First Int'l. Orbital Debris Conf. (2019) 6110.pdf Space Sustainability in Martian Orbits — First Insights in a Technical and Regulatory Analysis Isabell Suchantke(1), Alexander Soucek2), Francesca Letizia3), Vitali Braun4), Holger Krag5) (1) M.Sc. cand. at TU Berlin, Straße des 17. Juni 135, 10623 Berlin, DE, [email protected] (2) ESA International Law Division, Keplerlaan 1, POBox 299, 2200 AG Noordwijk, NL, [email protected] (3) IMS Space Consultancy GmbH, Robert-Bosch-Str. 5, 64293 Darmstadt, DE, [email protected] (4) IMS Space Consultancy GmbH, Robert-Bosch-Str. 5, 64293 Darmstadt, DE, [email protected] (5) ESA Space Safety Programme Office, Robert-Bosch-Str. 5, 64293 Darmstadt, DE, [email protected] ABSTRACT Hazards from the outer space environment either natural (space weather and asteroids) or artificial (space debris and the growing number of satellites launched to orbit) pose a rising risk to space flight activities. The awareness for space sustainability and space safety has seen a continuous increase in recent years and does not stop at the Earth's sphere of influence. The first spacefaring nations start thinking about sustainability in cislunar space and the Martian environment. This work deals with the issue of space debris in Martian orbits in the light of planetary protection. A Mars Sustainability Framework has been developed. This includes a study on the orbital and regulatory environment of Mars, long-term propagation of orbits of artificial objects and the two natural moons, and the analysis of objects evolution and first approaches for collision probability computation. With this work, the issue of space debris beyond Earth orbit is analysed at an early stage. 1 INTRODUCTION AND MOTIVATION Outer space became a fundamental resource to human beings, which needs to be protected to preserve its benefits. Current space flight activities are mainly concentrated in Earth orbits. However, deep space exploration has been of interest to space nations since the dawn of the space age. And in recent years, new initiatives focusing on extraterrestrial exploration have risen again also with focus on Mars. Missions to Mars could not only help in searching for life [1], but also represent a deep interest of human beings to explore and expand. Though most of the space flight activities already respect the Principle of Planetary Protection (PPP) (to avoid forward and backward contamination), debris is produced all over our solar system and beyond (Voyager space probes [2]). Human-made objects in orbit sooner or later become debris unless they are disposed of right away. Around Earth we are capable of monitoring the orbital environment using telescopes and radars. Reference [3] describes the space situational awareness in Earth’s vicinity at its current status and argues for an international cooperation for deep space traffic management. As there is no observation infrastructure in the Martian environment, the position of objects around Mars is only known by telemetry data (if still available). Together with increasing interest in Mars exploration, this raises the question about sustainability. Especially in recent years, sustainable space flight activities became a major point in international discussions. Corresponding guidelines have been established. It needs to be investigated whether the existing guidelines also apply to Mars, or whether new or different guidelines need to be established. And how would they look like? Would they interfere with existing principles? For that an analysis of the Martian orbital environment is required. What is the current orbital status? What might it look like in some decades from now? Does orbital congestion pose potential risk to the future of Mars exploration? Until now, questions about space debris beyond Earth orbit mainly addressed the Lagrange points [4], or lunar orbits [5]. However, the orbital debris situation around Mars has not yet been investigated in detail. This work represents first insights in the topic of Martian sustainability, thus the focus for now lies on space debris and orbital congestion even though sustainability includes much more than that. Section 2 provides an overview of the orbital-, section 3 of the regulatory environment of Mars. A complete framework has been developed for the investigation of the orbital environment around Mars with regard to the problem of space debris, including an atmosphere model and potential break-up modelling. The Martian Sustainability Framework and a perturbation analysis are presented in section 4. The preliminary results are detailed in section 5. The on-going and future work will be addressed in the outlook (section 6). First Int'l. Orbital Debris Conf. (2019) 6110.pdf 2 ORBITAL ENVIRONMENT OF MARS 2.1 Historic, current and future launch traffic Mars is the most Earth-like planet in our solar system. It provides reasonable gravitation, a thin atmosphere and temperature and weather conditions that could eventually enable living conditions through technology in the upcoming century. The peak of Mars’ interest was in the 70s, but current plans and ideas indicate an increasing number of future Mars missions. Also, Mars’ moons (Phobos and Deimos) are a target of future science missions, and may also serve as communication or infrastructure spots towards human exploration of the red planet [6]. Figure 1 shows the space actors that successfully placed orbiters around Mars in the specific epochs since the dawn of the space age. The dashed bar on the right indicates the planned orbiters to be launched in the 2020s. It can be seen that there is a slight increase since the 1990s. Here, the emphasis is on the fact that these are only Fig. 1. Successful placement of orbiters in Martian orbit by different states or space agencies over the decades from the 1960s until 2020s. confirmed figures for the future trends. News and headlines cover many more ideas from small telecommunications orbiters in areostationary orbit (the Mars counterpart of Earth’s geostationary orbit) [7] and joint projects for sample return [8] all the way to prepare human settlement on Mars [9]. Also, more and more countries indicate interest in Mars exploration: e.g. the United Arab Emirates [10], Japan [11], China [12] and Finland (just proposal status until now) [13]. The trend of miniaturisation has also been developing for missions to Mars, with recent proposals that include CubeSats [14], [15]. Eight out of 14 orbiting man-made objects in Martian orbit are inactive, so one could say more than half of the population around Mars is already debris. Table 1 compares these numbers with respect to the ones referred to the Earth orbit. Tab. 1. Satellites in numbers in Earth and Mars orbit, correct information as of January 2019. Earth orbit [16] Mars orbit Spacecraft placed in orbit since 1957 8,950 14 Number of defunct spacecraft 3,050 8 Debris objects tracked and monitored 22,300 0 Mass in orbit (tons) > 8,400 < 27 The position of active satellites in Martian environment is known through telemetry data, however the position of by now inactive probes is not known, nor what happened to them after completion of operation. Last available orbit data are considered here for defunct spacecraft and mission-related objects, orbit data from 1st of January 2019 for all active probes. Figure 2 and 3 show the inclinations and eccentricities respectively over semimajor axis for all objects in Mars orbits. These include all orbiters and mission-related objects (e.g. the autonomous propulsion unit (ADU) of Fobos-2, the bioshield bases of the Viking orbiters, MAVEN’s break-off cap) [3]. The majority of objects First Int'l. Orbital Debris Conf. (2019) 6110.pdf is located in eccentric (0.4-0.9) low Mars orbits (LMO) with inclinations between 30° and 100°. Only a few objects move on circular orbits; only two in the equatorial plane. For better visualisation of all objects, the axes are interrupted. Figure 2 provides a colour code depending on the orbiter mass. Only one orbiter is below 1 ton. So far there are no micro or nano satellites in Mars orbits (no spacecraft up to 150 kg). Figure 3 presents the colour-coded area-to-mass ratio of the objects. Fig. 2. Inclination over semimajor axis of Mars Fig. 3. Eccentricity over semimajor axis of Mars objects. The colour code indicates the mass. objects. The colour code indicates the area-to-mass ratio. The different physical and orbital characteristics of Mars lead to a slightly different behaviour of orbiting objects. The smaller gravitational force (at Mars’ surface about one third of the Earth’s surface gravity field) leads to smaller orbital velocities for similar orbital altitudes (2.8-3.5 km/s in LMO). This becomes important when talking about potential collisions of objects in Martian orbits and the resulting impact velocity. As Mars orbits the Sun in about 1.5 AU distance, the solar radiation pressure is about 45% of the solar radiation pressure in Earth’s vicinity. This means that its perturbing effect on trajectories of space objects in orbit around Mars has a different relative importance with respect to Earth orbits. Similar reasoning applies to the effect of atmospheric drag. The Martian atmosphere’s surface pressure at mean radius ranges from 4 mbar up to 9 mbar [17] whereas the Earth’s atmosphere has 1,013 mbar [18], so the Mars atmosphere is roughly 100-250 times thinner. That leads to a significantly smaller influence of the atmospheric drag on spacecraft trajectories, resulting in about 200 times longer orbital lifetimes compared to Earth satellites at similar height between 300–500 km at peri centre and the same eccentricity � (� > 0.01). For circular orbits the lifetime can be 300-800 times longer around Mars for low solar activity [19]. 2.2 Space debris sources comparison Since the launch of Sputnik-1 in 1957, many different sources have led to an increasing number of debris objects in the Earth’s orbital environment.