Paul Marks, chief technology correspondent
The Dragon capsule coupled to the ISS (Image: NASA)
For the second time this year, a commercial space freighter has docked at the International Space Station. The successful arrival of SpaceX's Dragon cargo vessel is a fresh shot in the arm for privately funded space flight - but there's a catch.
The mission moved forward at the expense of an Orbcomm communications satellite, which was riding in the "trunk" of the Falcon 9 rocket. An engine failure on the way up meant that the satellite was released in too low an orbit, and some industry experts think the stranded probe has now re-entered Earth's atmosphere. (Update: Orbcomm has confirmed that the satellite has deorbited.)
The episode provides a salutary lesson for space entrepreneurs hoping to launch small satellites as secondary payloads on Falcon 9 flights. Emerging commercial space-flight firms such as Planetary Resources, the wannabe asteroid miners, believe such ride shares will be one economic way to deploy their constellations of rock-spotting telescopes and deep-space probes.
But as the name suggests, secondary payloads will take a back seat to the primary mission, SpaceX has confirmed. That's especially so when the main mission involves approaching a crewed spacecraft like the ISS, where safety is paramount.
The Orbcomm loss comes on the heels of a SpaceX presentation on secondary payloads given last week at the International Astronautical Congress in Naples, Italy. There, mission manager Dustin Doud gave a paper on the dedicated payload bay beneath the Dragon capsule and what people can expect from a secondary satellite launch.
In his paper, Doud says:
Falcon 9 offers breakthrough reliability because the nine Merlin engines that power the first stage offer engine-out redundancy. In fact, Falcon 9 is the first American launch vehicle since the Saturn V to offer such redundancy and reliability.
Flying unperturbed when an engine fails is a marvel of engineering - something that would have brought down earlier rockets.
But when redundancy was put to the test in this week's launch, the outcome was less than ideal. A couple of minutes after the nine-engine Falcon 9 rocket lifted off, a pressure drop on one engine caused it to shut down. Falcon 9's software automatically recomputed its ascent profile to cope with having just eight engines. That meant it burned the remaining engines for longer than planned to stay on course and intercept the space station, flying at around 410 kilometres above Earth.
The new trajectory used slightly more fuel, and the second-stage burn that was supposed to loft Orbcomm's satellite into its operational orbit of 750 kilometres was called off, to protect the space station.
"While there was sufficient fuel on board to [lift the satellite], the liquid oxygen on board was only enough to achieve a roughly 95 per cent likelihood of completing the second burn, so Falcon 9 did not attempt a restart," says SpaceX spokesperson Katherine Nelson.
The event highlights the fact that redundancy measures are no guarantee that firms hitching future rides won't find their equipment lost in space - at least when crewed spacecraft are involved.
"The priority here was to protect the space station," says Nelson of this week's event. "Very few secondary missions will be space station missions."
Update (19:20 BST) Since posting, Nelson of SpaceX has elaborated on the conditions presented to Orbcomm to fly as a secondary payload during this week's ISS mission:
While you rightfully point out that the second stage burn did not happen because of pre-planned NASA safety gate designed to protect the space station, it is also important to note that Orbcomm understood and accepted from the beginning that there was a high risk of their satellite remaining at the Dragon insertion orbit. Orbcomm requested that SpaceX carry one of their small satellites (weighing a few hundred pounds vs Dragon at over 12000 pounds) on this flight so that they could gather test data before we launch their full constellation next year. The higher the orbit, the more test data they can gather, so they requested that we attempt to restart and raise altitude. NASA agreed to allow that, but only on condition that there be substantial propellant reserves, since the orbit would be close to the Space Station. SpaceX would not have agreed to fly their satellite otherwise, since this was not part of the core mission and there was a known, material risk of no altitude raise.