Posted on 26/06/2015 by emily
Rosetta and Philae: Searching for a good signal
After seven months in hibernation on the surface of Comet 67P/Churyumov-Gerasimenko, Rosetta’s lander Philae communicated with Earth via the orbiter on 13 June. Since then, seven periods of connection have been confirmed between the orbiter and lander, but all have been intermittent. One of the key issues being worked on is to adjust Rosetta’s trajectory to see whether a more reliable communications link can be established with Philae. This report describes the status of those efforts as of 26 June, and has been prepared with inputs from ESA’s Rosetta Science Ground Segment team at ESAC and the flight control team at ESOC, along with the Lander Control Centre at DLR.
When have contacts been made?
Confirmed contacts between Rosetta and Philae have been made on 13, 14, 19, 20, 21, 23, and 24 June, but were intermittent during those contact periods. For example, the contact on 19 June was stable but split into two short periods of two minutes each. Conversely, the contact on 24 June started at 17:20 UT (on board Rosetta) and ran for 20 minutes, but the quality of the link was very patchy and only about 80 packets of telemetry were received. Prior to this, on Tuesday, 23 June, there was a 20-second contact, but no stable link was established and consequently no telemetry data were received.
How frequently do Rosetta and Philae try to make contact?
Comet 67P/C-G rotates with a 12.4 hour period and thus Philae’s location is not always visible to Rosetta. Roughly speaking, there are two opportunities for contact between the two spacecraft each Earth day, but their duration depends on the orientation of the transmitting antenna on Philae and the location of Rosetta along its trajectory around the comet. Similarly, as the comet rotates, Philae is not always in sunlight and thus not always generating enough power via its solar panels to receive and transmit signals. At the moment, the predicted contact windows vary between a few tens of minutes and up to three hours. During these contact windows, the ideal situation would be that a powered-up Philae hears Rosetta’s calling signal and responds by establishing a link back to the orbiter, then transmitting the data stored on-board via that link.
Why do we care about a stable connection?
Data are stored in two mass memories on-board Philae, and in order to download the data in the most efficient way possible, a stable ‘call’ duration of about 50 minutes is desired. It can take around 20 minutes for the data to be dumped from each one to the Rosetta orbiter, and additional time is needed to confirm that a stable link has been acquired in the first place, and also for uploading new commands.
Can the lander still be operated with short communications links?
Yes, but this situation is not ideal because it has an impact on the overall time available to perform scientific operations in the long term. That’s because each time a new science sequence was initiated, it would take longer to get the accumulated science data back and free up on-board storage before new commands could be uploaded and subsequently executed.
What might be affecting the link from the lander’s point of view?
A number of factors regarding the lander’s current status may contribute to the quality of the communications links observed so far. These include:
- Lander power availability: the orbiter needs to be flying overhead the lander’s position when the lander is ‘awake’, that is, when it is generating enough power to have its receivers and transmitter switched on.
- Lander location and orientation: the orientation of the lander on the surface of the comet determines how its antenna pattern is projected into space, and the rugged topography immediately surrounding Philae can also distort that antenna pattern.
- Lander health status: errors in the various on-board units could also affect the chances of making a stable link.
What might be affecting the link from the orbiter’s point of view?
Equivalently, there are a number of parameters related to the orbiter that could be influencing the quality of the communications link observed so far:
- Distance to the comet: the strength of the signal received by the orbiter diminishes as the square of the distance between the orbiter and the lander, and thus the chances of a stable link are reduced if Rosetta is too far from the comet.
- Trajectory of the orbiter: to make a link, the antenna pattern of the lander must overlap with that of the orbiter, and given the constraints set by the lander antenna pattern, certain trajectories of the orbiter around the comet will be more effective at seeing a ‘clean’ lander signal than others.
- Pointing of the orbiter: the exact orientation of the orbiter in space plays a role, because if the dedicated, non-steerable antenna used to communicate with the lander is not pointed directly at the comet, the strength of the signal received from the lander will be reduced. Some science observations being made by Rosetta require the orbiter to be pointed off the nucleus, but steps are being taken to avoid that situation during potential contacts with Philae.
Can any of these factors be changed?
Until a stable link is achieved between the orbiter and the lander and new commands uploaded, it is obviously not possible to try and ‘tune’ the parameters on-board the lander. Thus, present efforts are focused on improving the factors related to the orbiter. However, this is not straightforward, as the spacecraft operations team must keep the safety of the orbiter as their highest priority at a time when the comet is becoming more and more active.
How close can Rosetta get to the comet and still remain safe?
In order to navigate around the comet, Rosetta uses its star trackers to determine its orientation in space, and thus keep its instruments and high gain antenna pointed in the right directions. However, in the dusty environment of a comet, individual dust particles can mimic stars, making it difficult for the star trackers to operate effectively. If the star trackers are unable to determine the spacecraft’s orientation, it will go into safe mode, as experienced during one of the March close fly-bys of the comet. In the worst case, contact with Earth may be lost, which would lead to the spacecraft entering an autonomous mode that could take days or weeks to recover from.
The increasingly-active environment of Comet 67P/C-G is proving to be dustier than planned for when Rosetta was built and thus since March, the spacecraft has been flying at safer distances of roughly 200 km from the comet to avoid similar issues occurring. It has also been moved into a so-called ‘terminator trajectory’ around the comet, also aimed at reducing the impact of the dusty environment on the star trackers.
The spacecraft operations team are slowly edging Rosetta closer to the comet in this terminator trajectory, closely monitoring the performance of the star trackers in ‘continuous tracking mode’, and planning the trajectory for the days ahead. At the moment, Rosetta is following a trajectory scheme that allows it to come as close as 165 km from the comet, and there are signs that dust interference is becoming an issue again. A manoeuvre planned for Saturday morning will move the spacecraft to 160 km by 30 June and the team will assess the star tracker performance at that time in order to determine if even closer orbits are possible, or if Rosetta needs to be moved further away again.
How is the trajectory being changed?
The current terminator trajectory of Rosetta has it flying over the boundary between comet day and night. The main change that can be made within this scheme is to the latitude of the ground track of the orbit on the surface of the comet. This is currently being stepped down from +55 degrees (on 24 June) to –8 degrees (on 26 June), with a better quality signal between Rosetta and Philae being detected at lower latitudes. For comparison, just after landing in November 2014, Rosetta was flying over latitudes of +15 to +25 degrees. In the coming week, the latitudes will slowly be stepped back up again from –8 to +50 degrees, with a careful assessment being made of signal strength at low latitudes again.
How long is it going to take to resolve the situation?
This is a very dynamic, real-time process, and thus it is hard to predict when a stable link might be made between Rosetta and Philae. The mission teams are working on a short-term trajectory planning schedule, which is updated every Monday and Thursday morning. Changes to Rosetta’s trajectory are made depending on the latest information with regards lander communications and the performance of the orbiter’s star trackers in the days between each decision point. In addition, representatives from ESA’s Rosetta team, the Lander Control Centre at DLR in Cologne, and the Lander Science Operations and Navigation Centre at CNES, Toulouse discuss daily the latest status of any lander communication events.
We will keep you posted as further information becomes available.