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Media propagation delay and delay-rate induced by the water vapor within the Earth's troposphere represent one of the main error sources for radiometric measurements in deep space. In preparation for the BepiColombo and JUICE missions, the European Space Agency has installed and operates the prototype of a tropospheric delay calibration system (TDCS) at the DSA-3 ground station located in Malargüe, Argentina. An initial characterization of the TDCS performance was realized using two-way Doppler measurements at X-band to perform the orbit determination of the Gaia spacecraft. This work will further characterize the system by analyzing two-way Doppler and range data at X- and Ka-band for 31 tracking passes of the BepiColombo spacecraft, which were recorded between March 2021 and February 2022 during the first two solar conjunction experiments. The performance exceeds the expectations based on the previous analysis, with a reduction of the Doppler noise of 51% on average and up to 73% when using the TDCS measurements in place of standard calibrations based on global navigation satellite system data. Furthermore, the campaign serves as validation of the TDCS operations during superior solar conjunctions, with most of the tracking passes at low elongation now satisfying the Mercury orbiter radioscience experiment requirements on two-way Doppler stability. These results, which are in line with those of similar instruments installed at other Deep Space Network antennas, are obtained using a commercial microwave radiometer with significantly lower installation and maintenance costs.

Hera is a European Space Agency (ESA) space mission, part of the Asteroid Impact and Deflection Assessment (AIDA) international collaboration with NASA, with a targeted launch to the Didymos system next October 2024. Following last 26 September 2022 extremely successful NASA’s DART impact on Dimorphos, the secondary body of the Didymos binary system, all the attention is now focused on the Hera mission. The main goals of Hera are the detailed study and characterization of DART’s crater, analysis of the impact in terms of momentum transfer efficiency, and accurate estimation of the physical properties of Didymos and Dimorphos, in order to validate and demonstrate the kinetic impactor technique to deflect potentially hazardous asteroids in the future. In this context, one of the main goals is to accurately estimate the mass and mass distribution of Didymos and Dimorphos, by means of radio science investigations. In particular, one of the very few direct measurements of the internal mass distribution of planetary bodies is the determination of their gravity field. The gravity of Didymos and Dimorphos will be estimated by precisely reconstructing the trajectory of Hera and the two companion CubeSats (Juventas and Milani) during a selected number of close encounters, by performing an orbit determination process. In particular, Hera will make use of an Inter-Satellite Link system (ISL) to track Juventas and Milani, measurements which will further improve Didymos’ system extended gravity field estimation. Furthermore, the Hera spacecraft is equipped with a Light Detection and Ranging instrument (LIDAR), a time-of-flight Planetary ALTimeter (PALT) that will measure the distances from the Hera spacecraft to the target body surfaces. The PALT altimetric measurements can be combined with Earth-based radiometric, ISL radiometric, and Hera optical observables to enhance the gravity science scientific parameters. This work discusses a covariance study of the Hera radio science experiment including crossovers estimation to the PALT LIDAR altimetry data. The trajectory constraints obtained from the radiometric tracking and optical data can be supplemented by altimetric crossovers, to further improve the reconstruction of the spacecraft trajectory with respect to the case without crossovers (i.e. PALT LIDAR altimetry data considered as single measurements, without estimating the surface landmarks probed multiple times by the LIDAR swaths). As a consequence of a better knowledge of Hera’s trajectory, the formal uncertainties of the scientific parameters of interest decrease, too. In particular, there is a potential further improvement of Dimorphos’ relative orbit estimation, as well as Didymos and Dimorphos gravity field, and their rotational state, with respect to the case without altimetric crossovers. © 2023 EG, RLM, MZ, and PT wish to acknowledge Caltech and the NASA Jet Propulsion Laboratory for granting the University of Bologna a license to an executable version of MONTE Project Edition S/W. Italian Space Agency (ASI) sponshorship acknowledged, Agreement No. 2022-8-HH.0 in the context of ESA’s Hera mission. EG is grateful to Fondazione Cassa dei Risparmi di Forlì for financial support of his PhD fellowship. This project has received funding from the NEO-MAPP project (European Union’s Horizon 2020 programme, agreement No 870377).

Comets, thought to be amongst the most primordial of Solar System objects, are distinguished by their activity, i.e. the insolation driven ejection of gas and dust from their surfaces. The exact mechanisms of the outgassing and dust ejection remain an important open question in planetary science, relating as it does to the structure, composition, and thermophysical properties of the surface material. Various thermophysical models (see e.g. [1,2,3]) have been proposed to explain the activity seen by Rosetta at 67P/Churyumov-Gerasimenko, with these often compared to the total outgassing rate. Another directly observable effect of the activity, however, is the resultant non-gravitational force and torque on the cometary nucleus, which can alter its trajectory and rotation state. Understanding the effect of non-gravitational forces on the dynamics of a particular comet therefore gives us a powerful additional tool to investigate its activity and surface properties.Here we present the latest work in an ongoing project, testing various activity distributions in an effort to fit the combined Rosetta outgassing, trajectory, and rotation data. We test a number of different activity distributions over the surface of the comet by varying the Effective Active Fraction (EAF) of facets on a shape model, relative to a pure water-ice emission. We investigate different spatial patterns in EAF, and attempt to correlate them to physical features on the cometary surface. In addition to the changes in rotation period examined in [4], we also compute changes in the rotation axis, using a method based on [5] in order to compare with the observations. This provides an additional constraint on the spatial distribution of activity. Trajectory information, in the form of Earth-to-comet range, is also compared with a recent re-analysis of Rosetta radio-tracking data [6].We are able to achieve a reasonable fit to the combined Rosetta data by parameterising EAF in terms of the different geological unit types on 67P (Fig. 1). This will have important implications for understanding how activity works on the different types of surface observed on cometary nuclei, including ‘rough’, ‘smooth’, ‘dusty’ and ‘rocky’ surface morphologies. The objective here is to constrain the shape of the activity curve on these various surfaces that a more detailed thermal model must produce in order to fit the data. We are in the process of implementing these more advanced thermal models [1,3] in our code, and will present results of an analysis of the non-gravitational forces and torques generated by [1].Fig. 1. Peak Effective Active Fraction over the surface of 67P for a model fit to geological unit types.ReferencesHow comets work. Fulle, M. Blum, J., Rotundi, A., 2019. ApJ Letters, 879:L8 (3pp). Near-perihelion activity of comet 67P/Churyumov–Gerasimenko. A first attempt of non-static analysis. Yu. Skorov, H. U. Keller, S. Mottola and P. Hartogh. Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 3, May 2020, Pages 3310–3316 On the activity of comets: understanding the gas and dust emission from comet 67/Churyumov-Gerasimenko’s south-pole region during perihelion. B. Gundlach, M. Fulle, J. Blum. Monthly Notices of the Royal Astronomical Society, Volume 493, Issue 3, April 2020, Pages 3690–3715 Constraining models of activity on comet 67P/Churyumov-Gerasimenko with Rosetta trajectory, rotation, and water production measurements. N. Attree, L. Jorda, O. Groussin, S. Mottola, N. Thomas, Y. Brouet, E. Kührt. Astronomy & Astrophysics 630, A18 Kramer, T., Laeuter, M., Hviid, S., et al. 2019. A&A, Volume 630, id.A3, 11 pp. Lasagni Manghi R., Zannoni M., Tortora P., Kueppers M., O'Rourke L., Martin P., Mottola S., et al., 2020, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19173

Media propagation noises are amongst the main error sources of radiometric observables for deep space missions, with fluctuations of the tropospheric excess path length representing a relevant contributor to the Doppler noise budget. Microwave radiometers currently represent the most accurate instruments for the estimation of the tropospheric delay and delay rate along a slant direction. A prototype of a tropospheric delay calibration system (TDCS), using a 14 channel Ka/V band microwave radiometer, has been developed under a European Space Agency contract and installed at the deep space ground station in Malargüe, Argentina, in February 2019. After its commissioning, the TDCS has been involved in an extensive testbed campaign by recording a total of 44 tracking passes of the Gaia spacecraft, which were used to perform an orbit determination analysis. This work presents the first statistical characterization of the end-to-end performance of the TDCS prototype in an operational scenario. The results show that using TDCS-based calibrations instead of the standard GNSS-based calibrations leads to a significant reduction of the residual Doppler noise and instability.

Enceladus is a Saturn’s icy moon that possesses a sub-surface ocean and a complex geological activity, with implications for its potential habitability. Gravity investigations at Enceladus could provide information on its internal structure including the radial density distribution, gravity anomalies, ice shell thickness and properties of the sub-surface ocean (mean density). In the recent years it was proposed to use SmallSat companion missions, dedicated only to specific investigations, to complement the science observations of a traditional larger exploration spacecraft. This represents a promising option since SmallSats provide relatively low-cost and versatile platforms for scientific observations. To investigate Enceladus’ gravity a SmallSats companion mission is proposed. The mission concept foresees two SmallSats deployed in orbit around Enceladus by a main Saturn’s orbiter. The objective is to globally characterize Enceladus’ gravity field through radio science investigations based on the Satellite-to-Satellite Tracking (SST). The observables are generated and decimated on-board by the SmallSats and transmitted back to Earth for data analysis and gravity field reconstruction, using the main spacecraft as a relay. However, Saturn proximity to Enceladus lead to major orbital instabilities for scientific near polar orbits. Moreover, the great distance from the Sun is a major drawback for the SmallSats platforms. Therefore, a mission feasibility study has been conducted by analyzing three-body stable orbits, proposing different SmallSats system budgets and concept of operations, and finally, by performing a radio science experiment performance preliminary assessment. ORBITAL ANALYSIS Due to the third body perturbation exerted by Saturn, near polar orbits are then highly unstable with a characteristic impact times up to 3 days [1]. Therefore, even if such orbits are optimal for scientific observations, they are too risky to be considered in this case. Periodic stable solutions are then searched by studying the Circular Restricted Three Body Problem (CRTBP) based on Saturn, Enceladus and one SmallSat. The research was mainly focused on 3D long period stable solutions with maximized inclination to improve the gravity science experiment performance. The search algorithm is based on a dedicated differential corrector [2] and the stability of the solution is evaluated through an index equal to the maximum of the eigenvalues of the monodromy matrix. The preliminary research has found 207505 periodic stable solutions from a search grid of 8000000 initial conditions (with minimum altitude above Enceladus set to 50km for planetary protection purpose). Currently, no stable solutions have found with average inclination greater than 75 degrees, in accordance with other studies [2][3]. Moreover, it has been seen that stable solutions with lower altitude tends to have a smaller average inclination. An example of a potential optimal solution (chosen from a tradeoff between eccentricity, altitude, and inclination) is represented in Figure 1. SYSTEM ANALYSIS Two different mission concepts have been analyzed, both consisting of two 12U CubeSats performing SST while orbiting Enceladus. In the first mission concept, the feasibility of a short mission employing CubeSats relying solely on battery cells stored energy is explored. The second concept evaluates equipping the CubeSats with solar arrays to extend the mission duration. Since the solar flux at Enceladus is 100 times smaller than the one experienced in Earth orbit, the expected solar panels power output is so low that, in this latter concept, scientific observation phases needs to be alternated to CubeSats hibernation phases in which batteries are recharged. The SST payload features an S-band transceiver, two monopole antennas for RX/TX scientific operations and an S-band patch antenna coupled with an HPA for data relay to mothercraft. The raw data are preprocessed on board. The expected scientific data output is in the order of 300 kb/day accounting for ranging and doppler measurements. Due to strict planetary protection requirements, the two CubeSats are equipped with a camera for autonomous Optical Navigation and a green chemical hybrid propulsion system for station-keeping, orbital disposal and momentum unloading. Environment related constraints, as thermal control management and radiation environment are also evaluated and addressed. RADIO SCIENCE EXPERIMENT PERFORMANCE Orbital stability and system analysis provided solutions have been then injected to the radio science experiment performance preliminary assessment. The expected performances of the gravity field reconstruction were evaluated through multi-arc numerical simulations, using realistic assumptions on the noise level. The dynamical model is based on gravitation accelerations of all planets and Saturn’s satellites plus the Enceladus extended gravity field 50x50 simulated with Kaula’s Rule (K=1.3E-3)[4], and the Solar Radiation Pressure (SRP). The simulated measurements are Two-way S/S Doppler measurements between the SmallSats. The angular separation between the SmallSats has been set to 5 degrees, as it was found that higher angles are less effective for the experiment performance. Simulations results show the scientific phase duration to be the main performance driver for the experiment. Therefore, a comparison of the different scientific phase durations has been conducted (potential durations have been provided by the system budget analysis). An example of the results of the radio science experiment performance assessment for the stable solution of Figure 1 are presented in Figure 2. In the gravity spectra plot, the black line identifies the simulated gravity field, therefore when an uncertainty line goes above it then the maximum observable degree is depicted. REFERENCES Russel, M. Lara, “On the Design of an Enceladus Science Orbit”, AIAA 2008-7072, 28 August 2008. Russel, “Global Search for Planar and Three-Dimensional Periodic Orbits Near Europa”, The Journal of the Astronautical Sciences, Vol. 54, No. 2, April–June 2006, pp. 199–226. S. Parker, R. L. Anderson, “LOW-ENERGY LUNAR TRAJECTORY DESIGN”, JPL, California, July 2013. Kaula, W, M., “Theory of Satellite Geodesy”, Blaisdell, Waltham, 1966.

Understanding cometary activity is key to exploring their materials and surface physics, with implications for comet formation and subsequent evolution. Outgassing produces a reaction force on nuclei that can alter their trajectory and spin, allowing constraints to be placed on comet models by measuring the observed orbit and rotation changes. For comet 67P/Churyumov-Gerasimenko, detailed measurements have been made by the Rosetta spacecraft and various attempts have been made to model the activity (see, e.g. [1,2]). Here we will present ongoing work, testing various activity distributions in an effort to fit Rosetta outgassing, trajectory, and rotation data using the activity model of [2]. We test a number of different activity distributions over the surface of the comet by varying the Effective Active Fraction (EAF), relative to pure water ice, of facets on a shape model. We investigate different spatial patterns in EAF, and attempt to correlate them to physical features on the cometary surface. Here we are able to achieve a good fit to the Rosetta data by parameterizing EAF in terms of the different geological unit types on 67P (Fig. 1). This will have important implications for understanding how activity works on the different types of surface observed on cometary nuclei, including ‘rough’, ‘smooth’, ‘dusty’ and ‘rocky’ surface morphologies.. The objective here is to constrain the shape of the activity curve on these various surfaces that a more detailed thermal model (see, for example [3,4]) must produce in order to fit the data. In addition to the changes in rotation period examined in [2], we also compute changes in the rotation axis, using a method based on [5] in order to compare with the observations. This provides an additional constraint on the spatial distribution of activity. Finally, we also compare the trajectory information, in the form of Earth-to-comet range, with the new analysis of Rosetta radio-tracking data performed in [6]. Fig. 1. Peak Effective Active Fraction over the surface of 67P for a model fit to geological unit types. References Nongravitational Effects of Cometary Activity. S. Mottola, N. Attree, L. Jorda, H.U. Keller, R. Kokotanekova, D. Marshall. Space Science Reviews 216 (1), 1-20 Constraining models of activity on comet 67P/Churyumov-Gerasimenko with Rosetta trajectory, rotation, and water production measurements. N. Attree, L. Jorda, O. Groussin, S. Mottola, N. Thomas, Y. Brouet, E. Kührt. Astronomy & Astrophysics 630, A18 On the activity of comets: understanding the gas and dust emission from comet 67/Churyumov-Gerasimenko’s south-pole region during perihelion. B. Gundlach, M. Fulle, J. Blum. Monthly Notices of the Royal Astronomical Society, Volume 493, Issue 3, April 2020, Pages 3690–3715 Near-perihelion activity of comet 67P/Churyumov–Gerasimenko. A first attempt of non-static analysis. Yu. Skorov, H. U. Keller, S. Mottola and P. Hartogh. Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 3, May 2020, Pages 3310–3316 Kramer, T., Laeuter, M., Hviid, S., et al. 2019. A&A, Volume 630, id.A3, 11 pp. Lasagni Manghi R., Zannoni M., Tortora P., Kueppers M., O'Rourke L., Martin P., Mottola S., et al., 2020, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19173

Nanosatellite missions represent a promising option for the exploration of the near-Earth asteroid population since they provide low-cost versatile platforms for scientific observations. This paper describes the preliminary orbital and navigation analyses for the DustCube mission, which was pre-selected to reach the binary asteroid system Didymos on-board ESA’s AIM spacecraft. Possible candidate orbits that exploit the binary nature of the system are identified and traded off to produce a preliminary concept of operations. The overall feasibility of the proposed scenario is then addressed by integrating the spacecraft trajectories in a realistic dynamical environment, evaluating their sensitivity to state errors, and estimating the accuracy of the orbit determination system. Preliminary results suggest that autonomous navigation of a Cubesat platform within a binary asteroid system is technically feasible. The proposed solution, which combines an initial parking orbit at the L4 equilibrium point with a Distant Retrograde Orbit for proximity operations, is shown to be consistent with the estimated orbit determination accuracy and allows to fulfil the mission requirements.

The Asteroid Impact and Deflection Assessment (AIDA) is an international collaboration supported by ESA and NASA to assess the feasibility of the kinetic impactor technique to deflect an asteroid, combining data obtained from NASA’s DART and ESA’s Hera missions. Together the missions represent the first humankind’s investigations of a planetary defense technique. In 2022, DART will impact Dimorphos, the secondary of the binary near-Earth asteroid (65803) Didymos. After 4 years, Hera will follow-up with a detailed post-impact survey of Didymos, to fully characterize and validate this planetary defense technique. In addition, Hera will deploy two CubeSats around Didymos once the Early Characterization Phase has completed, to augment the observations of the mother spacecraft. Juventas, the first Cubesat, will complete a low-frequency radar survey of the secondary, to unveil its interior. Milani, the second Cubesat, will perform a global mapping of Didymos and Dimorphos, with a focus on their compositional difference and their surface properties. One of the main objectives of Hera is to determine the binary system’s mass, gravity field, and dynamical state using radio tracking data in combination with imaging data. The gravity science experiment includes classical ground-based radiometric measurements between Hera and ground stations on Earth by means of a standard two-way X-band link, onboard images of Didymos, and spacecraft-to-spacecraft inter-satellite (ISL) radiometric tracking between Hera and the Cubesats. The satellite-to-satellite link is a crucial add-on to the gravity estimation of low-gravity bodies by exploiting the Cubesats’ proximity to the binary, as the range-rate measurements carried out by the inter-satellite link contain information on the dynamics of the system, i.e., masses and gravity field of Didymos primary and secondary.We will describe the updated mission scenario for the Hera radio science experiment to be jointly carried out by the three mission elements, i.e., Hera, Juventas and Milani. To conclude, our updated analysis and latest results, as well as the achievable accuracy for the estimation of the mass and gravity field of Didymos and Dimorphos, are presented.

Following its arrival at 67P/Churyumov-Gerasimenko in August 2014, the Rosetta spacecraft successfully navigated in proximity of the comet for two years, using a combination of radiometric measurements and optical images collected by the onboard navigation cameras.The reconstructed spacecraft and comet trajectories were obtained combining several long-arc and short-arc orbit determination solutions generated by ESOC Flight Dynamics during the Rosetta operations. Several discontinuities are present within these trajectories, due to the lack of a dynamical model for the representation of the comet Non-Gravitational Accelerations (NGA).The work presented in this study represents an effort to produce an accurate and continuous ephemeris reconstruction for comet 67P/Churyumov-Gerasimenko for the period between July 2014 and October 2016, through a complete reanalysis of the Range and ΔDOR measurements collected by Rosetta during its proximity phase with the comet.Using as input the reconstructed relative orbit of Rosetta, the radiometric observables were mapped to the comet nucleus and used to estimate the comet state and some key physical and observational parameters within a Square Root Information batch filter implemented in MONTE, most notably the NGA acting on the comet nucleus due to surface outgassing.Several orbit determination solutions were generated by varying the model used to represent the NGA. More specifically, empirical and stochastic models were compared by evaluating the reduced χ2 statistics of the measurement residuals to identify the most suitable trajectory estimations for each of the proposed models. From this narrow list of solutions, a preliminary selection for the final ephemeris reconstruction is proposed, based on its adherence to the original ESOC trajectory and on the consistency of the formal state uncertainties with the estimated solutions.It will be shown that the selected ephemeris solution, using a piecewise linear stochastic NGA model with intervals between 3 and 4 weeks, produces a continuous ephemeris reconstruction for 67P/Churyumov-Gerasimenko with maximum formal uncertainties around perihelion of σpos ≅ [20 km, 30 km, 200 km] in the Radial-Tangential-Normal reference frame. The advantage of using simple stochastic models, with limited a-priori assumptions on the involved physical processes, is that they allow to produce an unbiased estimation of the NGA variations around perihelion, which represent a valuable input for further investigations involving detailed physical models of the cometary activity.

The Asteroid Impact and Deflection Assessment (AIDA) is an international collaboration supported by ESA and NASA to assess the feasibility of the kinetic impactor technique to deflect an asteroid, combining data obtained from NASA’s DART and ESA’s Hera missions [1, 2]. In 2022, DART will perform a kinetic impact on the secondary of the binary near-Earth asteroid (65,803) Didymos, recently named Dimorphos. After 2 years, Hera will follow-up with a detailed post-impact survey of Didymos, to fully characterize this planetary defense technique. Additionally, Hera will deploy two CubeSats around Didymos once the Early Characterization Phase has completed, to complement the observations of the mother spacecraft and increase the scientific return of the mission. The first Cubesat, called Juventas, will complete a low-frequency radar survey of the secondary, to unveil its interior, while the second one has not yet been selected.One of the main objectives of Hera is to characterize the mass and mass distribution of both Didymos primary and secondary by means of radio science investigations. This paper describes the concept of the gravity science investigations to be jointly carried out by the three mission elements, i.e. Hera, Juventas and CubeSat-2. The experiment will combine classical ground-based radiometric measurements, spacecraft-based optical images of Didymos, and Satellite-to-Satellite radiometric tracking between Hera and the Cubesats. Finally, our results and achievable accuracy for the estimation of the mass and gravity field of Didymos and Dimorphos are presented. 1. IntroductionMost of the information about the formation processes of an asteroid lies in its interior structure. One of the very few constraints of the internal mass distribution of a celestial body is given by its gravity field, even if the inversion process is not unique. First, the bulk density can be inferred by measuring the mass of the body, combined to the volume estimated from optical images. In addition, the higher degrees of the gravity field provide information about the internal distribution of mass, such as the moments of inertia.The main scientific goals of the Hera radio science investigations are: Determine the mass and gravity field of Didymos and Dimorphos; Reconstruct the motion of Dimorphos around Didymos; Contribute to the characterization of the energy transfer between DART and Dimorphos. Such objectives are a valuable contribution to the Hera mission objectives, leading to a better understanding of the formation and evolution processes of the Didymos system. 2. TechniqueThe determination of the gravity field of a celestial body is an application of the orbit determination process of deep space spacecraft. In particular, the gravity of Didymos can be estimated precisely reconstructing the trajectory of Hera during a selected number of close encounters (about 10 km at closest approach). The classical observables used in the orbit determination are obtained from the X-band radio link between the spacecraft and the Earth. The microwave signal is sent to spacecraft from a ground antenna and coherently retransmitted back to Earth, where Doppler and range measurements are obtained. A previous study performed for the AIM proposed mission [4] demonstrated that gravity science at Didymos is feasible using radio tracking data only, under realistic assumptions on the technological capabilities of the space and ground segment. Shorter pericenter distances increase the attainable accuracy. However, a significant improvement can be obtained even at relatively large distances processing also optical images of Didymos and Dimorphos taken by the spacecraft.In addition, Hera may track Juventas and CubeSat-2 by means of a space-to-space inter-satellite link (ISL), capable of determining the relative distance (ranging) and the relative line-of-sight velocity (Doppler) between the two bodies. In particular, the latter is expected to represent a very nice add-on to the gravity investigation carried out by the Hera mission, as the Doppler shift that affects the inter-satellite link contains the information on the dynamics of the system, i.e. masses and gravity field of Didymos and Dimorphos.The expected accuracy in the estimation of Didymos gravity fields were obtained through numerical simulations of the orbit determination of Hera and the two Cubesats. Conservative assumptions were made in terms of both radiometric and optical measurement noises, and large a-priori uncertainties for the estimated parameters were used. 3. ResultsAs a result of the numerical simulations, the masses of Didymos and Dimorphos are expected to be estimated with relative uncertainties less than 10-4 and 10-3, respectively. The addition of the ISL measurements improves the achievable accuracies but it is not required to estimate the masses. However, given the relatively large distance of Hera from the system, the higher degree gravity of Didymos and Dimorphos can be estimated only adding the ISL Doppler measurements between the Cubesats and the mother spacecraft. In this case, the gravity field of Didymos can be estimated to at least degree 3, depending on the assumptions about the ISL operations and performance. Similarly, ISL Doppler measurements allows to estimate the extended gravity field of Dimorphos up to degree 2, with an uncertainty of about 10%. Acknowledgements This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870377 (project NEO-MAPP). MZ, IG, ML, EG, RLM, and PT wish to acknowledge Caltech and the Jet Propulsion Laboratory for granting the University of Bologna a license to an executable version of MONTE Project Edition S/W. References[1] Cheng A. F., et al., “AIDA DART asteroid deflection test: Planetary defense and science objectives,” Planet. Space Sci., vol. 157, no. February, pp. 104–115, 2018.[2] Michel, P., et al. “European component of the AIDA mission to a binary asteroid: Characterization and interpretation of the impact of the DART mission”. Adv. Space Res. (2018), Volume 62, Issue 8, pp. 2261-2272.[3] Lasagni Manghi, R., Modenini, D., Zannoni, M., Tortora, P., “Preliminary orbital analysis for a CubeSat mission to the Didymos binary asteroid system”, Adv. Space Res. (2018), Volume 62, Issue 8, pp 2290-2305[4] M. Zannoni, G. Tommei, D. Modenini, P. Tortora, R. Mackenzie, M. Scoubeau, U. Herfort, I. Carnelli, “Radio science investigations with the Asteroid impact mission”, Adv. Space Res. (2018), Vol. 62, Issue 8, pp. 2273-2289.

Following its arrival at comet 67P/Churyumov-Gerasimenko in August 2014, the Rosetta spacecraft successfully navigated in its proximity for two years, using a combination of Earth-based astrometric and radiometric tracking data as well as space-based optical navigation data. Depending on the mission phase, the orbital navigation system was tasked with the simultaneous estimation of both spacecraft and comet state, in addition to several other physical parameters including amongst others the comet rotational state, its gravitational field and the body-fixed coordinates of surface landmarks.Estimating the heliocentric trajectory of 67P has proven to be challenging, due to the lack of reliable models to take into account the non-gravitational accelerations acting on the comet (particularly close to perihelion) and to occasional degradations of the ranging observables caused by geometrical constraints (solar conjunctions).The accuracy of the resulting comet heliocentric trajectories, which show discontinuities in the order of tens of kilometers between consecutive short-arc solutions, was sufficient for spacecraft proximity operations, where navigators are mostly concerned by the relative comet/spacecraft position. However, a continuous and more accurate orbital solution is strictly coupled with the development of analytical models for non-gravitational accelerations and comet outgassing for which the Rosetta mission represents an ideal test case.The work presented here represents a joint effort between academic institutions and ESA’s Flight Dynamics team to improve the accuracy of 67P’s orbit, by re-analyzing the radiometric data over long time scales for the whole duration of Rosetta proximity operations at comet 67P.Details on the orbit determination process and filter implementation will be presented, together with a discussion on the achieved formal uncertainties and on the observables’ residuals.

To accurately estimate the mass of a comet, is a challenging task, given the complexity of its dynamical environment, and particularly so while close to perihelion, where the coma emissions are stronger. This study describes an operational approach for simultaneous orbit determination and mass estimation of a cometary target, using detailed dynamical models and combining measurements from multiple instruments. The testbed for the proposed approach is represented by the Proteus mission towards the main belt comets 288P and 238P-Read, for which formal uncertainties for the mass and other parameters of interest are estimated though a covariance analysis. It will be shown that the required relative accuracy of 10% in mass estimation can be achieved for both cometary targets under realistic operational conditions.

Nanosatellites are a promising option for the exploration of the solar system and near-Earth objects, providing an agile, reduced cost and mass solution for interplanetary missions. This paper summarises the efforts done during the Phase A study aimed at assessing the feasibility of a nanosatellite-based mission concept for a 3U CubeSat to the vicinity of the binary asteroid 65803 Didymos. The nanosatellite is meant to be part of the Asteroid Impact Mission (AIM) which is the European element of the NASA-ESA jointly developed Asteroid Impact & Deflection Assessment (AIDA) mission. The inclusion of a scientific optical payload based on a Nephelometer, the selected orbital and navigation strategy, together with the differences found between the Beyond-Earth space environments (radiation, micrometeoroids, ejected material, illumination conditions, communications access) and a typical LEO polar environment, unveils new technical challenges to be faced along interplanetary missions. These challenges have been identified and analysed to be included within the DustCube concept of operations, are described in this manuscript.

Aerosols are able to affect climate in two major ways, namely direct and indirect process. The direct effect is the one exerted by aerosols themselves on the radiative balance of the Earth through a combination of scattering and absorption of radiation. The indirect effects are a suite of possible impacts on aerosols through the modification of cloud properties interaction of aerosols on clouds (IPCC, 2013). Clouds and aerosols still contribute the largest uncertainty to the correct assessment and understanding of “climate change” (IPCC, 2013). Ion-induced nucleation has been proposed to act in addition to the classical homogeneous nucleation (Iida et al., 2006; Hirsikko et al., 2007; Laakso et al., 2007). In particular, possible links between cloud formation processes and the ionization of atmospheric particles caused by Cosmic Rays have been proposed. The most widely studied mechanism proposed to explain the possible influence of the cosmic ray flux on cloudiness is the “ion aerosol clear air” mechanism (Carslaw et al., 2002; Usoskin and Kovaltsov, 2008). Our current understanding of these phenomena remains very low, mostly due to the reduced number of conclusive observations and our dependence on model simulations of these phenomena. The “A5-Unibo” experiment flown in BEXUS18 stratospheric balloon has been developed by the University of Bologna to this aim. The primary objective of the experiment was to collect vertical profiles of different atmospheric parameters involved in the above-mentioned processes, while the secondary one was the sampling of stratospheric aerosols for a post-flight analysis. In fact, A5-Unibo experiment measured profiles of particles size distribution with an innovative aerosol counter (LOAC “Light Optical Particle Counter”, MeteoMODEM,), positive and negative ion densities (Air Ion Counter, AlphaLab. Inc.), together with key atmospheric parameters (such as temperature, humidity, and pressure). In this way, a correlation between ionization and aerosol formation processes has been investigated through a rather simple multi-instrument approach. Our preliminary results have identified a possible role of positive ions in condensation, as well as a larger importance of humidity with respect to temperature.

Nanosatellite missions represent a promising option for the exploration of the near-Earth asteroid population since they provide low-cost versatile platforms for scientific observations. This paper describes the orbital analysis for the DustCube mission, which was preliminarily selected to reach the binary asteroid system Didymos on-board ESA’s AIM spacecraft. Possible candidate orbits that exploit the binary nature of the system and satisfying the mission requirements and constraints were identified. The overall feasibility of the proposed Concept of Operations was then addressed by integrating the spacecraft trajectories in a realistic dynamical environment, evaluating their sensitivity to state errors, and estimating the accuracy requirements of the Orbit Determination and Control Subsystem (ODCS). The proposed operational concept proved consistent with the maneuvering capabilities of a CubeSat platform in terms of frequency of the station-keeping maneuvers and total delta-v budget. The proposed solution combines an initial parking orbit at the L4 equilibrium point with a Distant Retrograde Orbit for proximity operations, allowing to fulfil all mission requirements. However, this result is strictly coupled with the overall performance of the orbit determination system, which is required to allow for safe operations.