ON THE NECESSITY AND FEASIBILITY OF A MULTIPROBE MISSION TO STUDY THE EARTH'S MAGNETOSPHERE

OBJECTIVES OF THIS WRITEUP:

Synthesizing multi-point observations from the magnetosphere is the only way that we can "image" its vast volume, monitor its rapid topological reconfigurations while at the same time retain accurate knowledge of its local properties. Such observations hold the key to resolving the outstanding problems in the astrophysical laboratory closest to human space activity. It is the objective of this WEB page to convince the reader that such a mission is not only necessary but imminent with current technology, or technology that will be available in the next two years.

It is the contention of this writeup that the major scientific objectives of the field of magnetospheric physics can be accomplished with the deployment of a large but not inordinate number automomous micro-satellites (probes) to monitor the Earth's magnetosphere. A preliminary estimate puts the number of probes necessary to answer the major questions in magnetospheric physics to ~60, but this number is naturally subject to debate and change based input from the scientific community. The probes can comprise a magnetospheric laboratory whose data will be shared by the community in an automated fashion and in common format. They will perform plasma and magnetic field measurements that are necessary and sufficient for the determination of the energy and magnetic flux transport through the magnetospheric system. More instruments may need to be included if deemed necessary by the community and if possible without jeopardising the primary goal of the mission to simultaneously probe with sufficient resolution many parts of the system. Global information on particle flow, energy flow density, and magnetic flux transport can be obtained by inverting the instantaneous point measurements as "pixels" of a three-dimensional "image".

The zero level mission is the study of the most probable (quiescent) and the most dynamic (substorm) states of Earth's magnetotail but with significant scientific return also on the physics of the magnetopause, magnetosheath and boundary layers. The probes can be raised in their final elliptical orbits with perigee ~3 Re and apogee ranging from 12 to 42 Re in two stacks of 30 probes each by two ion engines. Each probe will weigh ~5 kg. Each stack of probes, including the ion engine will weigh ~240 kg. This strawman mission is a MIDEX-class mission (~$47 M) which can be launched on a Med-Lite launch vehicle.

RELEVANCE TO NASA'S SPACE PHYSICS PROGRAMS:

A multiprobe mission is directly aligned with all three stated NASA goals:

(1) "Global magnetospheric imaging". It will provide images of the magnetospheric system, that will represent in situ measurements of the quantities of interest.

(2) "Explore, use and enable the development of space for human enterprise" It will provide the vital link between the present, statistical description of the magnetosphere and the needed time dependent, 3-dimensional description. It will provide the necessary datasets to lead to a predictive capability of the system's behavior.

(3) "Research, develop, verify and transfer advanced technologies" The multiprobe strawman mission described in this writeup is the beneficiary of concurrent development by industry and NASA on ion propulsion, solar power and battery technology. It provides a new approach to mission design, whereby spacecraft components and instrument complements are viewed in an integrated fashion.

THE MAJOR STEPS BETWEEN THIS WRITEUP AND A FUTURE MISSION:

It is necessary to bring together expertise from academia, industry and NASA centers to perform a feasibility and cost-benefit analysis of the proposed mission and mission alternatives. Can this idea materialize with current or near-future software and technology? Here are the steps that can be taken to address questions that go beyond this writeup.

* Use magnetospheric field modelling and 3-dimensional MHD simulations of the magnetosphere under different solar wind conditions to determine the optimum probe number density and orbital configuration that can achieve the scientific objectives at a minimum cost.

* Utilize existing data assimilation techniques to invert the probe measurements of the simulated magnetosphere and produce a 3-dimensional reconstruction of the magnetospheric magnetic field, flow field and the resulting energy and flux transport. In the process develop data visualization tools that borrow extensively from simulations, meteorology and oceanography.

* Decide on which instruments are absolutely necessary to perform basic characterization of the magnetospheric medium and energy transport/enegrgization processes? (Are electrons, heavy ions, energetic particles necessary? For what mass/telemetry/$ cost ?)

* Although minimal control operations are required, show how the probes can be commanded in the event of a possible mulfunction. Determine most vulnerable parts of the system and design a method to restart operation in the event of a problem, at a minimal cost. Even with zero-to-minimal attitude/orbit control, the subliminal question remains: can one REALLY control 60 multiprobes at a non-prohibitive cost??

* Simulate data acquisition and dissemination process.

* Determine the components, weight, mass, power and design cost of a fully integrated probe. Design the bus around the instrument complement, rather than fitting the instruments on a generic bus.

* Determine the preliminary design and cost of a probe dispenser that fits the mission specifications, the ion engine and the possible launch vehicle fairings.

* Determine launch vehicles that can achieve the mission requirements but minimize total mission cost, by using realistic numbers on fairing and payload configuration.

* Perform similar cost-benefit analysis of alternative missions.