The Lander That Landed Three Times

1. A small explorer

Rosetta carried the Philae lander to Comet 67P. On 12 November 2014, the lander descended toward the small body’s surface. Approaching a comet is different from approaching a large planet. A gentle-looking contact could still leave the lander moving away again.

The destination had very little gravity to keep a visitor down.

2. A surface that could not hold it easily

On a small comet, the strength of gravity changes what a landing requires. Making contact is not enough if the spacecraft cannot remain where it touches down. The planned anchors were therefore central to the operation, intended to turn a brief collision with the surface into a stable place to conduct science.

Touching down and staying down were different achievements.

3. The anchors fail

Philae’s harpoons did not secure it on first contact. Without its intended anchoring, the spacecraft bounced away from the landing site. The intended anchor system was part of the landing plan. Its failure changed the trajectory rather than simply a detail in the photographs.

Touchdown had turned into a departure.

4. A second bounce

The lander contacted the surface again and eventually settled after a third touchdown. The unintended journey changed its final position. The controllers did not choose each successive contact point. The final site was a consequence of a landing that continued unexpectedly.

Three landings had produced one awkward parking space.

5. In the shade

Philae came to rest where limited sunlight reached its solar panels. Its available battery power became a deadline for scientific operations. Stored energy allowed work even without ideal sunlight. The available power shaped which tasks had to be completed first.

A successful arrival had acquired a charging problem.

6. A deadline written in stored power

Once the lander had settled in a poorly lit position, its available energy became a practical limit. The team had to make useful observations and return the results before that supply ran down. The location could not be improved by wishing for sunlight, so the scientific work had to fit the situation that existed.

The next sunrise was not a reliable charging appointment.

7. Science before silence

The team collected and transmitted data before the lander entered hibernation. The difficult landing still delivered direct measurements from a comet. Scientific measurements survived the imperfect arrival. A mission can deliver useful evidence even when its original landing plan does not work.

The parking was poor. The scientific delivery was real.

Sources

• ESA — Philae completes main mission