Cross-Scale

Mission: Overview

The science requirements of the Cross-Scale concept, combined with the aim of selection as a Cosmic Vision mission, lead to constraints on the mission design. Nevertheless, many options remain, as discussed below. All of these issues are discussed in more detail in the Science Priorities Document.

Orbit

The likely ESA launch vehicle is a Soyuz-Fregat from Kourou, which results in a larger in-orbit mass for an equatorial orbit than an inclined one. Apogee of at least 25 Earth radii would allow the spacecraft to pass through all regions of interest (magnetotail, magnetopause, magnetosheath, bowshock, solar wind). A 10 Earth radii perigee would allow magnetopause skimming, but at the expense of a lower mass than a 500 km perigee - while the latter would allow measurements in the radiation belts and an increase in telemetry.

Unlike Cluster or MMS, Cross-Scale would not require significant manoeuvers after the spacecraft are in orbit.

Mass

Expected on-orbit dry mass for a 10x25 Earth radii orbit is around 1050kg: for 12 spacecraft, this results in around 85kg per spacecraft, without propellant - obviously, this figure rises with fewer spacecraft.

Instruments

With more than one spacecraft design, it is possible to have several lighter spacecraft with fewer instruments (typically at large separations) and a smaller number of more comprehensively instrumented ones near the formation centre. In this way, the most detailed measurements can be made where they are needed most (for example, high time resolution electron distributions on electron scales). Indeed, it may even be possible to use a "plug and play" approach to instrument/spaceraft interfaces, allowing different instruments to be flown on different spacecraft of the same basic design.

Communications

The science goals of Cross-Scale require prodigious data downlink rates. It is not possible to sustain these rates and therefore a strategy of either selective download or selective measurement ("burst mode") will be required. We estimate that around 10% of data from each orbit will be returned.

Inter-spacecraft communication would allow accurate range estimates to be made, as well as facilitating downlink from a subset of spacecraft with higher telemetry rates. It would also allow formation-wide burst mode triggering based on real, rather than predicted, event occurrence.

Operations

With such a large number of spacecraft, it will be essential to streamline mission operations. Spacecraft commanding can be reduced by the enhancement of spacecraft autonomy. Manoeuvers and station-keeping will be required infrequently and large-scale orbit changes will never be needed.

Instrument operations must be reduced - ideally, instruments should have states of "On" and "Off."

The selection of data to downlink could be selected by pre-determined burst mode triggering, onboard triggering, or ground selection from a low resolution telemetered data set. Data selection in the latter case would be performed by scientists, probably daily.