1 August 2012—Today a Russian space experiment is testing a new flight plan for visits to the International Space Station (ISS) that harks back to the very first rendezvous missions of the 1960s. If it works, it could become the standard, not only for all Russian spacecraft but also for Western vehicles from both government and private operators. And that might be a problem.
Called a fast rendezvous scheme, the time from launch to docking is about 6 hours, as opposed to the current time frame of two days. According to Rafail Murtazin, one of the designers of the new scheme, the two-day plan “is one of the most stressful parts of the space flight,” and shortening it will make the trip more tolerable for flight crews. “In that case, trios of astronauts will not have to spend two days in tight quarters suffering from vertigo and nausea,” a Russian space program spokesman told the news agency Interfax earlier this month.
In the April 2012 issue of Acta Astronautica, Murtazin and Nikolay Petrov, his associate from Moscow’s manned spacecraft builder Rocket and Space Corp. Energia, explain that the new plan involves four fast-sequence rocket burns right after launch, leading to a quick docking. Murtazin later said by e-mail that the plan has been revised to include six burns in that period, thus improving its flexibility.
But there’s a price to be paid for the express ticket. The destination in space, usually the ISS, must be lined up much more precisely in a narrow “slot” in the sky. With the two-day profile, that destination could be anywhere halfway along the ISS’s round-the-world orbital track. But with the fast-track rendezvous, the target must be maneuvered in advance into a segment of the target’s orbit that is only 20 degrees wide at the moment of the spacecraft’s launch.
That segment is determined by the location of the launch site—Kazakhstan in the case of this test. For a launch into that same “sweet slot” from any other site, the quick-rendezvous scheme would be unlikely to succeed. To adjust the ISS orbit for a fast rendezvous from launch sites other than Kazakhstan would require time-consuming and fuel-expensive maneuvers of the massive space station. That may be something space station operators would be unwilling to do for a private craft or an upstart spacefaring nation—possibly forcing them to perform the longer two-day maneuver.
You might expect that the actual orbit speed of the fast-rendezvous vehicles would be, well, faster. But in fact it’s the same as in the two-day scheme. Nevertheless, those spacecraft will reach the target much more quickly. Exactly how they do that involves some new navigation systems and procedures, but it’s mainly associated with the esoteric art of orbital mechanics—the motion of satellites through space.
For a launch into orbit aimed at a specific destination, liftoff must occur in the brief moment when the Earth’s rotation carries the launch site through the target satellite’s orbital plane. If it occurs at any other time, it will lead to an orbital path that crisscrosses the target’s orbit at too steep an angle for onboard rocket engines and their limited supply of propellant, and the craft won’t be able to steer into the necessary parallel path.
This is called the planar launch window, and it can be stretched to a few minutes if the launch vehicle can perform a “sideslip” during ascent and align itself precisely with the desired plane. When calculating the aimed-for plane, orbital mechanics experts take into account that the spacecraft’s orbital plane will be twisted slightly more strongly by the Earth’s equatorial bulge than the space station’s orbital plane. Ordinarily, it takes a series of small adjustments to overlay the orbits precisely.
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.
The Progress M-16M launched at 19:35 GMT, and docking should follow 5 hours and 49 minutes later. Even though that’s overnight for the space station crew, who normally sleep on a Greenwich mean time schedule, they will be awake for this docking. (If the vehicle had been aimed for the old 50-hour flight, launch time would be just 3 minutes later.) If things go wrong early on, the vehicle can still go into a one-day or even a two-day profile. If they go right, look for a cargo launch this fall using the fast rendezvous and a possible manned mission in early 2013. It’s an unearthly ballet with celestial choreography, but the rules are strict and clear.
UPDATE: The Progress spacecraft arrived safely at the International Space Station on schedule.
This article was modified on 2 August 2012.
About the Author
James Oberg worked as an aerospace engineer at NASA for 22 years. He switched to journalism in the late 1990s and now makes his living reporting on space for such outlets as Popular Science, NBC News, and IEEE Spectrum. In the June 2011 issue of Spectrum, he looked at some of the gutsier (and goofier) shuttle missions that never flew