February 1980:Soviet spacecraft orbits

A number of Soviet satellites flown in recent years in the 51.6 degree inclination slot have used orbits which produce repeated ground tracks after a few days. The usual reason for this type of orbit is to ensure several opportunities for rendezvous attempts in case of launch delays with the second craft.

Daily Track Repeats

The low orbital periods of the early Soyuz rendezvous flights and their Cosmos precursors gave ground tracks which repeated from day to day, every 16 orbits. The purpose of this was to allow a first orbit rendezvous with the second or third vehicles.

Salyut 1 (1971-32A) used an orbit with daily track repeats to provide launch windows for both Soyuz 10 and Soyuz 11. The nodal period (time between successive northbound equator crossings) to achieve this is 88.5 minutes.

Launched on 19 April 1971, Salyut 1 soon manoeuvred to its stabilised orbit. On 26 April, three days after the departure of Soyuz 10, the orbit had been raised to 89.7 minutes. From then until 2 June, the orbit decayed naturally to 88.5 minutes again and a couple of minor manoeuvres opened a series of launch windows for Soyuz 11. It was the detection of radio transmissions during this operation which allowed the Kettering Group to be waiting for the launch of Soyuz 11 on 6 June,and thus receiving signals as the craft went into orbit nine minutes after launch. After the docking, Salyut’s orbit was again raised to prolong its lifetime; none of its later manoeuvres reproduced the daily track repeats.

Higher Orbits

One disadvantage of an orbit of 88.5 minutes is that the height of around 200 km means that costly, fuel-using manoeuvres are constantly needed to avoid rapid decay due to air drag. An orbit giving ground track repeats every day after 15 orbits has a period of 94.4 minutes, much more economical in fuel use but imposing severe penalties on satellite masses.

Salyut 4 (1974-104A), and later Salyut 6 (1977-97A), used orbits with rendezvous launch windows occurring every two days as their tracks repeated after 31 orbits. Some of the manoeuvres of the latter have already been documented.

Salyuts 3 and 5 (1974-46A and 1976-57A) operated at heights around 260 km with periods near to 89.8 minutes. During rendezvous operations with the Soyuz ferries, the orbit was slightly lower with an 89.6 minute period; this is the lowest of a series of orbits giving repeated tracks after five days.The 89.6 minute track repeats every 79 orbits. The next in the series with 78 orbits between repeats has a period of 90.75 minutes. Interestingly, this is one of the orbits used by Cosmos 929 (1977-66A). Cosmos 929 may have been set up as a rendezvous target for a simulated second launch or may have rendezvoused itself with an imaginary target vehicle. Weight is added to this theory by the fact that it made a series of minor orbit trimming manoeuvres as if station keeping with or approaching another object.

Later Developments

Two other recent launches have used orbits with periods around 90.75 minutes and almost identical elements to Cosmos 929. These are Cosmos 1001 (1978-36A) and Cosmos 1074 (1979-8A). Both of these were A-2 vehicle launches, as opposed to Cosmos 929’s D-1.

Cosmos 1001, launched on 4 April 1978 was initially discovered to be transmitting on a VHF channel normally used for man related craft. Later, it was discovered to be transmitting CW/PDM signals on the primary Soyuz HF channel. Initially flying in an 89 minute orbit. Cosmos 1001 moved up to its five day stabilised one after six days. After a total of 11 days in orbit, it was commanded to re-enter and was recovered.

Cosmos 1074, launched on 31 January 1978 had the same radio transmission characteristics as Cosmos 1001. Injected into a similar initial orbit, the move up to the stabilised one came three days later. Four days after this, on 7 February a further manoeuvre resulted in a still higher orbit with a period of 92.0 minutes, still repeating its tracks every five days but now at only 77 orbit intervals!

Unlike Cosmos 1001, it was not recovered after a short flight but continued in a slowly decaying orbit until 1 April when recovery took place after 60 days in flight. This mission profile is reminiscent of that of Cosmos 613 (1973-96A), an unmanned Soyuz test.

Cosmos 613 orbited for 60 days from the end of November 1973 to the end of January 1974 as a systems test of Soyuz in preparation for the upcoming Salyut 4 long-duration flights.Presumably, it was powered down for most of the mission and then its batteries recharged from a device launched already docked with it to simulate Salyut. This device was probably the large object (1973-96C) left in orbit after recovery. Both Cosmos 1001 (4 catalogued objects) and Cosmos 1074 (5 objects) left debris in orbit after recovery.