I've just spent a pleasant morning reading through two months' worth of the Rosetta blog to catch up on how the mission has evolved. Until January, Rosetta was flying in circular, "bound" orbits, with the comet holding the spacecraft to its path. But as the comet has gotten more active and begun to produce more gas and dust, that gas and dust exerts drag on the spacecraft, preventing Rosetta from being able to maintain these bound orbits. Since February, though, it has flown further away, on "unbound" orbits, meaning that the comet's gravity no longer constrains the spacecraft's path. With these unbound orbits, Rosetta must perform regular trajectory correction maneuvers to maintain proximity to the comet.

When your spacecraft's orbit is unbound, a safe mode can mean a missed trajectory correction maneuver and an unplanned excursion. That's what happened during the March 28 flyby, summarized in detail in this blog post. An update from April 10 explains that Rosetta wound up in an escape trajectory and traveled as far as 400 kilometers away from the comet. There wasn't any real danger of Rosetta losing the comet, though, because the speeds involved are very slow and well within the capability of Rosetta's thrusters.

On April 1, a rocket firing began to return the spacecraft toward the comet, and it reached its target distance of 140 kilometers altitude on April 8. It is a terminator orbit, meaning that the spacecraft travels around the comet in an orbital plane that coincides with the night-day boundary, and it sees the comet at roughly half phase at all times. As of that update, the plan was to go into pyramid orbits with minimum altitudes of 100 kilometers throughout the rest of April.

The change in Rosetta's trajectory unfortunately meant a complete replanning of targeted science observations and a reconsideration of the entire orbit strategy, which involved a few more of these close flybys. As of April 10, not all the instruments had been returned to normal operation yet.

Meanwhile, the unplanned excursion from the comet has not affected Rosetta's plans to listen for the Philae lander. Philae mission manager Stefan Ulamec still does not expect conditions to favor Philae's waking until at least next month, but listening campaigns were conducted over a period of about a week beginning March 12 and April 12, just in case.

Throughout all of this work, the NavCam is continuing to photograph the comet to good effect. ESA has begun to release NavCam data using its archive image browser, which presently only contains Rosetta data. It includes NavCam data from the cruise phase of the mission (which was already public; I worked through that archive and blogged about some of the images a while ago), and also includes early images from the comet phase of the mission. As of this writing, it contains images taken through August 1, when Rosetta had approached to within 850 kilometers of the comet. They plan to release images on a monthly basis, eventually catching up to the point that they release pictures approximately six months after the images were originally taken.

I can't possibly include in this post all the great photos they've released in the last two months. I'll pick a few favorites, but also provide links to help you browse. First, some oldies but goodies: photos from October "bound orbits" at 10 kilometers: Three super close up images and another one. The image below contains both "dunes" and a funky sort of foldy lineament. All of the closeup images contain bizarre morphology.