If you’re preparing for a late night and early morning for the Rosetta lander launch you might be interested in the sequence of critical steps necessary to land Philae on comet 67P.

The first five steps are called a “Go” – “No Go” because there is at these points in the sequence the option to abort the separation of Philae from Rosetta. Once the lander has separated after step 5 there is no turning back and either a successful touch down will be achieved – or it will not.

1. FIRST GO/NO GO 19:30 UTC (Wed 12 Nov at 8.30am NZDT)
Rosetta is on track: Following a full orbit determination before separation, the Flight Dynamics Team at ESOC confirm that Rosetta and Philae are on the correct orbit and that Rosetta can achieve the delivery trajectory, i.e. it can be at the correct point in space at the correct time so as to deliver the lander onto the surface as planned.
This is the last activity required to plan and command the delivery manoeuvre (i.e. a thruster burn) planned for the morning of 12 November, which will shift Rosetta and Philae onto the correct lander delivery trajectory.

2. SECOND GO/NO GO 00:00 UTC (Wed 12 Nov at 1.00pm NZDT)
Telecommands to control separation and delivery are complete: This GO/NOGO happens in two steps. (A) By this time, the Flight Dynamics Team at ESOC must have prepared the final instructions to be uploaded to Rosetta to command the delivery operations. (B) The Flight Control Team at ESOC will also check and verify the overall health of the spacecraft, and ensure it’s ready to perform. By this time, the Flight Dynamics Team must have processed the results of the orbit determination and generated a set of telecommands that will control the delivery operations. These must be made available to the Flight Control Team for merging with the rest of the Rosetta and Philae commands and be uploaded. If for any reason they are not available, lander delivery cannot continue. At the same time, Rosetta has to be ready to proceed with the delicate and critical operations coming up.

3. THIRD GO/NO GO 01:35 UTC (Wed 12 Nov at 2.35pm NZDT)
Confirm status and health of Philae: The Philae Control Team at the Lander Control Centre (DLR/Cologne) will do a final check and verification of the lander’s health. Note that, when in contact, both the Rosetta and Philae control teams will be monitoring their respective spacecraft status constantly throughout the day.This is the final verification that the Philae lander is ready for one of the most spectacular missions in spaceflight history.

(Wed 12 Nov between 7.03pm and 8.03pm NZDT)
Rosetta performs the final lander pre-delivery manoeuvre: This is expected to last about 6 minutes and is planned to place the pair onto the correct delivery trajectory. This new orbit will take Rosetta and Philae to a point about 22.5 km from the comet’s centre for separation. Since the separation point is fixed in space and time, the exact time, size and direction of this manoeuvre will only be resolved with the last orbit determination (GO/NOGO No. 1) so as to be able to take into account the exact position where Rosetta was in its orbit.
The burn must be done at the correct time and give the right amount of ‘push’ (change in speed and direction) to Rosetta. If the thrusters over or under perform, or if there is a malfunction on the spacecraft that interferes with the burn, Rosetta may not release Philae at the right point in space, and thus it might not land on the comet in the desired location.

5. FOURTH GO/NO GO 06:35-07:35 UTC
(Wed 12 Nov between 7.35pm and 8.35pm NZDT)
ESOC will make final GO/NOGO decision on lander separation. There is no way for Philae to return to Rosetta, so separation is a one-time event.

If the pre-delivery manoeuvre is not within certain accuracy limits, then Philae would not land in the desired area on the comet. This is also the final verification that everything – the two spacecraft, the orbit, the ground stations, the ground systems and the teams – are ready for landing.

(Wed 12 Nov at 10.03pm NZDT)
Finding out if separation happened as planned: After separation occurs in space at 08:35 UTC, it will take the radio signals from the transmitter on Rosetta 28 minutes and 20 seconds to reach Earth and be transferred to the Rosetta Mission Control Centre at ESOC. Therefore, we must wait until about 09:03 UTC / 10:03 CET for confirmation the separation has happened correctly. If all goes well, there is a 90-second window for receipt of this signal.
Any incorrect performance during separation could affect the lander’s journey to the comet surface. Any problem with Rosetta during separation might affect Philae’s capability to reach the comet or hamper the possibility of following it during its descent and touch-down (Rosetta must serve as a communication relay platform connecting Philae with Earth).

(Wed 12 Nov at 11.53pm NZDT)
First receipt of signals from Philae via Rosetta during its descent: This AOS means that Rosetta and Philae have successfully established a communication link during the descent, and Rosetta can now relay data from Philae to Earth. Philae will start transmitting telemetry – health & status information – as well as science data obtained so far.
Philae cannot send its data to Earth directly, and must establish a link with Rosetta.

Thu 13 Nov 5.02am NZDT
Receipt of touchdown confirmation: This signal means that Philae has landed on the comet; anchoring operations would then start (harpoon firing, etc.).
Receipt of signals from Philae after touchdown: This signal means that Philae has successfully survived landing on comet 67P/C-G and is transmitting signals from the surface via Rosetta to Earth.
Note: there is a 40-minute window for receipt of these.
This will indicate that Philae has successfully landed. The touchdown is risky because the lander might land on a boulder or a very steep surface and be unable to anchor itself. If Philae can’t anchor then even at the very gentle touchdown speed, it will rebound off the surface and be lost. Anchoring is critical to success.

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Source: European Space Agency