The European Space Agency will launch a mission late this year to demonstrate precision formation flying in orbit to create artificial solar eclipses. In a press conference last week, the agency announced details of the mission and the technology the orbiters will use to pull off its exquisitely-choreographed maneuvers.
ESA’s Proba-3 (PRoject for On-Board Autonomy) consists of a pair of spacecraft: a 300-kilogram Coronagraph spacecraft and a 250-kilogram Occulter. The pair are now slated to launch on an Indian PSLV rocket in September and ultimately enter a highly elliptical, 600-by-60,530-kilometer orbit. The aim, the agency says, is to move the separate spacecraft to some 144 meters apart, with the Occulter, as a disc, blocking out the sun.
Achieving this formation will allow the Coronagraph to study our star’s highly ionized, extremely hot atmosphere—but also demonstrate the technology as a precursor for more ambitious, future, formation-flying endeavors.
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The two spacecraft will align to create eclipses for around six hours per orbit, starting on approach to its farthest point from Earth (apogee). The two spacecraft will use radio inter-satellite links to communicate, and star trackers will be used by both craft for determining their attitude.
Global navigation satellite system (GNSS) receivers will pick up GPS signals around perigee, or the closest point to Earth, which, when combined with a dedicated relative navigation algorithm, will allow regular determination of their relative positions. Optical sensors on the Occulter will view pulsing LEDs on the Coronagraph to provide data for finer measurements. But greater, millimeter-scale precision requires more technological wizardry still.
For this, the Occulter will ping a laser at a corner cube retro-reflector mounted on the Coronagraph spacecraft, which will bounce the beam back. This act of metrology will allow precise tracking of the relative position and orientation of the two spacecraft. Using the data gathered, the craft can, the ESA says, control and maintain millimeter-level accuracy using 10 millinewton-scale cold gas thrusters aboard the Occulter.
The occulter (OSC) uses cold gas thrusters. These work by opening a valve to release a high-pressure propellant, where the mass flow itself accelerates the spacecraft. (I'd expect the gas to be...
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The occulter (OSC) uses cold gas thrusters. These work by opening a valve to release a high-pressure propellant, where the mass flow itself accelerates the spacecraft. (I'd expect the gas to be something like nitrogen, but I couldn't find any details.) These are pretty much the simplest possible thruster: a tank and a valve. ("Cold" here refers to there being no ignition, not cryogenic temperatures; the OSC's propellant tanks run at a modest 14°C–34°C.)
The other spacecraft (the coronagraph, or OSC) uses hydrazine thrusters, which run their single propellant over a catalyst to release energy.
Cold gas thrusters are relatively inefficient, with a specific impulse of around 70, limiting the total delta-v of the spacecraft. This tradeoff is perfectly okay for stationkeeping because there's less atmospheric drag at these altitudes.
From the article:
The CSC is responsible of performing the main orbital maintenance impulsive manoeuvres with monopropellant thrusters.
To me, this reads as the CSC using hydrazine thrusters for coarse alignment (orbital parameters, keeping the right distance away) to the the occulter, which is then only responsible for fine alignment.
Crewed spacecraft tend to use hypergolic propellants which are relatively efficient, in the ballpark of 300 seconds of specific impulse. These propellants are also storable at ambient temperatures, and hypergolic thrusters are easy to scale up to high-acceleration burns.
Long-duration stationkeeping in low earth orbit, or (more famously) interplanetary probes, can use ion thrusters which are highly efficient (thousands of seconds of specific impulse, reducing fuel mass requirements for the same delta-v), but use so much electricity that high-acceleration burns aren't feasible.
For short-lifetime spacecraft (upper stages, orbital transfer vehicles), cryogenic propellants are possible. These run at very low temperatures (ballpark: -200°C), and at least in the case of upper stages, require high thrust to avoid falling back into the atmosphere.
From the article:
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Does anyone know what kind of propulsion and fuel are used by craft like this?
The occulter (OSC) uses cold gas thrusters. These work by opening a valve to release a high-pressure propellant, where the mass flow itself accelerates the spacecraft. (I'd expect the gas to be something like nitrogen, but I couldn't find any details.) These are pretty much the simplest possible thruster: a tank and a valve. ("Cold" here refers to there being no ignition, not cryogenic temperatures; the OSC's propellant tanks run at a modest 14°C–34°C.)
The other spacecraft (the coronagraph, or OSC) uses hydrazine thrusters, which run their single propellant over a catalyst to release energy.
Cold gas thrusters are relatively inefficient, with a specific impulse of around 70, limiting the total delta-v of the spacecraft. This tradeoff is perfectly okay for stationkeeping because there's less atmospheric drag at these altitudes.
From the article:
To me, this reads as the CSC using hydrazine thrusters for coarse alignment (orbital parameters, keeping the right distance away) to the the occulter, which is then only responsible for fine alignment.
Crewed spacecraft tend to use hypergolic propellants which are relatively efficient, in the ballpark of 300 seconds of specific impulse. These propellants are also storable at ambient temperatures, and hypergolic thrusters are easy to scale up to high-acceleration burns.
Long-duration stationkeeping in low earth orbit, or (more famously) interplanetary probes, can use ion thrusters which are highly efficient (thousands of seconds of specific impulse, reducing fuel mass requirements for the same delta-v), but use so much electricity that high-acceleration burns aren't feasible.
For short-lifetime spacecraft (upper stages, orbital transfer vehicles), cryogenic propellants are possible. These run at very low temperatures (ballpark: -200°C), and at least in the case of upper stages, require high thrust to avoid falling back into the atmosphere.