# PROBA-3

PROBA-3 is a dual-probe technological demonstration mission by the European Space Agency devoted to high-precision formation flying to achieve scientific coronagraphy. It is part of the series of PROBA satellites that are being used to validate new spacecraft technologies and concepts while also carrying scientific instruments. It lifted off aboard ISRO's PSLV-XL rocket from Satish Dhawan Space Center in Sriharikota, India. The project is managed by Damian Galano.

## Timeline

* The mission concept dates to 2005, when a study was performed in the ESA CDF.
* After several phase-A studies and a change of industrial organisation at the beginning of the phase B, the mission's implementation phase (phases C/D/E1) eventually began in July 2014.
* The system critical design review was completed in 2018.
* The two spacecraft were integrated before environmental campaign was completed as of March 2023.
* The twin spacecraft reached the launch site, Satish Dhawan Space Center on the Eastern coast of India on 3 November 2024.
* Following check out, the spacecraft were stacked into their launch configuration, fueled with hydrazine, and successfully launched on 5 December 2024.
* While the two spacecraft were still attached to each other, the ASPIICS coronograph took its very first in-space image (a star field in the constellation of Ophiuchus) in order to test the accuracy of the mission’s pointing.
* On 14 January 2025, the two spacecraft successfully separated from each other and on 24 January 2025, they were again brought closer together by a series of maneuvers that resulted in them maintaining a separation of minimum 1000 meters.
* On 7 February 2025, the Occulter Spacecraft took the first picture of the Coronagraph Spacecraft while at a distance of several kilometers.
* In March 2025, the two spacecraft were brought to a distance of 200 metres to test target pointing and the Visual Based System on the Occulter Spacecraft took a series of images of the Coronagraph Spacecraft. This was the 'first light' of the Visual Based System. Then the spacecraft were commanded to switch to autonomous pointing mode for the first time.

## Mission concept

PROBA-3 consists of two independent, three-axis-stabilized spacecraft: the Coronagraph Spacecraft (CSC) and the Occulter Spacecraft (OSC). The spacecraft will fly close to each other on a highly elliptical orbit around the Earth, with an apogee at 60,500 km altitude.
ESA said that by flying in tight formation about 150 metres apart, the Occulter will precisely cast its shadow onto the Coronagraph’s telescope, blocking the Sun’s direct light. This will allow the Coronagraph to image the faint solar corona in visible, ultraviolet and polarised light for many hours at a time.
Along the apogee arc, when the gravity gradient is significantly smaller, the two spacecraft will autonomously acquire a formation configuration, such that the CSC remains at a fixed position in the shadow cast by the OSC. The CSC hosts a coronagraph, which will then be able to observe the Sun's corona without being blinded by the intense light from the photosphere. Given the diameter of the occulter disk on the OSC and the intended corona observation regions, the CSC must be approximately 150 meters from the OSC and maintain this position with millimetric accuracy, both in range and laterally. The scientific objective is to observe the corona down to about 1.1 solar radius in the visible wavelength range.
Besides formation flying for coronagraphy, some demonstration manoeuvers (retargeting and resizing) will be attempted during the apogee phase of the orbit, as well as a space rendezvous experiment.
The formation acquisition and control is performed on-board by metrology equipment and actuators. The metrology equipment comprise a laser-based system providing high-accuracy relative position estimate, a video-based sensor with a coarser precision but wider field of view, and a shadow position sensor providing finest precision when the CSC is in the vicinity of the target position in the shadow cone.
After the apogee arc, the formation is broken by impulsive manoeuvers executed by the spacecraft. The two spacecraft are placed on a relative trajectory that passively ensures no risk of collision during the perigee passage, when the spacecraft altitude goes down to 600 km. Along the perigee phase of the orbit, the two spacecraft acquire GNSS data to derive a precise estimation of the relative position and velocity, which is propagated for a few hours up to the reacquisition of the metrology before the next apogee arc.
The CSC and OSC exchange sensor data and commands through a radio-frequency inter-satellite link to coordinate their activities. Scientists hope that PROBA-3’s unique vantage point will provide new insights into the origins of coronal mass ejections (CMEs) – eruptions of solar material that can disrupt satellites and power grids on Earth. The mission will also measure total solar irradiance, tracking changes in the Sun’s energy output that may influence Earth’s climate.

## Design
### CSC and OSC spacecraft

The CSC is a 300 kg mini-satellite, hosting the coronagraph ASPIICS and the shadow position sensors. It is equipped with a monopropellant propulsion system to perform the large-delta-V manoeuver necessary for formation acquisition and breaking. It also hosts the targets used by the metrology optical heads on board the OSC.
The OSC is a 250 kg mini-satellite, hosting the laser and visual metrology optical heads. It features the occulter disk that is 1.4 meters in diameter. The shape of its rim is intended to reduce the amount of diffracted sunlight entering the coronagraph. The OSC uses a low-thrust cold-gas propulsion system that enables the fine position control required for the formation flying.

### Scientific payloads

The primary payload is the ASPIICS coronagraph. It follows the design concept of a classical externally occulted Lyot coronagraph, with the external occulter physically attached to the OSC, while the rest of the instrument is on the CSC.
ASPIICS will observe the solar corona through refractive optics, able to select 3 different spectral bands: Fe XIV line at 530.4 nm, He I D3 line at 587.7 nm, and the broad spectral band 540–570 nm.
It is expected that the data from ASPIICS will fill the gap in term of field of view between EUV imagers and externally occulted coronagraphs, when the latter are monolithic instruments that don't benefit from the longer distance enabled by formation flying.
The principal investigator for the coronagraph instrument is Andrei Zhukov from Royal Observatory of Belgium.
A secondary scientific payload (DARA) is hosted on the OSC. DARA stands for "Davos Absolute Radiometer" and is an absolute radiometer for measuring total solar irradiance (TSI).

### Ground segment and operations

Like the other Proba satellites, PROBA-3 will be operated from the ESA center in Redu, Belgium.

## Project development

PROBA-3 is a project managed by the European Space Agency. The industrial development of the S/C and the ground segment is led by SENER Aerospace, which coordinates the work of a core team with Airbus Defence and Space, QinetiQ Space, GMV, Celestia Antwerp BV and Spacebel.
The coronagraph payload is developed for ESA by a consortium led by Liège Space Center (CSL) in Belgium, made up of 15 companies and institutes from five ESA member states.
DARA is provided by the PMOD institute in Switzerland.
Testing of the mission's vision-based sensor system was performed at ESA's ESTEC technical centre in the Netherlands in March 2021. The system will enable the two spacecraft to fly in a precise formation. The testing reportedly yielded promising results. The miniature satellites recently underwent final integration and were viewed in person by PROBA-3’s Science Working Team. Members of the team plan to test flight hardware during April's total solar eclipse over North America, gaining valuable experience for interpreting PROBA-3’s future results.