Cross-Scale

Mission scenarios

Meeting the science objectives of multi-scale coupling requires multiple spacecraft on multiple scales. This poses numerous challenges in terms of launch, orbit, spacecraft bus, payload, and operations, all of which have cost implications. With a goal of an ESA Cosmic Vision Small to Medium size mission (300M Euros) the Cross-Scale team has considered a mission based on a Soyuz-Fregat launch vechicle. To reach the important regions of the magnetosphere and solar wind where shocks, reconnection and turbulence occur requires an orbit apogee of 25 Earth radii. Estimates then suggest that the available total dry spacecraft mass, excluding propulsion and dispenser system, is roughly 1050kg. (see the Science Priorities Document for this and related estimates).

The simple concept

A simplistic interpretation of the need to resolve the fluid, ion, and electron scales simultaneously and separate length and time results in a set of four spacecraft at each scale, as demonstrated by Cluster and adopted by MMS. Full contextual information at each level in the hierarchy suggests that each scale is nested within the larger scale, thus requiring 12 spacecraft as illustrated. Launched on a single Soyuz-Fregat, these would have an average dry mass of 85kg. Making all the necessary measurements, especially the detailed measurements at the smallest scales, within this mass budget would be challenging. Acquiring a second Soyuz-Fregat launch would solve this problem and provide perhaps the possibility to increase beyond the minimal 12-spacecraft configuration to one that provides elements of redundancy and places fewer demands on maintaining specific constellations of the spacecraft. However, a dual Soyuz-Fregat launch almost certainly pushes the mission envelope above the Cosmic Vision Small or Medium class.

A first-order option

Some respite would be offered by re-using a spacecraft at more than one scale. This concept, also illustrated, reduces the number of spacecraft to 10, at the expense of moving the smallest scale measurements to one corner of the configuration. This reduces the level and confidence of the contextual data taken at the larger scales. Preliminary considerations suggest this scenario is feasible and would achieve many of the science objectives.

A hybrid approach

Other communities are also interested in the physics of cross-scale coupling in collisionless plasmas. A Japanese proposal, SCOPE, involves a highly capable mother ship surrounded by smaller daughters. Such a mission, in partnership with an ESA Cross-Scale complement of spacecraft, provides benefits in terms of both science capabilities and spacecraft numbers, as the Japanese spacecraft would be launched separately. This attractive scenario might meet the requirement of 12 spacecraft and comprehensive payload through careful planning and inter-agency (ESA-JAXA) cooperation. For example, JAXA might be able to launch the mother, a daughter, and perhaps up to two ESA-built daughters. This would leave 8 further spacecraft to be launched aboard a Soyuz-Fregat.