Planning the end of the Aura mission

OMI is one of four instruments on the Aura satellite, which was launched in 2004. After all these years two instruments are still going strong: MLS and OMI. Whilst MLS is a unique instrument without a successor OMI does have a successor: TROPOMI. Prolonging the mission will extend the lifetime of MLS as well as increase the data record of OMI. In order to prolong the mission there are several items that must be considered.

First a few facts about the Aura mission.

• Aura orbits the Earth in a sun-synchronous orbit over the poles while the Earth revolves around it’s axis underneath. In this way the satellite and its instruments cover the entire (illuminated) surface of the Earth every 24 hours.?
• Together with other satellites Aura flies in the Afternoon Constellation (A-train), so called because the overpass time of the satellites in the A-train is near 13:30. The satellites in the A-train pass the same scene within minutes from each and thus measure the ever-changing Earth atmosphere under nearly identical conditions.?Therefore, measurements from instrument on different satellites can be combined to gain a better understanding of the Earth atmosphere.
• Regular Inclination Adjust Maneuvers (IAMs) are needed to keep the overpass time from drifting. An IAM is a thruster maneuver?and expends fuel.
• For Aura a science range for overpass time is defined of +/- 15 minutes.
• A much tighter range is defined for to phasing with Aqua, another satellite in the A-train. But Aqua has entered its last stage and will soon leave the A-train. So this tight boundary does not need to be kept much longer.
• Without IAM’s the satellite would drift to an overpass time later in the afternoon. This has consequences for the power generated by the solar panels.
• Eventually Aura will need to leave the A-train by performing a special retrograde maneuver.
• To avoid the satellite ending up as space debris at the end of the mission there is a 25 year reentry constraint: the satellite must safely reenter the Earths atmosphere within 25 years of the end of its operational lifetime.

Saving Fuel

The mission can be prolonged by saving fuel. Fuel can be saved by performing less thruster manoeuvres. However, less IAMs implies that the satellite will start drifting in overpass time. It has been decided that the IAMs performed this fall will be the final IAMs performed for this mission.

There were two options to choose between. At the beginning of August a meeting was organised to discuss these options. The parties involved in the Aura mission (Aura Project Science Team, MLS Instrument Operations Team, MLS Science Team, OMI Instrument Operations Team, OMI Science Team, Aura Flight Operations Team, Aura Flight Dynamics System Team, Earth Science Constellation Team and ESMO Management) were invited. The Flight Dynamics team explained both options and the advantages and disadvantages of each option.

• Option 1: perform 4 IAMs this fall. Allow the satellite to slowly drift in overpass time until it leaves the defined science range (+/- 15 minutes). At this point Aura leaves the A-train and the satellite overpass time will start drifting faster. In addition, orbit parameters like ground track error, semi-major axis, eccentricity, perigee and orbital period will no longer remain stable. In August 2025 the mission will have to end to meet the 25 year reentry constraint.
• Option 2: perform 2 IAMs this fall. The satellite will drift a bit faster in overpass time than with option 1 in the A-train, but the satellite will remain in the A-train even after moving out of the previous defined science range of +/- 15 minutes. July 2025 Aura will have to leave the A-train and one month later the mission needs to be terminated.

An obvious reason to base a decision on would be the duration of the mission. But this is the same for both options. Both options have their advantages and disadvantages. What is more important for the remaining mission duration? A slightly more stable overpass time or stable orbit parameters? People present in the meeting did not express a clear preference for either option. At the end there was a slight preference for option 2, because in that scenario Aura remains in the A-train and the orbit parameters remain stable for longer. From an operations point of view (my view) the drift in overpass time does not present a problem. We took this into account when making changes to the OMI baseline last year to ensure the quality of the (background) data even for overpass times in late afternoon.

But the question remained, what is best for the science objectives? The OMI science team announced?two days later a (strong) preference for option 1, because in this option the overpass time changes slower. Since it is uncertain for how long the OMI instrument will remain?functional it seemed the better choice to remain consistently in the already defined physics range. The MLS science team announced a week later that they had a slight preference for option 2 after careful consideration, but considering OMI’s stronger preference for option1, were OK with option 1 as well.

And thus… The Aura Project Science Team has decided to go instead with option 3. With this option 2 IAMs will be performed this fall with the possibility to perform two additional IAMs in spring of 2022 if requested. The choice is delayed and there is thus no need to make any decisions at this point in time.

All these discussions demonstrate that Flight Dynamics is quite a ‘vak apart’. It is not so much the number of IAMs that makes a difference, but much more the timing and duration of these IAMs. Thanks for the FDS team for calculating all these different options and providing these choices, and clarifying the consequences of the different scenarios.?