were launched in March 2011 as secondary payloads on the ill-fated Glory launch, and several other ELaNa follow-on missions are in the launch queue. To date 35 satellite projects have been selected for manifesting as secondary payloads under the ELaNa program. A new call for proposals was announced (July 2011) that entertains flight opportunity proposals for 6-U CubeSats, in addition to the traditional 1-U, 2-U, and 3-U form factors. Launch opportunities in the future are expected to increase, especially for certain orbits. Two large spacecraft are under development that will resupply the International Space Station (ISS)—the Dragon spacecraft of Space-X (Space Exploration Technologies Corporation) and the Cygnus spacecraft of Orbital Sciences Corporation. Both Space-X and Orbital have initiated programs to carry CubeSats on these ISS resupply missions. Indeed, the ISS itself could be used to store and launch on demand any number of CubeSats. Such a capability would allow quick responses to geophysically interesting events, or, for example, the measured deployment of CubeSats to form a thermosphere constellation. The feasibility of launching CubeSat-like satellites from the ISS has been demonstrated multiple times through the micro-electromechanical systems-based PicoSat Inspector program for the Aerospace Corporation.
Satellites with bigger volumes than CubeSats, but still well below a standard Explorer size, could revolutionize research in AIM and other science disciplines. Because of their substantially smaller cost, such tiny Explorers could be built with a much higher risk-tolerance and could carry a small number of instruments, or novel low-weight sensors, into interesting locations of the space environment. Tiny Explorers would still be carried as secondary payloads and would share some commonalities. However, they would open a new set of opportunities for longer-duration measurements.
Constellations of measurement platforms of various sizes have the potential to take advantage of miniaturization technologies, enhanced computational capabilities, and autonomous systems, as well as novel system and network approaches. The value of many breakthrough solar and space physics constellations strongly depends on the number of elements in the system. With the advent of small CubeSats and tiny Explorers, novel means of investigations are enabled that have the potential to provide an unprecedented density of measurements in the upper atmosphere and elsewhere.
Illustrative Examples of Newly Enabled Constellations
Magnetosphere Radiation Belt Constellation
A 6- to 12-month database of continuous ultralow-frequency measurements of the azimuthal mode number spectrum would provide a critical missing piece of information for radiation belt modelers. A constellation of ~30 CubeSats equipped with fluxgate magnetometers in a circular orbit near the geomagnetic equatorial plane (e.g., geosynchronous) would be an ideal platform for such a radiation belt study.
Ionosphere-Magnetosphere Coupling Constellation
A constellation mission utilizing small satellites would radically improve understanding of the dynamics of the coupled thermosphere-ionosphere system. In order to achieve this goal, the following top-level requirements must be met:
1. Each satellite needs to be complemented with instrumentation that is capable of measuring both the neutral and ion state variables, such as density, temperature, and winds. There are a few different instru-