The main benefit of an interstellar mission is to carry out in-situ measurements within a target star system. To allow for extended in-situ measurements, the spacecraft needs to be decelerated. One of the currently most promising technologies for deceleration is the magnetic sail which uses the deflection of interstellar matter via a magnetic field to decelerate the spacecraft. However, while the magnetic sail is very efficient at high velocities, its performance decreases with lower speeds. This leads to deceleration durations of several decades depending on the spacecraft mass. Within the context of Project Dragonfly, initiated by the Initiative of Interstellar Studies (i4is), this paper proposes a novel concept for decelerating a spacecraft on an interstellar mission by combining a magnetic sail with an electric sail. Combining the sails compensates for each technologys shortcomings: A magnetic sail is more effective at higher velocities than the electric sail and vice versa. It is demonstrated that using both sails sequentially outperforms using only the magnetic or electric sail for various mission scenarios and velocity ranges, at a constant total spacecraft mass. For example, for decelerating from 5% c, to interplanetary velocities, a spacecraft with both sails needs about 29 years, whereas the electric sail alone would take 35 years and the magnetic sail about 40 years with a total spacecraft mass of 8250 kg. Furthermore, it is assessed how the combined deceleration system affects the optimal overall mission architecture for different spacecraft masses and cruising speeds. Future work would investigate how operating both systems in parallel instead of sequentially would affect its performance. Moreover, uncertainties in the density of interstellar matter and sail properties need to be explored.
The Msail (Magnetic Sail) consists of a superconducting coil and support tethers which connect it to the spacecraft and transfer the forces onto the main structure. The current through the coil produces a magnetic field. When the spacecraft has a non-zero velocity, the stationary ions of the interstellar medium are moving towards the sail in its own reference frame. The interaction of ions with the magnetosphere of the coil leads to a momentum exchange and a force on the sail, along the direction of the incoming charged particles.
According to Zubrin, the current densities of superconductors can reach up to jmax = 2 · 10^10A/m2 and this is the value used in the analysis. For the material of the sail, the density of common superconductors like copper oxide (CuO) and YBCO was used, with ρMsail = 6000 kg/m3.
The main disadvantage of the magnetic sail is also evident when taking the force formula into account. At lower speeds, the force keeps getting reduced asymptotically, and hence the effect of the Msail at these velocities becomes negligible. This has as consequence that reaching orbital speeds (10-100 km/s) requires long deceleration duration.
In the present work, the power system for the Esail was modeled with a specific power supply of 50 W/kg. Although the details of the power system were not part of this analysis, photovoltaic cells could be used, utilizing the laser beam power in combination with radioisotope thermoelectric generators and batteries. Another option is the use of electromagnetic tethers as an energy source, by means of electromagnetic induction.
It becomes clear that the Esail has a disadvantage when dealing with high speeds, because of the very high voltage and consequently system mass needed. For that reason, an additional system would be necessary for the initial deceleration from the high cruising speeds until the point where an optimally designed Esail can take over.
Missions to neighboring star systems require high cruising speeds in order to reduce the total trip duration. There have been proposals based on fusion propulsion that aim to keep the total mission duration underneath 100 years, which means that an average speed bigger than 0.0435 c is necessary in the case of Alpha Centauri. The present analysis focuses on missions with the objective of performing scientific measurements in the target system, hence requiring orbital insertion around a star or a planet. In this context, the combination of Msail and Esail seems to be an elegant solution.
Starting the deceleration phase of the mission with the use of a magnetic sail is beneficial, due to the high forces produced in the large velocity range. As the velocity decreases, the force drops also and the Msail starts being ineffective. At this moment (which has to be optimally chosen as described later), the Msail can be switched off and detached from the spacecraft and the Esail can start operating. The electric sail must be designed to perform optimally in this velocity region and can decrease the velocity of the spacecraft further, until the required value for orbital insertion is achieved.
Electric space Sail
Magnetic Space Sail