  Astrophysics spans an enormous range of questions on scales from individual
planets to the entire cosmos. To address the richness of 21st century
astrophysics requires a corresponding richness of telescopes spanning all bands
and all messengers. Much scientific benefit comes from having the
multi-wavelength capability available at the same time. Most of these bands,or
measurement sensitivities, require space-based missions. Historically, NASA has
addressed this need for breadth with a small number of flagship-class missions
and a larger number of Explorer missions. While the Explorer program continues
to flourish, there is a large gap between Explorers and strategic missions. A
fortunate combination of new astrophysics technologies with new, high capacity,
low dollar-per-kg to orbit launchers, and new satellite buses allow for cheaper
missions with capabilities approaching strategic mission levels. NASA has
recognized these developments by calling for Probe-class mission ideas for
mission studies, spanning most of the electromagnetic spectrum from GeV
gamma-rays to the far infrared, and the new messengers of neutrinos and
ultra-high energy cosmic rays. The key insight from the Probes exercise is that
order-of-magnitude advances in science performance metrics are possible across
the board for initial total cost estimates in the range 500M-1B dollars.
