## 1 Introduction  
Exploration of Mars with a human crew is widely viewed as the ultimate culmination of planetary exploration [1]. [PERSON] (2001) wrote a superb history of planning activities for missions to send humans to Mars [2]. In addition, [PERSON] (2001) wrote a history covering 1952 to 1970 [3]. According to [PERSON]: \"More than 1000 piloted Mars mission studies were conducted inside and outside NASA between about 1950 and 2000. Many were the product of NASA and industry study teams, while others were the work of committed individuals or private organizations. Due to space limitations, only 50 mission studies (one per year, or less than 5 percent of the total) are described in this monograph. The studies included are believedto be representative of most of the technologies and techniques associated with piloted Mars exploration.\"  
[PERSON] (2023) extended [PERSON]'s history another twenty years, from 2001 through 2022 [1]. As of about 2020, the consensus was that a long-stay human mission to Mars was the most feasible approach and provided by far the greatest return on investment. [PERSON], [PERSON], and [PERSON] (1978) published a pivotal paper in which they pointed out the potential savings by producing rocket fuel in situ from Mars resources for ascent from Mars, rather than bringing the rocket fuel from Earth [4]. They coined the phrase \"ISPP\" (In Situ Propellant Production), that was commonly used until about 2000. In the late 1990s (and beyond), several technology tasks were carried out related to \"ISPP\" and Mars mission planners typically incorporated ISPP in their plans for long-stay Mars missions [5]. Starting somewhere around the year 2000, [PERSON] at NASA JSC emerged as a leading NASA advocate for ISPP, and he produced a series of presentations over the years in which he broadened the concepts for utilization of in situ resources beyond propellant production, and he coined the now universally accepted phrase: \"In Situ Resource Utilization\" (ISRU). He and co-workers continue to advocate for a wide realm of ISRU activities on the Moon and Mars [6]. While [PERSON] continued to inform NASA on a wide array of potential ISRU opportunities, NASA did not develop Mars ISRU in a systematic way, and funding for specific tasks over the years was sporadic. That was until a major new investment was decided upon in 2014, leading to the MOXIE Project, which reached completion 30 September 2023 [7]. Despite the change in reference from ISPP to ISRU, the reality is that propellant production on Mars offers the most practical near-term advance in ISRU with significant demonstrable benefits, and essentially all the technical accomplishments in Mars ISRU (including MOXIE) since the original paper by [PERSON], [PERSON], and [PERSON] were aimed at propellant production on Mars. In the period 2020-2023, NASA revised its previous concepts for human exploration of Mars, substituting a speculative short-stay mission concept for accepted long-stay mission concepts. In the process, the Mars ISRU seems to have been sidestepped. It is not clear whether this mission concept will stand the test of time or be a temporary phase [8]. ISRU has much greater leverage for ascent propellants than lunar ISRU [1]. The \"gear ratio\" (mass in LEO required to deliver one mass unit of payload to destination) for transport of mass from Earth LEO to the lunar surface is about 2.5; each mass unit saved on the lunar surface saves 2.5 mass units in Earth LEO. The gear ratio for transport of mass from Earth LEO to Mars' surface is about 8-10; each mass unit saved on the lunar surface saves 8-10 mass units in Earth LEO. Ascent from the Moon requires about 4-6 tons of O\({}_{2}\) propellant, while ascent from Mars requires about 30 tons of O\({}_{2}\) propellant.