  Aerospace technologies are crucial for modern civilization; space-based
infrastructure underpins weather forecasting, communications, terrestrial
navigation and logistics, planetary observations, solar monitoring, and other
indispensable capabilities. Extraplanetary exploration -- including orbital
surveys and (more recently) roving, flying, or submersible unmanned vehicles --
is also a key scientific and technological frontier, believed by many to be
paramount to the long-term survival and prosperity of humanity. All of these
aerospace applications require reliable control of the craft and the ability to
record high-precision measurements of physical quantities. Magnetometers
deliver on both of these aspects, and have been vital to the success of
numerous missions. In this review paper, we provide an introduction to the
relevant instruments and their applications. We consider past and present
magnetometers, their proven aerospace applications, and emerging uses. We then
look to the future, reviewing recent progress in magnetometer technology. We
particularly focus on magnetometers that use optical readout, including atomic
magnetometers, magnetometers based on quantum defects in diamond, and
optomechanical magnetometers. These optical magnetometers offer a combination
of field sensitivity, size, weight, and power consumption that allows them to
reach performance regimes that are inaccessible with existing techniques. This
promises to enable new applications in areas ranging from unmanned vehicles to
navigation and exploration.
