Very low frequency (VLF) waves (about 3–30 kHz) in the Earth’s magnetosphere interact strongly with energetic electrons and are a key element in controlling dynamics of the Van Allen radiation belts. Bistatic very low frequency (VLF) transmission experiments have recently been conducted in the magnetosphere using the high-power VLF transmitter on the Air Force Research Laboratory’s Demonstration and Science Experiments (DSX) spacecraft and an electric field receiver onboard the Japan Aerospace Exploration Agency’s Arase (ERG) spacecraft. On 4 September 2019, the spacecraft came within 410 km of each other and were in geomagnetic alignment. During this time, VLF signals were successfully transmitted from DSX to Arase, marking the first successful reception of a space-to-space VLF signal. Arase measurements were consistent with field-aligned propagation as expected from linear cold plasma theory. Details of the transmission event and comparison to VLF propagation model predictions are presented. The capability to directly inject VLF waves into near-Earth space provides a new way to study the dynamics of the radiation belts, ushering in a new era of space experimentation. Graphical Abstract

Fundamental big-picture challenges face the enterprise of burgeoning space test, which may also have an impact on the syllabus and training of a possible future Space Test Pilot School. Test fundamentals and test conduct under stresses of time and fuel will be where the rubber meets the road. This paper specifically discusses four of those challenges, and their impact on US Space Force operators. (1) Testing limits of space capabilities. Space vehicles have traditionally imposed very strict operating limits. A culture shift from the top-down will be required before space testers are allowed to test spacecraft limits as freely as air-breathing test pilots do today. (2) Permitted procedures. There are a limited number of contingency procedures at a space operator's disposal, further constrained within themselves. (3) Safety nets. A discussion of space-specific methods that mitigate risks in operations, and the need for more agile fault-detection. (4) Operator proficiency. The qualifications and recency of a tester at the controls must be considered in the context of a single misstep potentially resulting in the entire asset being lost. We suggest that hours in high-risk test per spacecraft may become the defining factor for high-risk space test execution.