The European Space Agency (ESA) wants to bring the sort of precision normally associated with Swiss watch making to satellite navigation. When it launches in 2017, ESA’s Proba-3 mission will incorporate the first satellite pair capable of flying in formation to within a tolerance of a millimeter to one another. It's part of a demonstration technology that could one day be used to build space telescopes using formation-flying satellites as a “rigid structure” that would be impossibly large to achieve in a single spacecraft.
Led by SENER of Spain, the “Proba” project stands for PRoject for OnBoard Autonomy – a name that highlights the fact that the ground control team based in Redu, Belgium, only need to monitor the spacecraft during working hours. The mission’s basic task is flying in formation to form a 150-meter (492-ft) long solar coronagraph to study the Sun, with the size of the “instrument” and the vacuum of space providing enough clarity and resolution to allow Proba-3 to see closer to the solar rim than ever before.
To carry out such a task, the two Proba-3 craft must fly in tighter formation than satellites have managed until now – to within a millimeter and one second of arc in precision over a distance of 150 meters. Demonstration of this sort of precise maneuvering to turn two small satellites into one giant instrument is the main purpose of the Proba-3 mission, outside of its solar study.
This sort of precision is needed because space-based instruments will need to be ever larger if they are to increase their ability to study the universe. Aside from the logistical problems of building giant telescopes and other instruments, very large constructs in space soon become their own worst enemies as temperature variations, machine vibrations and other factors interfere with observations. By using formation flying, the satellites not only make building such instruments cheaper and easier, but also greatly reduce interference. In addition, such precision flying is a great asset in building space stations or assembling satellites in orbit.
Proba-3 consists of two solar-powered spacecraft based on ESA’s standard Proba platform. The larger of the two is the Coronagraph. Weighing 340 kilograms (750 lb) and measuring 1.1 x 1.8 x 1.7 meters (3.6 x 5.9 x 5.5 ft), it’s the active member of the pair that carries out most of the maneuvers. As the name suggests, it contains the coronagraph for observing the Sun, and optical metrology sensors. For maneuvering, there are reaction motors using cold gas thrusters, three gyroscopes, a three-headed star tracker, six sun sensors and two GPS receivers.
The other satellite is the Occulter. This one is smaller at 200 kilograms (440 lb) and a more compact 0.9 x 1.4 x 0.9 meters (2.9 x 4.5 x 2.9 ft). It has a similar maneuvering package to the Coronagraph, and its job is to block the Sun for the Coronagraph with a 1.4-meter (4.5 ft) disk.
The planned Proba-3 mission is scheduled for launch in 2017 when the pair will be set in a highly elliptical orbit at a 60-degree inclination, where they will separate and fly in tandem, circling the earth every 19.7 hours. As they fly in formation, the pair will line up with the Sun, with the Occulter forming artificial eclipses.
Because of the need to conserve fuel, the satellites won’t fly in formation all the time. Instead, they will go into formation while approaching apogee (60,530 km/37,611 mi), when they're traveling slowest, to carry out coronagraph observations. They will then maneuver to avoid colliding with each other as they break formation, while approaching perigee (600 km/372 mi).
The Proba-3 craft will form and un-form formation until the end of the mission, when the elliptical orbit will quickly decay and the probes burn up in the atmosphere. Previous missions have used coronagraphs, but these were internal devices with a very short length and therefore with limited resolution.
Oddly, using two spacecraft as a coronagraph isn’t a first for Proba-3. That honor goes to the Apollo-Soyuz mission in 1975, when the Apollo spacecraft blocked the Sun for observation by the Soyuz capsule. What is new is the increase in precision from being able to observe the Solar corona out to three solar radii down to 1.04 radii.
During the mission, Proba-3 will demonstrate a suite of technologies, such as algorithms and sensors that could be used for a Mars sample return mission, inter-satellite RF links, optical tracking systems, and the satellite pair’s ability to carry out maneuvers without ground control.
The Proba-3 mission is discussed in the journal Acta Astronautica.