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While looking for the Earth satellites at the highest orbit I found mention of the Vela 1A.

Wikipedia's High Earth Orbit page indicates a perigee of 101,925km and apogee of 116,528km, however heavens-above indicates a perigee of 43,087km and an apogee of 174,729km.

I can understand that different sources have numbers that vary a bit, but in this case the ellipse is very different, I can't figure out why. So which one (if there's one) is right, and optionally why is the other wrong ? Has the initial orbit been modified ?

Note that I've already looked at the NASA page but didn't found information about the orbit.

Tim
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2 Answers2

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Interestingly I can't find any TLEs for Vela 1A (1963-039C, 00692) between April 1968 and October 2014! It was inserted into the nearly circular orbit described in your Wikipedia article, but when it re-appeared in the catalog it had an eccentricity in the 0.54 to 0.57 ballpark, and by 2018 this had risen to 0.64.

The Heavens-Above site is using the last available TLE from August 2018, and this will be the best set of values to use. Although bear in mind that the orbit is changing over time and even this TLE is already out-of-date. So the information therein will not be entirely accurate.

Prior to this, Heavens-Above used TLEs dated July 2017, March 2016 and March 1968. Data from all of these TLEs is given below and we can see from these how the eccentricity of Vela 1A's orbit increased over time.

As an aside, there seems to be two naming conventions. Two spacecraft were launched each time. One convention calls them A and B, the other n and n+1. So Vela 6A and 6B are also called Vela 11 and 12.

**TLE dated 24th August 2018:**

The orbit data is extracted from the following two-line orbital elements,

1   692U 63039C   18236.14976426 -.00001479 +00000-0 +00000-0 0  9999
2   692 027.2026 342.0351 6396976 210.0920 087.3147 00.22178276044278

Epoch (UTC):                        24 August 2018 03:35:39
Eccentricity:                       0.6396976
inclination:                        27.2026°
perigee height:                     35160 km
apogee height:                      182657 km
right ascension of ascending node:  342.0351°
argument of perigee:                210.0920°
revolutions per day:                0.22178276
mean anomaly at epoch:              87.3147°
orbit number at epoch:              4427

Heavens-Above TLE for Vela-1A

**TLE dated 3rd March 2016:**

The orbit data is extracted from the following two-line orbital elements,

1 00692U 63039C   16063.84064408 -.00001506  00000-0  00000+0 0  9990
2 00692  37.4196 358.2839 5630613 170.2010 359.2879  0.22145093 42276

Epoch (UTC):                        03 March 2016 20:10:31
Eccentricity:                       0.5630613
inclination:                        37.4196°
perigee height:                     44045 km
apogee height:                      174002 km
right ascension of ascending node:  358.2839°
argument of perigee:                170.2010°
revolutions per day:                0.22145093
mean anomaly at epoch:              359.2879°
orbit number at epoch:              4227

**TLE dated 24th March 1968:**

The orbit data is extracted from the following two-line orbital elements,

1 00692U 63039C   68084.10555552 -.00000374 +00000-0 +00000-0 0 00000
2 00692 036.7283 179.1791 0852721 237.5470 072.7561 00.22091344003596

Epoch (UTC):                        24 March 1968 02:32:00
Eccentricity:                       0.0852721
inclination:                        36.7283°
perigee height:                     99355 km
apogee height:                      119068 km
right ascension of ascending node:  179.1791°
argument of perigee:                237.5470°
revolutions per day:                0.22091344
mean anomaly at epoch:              72.7561°
orbit number at epoch:              359
Astrid_Redfern
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uhoh
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    Thank you for this detailed answer. Any idea how it went from a ~0.07 to a ~0.6 orbit eccentricity? – Tim Feb 13 '18 at 13:17
  • @Tim All twelve of them have behaved similarly. I don't know yet but I'd like to find out. My guess is that it is a natural process, but I'm not sure if its related to the shape of the Earth, or to effects of the Sun and Moon. I was going to post another question about this in a little while. – uhoh Feb 13 '18 at 13:22
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    @Tim I've just asked What is the basic mechanics behind the way the orbits of the twelve Vela spacecraft evolved over decades? I am not sure if it will receive a helpful answer, or if it will need more work, but let's see what happens! This has turned out to be a really interesting question! – uhoh Feb 13 '18 at 14:30
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    Thank you for that, I will follow your question attentively, it's really bugging me now. – Tim Feb 13 '18 at 14:32
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    For anyone who's interested - heavens-above.com has updated its information. The current entry for Vela 1A is dated 24 August 2018. The new figures include a perigee of 35160 km and apogee of 182657 km. – Astrid_Redfern Dec 27 '19 at 17:23
  • @Astrid_Redfern you can consider posting an additional answer with that; it certainly addresses Where is Vela 1A? Is there any information what its orbit would be like now? – uhoh Dec 28 '19 at 00:03
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    @uhoh With your permission, I think it would be better if I edited your answer to include both sets of figures. It would help to illustrate how the orbit's eccentricity has increased over time. What do you think? – Astrid_Redfern Dec 28 '19 at 12:58
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This object was "lost" for some decades. In October 2014, the Palomar Transient Factory (which was looking for near-earth asteroids that might hit us) recovered it. That was apparently enough to get the Space-Track folks back on it again.

The near-earth asteroid folks stumbled across it again in November 2017, so we now have a pretty good orbit for it.

Further details, and a plot of perigee height and inclination vs. time from 1969 to the present, are at

https://www.projectpluto.com/pluto/mpecs/63039c.htm

Bill Gray
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