Vega C returns to flight, putting Sentinel-1C into orbit!

On December 5th, 2024, French Guyana got shaken by the power of the Vega C, a small-satellite launcher, returning to flight after a catastrophic failure in 2022! This launch put Sentinel-1C into orbit to extend Europe’s space capabilities in Earth observation.

Sentinel-1 family

The Sentinel-1 family is a category of satellites belonging to the Sentinel program, which is included in the bigger Copernicus program, Europe’s satellite constellation for Earth observation. Sentinel-1 is designed to be a 2-satellite mission, but one of the satellites failed in 2022, and that’s why Sentinel-1C is going to replace it. Sentinel-1A was launched on April 3rd, 2014, while Sentinel-1B was launched on April 25th, 2016. Both satellites orbited the Earth 180° apart in a polar orbit at 693 km of altitude and 98.18° of inclination, with an orbital period of 98.6 minutes; this configuration allowed them to have a repeat frequency (how often they’d cover all of an Earth’s zone) of less than 1 day over the Arctic, less than 3 days over Europe, Canada, and main shipping routes, and 6 days for all the world. All was working well until December 23rd, 2021, when Sentinel-1B started experiencing an electrical anomaly related to data transmission, which ultimately led to a loss of data; on August 3rd, 2022, ESA (European Space Agency) concluded that a power issue didn’t allow recovery of the 28V regulated bus on the C-band SAR (Synthetic Aperture Radar) antenna power supply unit; this was responsible for providing power to the radar electronics, meaning the satellite couldn’t gather data anymore. Plans to launch Sentinel-1C began as soon as possible, and Sentinel-1B disposal operations began in September 2022 and culminated on September 12th, 2024, when the satellite was passivated in its final orbit (about 580 km) with an estimated reentry time of not more than 25 years. Since 2022, Sentinel-1A has been working alone, and the time needed to cover all of the Earth has passed from 6 to 12 days. Hopefully, with the launch of Sentinel-1C, the capability will return to normal; this shows Europe’s resilience, managing to design, develop, and launch a replacement satellite within 3 years of the failure; it’s also worth noting that the main delay reason of the launch was the failure and grounding of Vega C. But now, let’s take a deep look at Sentinel-1C.

Sentinel-1C

Sentinel-1C render. Credit: Thales Alenia Space

All the Sentinel-1 satellites are basically identical, so the description of Sentinel-1C also applies to Sentinel-1A, B, and D (which is going to replace Sentinel-1A next year): it hosts a C-band Synthetic Aperture Radar, or SAR, at 5.405 GHz; SARs are really advanced instruments, since they can take high-resolution data of the Earth’s surface regardless of the time of day and of the weather conditions; this is because it is not an optical system, which relies on the sunlight’s reflections to see, but it’s a radar system, which sends waves as signals and then receives them back, gathering data. This radar can operate in several modes, modifying the swath from 20 to 400 km and its spatial resolution (range X azimuth) from 5X5 m to 20X40 m, and it can also operate in an optical mode rather than a wave mode for 25 minutes/orbit. The satellite, like all previous ones, has a mass of 2185 kg, including 945 kg of instrumentation and antennas, with an additional 130 kg of fuel; its 2 solar panels are 10 m long, and its antenna is 12 m long; all of these are folded during launch, and then they’re deployed once in orbit during a 10-hour period after separation. Additionally, this satellite has something that the previous satellites did not have: the first new feature is an advanced separation mechanism on the radar, consisting of soldered joints installed at the main connection points to the satellite, which melt under strong heating and separate the antenna from the main platform; this feature will allow for an earlier, faster, and more accurate burn-up in the atmosphere at the end of the satellites’ life (which, speaking of, is 7-12 years of operations before orbital decay), ensuring less debris reaches the ground. The other feature is the Automatic Identification System, which allows tracking ships using their individual ID to help avoid collision risks and detect illegal maritime traffic; the other purposes of the Sentinel-1C satellite are to monitor sea ice and glacier melting, sea level rise, oil spills, land deformation, sea winds and currents, and to help during natural disasters. Airbus has built the radar while Thales Alenia Space has built the satellite and the PRIMA satellite bus (Piattaforma Riconfigurabile Italiana Multi Applicativa), integrated it with the radar, and conducted testing. The satellite is the first of the Sentinel-1 family to be launched using a Vega-C, since the previous 2 were launched with a Soyuz rocket. 

Sentinel-1 schemes. Credit: https://sentiwiki.copernicus.eu/web/sentinel-1

Vega C: the return to flight

Vega C on the launch pad a few hours before launch. Credit: European Space Agency

This launch not only carries a satellite to orbit but also marks an important milestone for European spaceflight and for Vega: the Vega launch family included the Vega launch vehicle (last launch on September 5th, 2024) and the Vega C launch vehicle; this will be the third launch of the latter and its return to flight after a failure that occurred on December 21st, 2022, when the second stage Zefiro 40 motor had an issue in its nozzle; since then, some corrective actions have been taken, including a redesign and 2 successful hot fire tests that confirmed flight readiness. But let’s explore Vega C in detail: Vega C is a vehicle mainly developed by Avio in Italy (which provides 65% of the funding) along with many other European countries; it stands nearly 35 m tall and has 4 stages, of which the last one uses liquid propellant: the P120C first stage uses solid fuel, which gives the characteristic instantaneous liftoff thrust of 4500 kN; these first stages are also used as side boosters of the Ariane 6. The second stage, Zefiro 40, provides 1304 kN of thrust and uses solid fuels as well; the small third stage, Zefiro-9, burns 10t of solid propellant, and then comes the 4th stage: AVUM+ (Attitude Vernier Upper Module); it’s the only module that uses liquid propellant on Vega C (UDMH as fuel and N₂O₄ as oxidizer), and its engine can reignite multiple times (up to 5) to place the payload in a specific orbit. It can also conduct a deorbit burn after payload separation to accelerate its disposal into Earth’s atmosphere and lower the number of space debris. With the return to flight of Vega C, Europe demonstrates its capabilities of launching payloads into orbit and ignites its future ambitions.

Launch

All times in UTC except where otherwise specified.

Launch of VV25-Sentinel-1C. Credit: ESA Space Transport.

Launch, initially scheduled for December 3rd at 21:20 UTC, was delayed 24 hours to allow for more thorough checks from the teams; at 17:30 on December 4th, the mobile gantry, the 50-m tall structure that weighs over 1 million kg and covers the rocket until a few hours from launch, started retracting to expose the launch vehicle to the skies; however, a mechanical issue prevented it from fully retracting, and the teams had to scrub the launch for the day and postpone it 24 hours later. On December 5th, thanks to the hard work of the teams, the mobile gantry retracted flawlessly, and the weather was looking good for launch. Step by step, the launch countdown continued flawlessly, and all systems were GO for launch; finally, at T0, the P120C ignited and lifted off at full thrust to the skies! Once the thicker parts of the atmosphere were left behind, the first stage separated, and the second stage, Zefiro 40, ignited at T+2m30s; this stage runs on solid fuel as well and was the cause of the last Vega C failure, so it was a relief when it completed its burn and separated successfully, after which the Zefiro 9 third stage engine ignited at T+4m38s! At T+5m4s, the fairings separated, exposing the satellite to space for the first time. At T+7m8s, the third stage separated, and more than a minute later, at T+8m16s, AVUM+ lit its engine for the first of 4 planned burns: the first burn lasted a lot, until T+16m45s, after which AVUM+ shut down its engine at a speed of 7.85 km/s. This first burn was followed by a coast phase that was interrupted by the second engine ignition from T+53m27s to T+56m10s, boosting the speed by 0.12 km/s, after which another coast phase began!

 And then, after a long wait, the third burn occurred, from T+1h40m27s to +1h42m15s, increasing its speed by another 0.09 km/s; this put Sentinel-1C in its final nominal orbit, and after about 1 minute, at T+1h43m49s, the satellite successfully separated and began its on-orbit life! But the mission wasn’t finished yet, because one last burn had to occur, because AVUM+ conducted a deorbit burn to lower its orbit and accelerate reentry times, contributing to space debris’ lowering and removal. The burn was conducted from T+1h49m47s to 1h50m52s and was successfull. A few minutes after separation, the signal was received from Sentinel-1C, confirming it is operational! It will now spend the next 10 hours deploying the solar panels and antenna, and this officially concluded the Sentinel-1C launch and Vega C’s return to flight with the VV25 launch, completed successfully!

Sentinel-1C after separation in the mission animation. Credit: Arianespace