The Future of Sentinel-2A: Should It Continue After Sentinel-2C's Launch?
With Sentinel-2C Launched Into Orbit, What Role Could Sentinel-2A Still Play?
Sentinel-2C launched into orbit on 5 September at 03:50 CEST, marking a significant milestone for the Copernicus program. In the lead-up to this launch, there has been considerable discussion within the Earth observation community, even sparking a petition advocating for the continued operation of Sentinel-2A.
This satellite, which has outperformed expectations, now faces potential decommissioning as Sentinel-2C prepares to assume its duties.
This raises a critical question:
Should the successful launch of Sentinel-2C automatically lead to the decommissioning of a fully functional and still valuable Sentinel-2A?
Follow the discussion taking place on LinkedIn here and here.
Edit: As Mark Drinkwater, Head of the Earth & Mission Science Division at the European Space Agency (ESA), pointed on the LinkedIn discussion, figures fpr operational costs are listed here.
"The costs for the construction of Sentinel-2C amount to a total of 200 million euros. The annual operating costs are around 25 million euros."
The Case for Extending Sentinel-2A's Mission
One compelling argument for retaining Sentinel-2A in orbit is the enhanced capability of a constellation with three active Sentinel-2 satellites. This configuration would significantly increase revisit frequency, improving the temporal resolution of the data.
Upon launch, Sentinel-2C will enter a 3-6 month commissioning phase, during which its instruments will be calibrated and validated. Once fully operational, it is intended to replace Sentinel-2A, taking over its position in the constellation alongside Sentinel-2B.
Why Keep Sentinel-2A Operational?
Sentinel-2 satellites are designed for an initial mission life of seven years, with the potential to extend operations up to 12 years, depending on fuel reserves. These reserves include fuel designated for a controlled deorbit at the end of the satellite’s mission.
Sentinel-2A, launched in 2015, is therefore theoretically capable of functioning until mid-2027. However, the exact status of its fuel reserves is not publicly disclosed, adding a layer of uncertainty to predictions about its operational longevity. The satellite might require deorbiting earlier than 2027 if the remaining fuel is insufficient to continue operations safely.
One factor influencing the decision to decommission could be the cost of maintaining the satellite. While the exact annual operational costs for Sentinel-2A are not public, they are likely substantial. The petition estimates that these costs could range from several million to a few dozen million euros per year.
Despite these expenses, the petition argues, continuing operations of an existing satellite is generally more economical than building and launching a new one. For instance, Sentinel-2B, which is similar in design, cost several hundred million euros to develop and launch.
Is Retirement the Best Option?
The decision to retire Sentinel-2A is likely driven by a combination of factors, including not only operational costs but also strategic considerations. Extending Sentinel-2A's mission by a mere 1.5 to 2 years might not justify the expense and effort of supporting three satellites in the constellation—unless the benefits of enhanced data collection significantly outweigh these costs.
The Landsat 5 Comparison: A Useful Benchmark?
While some point to the extraordinary 29-year operational life of Landsat 5 as a precedent, it’s important to recognize the differences in mission design and requirements. Landsat 5, launched in 1984, was initially planned as a 3-year mission but far exceeded its expected lifespan due to strategic operational adjustments that conserved fuel and extended its life.
Sentinel-2 is designed to revisit the same location on Earth every 5 days, often in tandem with its twin, Sentinel-2B. To achieve this frequent revisit rate, Sentinel-2A performs regular attitude control maneuvers to ensure its instruments remain precisely aligned with their targets. It also maintains a highly stable orbit to guarantee consistent image quality and calibration throughout its mission.
In contrast, Landsat 5 prioritized long-term data continuity over frequent, precisely-timed revisits. Consequently, it required fewer attitude and orbit adjustments, as its mission did not necessitate the same level of precision as Sentinel-2A.
Moreover, as Landsat 5 aged, NASA and the USGS adapted their operations to conserve fuel, reducing the frequency of certain maneuvers to prolong its operational life.
Finally, it's worth noting that while Sentinel-2 satellites are equipped with specific fuel reserves for a planned end-of-life deorbit maneuver, Landsat 5 was not designed with a deorbit plan, allowing it to continue operations for as long as possible.
Reimagining Sentinel-2A’s Role: A New Mission
One possible avenue for extending Sentinel-2A's mission could involve redefining its operational objectives. If ESA were to adjust its mission profile—perhaps by reducing the frequency of maneuvers and focusing on less frequent, long-term data collection—Sentinel-2A could continue to deliver valuable data for several more years. This approach, similar to the later years of Landsat 5, would conserve fuel and potentially allow Sentinel-2A to operate beyond its current projected end-of-life.
However, such a shift would require careful consideration of the trade-offs involved, particularly regarding data precision and the overall objectives of the Copernicus program.
Conclusion: A Strategic Decision for the Future
The fate of Sentinel-2A is not just a matter of operational capability or cost—it’s a strategic decision that will impact the Copernicus program's ability to provide critical Earth Observation data. Whether to decommission or extend the mission of Sentinel-2A involves balancing the immediate benefits of enhanced data collection against the long-term sustainability of the program.
As Sentinel-2C begins its mission, it's crucial for ESA and the broader Earth observation community to carefully weigh the benefits of extending Sentinel-2A's mission against the program's broader goals.
Follow the discussion taking place on LinkedIn here and here.
From Mark Drinkwater's LinkedIn comments, I found this point quite telling:
"The 12 yrs propellant includes a strict requirement for EOL deorbiting, whilst this potential lifespan is foreshortened by all additional orbit manouvers which have been required to avoid space debris throughout its operations."
Space Junk strikes again! Landsat 5 didn't have to dodge the quantity of space junk we have now.