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[i]...but there is a second, less well-known launch window that has been the primary driver behind a standard two-day (sometimes three-day) rendezvous. It is connected to the awkward fact that at the precise moment the spacecraft must launch to hit the right orbital plane, the target satellite can be anywhere along its near-circular, approximately 92-minute-long orbit around Earth. The difference in the position of the spacecraft and target along the same track is called the phase angle. You make up for the phase angle by launching into a lower orbit, which has a speed slightly greater than that of the target. As the chasing vehicle climbs into the target’s orbit, it closes the distance between them. The lower the initial orbit, the faster the chaser catches its target. With two days to catch the space station, you’ve got a lot of choices for your initial altitude, a wide range of possible overtaking speeds, and therefore a big choice of phase angles to work with. According to Murtazin's recent paper, Soyuz can catch the ISS as long as it gets anywhere within 42 percent of the station's entire orbital arc (about 150 degrees). This gave Russia's space agency opportunities to launch as often as every other day. But for the fast rendezvous, there is a greatly diminished capability to overtake the station. Even with the most recent tweaks to the plan, less than 6 percent of the orbital arc is accessible. Waiting for the right slot is frustrating — weeks could go by between opportunities. So throughout July, the station's orbit has been gently tweaked, with rocket firings to nudge it into the sweet slot for Kazakhstan launches...[/i]
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