European Solar Polar Orbiter Mission

Science Instruments

During this TRS a representative set of instruments for the mission were defined, together with basic system-level requirements. Thus, a strawman payload was implemented to allow mission analysis. The current strawman payload comprises six instruments, detailed in Table 2. A relatively simple package of plasma instruments is required to relate the properties of the solar wind at 0.48 AU to the solar and coronal features studied with the remote sensing instruments. However, the spacecraft field must be low (preferably < 1 nT), known and constant so the magnetometer data can be corrected on-board and used in reducing the plasma distributions from the three-dimensional measured distributions to the two-dimensional, field-aligned distributions to be returned to Earth. A deployable 2.5-m boom from the spacecraft was selected.
Table 2 Strawman payload budget

Component	CBE Mass (kg)	DMM (%)	Total Mass (kg)	Average Power (W)
Coronagraph	10.0	10.0	11.0	10
Extreme Ultraviolet Imager	10.0	10.0	11.0	10
Velocity and Magnetograph Imager	10.0	15.0	11.5	10
Plasma Analyzer	3.0	10.0	3.3	4
Magnetometer	2.0	10.0	2.2	4
Energetic Particle Telescope	2.0	10.0	2.2	4

We recall the transfer trajectory mission phase lasts approximately 5 years. The mission can thus be significantly enhanced if the science suite can be utilized during this period, while the spacecraft retains the sail. Use of the science suite during the transfer trajectory would enable a full comparison of the solar environment at all latitudes. It is anticipated that the solar sail will have pointing accuracy of approximately 1 deg due to sail flexing, while pointing knowledge of the science suite would be high as this can be determined by the spacecraft’s attitude and orbit control system (AOCS). Pointing stability of the science suite is difficult to determine, although we note that the lowest structural mode frequency of a solar sail is typically below 0.1 Hz.^{7, 8} It is thus feasible that while attached to the sail the pointing stability over very short integration times may be compatible with the required instrument stability, allowing some low quality data to be generated by the imaging instruments during the transfer trajectory through the use of shortened integration times. Note however, on-board autonomy would be required to select the images which are of scientific use as sail flexing means the instrument field-of-view may be sub-optimally orientated. The pointing requirements of the plasma analyzer and magnetometer match the sail design specifications, however it is unclear if the local spacecraft environment will be suitable for use of such instruments.⁹ We note that the analysis within this paper assumes no science during the transfer trajectory.

Attitude and Orbit Control System

The AOCS is defined for the two distinct phases of being attached to the sail and then following sail jettison. The solar polar spacecraft is three-axis stabilized at all times, with a baseline pointing control of 360 arcseconds following sail jettison provided for the payload instruments that are considered to be always-on, such as the plasma analyzer and magnetometer. Higher, short term pointing stability is provided for the imagers. It was found that little gain was made by relaxing the nominal pointing requirement since the magnetometer has a required pointing stability of 360 arcseconds per second. Furthermore, maintaining pointing control at 360 arcseconds allows the X-Band HGA to be used at any time through the mission, for example in space weather applications, with no impact on the AOCS hydrazine budget used for attitude control following sail jettison.
Pointing knowledge is maintained by a combination of coarse Sun sensors, gyroscopes and star sensors, while attached to the sail. Recall, the lowest structural mode frequency of a solar sail is typically below 0.1 Hz,Error: Reference source not found^{, Error: Reference source not found} thus as the sail flexes the orientation of the thrust vector is fairly uncertain and highly accurate guidance becomes difficult, meaning that the sail will require many course corrections over the period of the transfer trajectory due to thrust vector misalignment errors. Pointing knowledge is maintained by star sensors and gyroscopes once the sail has been jettisoned and continuing through the science operations phase. Reaction wheels, which are unloaded through the use of a monopropellant system, maintain spacecraft pointing stability. It is not possible to utilize all instruments all of the time due to pointing requirements, however we wish to maximize the use of all instruments. Analysis of propellant requirements leads to the conclusion that a hydrazine system is preferable to a cold-gas system due to the reduction in propellant mass. We note that part of the AOCS propellant mass budget is a contingency propellant budget which is provided for use in the sail separation and avoidance maneuver, the specifics of which will be discussed later within this paper. The AOCS propulsion assumes a specific impulse of 200 seconds for pulse maneuvers and 230 seconds for longer duration burns, such as the sail separation and avoidance maneuver. The AOCS propellant mass total is 10.8 kg, as seen in Table 1.