  Fifty years after the Apollo program, space exploration has recently been
regaining popularity thanks to missions with high media coverage. Future space
exploration and space station missions will require specific networks to
interconnect Earth with other objects and planets in the solar system. The
interconnections of these networks form the core of an Interplanetary Internet
(IPN). More specifically, we consider the IPN as the combination of physical
infrastructure, network architecture, and technologies to provide communication
and navigation services for missions and further applications. Compared to the
current implementation of the Internet, nodes composing the core of the IPN are
highly heterogeneous (base stations on planets, satellites etc.). Moreover,
nodes are in constant motion over intersecting elliptical planes, which results
in highly variable delays and even temporary unavailability of parts of the
network. As such, an IPN has to overcome the challenges of conventional
opportunistic networks, with much higher latency and jitter (from a couple of
minutes to several days) and the additional constraint of long-term autonomous
operations. In this paper, we highlight the challenges of IPN, demonstrate the
elements to deploy within the areas of interest, and propose the technologies
to handle deep space communication. We provide recommendations for an
evolutionary IPN implementation, coherent with specific milestones of space
exploration.
