To address the Daedalus Science questions, there is a need for measurements at different altitudes throughout the Lower Thermosphere-Ionosphere and down to altitudes where key electrodynamics processes such as Joule heating and Energetic Particle Precipitation maximize, in the 100-200 km region, for an extended time period. Thus the Daedalus satellite will sample the upper atmosphere at altitudes that are extremely low for a satellite. But due to air drag, it can not stay at those altitudes all the time. The satellite will therefore have an elliptical orbit, with a perigee that reaches as low as possible in the 100-200 km region; preliminary orbital simulations indicate that a nominal perigee of 140 km is feasible for a prolonged mission. In order to perform measurements below the “observation barrier” of 140 km, in the preliminary system concept the spacecraft performs several perigee descents to lower altitudes, down to altitudes below 120 km by use of propulsion. A mission lifetime of at least three years is baselined. Most dynamic processes in the LTI, and in particular Joule heating, maximize at high latitudes; thus, a high inclination orbit is preferred.
The most important measurements of Daedalus will be made around the lowest point in its orbit, the perigee, indicated in the animations below in green. In order to cover all local times and latitudes, we will make use of the natural precession of the perigee, which is caused by the Earth's gravity field.
This animation shows an example of how the sampling close to perigee relates to the variations of the seasons, as well as the local-time- and latitude-dependent illumination of the upper atmosphere.
This animation shows the same simulation, but now constantly viewed from the dawn side of the Earth, instead of by keeping the perigee always in view of the camera.
The exact parameters of the Daedalus orbit, and how they affect the sampling, are currently still under study.