Coming now to the more technical challenges, propulsion systems performance is a significant hurdle to overcome in the space sector (Turner et al., 2009).
Starting with the launch vehicles, their capabilities (broadly speaking, payload size, and thrust) have essentially plateaued, as only relatively minor incremental progress has been made in the last decades. Indeed, materials have improved with the introduction of composites with mechanical properties far superior to the typical alloys that were used at the beginning of the space age. Design and manufacturing techniques have also improved, with progress in software simulation enabled by the extraordinary growth of computer power, or new manufacturing methods such as additive manufacturing. Guidance and control systems have also improved thanks to advances in electronics and software. However, besides the push toward green propellants (Gohardani et al., 2014), nothing substantial has changed with the solid or liquid propellants performance and related technologies, which are key to overall launcher capability (Devezas, 2018). Reusable launchers are being used by a few companies in order to reduce costs or increase launch frequencies, and it is undeniable that costs have slowly come down, although this is largely due to combinations of countries' policies and market forces, but truly economic access to space is yet to be achieved.
On a longer timescale, the challenge is to develop and implement technologies, such as hypersonic air breathing rocket engines2, to be used in hybrid launchers to cut the need for large amounts of oxygen that have to be carried by current vehicles. Launch vehicles that could take off and land as aircraft, without the need for extensive and expensive service between missions, should also be developed. Similarly, in-space propulsion offers opportunities for improvement, in particular on Electric propulsion systems, which are also hybrid systems that would utilize different modes of operation (Levchenko et al., 2020).
Propulsion systems performance are also fundamental to interplanetary missions in terms of enabling faster travel and larger payloads to be delivered where required. Our current limitations in the manned exploration of the solar system are mainly due to the length of travel, which is directly related to the level of performance that is available from existing propulsion systems. Similarly, propulsion systems performance limits our exploration and exploitation capabilities (also for robotic activities) as it is a significant limitation to the mass of payload that can be safely transported to/from other celestial bodies.
Protection of Humans
Strictly related to human exploration are space health and medicine (Hodkinson et al., 2017), to enable humans to withstand the space environment for long periods, and the creation of artificial habitats in space and on other planets to support a reasonable quality of human life. The challenge here consists in the creation of a whole artificial environment to support people's well-being and physical and mental health, with means protecting against the negative effects of the space environment (Grimm, 2019). Whether we are considering a man-made vessel for long distance space travel or a space platform for large-scale human inhabitation or a planetary colony, some challenges overlap. These overlapping challenges include the need to create efficient closed loop systems to replenish resources and minimize waste, with the common goal to generate an artificial ecosystem for the long-term support of human life 3.