Operating Spacecraft Around Comets: Evaluation of the Near-Nucleus Environment
C. M. Lisse, M. R. Combi, T. L. Farnham, N. Dello Russo, S. Sandford, A. F. Cheng, U. Fink, W. M. Harris, J. McMahon, D. J. Scheeres, H. A. Weaver, J. Leary
备注：
38 Pages, 15 Figures, 1 Table; accepted for publication in Acta Astronautica 25-Nov-2021
We present a study of the current state of knowledge concerning spacecraft operations and potential hazards while operating near a comet nucleus. Starting from simple calculations comparing the cometary coma environment to benign conditions on Earth, we progress to sophisticated engineering models of spacecraft behavior, and then confront these models with recent spacecraft proximity operations experience. Finally, we make recommendations from lessons learned for future spacecraft missions that enter into orbit around a comet for long-term operations. All of these considerations indicate that, with a proper spacecraft design and operations planning, the near-nucleus environment can be a relatively safe region in which to operate, even for an active short period comet near perihelion with gas production rates as high as 1e29 molecules/s. With gas densities similar to those found in good laboratory vacuums, dust densities similar to Class 100 cleanrooms, dust particle velocities of 10s of m/s, and microgravity forces that permit slow and deliberate operations, the conditions around a comet are generally more benign than a typical day on Mars. Even in strong dust jets near the nucleus surface, dust densities tend to be only a few grains/cm3, about the same as in a typical interior room on Earth. Stochastic forces on a modern spacecraft with tens of square meters of projected surface area can be accounted for using modern Attitude Control Systems to within tens of meters navigation error; surface contamination issues are only important for spacecraft spending months to years within a few kilometers of the nucleus surface; and the issues the Rosetta spacecraft faced, confusion of celestial star trackers by sunlit dust particles flying past the spacecraft, will be addressed using the next generation of star trackers implementing improved transient rejection algorithms.
We present a statistical approach to assess the dust environment for a yet unknown comet (or when its parameters are known only with large uncertainty). This is of particular importance for missions such as ESA's Comet Interceptor mission to a dynamically new comet. We find that the lack of knowledge of any particular comet results in very large uncertainties (~3 orders of magnitude) for the dust densities within the coma. The most sensitive parameters affecting the dust densities are the dust size distribution, the dust production rate and coma brightness, often quantified by Af$ρ$. Further, …
During March-April 2002, while between the orbits of Jupiter and Saturn, the Cassini spacecraft detected a significant enhancement in pickup proton flux. The most likely explanation for this enhancement was the addition of protons to the solar wind by the ionization of neutral hydrogen in the corona of comet 153P/Ikeya-Zhang. This comet passed relatively close to the Sun-Cassini line during that period, allowing pickup ions to be carried to Cassini by the solar wind. This pickup proton flux could have been further modulated by the passage of the interplanetary counterparts of coronal mass ejec…
Comet C/2014 UN$_{271}$, alternative designation 'BB' after its discoverers 'Bernardinelli/Bernstein', and commonly referred to as UN$_{271}$, is an extreme case on two fronts, firstly its solar distance on discovery ($>$ 29 au) and secondly the size of its nucleus (137$\pm$ 15 km). With an aphelion distance of $\sim$33,000 au (w.r.t. the solar system barycentre) and an orbital period $\sim$2 million years, it is definitely an object from the solar system's Oort cloud, and also by a good measure the largest Oort cloud object ever observed. In situ observation of UN$_{271}$ would be of consider…
Cometary meteoroid trails exist in the vicinity of comets, forming fine structure of the interplanetary dust cloud. The trails consist predominantly of cometary particles with sizes of approximately 0.1 mm to 1 cm which are ejected at low speeds and remain very close to the comet orbit for several revolutions around the Sun. When re-analysing the Helios dust data measured in the 1970s, Altobelli et al. (2006) recognized a clustering of seven impacts, detected in a very narrow region of space at a true anomaly angle of 135 deg, which the authors considered as potential cometary trail particles.…
Comet Interceptor is an ESA F-class mission expected to launch in 2028 on the same launcher as ESA's ARIEL mission. Comet Interceptor's science payload consists of three spacecraft, a primary spacecraft that will carry two smaller probes to be released at the target. The three spacecraft will fly-by the target along different chords, providing multiple simultaneous perspectives of the comet nucleus and its environment. Each of the spacecraft will be equipped with different but complementary instrument suites designed to study the far and near coma environment and surface of a comet or interste…
The structure of cometary dust is a tracer of growth processes in the formation of planetesimals. Instrumentation on board the Rosetta mission to comet 67P/Churyumov- Gerasimenko captured dust particles and analysed them in situ. However, these deposits are a product of a collision within the instrument. We conducted laboratory experiments with cometary dust analogues, simulating the collection process by Rosetta instruments (specifically COSIMA, MIDAS). In Paper I we reported that velocity is a key driver in determining the appearance of deposits. Here in Paper II we use materials with differ…
The Martian Moons Exploration (MMX) spacecraft is a JAXA mission to Mars and its moons Phobos and Deimos. MMX will carry the Circum-Martian Dust Monitor (CMDM) which is a newly developed light-weight ($\mathrm{650\,g}$) large area ($\mathrm{1\,m^2}$) dust impact detector. Cometary meteoroid streams (also referred to as trails) exist along the orbits of comets, forming fine structures of the interplanetary dust cloud. The streams consist predominantly of the largest cometary particles (with sizes of approximately $\mathrm{100\,μm}$ to 1~cm) which are ejected at low speeds and remain very close …
This white paper proposes that AMBITION, a Comet Nucleus Sample Return mission, be a cornerstone of ESA's Voyage 2050 programme. We summarise some of the most important questions still open in cometary science after the successes of the Rosetta mission, many of which require sample analysis using techniques that are only possible in laboratories on Earth. We then summarise measurements, instrumentation and mission scenarios that can address these questions, with a recommendation that ESA select an ambitious cryogenic sample return mission. Rendezvous missions to Main Belt comets and Centaurs a…
The European Space Agency's Solar Orbiter spacecraft will pass approximately downstream of the position of comet C/2019 Y4 (ATLAS) in late May and early June 2020. We predict that the spacecraft may encounter the comet's ion tail around 2020 May 31-June 1, and that the comet's dust tail may be crossed on 2020 June 6. We outline the solar wind features and dust grain collisions that the spacecraft's instruments may detect when crossing the comet's two tails. Solar Orbiter will also pass close to the orbital path of C/2020 F8 (SWAN) on 2020 May 22, but we believe that it is unlikely to detect an…
In August 2019, a second interstellar object 2I/Borisov was discovered 2 years after the discovery of the first known interstellar object, 1I/'Oumuamua. Can we send a spacecraft to this object, using existing technologies? In this paper we assess the technical feasibility of a near-term mission to 2I/Borisov. We apply the Optimum Interplanetary Trajectory Software (OITS) tool to generate trajectories to 2I/Borisov. As results, we get the minimal $ΔV$ trajectory with a launch date in July 2018. For this trajectory, a Falcon Heavy launcher could have hauled an 8 ton spacecraft to 2I/Borisov. For…
Comets hold answers to mysteries of the Solar System by recording presolar history, the initial states of planet formation and prebiotic organics and volatiles to the early Earth. Analysis of returned samples from a comet nucleus will provide unparalleled knowledge about the Solar System starting materials and how they came together to form planets and give rise to life: 1. How did comets form? 2. Is comet material primordial, or has it undergone a complex alteration history? 3. Does aqueous alteration occur in comets? 4. What is the composition of cometary organics? 5. Did comets supply a sub…
The OSIRIS/NAC camera onboard the Rosetta spacecraft acquired approximately 27000 images of comet 67PChuryumov-Gerasimenko at spatial scales down to a few centimeters. Numerous sequences of images separated by a few minutes suitable to stereo reconstruction allowed producing anaglyphs. They offer three-dimensional views complementary to other technics as a tool to understand the topography of the nucleus. Each one is documented by a set of 17 parameters. Over 1820 anaglyphs are available on a dedicated website at https://rosetta-3dcomet.cnes.fr
We present high resolution imaging observations of interstellar comet 2I/Borisov (formerly C/2019 Q4) obtained using the Hubble Space Telescope. Scattering from the comet is dominated by a coma of large particles (characteristic size 0.1 mm) ejected anisotropically. Convolution modeling of the coma surface brightness profile sets a robust limit to the spherical-equivalent nucleus radius r_n < 0.5 km (geometric albedo 0.04 assumed). We obtain an independent constraint based on the non-gravitational acceleration of the nucleus, finding r_n > 0.2 km (nucleus density 500 kg/m3 assumed). The profil…
Comet 162P/Siding Spring is a large Jupiter-family comet with extensive archival lightcurve data. We report new r-band nucleus lightcurves for this comet, acquired in 2018, 2021 and 2022. With the addition of these lightcurves, the phase angles at which the nucleus has been observed range from $0.39^\circ$ to $16.33^\circ$. We absolutely-calibrate the comet lightcurves to r-band Pan-STARRS 1 magnitudes, and use these lightcurves to create a convex shape model of the nucleus by convex lightcurve inversion. The best-fitting shape model for 162P has axis ratios $a/b = 1.56$ and $b/c = 2.33$, side…
The Rosetta mission delivered images of comet 67P's nucleus at unprecedented resolution which indicate the presence of a global layering system. By merging techniques of structural geology, statistical image processing, and solar system science, this thesis aims to contribute to the understanding of the formation of the layerings, and thus of cometary nuclei as a whole. I describe the two distinctive approaches to studying the layerings' orientation on comet 67P's nucleus. First, I mapped layering-related linear features on a 3D shape model of the nucleus, onto which I projected high-res OSIRI…
Anisotropic outgassing from comets exerts a torque sufficient to rapidly change the angular momentum of the nucleus, potentially leading to rotational instability. Here, we use empirical measures of spin changes in a sample of comets to characterize the torques and to compare them with expectations from a simple model. Both the data and the model show that the characteristic spin-up timescale, $τ_s$, is a strong function of nucleus radius, $r_n$. Empirically, we find that the timescale for comets (most with perihelion 1 to 2 AU and eccentricity $\sim$0.5) varies as $τ_s \sim 100 r_n^{2}$, wher…
In this work, a novel approach to explain the survival of sungrazing comets within the Roche limit is presented. It is shown that in the case of low tensile strength of the cometary nucleus, tidal splitting of the nucleus can be prevented by the reaction force caused by the sublimation of the icy constituents. The survival of Comet C/2011 W3 (Lovejoy) within the Roche limit of the Sun is, thus, the result of high tensile strength of the nucleus, or the result of the reaction force caused by the strong outgassing of the icy constituents near the Sun.
The measured nitrogen-to-carbon ratio in comets is lower than for the Sun, a discrepancy which could be alleviated if there is an unknown reservoir of nitrogen in comets. The nucleus of comet 67P/Churyumov-Gerasimenko exhibits an unidentified broad spectral reflectance feature around 3.2 micrometers, which is ubiquitous across its surface. On the basis of laboratory experiments, we attribute this absorption band to ammonium salts mixed with dust on the surface. The depth of the band indicates that semivolatile ammonium salts are a substantial reservoir of nitrogen in the comet, potentially dom…
Comet 41P/Tuttle-Giacobini-Kresak (41P), a Jupiter family comet with three discoveries over about 100 years, is in a short-periodic orbit around the Sun with the perihelion close to the Earth distance. The 2017 apparition of 41P offered a long-lasting visibility of the comet at a close distance to Earth. The four month-long imaging campaign with the 2 m telescope at the Mount Wendelstein Observatory was aimed at characterizing dust activity and nucleus properties of the comet. Using a new analysis method of the inner coma flux, we derived a small mean equivalent radius of about 600 m for the n…
Acceleration of cometary dust near the nucleus: Application to 67P/Churyumov-Gerasimenko
我们提出了一个模型的彗星尘埃能够模拟的动态范围内的彗星表面的前几十公里。罗塞塔上的GIADA和COSIMA仪器的最新测量表明，该核发出蓬松的尘埃颗粒，孔隙率超过50%，尺寸至少达到mm（Schulz et al. 2015，Rotundi et al. 2015，Fulle et al. 2015）。这些粒子的物理性质的检索需要一个有效的力量控制其动力学模型。在这里，我们提出了一个模型，能够模拟现实的，大的和多孔的颗粒使用分级聚集体，这表明以前的外推是不够的。我们的方法的主要优点是，我们可以模拟非常大的（毫米级）非球形团聚体，并可以准确地确定它们的1）有效截面和截面与质量的比率，2）气体阻力系数，和3）光散射特性。在实践中，我们发现，一个更详细的处理的尘埃结构的结果在3-5倍更高的速度为大的尘埃粒子在内部彗发比以前估计使用相同质量的球形粒子。我们将我们的模型应用于彗星67 P的核附近的尘埃动力学，并成功地再现了早期报告的尘埃速度，当时彗星距离太阳约3.5 AU。在这个阶段，我们采用一个简单的球形彗星核，我们的模型活动作为恒定速度的气体膨胀从一个均匀的活动表面，并使用米氏散射。我们讨论了未来改进这些简化的途径。
Cometary outgassing can produce torques that change the spin state of the nucleus, influencing the evolution and lifetimes of comets (1,2). If these torques spin up the rotation to the point that centripetal forces exceed the material strength of the nucleus, the comet may fragment (3). Torques that slow down the rotation can cause the spin state to become unstable, but if the torques persist, the nucleus may eventually reorient itself and start to spin up again (4). Simulations predict that most comets will go through a short phase of changing spin states, after which changes occur gradually …
We show that the scattering phase functions of the coma and the nucleus of the comet 67P/Churyumov-Gerasimenko measured by the Rosetta/OSIRIS instrument can be reproduced by a particle model involving clustered densely packed submicrometer-sized grains composed of organic material and larger micrometer-sized silicate grains. The simulated and measured coma phase functions suggest that near the nucleus scattering is dominated by large particles, and the size distribution of dust particles varies with time and/or local coma environment. Further, we show that the measured nucleus phase function i…
The Near-Earth Comet P/2016 BA$_{14}$ (PanSTARRS) is a slow-rotatating nearly-dormant object, a likely dynamical twin of 252P/LINEAR, and was recently shown to have a mid-infrared spectrum very dissimilar to other comets. BA$_{14}$ also recently selected one of the back-up targets for the ESA's \textit{Comet Interceptor}, so a clearer understanding of BA$_{14}$'s modern properties would not just improve our understanding of how comets go dormant, but could also aid planning for a potential spacecraft visit. We present observations of BA$_{14}$ taken on two dates during its 2016 Earth close app…
We present mid-infrared observations of comet P/2016 BA14 (PANSTARRS), which were obtained on UT 2016 March 21.3 at heliocentric and geocentric distances of 1.012 au and 0.026 au, respectively, approximately 30 hours before its closest approach to Earth (0.024 au) on UT 2016 March 22.6. Low-resolution ($λ$/$Δλ$~250) spectroscopic observations in the N-band and imaging observations with four narrow-band filters (centered at 8.8, 12.4, 17.7 and 18.8 $μ$m) in the N- and Q-bands were obtained using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) mounted on the 8.2-m Subaru telescope atop …
Context. Breathtaking imagery recorded during the European Space Agency's Rosetta mission confirmed the bilobate nature of comet 67P/Churyumov-Gerasimenko's nucleus. Its peculiar appearance is not unique among comets. The majority of cometary cores imaged at high resolution exhibit a similar build. Various theories have been brought forward as to how cometary nuclei attain such peculiar shapes. Aims. We illustrate that anisotropic mass loss and local collapse of subsurface structures caused by non-uniform exposure of the nucleus to solar irradiation can transform initially spherical comet core…
Data on perihelion fragmentation of the Great September Comet of 1882 (C/1882 R1), a prominent member of the Kreutz sungrazer system, are employed to estimate the size of the nucleus along the radius vector at the time of splitting. The prolate-spheroidal nucleus is assumed to fragment tidally at perihelion along planes normal to this direction. The relative velocities, derived by Sekanina & Chodas (2007) from revised positional-separation data on six fragments collected originally by Kreutz (1888) are interpreted as measures of the Sun's differential gravitational acceleration on the centers …
The ESA Rosetta mission has acquired unprecedented measurements of comet 67/P-Churyumov-Gerasimenko (hereafter 67P) nucleus surface, whose composition, as determined by in situ and remote sensing instruments including VIRTIS (Visible, InfraRed and Thermal Imaging Spectrometer) appears to be made by an assemblage of ices, minerals, and organic material. We performed a refined analysis of infrared observations of the nucleus of comet 67P carried out by the VIRTIS-M hyperspectral imager. We found that the overall shape of the 67P infrared spectrum is similar to that of other carbon-rich outer sol…
Comets hold the key to the understanding of our solar system, its formation and its evolution, and to the fundamental plasma processes at work both in it and beyond it. A comet nucleus emits gas as it is heated by the sunlight. The gas forms the coma, where it is ionised, becomes a plasma and eventually interacts with the solar wind. Besides these neutral and ionised gases, the coma also contains dust grains, released from the comet nucleus. As a cometary atmosphere develops when the comet travels through the solar system, large-scale structures, such as the plasma boundaries, develop and disa…
Photometric and Spectroscopic Observations of Comet 2P/Encke's Nucleus in the Mid-Infrared Wavelength Region
University of Occupational and Environmental Health, Japan（日本职业与环境大学）；Kyoto Sangyo University（京都产业大学）；National Astronomical Observatory of Japan（日本国立天文台）；Photocross Co. Ltd.（Photocross有限公司）
We present results of two-year photometric monitoring of Comet C/2011 J2 (LINEAR) that spans the time period from February 2013 through December 2014, before and after perihelion passage. The observations were conducted with broadband R filter. Analysis of observations of Comet C/2011 J2 (LINEAR) allows estimating the nucleus radius as function of geometric albedo and phase-darkening coefficient. Furthermore, our observations showed split of the comet. Fragment (B) additional to the primary C/2011 J2 nucleus was unambiguously detected; relative velocity is estimated to be near 0.17 ''/day. We …
