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Aviation History
1964
1964 - 0043.PDF
38 FLIGHT International, 2 January 1964 Missiles and Spaceflight ACCIDENTAL RENDEZVOUS By J. A. PILKINGTON, BSc IN their plans for manned lunar landing, both Soviet andAmerican scientists envisage the use of rendezvous in orbit—the joining together of two separate orbiting satellites as they circle the Earth. The last four Vostoks have shown by virtue of their orbits that the Russians are already attempting rendezvous, and the plans for American astronauts to join their Gemini spacecraft to an orbiting Agena D have received great publicity. Rendezvous is, of course, difficult enough to achieve between two co-operating spacecraft; near-rendezvous by two US satellites, however, will be unintentionally achieved next month. The minimum distance between them may be about 10km, which is approaching the prox- imity attained by the Vostoks. The two satellites in question, known as 1963-03A and 1963-27A, are part of a USAF project whose purpose is classified. Both are Agena D second-stage rockets measuring about 5ft in diameter and 30ft long, which were launched by Thor boosters from Vandenberg AFB, California, on January 16 and June 29, 1963. They may be part of either the Discoverer or the Samos programme, but have probably ceased transmitting by now. At 0824hr GMT on October 16, 1963, they had the orbital elements listed in the table below. Orbital Satellite Inclination (deg)Nodal period (min) Right ascension of ascending node Change per dayArgument of perigee Change per dayPerigee height (km) Apogee height (km) Elements 1963-03A 81.8894.60 218.12 -1.083167.1 -3.454460 545 1963-27A 82.3094.79 210.22 -1.022353.0 -3.476490 535 From the table of elements, we see that the difference between the right ascensions defining the orbital planes of the two objects was 7.9° on October 16. This difference is decreasing at a rate of 0.061° per day and will become zero 129.5 days later—on February 22, 1964. Although the ascending nodes of both satellites will be coincident on that date, the longitudes on their crossing latitude 50°N will be 0.5° apart because of the difference in inclinations. Coincidence of the orbital planes at this latitude will occur eight days later for Northbound and eight days earlier for Southbound passages. The two points of coincidence will be between 50°S and 50°N from February 15 to March 2. Having considered the orbital planes, the position of the satellites in their orbits is of some interest. The satellites were both at the same point in their orbits at noon on October 19,1963, but 1963-O3A was "catching up" 1963-27A at a rate of 2.92min per day. This means that 27A is lapped by 03A every 32.45 days and four laps will be completed 129.8 days later—in fact, by February 26, 1964. Both satellites are affected slightly by atmosphere drag, however, and these estimates, made some months in advance, may prove slightly inaccurate. The lapping nevertheless should occur between February 23 and 29, and will provide an interesting spectacle for all who care to watch it. To the observer on the Earth, both satellites appear to flash regularly, occasionally as bright as stellar magnitude +1.5. The average brightnesses are :- 1963-03A from +3.2 to +6.2, flashing every 2.1sec 1963-27A from +3.1 to +5.6, flashing every l.lsec. The fact that 1963-03A flashes more slowly than 27A may well be the only means of differentiating between the two when they are seen crossing the sky together. From the position of the orbital planes next February, the two satellites will be visible from the British Isles. In the evenings after twilight they will appear to travel from the southern sky to the northern sky, and in the morn- ings before sunrise in the opposite direction. The following table gives the times when the orbital plane will cross the Greenwich meridian at latitude 50°N:— Date 1964 Northbound GMT Southbound GMT 1963-27A Feb 21-2222-23 23-24 24-25 25-26 26-2727-28 28-29 29-1 Mar 19.4519.37 19.29 19.21 19.12 19.0418.56 18.48 18.39 06.2306.15 06.0705.59 05.5005.42 05.34 05.2605.17 precedes 03A b\about: — ^12 mins+9 +6+3 0-3 -6 9-12 Observers should make the above times 4min later for every degree they are west of Greenwich, to obtain the time the orbit crosses their longitude. The separation of the satellites may prove to be slightly different to that suggested in the last column. The satellites are unlikely to be overhead at the same time as the orbital plane, and so the observer would have to search for them as the plane moves westwards across the sky due to the Earth's rotation. However, individuals wishing to observe this rendezvous without undertaking a search may refer to a table to appear next month in an issue of Flight International. 45O 9O 18O 27O ANGLE ROUND ORBIT 36O The orbital heights of the satellites 1963-3A and 1963-27A Finally, let us examine exactly how close the two satellites will be. Although the orbits of both objects are fairly circular, the arguments of perigee will differ by about 176° in late February. This means, that while one is at its apogee, the other is at perigee. They will only be close at two -points in their orbits when both their heights are about 508km (see figure). Under the best circumstances possible, they would be within 9km of each other for only 6min since, at the two approaches, one satellite would be changing height at about 2km per minute and the other at about lkm per minute in the opposite direction. If the arguments of perigee had been almost equal, the satellites would have remained very close to each other for several revolutions, the maximum separation in height being 30km at perigee, and tne minimum perhaps only a few kilometres. So it appears that 1963-03A and 1963-27A will come together only to travel their separate ways in the years to come. Unless, of course, their restartable motors are still capable of bursting to life on a command from the Earth. . ..
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