Atmospheric Entry Probes in the Outer Solar System: Developing a Software Tool to Assess Trajectory Options

<p>The composition of outer planet atmospheres holds fundamental clues to understanding the formation and evolution of the solar system. Measurements of noble gas abundances and key isotope ratios help constrain formation models, and along with measurements of atmospheric structure and dynamics they reveal formation and evolutionary processes [1], [2]. These enable conclusions about giant planet formation and possible migration during the epoch of solar system formation. With the Galileo Probe laying the foundation of in situ atmospheric measurements of the outer planets by exploring Jupiter, entry probe missions to Saturn, Uranus and Neptune are essential to complete the picture of how our solar system evolved to its present state.</p> <p> </p> <p>During the development of entry probe missions, the interplanetary and probe approach trajectory, as well as the selection of the entry interface zone, are critical elements to mission success. Both elements are driven by considerations such as spacecraft safety (e.g. avoiding rings), while balancing science and engineering requirements at the same time (e.g. highly interesting science and entry zone vs. optimal communication geometry between probe and relay spacecraft). Due to the complexity of the problem, there is no analytical solution for finding the ‘best’ trajectory. Instead, one relies on the experience and intuition of mission designers to select a few possible interplanetary trajectories, which are then explored in detail to see how they meet science and engineering requirements. This approach leaves a huge trade space unexplored and may find a local, rather than a global optimum trajectory for the mission.</p> <p> </p> <p>We are addressing this gap by developing a software tool called <em>VAPRE</em> (<strong>V</strong>isualization of <strong>A</strong>tmospheric <strong>PR</strong>obe <strong>E</strong>ntry Conditions for different bodies and trajectories)<em> </em>[3], [4] that allows us to explore those previously unexplored trade spaces. <em>VAPRE</em> can process thousands of trajectories, significantly more than in the currently common mission design processes. Due to its flexible architecture, <em>VAPRE</em> can be adapted and extended to accommodate new science and engineering constraints for different or similar mission scenarios. In our talk, we will present an example of how the tool can be used to design a flyby mission to a giant planet that delivers an atmospheric probe considering opportunities between 2028 and 2042.</p>

Atmospheric Entry Probes for in situ Exploration of the Ice Giant Planets

<p>Understanding the formation and evolution of the solar system and the formation of the giant planets is constrained by inherent limitations in the capabilities of remote sensing. In situ exploration of planetary atmospheres provides key measurements not possible from remote observations, remarkably demonstrated at Jupiter by the Galileo probe, where key measurements included the determination of noble gas abundances and the precise measurement of the Jupiter helium mixing ratio. In this paper, we describe the primary scientific goals to be addressed by future in situ exploration of the ice giants Uranus and Neptune, placing in situ explorations of the gas giants, including the Galileo probe and a future Saturn probe, into a broader solar system context. An ice giant atmospheric entry probe reaching 10 bars would provide insight into both the formation history of the solar system and the giant planets, and the structure and composition of, and physical processes at play within ice giant atmospheres. An entry probe as an element of a future ice giant flagship mission would descend under parachute to measure the abundances and isotopic ratios of the noble gases, D/H in H<sub>2</sub> and <sup>13</sup>C/<sup>12</sup>C, and the thermal structure and dynamics from the upper atmosphere down to the deepest region from which the probe is able to return data, perhaps 10-20 bars or more. Probe data would be returned to Earth using a Carrier Relay Spacecraft as a relay station. The relay spacecraft, particularly if it is an orbiter with a suite of remote sensing instruments, can significantly enhance the science return from the probe; remote sensing provides the global context from which to understand the probe's local measurements of weather and cloud properties. One or more small atmospheric probes could represent a significant ESA contribution to a future NASA New Frontiers or Flagship Ice Giant mission.</p> <p> </p>

Entry Probe Radio Science for Measurements of Ice Giant Atmospheric Dynamics and Composition

<p>Planetary atmospheric winds, waves, tides, and turbulence represent a tie-point between planetary structure and processes, including atmospheric thermal and energy structure, cloud location and properties, and atmospheric composition and compositional gradients. The only direct means by which dynamics of an ice giant atmosphere can be measured along the probe descent path is via radiometric tracking of an ice giant entry probe. Additionally, measurements on an orbiter of the strength of a probe telecom signal can be used to provide the abundance of microwave absorbing molecules along the probe relay signal raypath, expected to be primarily ammonia (NH<sub>3</sub>) or hydrogen sulfide (H<sub>2</sub>S).</p> <p> </p> <p>Doppler tracking of a descent probe has been demonstrated with the Galileo probe at Jupiter and the Huygens probe at Titan. By including an ultrastable oscillator on both the transmit and receive sides of the probe telemetry relay signal, the time variation of the measured relay signal frequency provides a measure of wind speeds (via the Doppler effect). and the signatures of atmospheric waves, convection, and turbulence. In addition, other probe dynamical effects such as pendulum motion under the parachute, probe spin, and aerodynamic buffeting can be retrieved from careful analysis of the probe telecom signal Doppler residuals. Measurements made on board the orbiter of the time-varying received signal strength would provide a profile of microwave absorbing molecules along the probe radio signal raypath, complementing composition measurements made the probe mass spectrometer.</p> <p> </p> <p>The scientific objectives, measurement requirements, and expected measurement accuracies of the profile of zonal winds and atmospheric absorption will be discussed in this presentation, with a preliminary attempt to quantify the effect of uncertainties in the reconstruction of the probe descent and carrier overflight trajectories.</p>

Quadrupole Ion Trap Mass Spectrometer for Ice Giant Atmospheres Exploration

To date, a variety of different types of mass spectrometers have been utilized on missions to study the composition of atmospheres of solar system bodies, including Venus, Mars, Jupiter, Titan, the moon, and several comets. With the increasing interest in future small probe missions, mass spectrometers need to become even more versatile, lightweight, compact, and sensitive.For in situ exploration of ice giant atmospheres, the highest priority composition measurements are helium and the other noble gases, noble gas isotopes, including 3He/4He, and other key isotopes like D/H. Other important but lower priority composition measurements include abundances of volatiles C, N, S, and P; isotopes 13C/12C, 15N/14N, 18O/17O/16O; and disequilibrium species PH3, CO, AsH3, GeH4, and SiH4. Required measurement accuracies are largely defined by the accuracies achieved by the Galileo (Jupiter) probe Neutral Mass Spectrometer and Helium Abundance Detectors, and current measurement accuracies of solar abundances.An inherent challenge of planetary entry probe mass spectrometers is the introduction of material to be sampled (gas, solid, or liquid) into the instrument interior, which operates at a vacuum level. Atmospheric entry probe mass spectrometers typically require a specially designed sample inlet system, which ideally provides highly choked, nearly constant mass-flow intake over a large range of ambient pressures. An ice giant descent probe would have to operate for 1-2 hours over a range of atmospheric pressures, possibly covering 2 or more orders of magnitude, from the tropopause near 100 mbar to at least 10 bars, in an atmospheric layer of depth beneath the tropopause of about 120 km at Neptune and about 150 km at Uranus.The Jet Propulsion Laboratory’s Quadrupole Ion Trap Mass Spectrometer (QITMS) is being developed to achieve all of these requirements. A compact, wireless instrument with a mass of only 7.5 kg, and a volume of 7 liters (7U), the JPL QITMS is currently the smallest flight mass spectrometer available for possible use on planetary descent probes as well as small bodies, including comet landers and surface sample return missions. The QITMS is capable of making measurements of all required constituents in the mass range of 1–600 atomic mass units (u) at a typical speed of 50 mass spectra per second, with a sensitivity of up to $10^{13}$ 10 13 counts/mbar/sec and mass resolution of $m/\Delta m=18000$ m / Δ m = 18000 at m/q = 40. (Throughout this paper we use the unit of m/q = u/e for the mass-to-charge ratio, where atomic mass unit and elementary charge are $1~\text{u} = 1.66\times 10^{-27}~\text{kg}$ 1 u = 1.66 × 10 − 27 kg and $1\text{e} = 1.6\times 10^{-19}$ 1 e = 1.6 × 10 − 19 C, respectively.) The QITMS features a novel MEMS-based inlet system driven by a piezoelectric actuator that continuously regulates gas flow at inlet pressures of up to 100 bar.In this paper, we present an overview of the QITMS capabilities, including instrument design and characteristics of the inlet system, as well as the most recent results from laboratory measurements in different modes of operation, especially suitable for ice giant atmospheres exploration.

Science Goals and Mission Objectives for the Future Exploration of Ice Giants Systems - A Horizon 2061 Perspective

<p>The comparative study of the different planetary systems accessible to our observations is a unique source of new scientific insight: it can reveal to us the diversity of these systems and of the objects within them, help us better understand their origins and how they work, find and characterize habitable worlds, and ultimately, search for alien life in our galactic neighborhood. But, in the solar system itself, two of its secondary planetary systems, the ice giant systems, as well as their two main objects, Uranus and Neptune, remain poorly explored. We will present an analysis of our current limited knowledge of these systems in the light of six key science questions about planetary systems formulated in the “Planetary Exploration, Horizon 2061” long-term foresight exercise: (Q1) What is the diversity of planetary systems objects? (Q2) What is the diversity of their architectures? (Q3) What do we know of their origins and formation scenarios? (Q4) How do they work? (Q5) Do they host potential habitats? (Q6) Where and how to search for life?</p> <p>We will show that a long-term plan for the space exploration of ice giants and their systems, complemented by the combination of Earth and space-based observations, will provide major contributions to answers to these six questions. In order to do so, we identify the measurements that must be performed in priority to address each of these questions, the destinations to choose (Uranus, Neptune, Triton or a subset of them), and the combinations of space platform(s) (orbiter, atmospheric entry probe(s), lander…) and of  flight sequences needed.</p> <p>Based on this analysis, we look at the different launch windows available until 2061, using a Jupiter fly-by, to send a mission to Uranus or Neptune and find that:</p> <p>(1) a single mission to one of the Ice giants, combining an atmospheric entry probe and an orbiter tour starting on a high-inclination, low-periapse orbit, followed by a sequence of lower- inclination orbits, at least at one of the planets, will make it possible to address a broad range of these key questions;</p> <p>(2) a combination of two well-designed missions to each of the ice giant systems, to be flown in parallel or in sequence, will make it possible to address five out of the six key questions, and to establish the prerequisites for addressing the sixth one. The 2032 Jupiter fly-by window offers a unique opportunity to achieve this goal;</p> <p>(3) if this window cannot be met, using the 2036 Jupiter fly-by window to send a mission to Uranus first, and then the 2045 window for a mission to Neptune, will achieve the same goals. As a back-up option, the feasibility of sending an orbiter + probe mission to one of the planets and using the opportunity of a mission on its way to the interstellar medium to execute a close fly-by of the other planet and deliver a probe into its atmosphere should be studied carefully;</p> <p>(4) based on the expected science return of the first two missions, a third mission focusing on the search for life at a promising moon, namely Triton based on our current knowledge, or perhaps one of the active moons of Uranus after due characterization, can be properly designed.</p> <p>By the 2061 horizon, the first two missions of this plan can be implemented and the design of a third mission focusing on the search for life can be consolidated. Given the likelihood that such a plan may be out of reach of a single national agency, international collaboration is the most promising way to implement it.</p>

Quadrupole Ion Trap Mass Spectrometer for ice giant atmospheres exploration

<p>To date a variety of different types of Mass Spectrometers has been utilized on missions to study the composition of atmospheres of many solar system bodies including Venus, Mars, Jupiter, Titan, the moon and several comets. For in-situ exploration of ice giant atmospheres, the highest priority composition measurements are helium and the other noble gases, noble gas isotopes, and other key isotopes including <sup>3</sup>He/<sup>4</sup>He and D/H. Other important but lower priority composition measurements include abundances of volatiles C, N, S, and P, isotopes <sup>13</sup>C/<sup>12</sup>C, <sup>15</sup>N/<sup>14</sup>N, <sup>18</sup>O/<sup>17</sup>O/<sup>16</sup>O and disequilibrium species PH<sub>3</sub>, CO, AsH<sub>3</sub>, GeH<sub>4</sub>, and SiH<sub>4</sub>. Required measurement accuracies are largely defined by the accuracies achieved by the Galileo (Jupiter) probe Neutral Mass Spectrometer and Helium Abundance Detectors, and current measurement accuracies of solar abundances<sup>[1]</sup>.</p><p>The Jet Propulsion Laboratory’s Quadrupole Ion Trap Mass Spectrometer (QITMS)<sup>[2] </sup>is a compact, wireless instrument with a mass of only 7.5 kg, designed to meet these requirements and challenges specific to the planetary probe missions. It is currently the smallest flight MS available, capable of making measurements of all required constituents in the mass range 1-600Da, with a sensitivity of up to 10<sup>13</sup> counts/mbar/sec and resolution of m/∆m=12000 at 40Da.</p><p>During a fly-by or a descent mission, the time available to perform an in-situ measurement is usually short. This makes it challenging to measure the abundances of minor constituents for which long integration times are needed. Mass spectrometers largely employ a non-discriminatory electron impact ionization of sampled gas mixtures for creating ions, which means the probability to create and trap ion fragments of trace species is very low and further destabilized by space charge effects due to an excessive number of ions from dominant species. A selective resonant ejection technique was employed to lower the amount of major constituent species, while keeping the minor constituents intact, which resulted in higher accuracy measurements of minor species.</p><p>Another inherent challenge of planetary entry probe mass spectrometers is the introduction of material to be sampled into the instrument interior, which operates at vacuum. Atmospheric entry probe mass spectrometers typically require a specially designed sample inlet system, which ideally provides highly choked, nearly constant mass-flow intake over a large range of ambient pressures. An ice giant descent probe would have to operate over a range of atmospheric pressures covering 2 or more orders of magnitude, 100 mb to 10+ bars, in an atmospheric layer of ~120 km at Neptune to ~150 km at Uranus. The QITMS features a novel MEMS based inlet system driven by a piezo-electric actuator that continuously regulates gas flow at inlet pressures of up to 100 bar.</p><p>In this paper, we present an overview of the QITMS capabilities including instrument design and characteristics of the inlet system, as well as the most recent results from laboratory measurements in different modes of operation.</p><p>[1] Mousis, O., et al., Pl. Sp. Sci., 155 12–40, 2018.</p><p>[2] Madzunkov, S.M., Nikolic, D., J. Am. Soc. Mass Spectrom. 25(11), 2014.</p>

Accessible Latitudes for Planetary Entry Probe Missions to Saturn, Uranus or Neptune

<p>In-situ probe measurements of planetary atmospheres add an immense value to remote sensing observations from orbiting spacecraft or telescopes, as highlighted and justified in numerous publications [1,2,3]. Certain key measurements such as the determination of noble gas abundances and isotope ratios can only be made in situ by atmospheric entry probes, but represent essential knowledge for investigating the formation history of the solar system as well as the formation and evolutionary processes of planetary atmospheres. Following the above rationale, a planetary entry mission to one of the outer planets (Saturn, Uranus and Neptune) has been identified as a mission of high priority by international space agencies. In particular, an entry probe mission proposal to Neptune has been selected as a flagship mission study in the next NASA decadal survey.</p><p>Within the scientific frame of atmospheric planetary sciences, a two- to three-year research study called IPED (<strong>I</strong>mpact of the <strong>P</strong>robe <strong>E</strong>ntry Zone on the Trajectory and Probe <strong>D</strong>esign) investigates the impact of the interplanetary and approach trajectories on the feasible range of atmospheric entry sites as well as the probe design, considering Saturn, Uranus and Neptune as target bodies. The objective is to provide a decision matrix for entry site selection by comparing several mission scenarios for different science cases.</p><p>In this presentation, the focus is on approach circumstances of the planetary entry probe upon arrival at a normalized, spherical planet. Science objectives are organised in four (planetocentric) latitude ranges: (1) low latitudes < 15°, (2) mid latitudes between 15° and 45°, (3) high latitudes between 45° and 75° and (4) polar latitudes of > 75°. The latitude ranges are considered as potential entry zones for the implementation. The implementation strategy will be explained and discussed. Astrodynamically accessible latitudes are presented as a function of the approach velocity  vector v<sub>∞ </sub>(both declination of the approach asymptote and magnitude). A roadmap is shown that explains the next implementation step to include the physical characteristics of the destination planet such as the planet’s size, rotation period, shape, ring geometries and obliquity.</p><p>The presented research was supported by an appointment to the NASA Postdoctoral Program (NPP) at the Jet Propulsion Laboratory (JPL), California Institute of Technology, administered by Universities Space Research Association (USRA) under contract with National Aeronautics and Space Association (NASA). © 2020 All rights reserved.</p><p>[1] Mousis, O. et al., Scientific Rationale for Saturn’s in situ exploration, Planetary and Space Science 104 (2014) 29-47.</p><p>[2] Mousis, O. et al., Scientific Rationale for Uranus and Neptune in situ explorations, Planetary and Space Science 155 (2018) 12-40.</p><p>[3] Hofstadter, M. et al., Uranus and Neptune missions: A study in advance of the next planetary science decadal survey, Planetary and Space Science 177 (2019) 104680.</p>