the ISS, based on the elements above, is:
DATE UT DISTANCE (KM)
3 JUNE 2017 01:41:01 503.2
3 JUNE 2017 02:28:55 460.1
3 JUNE 2017 03:13:38 437.9
3 JUNE 2017 04:01:19 395.0
3 JUNE 2017 04:46:14 372.6
3 JUNE 2017 05:33:42 329.9
3 JUNE 2017 06:18:51 307.3
3 JUNE 2017 07:06:05 264.8
3 JUNE 2017 07:51:28 242.1
3 JUNE 2017 08:38:29 199.8
3 JUNE 2017 09:24:05 176.8
3 JUNE 2017 10:10:52 135.1
3 JUNE 2017 10:56:41 111.5
3 JUNE 2017 11:43:15 71.0
3 JUNE 2017 12:29:18 46.3
3 JUNE 2017 13:15:37 18.1 **
3 JUNE 2017 14:01:55 19.6 **
3 JUNE 2017 14:48:00 64.1
3 JUNE 2017 15:34:32 84.6
3 JUNE 2017 16:20:22 128.0
3 JUNE 2017 17:07:08 149.8
3 JUNE 2017 17:52:45 192.7
3 JUNE 2017 18:39:45 215.0
3 JUNE 2017 19:25:07 257.6
3 JUNE 2017 20:12:22 280.3
3 JUNE 2017 20:57:28 322.6
3 JUNE 2017 21:44:59 345.5
3 JUNE 2017 22:29:50 387.6
3 JUNE 2017 23:17:35 410.7
4 JUNE 2017 00:02:12 452.6
4 JUNE 2017 00:50:12 475.9
4 JUNE 2017 01:34:33 517.5
4 JUNE 2017 02:22:49 541.1
4 JUNE 2017 03:06:54 582.5
4 JUNE 2017 03:55:26 606.3
4 JUNE 2017 04:39:15 647.5
4 JUNE 2017 05:28:03 671.5
4 JUNE 2017 06:11:36 712.4
4 JUNE 2017 07:00:39 736.6
4 JUNE 2017 07:43:56 777.3
4 JUNE 2017 08:33:16 801.7
4 JUNE 2017 09:16:17 842.2
4 JUNE 2017 10:05:53 866.8
4 JUNE 2017 10:48:37 907.1
4 JUNE 2017 11:38:30 931.8
4 JUNE 2017 12:20:57 971.9
4 JUNE 2017 13:11:07 996.8
4 JUNE 2017 13:53:16 1036.7
4 JUNE 2017 14:43:43 1061.8
4 JUNE 2017 15:25:35 1101.5
4 JUNE 2017 16:16:20 1126.8
4 JUNE 2017 16:57:54 1166.2
4 JUNE 2017 17:48:57 1191.7
4 JUNE 2017 18:30:13 1230.9
4 JUNE 2017 19:21:34 1256.6
4 JUNE 2017 20:02:32 1295.6
4 JUNE 2017 20:54:10 1321.5
4 JUNE 2017 21:34:50 1360.2
4 JUNE 2017 22:26:47 1386.3
4 JUNE 2017 23:07:07 1424.8
4 JUNE 2017 23:59:24 1451.1
Distance variation over time in diagram form:
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| click diagram to enlarge |
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| click diagram to enlarge |
Update 4, 3 June 13:15 UT:
In a post on SeeSat-L, Ted Molczan has summed up the pro's and con's of the conjunction between ISS and USA 276 being coincidental or not. Like me, he does not really know what to think of it.
Update 5, 3 June 14:00 UT:
Updated elements based on adding observations from June 2:
USA 276 388 x 408 km
1 42689U 17022A 17153.82560337 0.00004761 00000-0 65966-4 0 09
2 42689 50.0075 114.1658 0015063 110.3625 249.8963 15.56237668 07
Updated list with times and distances of close approaches to the ISS:
DATE UT DISTANCE (km)
3 JUN 2017 01:40:57 505.7
3 JUN 2017 02:28:57 460.2
3 JUN 2017 03:13:35 440.4
3 JUN 2017 04:01:20 395.1
3 JUN 2017 04:46:12 375.1
3 JUN 2017 05:33:43 330.0
3 JUN 2017 06:18:49 309.8
3 JUN 2017 07:06:06 265.0
3 JUN 2017 07:51:26 244.5
3 JUN 2017 08:38:29 200.0
3 JUN 2017 09:24:03 179.3
3 JUN 2017 10:10:52 135.3
3 JUN 2017 10:56:40 114.0
3 JUN 2017 11:43:14 71.2
3 JUN 2017 12:29:17 48.8
3 JUN 2017 13:15:37 18.7 **
3 JUN 2017 14:01:54 17.4 **
3 JUN 2017 14:47:59 64.1
3 JUN 2017 15:34:31 82.1
3 JUN 2017 16:20:21 127.9
3 JUN 2017 17:07:08 147.3
3 JUN 2017 17:52:43 192.5
3 JUN 2017 18:39:46 212.5
3 JUN 2017 19:25:04 257.4
3 JUN 2017 20:12:23 277.8
3 JUN 2017 20:57:26 322.3
3 JUN 2017 21:45:00 343.0
3 JUN 2017 22:29:47 387.3
3 JUN 2017 23:17:37 408.2
4 JUN 2017 00:02:08 452.3
4 JUN 2017 00:50:14 473.4
4 JUN 2017 01:34:29 517.3
Distance to the ISS with time in diagram form:
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| click diagram to enlarge |
UPDATE 6, 6 June 15:25 UT:
A new blog post with a detailed post-event analysis of the close approach can be read here
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