The ambitious scientific goals of the Rosetta mission created the following requirements:
- A large number of complex scientific instruments are accommodated on one side of the spacecraft, which must permanently face the comet during the operational phase of the mission, including at 1 km proximity to the active comet. During the long cruise to the comet the dormant instruments must be heated to ensure their survival
- Complex spacecraft navigation at low altitude orbits around an irregular celestial body with weak, asymmetric, rotating gravity field, enveloped by dust and gas jets
- The Rosetta Lander has to be stowed to survive the cruise and eventually to self-eject from the spacecraft. The orbiter must navigate with 10 cm, 1 mm per second accuracy for the ejection, and then relay data from the SSP back to Earth
These primary mission requirements were design driving for most of the spacecraft layout and performance features, such as: the data rate needs to be as high as possible; given the limitations of the extremely long distance from Earth, the data must be highly compressed with a high pointing accuracy (a few arcseconds), in particular for the remote-sensing instruments. Further requirements are:
- thermal layout - the spacecraft needs to endure both extremes of temperature, from that of deep space to that within one kilometre of the active comet
- during the asteroid flybys, Rosetta must be able to track the asteroids autonomously (too fast and too far away for direct ground control)
- better than mm per second relative velocity accuracy for manoeuvring in the vicinity of comet 67P/Churyumov-Gerasimenko
- the spacecraft's mass limit was governed by the launch capability of the Ariane-5 which allowed for a maximum 'wet' (spacecraft and fuel) mass of ~3000 kg at launch, with a propellant portion of more than 50%
- nominal spacecraft lifetime is 11 years in heliocentric trajectory
- the achievement of this lifetime is helped by the long hibernation periods during cruise, where most of the electrical systems are not operational. This hibernation increases their lifetime by a factor of 10 but requires onboard autonomy to guarantee continued operation in all circumstances
- subsystem reliability is maximised by a comprehensive redundancy, including 'hot' redundancy (backup units actually on standby) for functions which are essential for a continuous, uninterrupted operation during critical mission phases
Mechanical Design Overview
The Rosetta design is based on a box-type central structure, 2.8 m × 2.1 m × 2.0 m, on which all subsystems and payload equipment are mounted. The two solar panels have a combined area of 64 m², with each extending panel measuring 14 m in length.
The 'top' of the spacecraft accommodates the payload instruments, and the 'base' of the spacecraft the subsystems. The spacecraft can be physically separated into two main modules:
- a Payload Support Module (PSM)
- a Bus Support Module (BSM)
The Lander is attached to the face opposite the two-axes steerable high-gain antenna. The two solar wings extend from the side faces. The instrument panel points almost always towards the comet, while the antennas and solar arrays point towards the Sun and Earth (at such great distances the Earth is relatively speaking in the same direction). The spacecraft attitude concept is such that the side and back panels are shaded throughout all nominal mission phases, offering a good location for radiators and louvres. This will normally be facing away from the comet, minimising the effects of cometary dust.
The spacecraft is built around a vertical thrust tube, whose diameter corresponds to the 1194 mm Ariane-5 interface. This tube contains two large, equally sized, propellant tanks, the upper one containing fuel, and the lower one containing the (heavier) oxidiser. At launch the total amount of stored propellant was roughly 1670 kg.