Rosetta—one comet rendezvous and two asteroid fly-bys

One of the two planetary cornerstone missions of the European Space Agency is the Rosetta mission to comet 67P/Churyumov-Gerasimenko. Rosetta is a rendezvous mission with a comet nucleus, which combines an Orbiter with a Lander. It will monitor the evolution of the comet nucleus and the coma as a function of increasing and decreasing solar flux input along the comet’s pre- and post-perihelion orbit. Different instrumentations will be used in parallel, from multi-wavelength spectrometry to in-situ measurements of coma and nucleus composition and physical properties. Rosetta will go in orbit around the nucleus of its target comet 67P/Churyumov-Gerasimenko, when it is still far from the Sun and accompany the comet along its way to perihelion and beyond. In addition the Rosetta Lander Philae will land on the nucleus surface, before the comet is too active to permit such a landing (i.e. at around r = 3 AU) and examine the surface and subsurface composition of the comet nucleus as well as its physical properties.     

73p/Schwassmann–Wachmann 3 – One Orbit after Break-Up: Search for Fragments

We report on our search campaign for the fragments of the Jupiter family cometand target of NASA's CONTOUR mission, Comet 3P/Schwassmann–Wachmann 3, one orbit revolution after the splitting of its nucleus in 1995 (Boehnhardt et al., 1995). Fragment C was found back with coma in November 1999 at 4 AU inbound and it continued to be active during the perihelion arc until at least December 2001 when we observed it last at 3.3 AU outbound. Fragment B was observed with coma between July and September 2001 when moving outbound from 2.35 to 2.75 AU. The search for other fragments including A, D (Boehnhardt et al., 1995) and E (Kodata et al., 2000a, b; Nakamura et al., 2000) in a search area of 15 × 4 arcmin starting at fragment C along direction of the extended radius vector was not successful in July and September 2001. The limiting magnitude of this search of ∼ 25 mag in R puts an upper limit on the radius of potential fragments of about 200 m (assuming albedo 0.04). The orbit deceleration parameter and the observed coma brightness of component C suggest that this object is the primary fragment that may contain a major part of the original nucleus.     

The flyby of Rosetta at asteroid Šteins - mission and science operations

The international Rosetta mission, a cornerstone mission of the european space agency scientific Programme, was launched on 2nd March 2004 on its 10 years journey towards a rendezvous with comet Churyumov-Gerasimenko (Gardini et al., 1999). During its interplanetary flight towards its target Rosetta crosses the asteroid belt twice with the opportunity to observe at close quarters two asteroids: (2867)-Šteins in 2008 and (21)-Lutetia in 2010. The spacecraft design was such that these opportunities could be fully exploited to deliver valuable data to the scientific community. The mission trajectory was controlled such that Rosetta would fly next to asteroid Šteins on the 5th of September 2008 with a relative speed of 8.6 km/s at a minimum distance of 800 km. Mission operations have been carefully planned to achieve the best possible flyby scenario and scientific outcome. The flyby scenario, the optical navigation campaign, and the planning of the scientific observations had to be adapted by the Mission and the Science Operations Centres to the demanding requirements expressed by the scientific community. The flyby was conducted as planned with a large number of successful observations.     

The Experiments Onboard the ROSETTA Lander

As a part of ESA's cornerstone mission ``ROSETTA' to comet 46P/Wirtanen a 100 kgLander will bring a scientific payload of almost 27 kg to the surface of the nucleus.After a first scientific sequence it will operate for a considerable fraction of thecometary orbit around the sun (between 3 AU and 2 AU). Ten experiments with a number of sub-experiments are foreseen; this paper presents the current status of the Lander development and reviews the scientific capabilities of each of the experiments at a time when the Flight Model (FM) of the Lander is already delivered.     

Comparison of some characteristics of comets 1P/Halley and 67P/Churyumov–Gerasimenko from the <Emphasis Type="Italic">Vega</Emphasis> and <Emphasis Type="Italic">Rosetta</Emphasis> mission data

On March 6 and 9, 1986, for the first time in the history of science, the Russian spacecraft Vega-1 and -2 approached the nucleus of comet 1P/Halley and flew by at a small distance. A while later, on March 14, 1986, the Giotto spacecraft (European Space Agency (ESA)) followed them. Together with the Japanese spacecraft Suisei (Japan Aerospace Exploration Agency (JAXA)), they obtained spaceborne investigations of cometary nuclei. Direct studies of cometary bodies that bear traces of the Solar System formation were continued in the next missions to comets. Starting from 2014 and up to 2016 September, the Rosetta spacecraft (ESA), being in a low orbit around the nucleus of comet 67P/Churyumov–Gerasimenko, has performed extremely sophisticated investigations of this comet. Here, we compare some results of these missions. The paper is based on the reports presented at the memorial conference dedicated to the 30th anniversary of the Vega mission, which took place at the Space Research Institute of the Russian Academy of Sciences in March, 2016, and does not pretend to comprehensively cover the problems of cometary physics.     

Venus Express—The first European mission to Venus

Venus Express is the first European mission to planet Venus. The mission aims at a comprehensive investigation of Venus atmosphere and plasma environment and will address some important aspects of the surface physics from orbit. In particular, Venus Express will focus on the structure, composition, and dynamics of the Venus atmosphere, escape processes and interaction of the atmosphere with the solar wind and so to provide answers to the many questions that still remain unanswered in these fields. Venus Express will enable a breakthrough in Venus science after a long period of silence since the period of intense exploration in the 1970s and the 1980s.The payload consists of seven instruments. Five of them were inherited from the Mars Express and Rosetta projects while two instruments were designed and built specifically for Venus Express. The suite of spectrometers and imaging instruments, together with the radio-science experiment, and the plasma package make up an optimised payload well capable of addressing the mission goals to sufficient depth. Several of the instruments will make specific use of the spectral windows at infrared wavelengths in order to study the atmosphere in three dimensions. The spacecraft is based on the Mars Express design with minor modifications mainly needed to cope with the thermal environment around Venus, and so a very cost-effective mission has been realised in an exceptionally short time.The spacecraft was launched on 9 November 2005 from Baikonur, Kazakhstan, by a Russian Soyuz-Fregat launcher and arrived at Venus on 11 April 2006. Venus Express will carry out observations of the planet from a highly elliptic polar orbit with a 24-h period. In 3 Earth years (4 Venus sidereal days) of operations, it will return about 2 Tbit of scientific data.Telecommunications with the Earth is performed by the new ESA ground station in Cebreros, Spain, while a nearly identical ground station in New Norcia, Australia, supports the radio-science investigations.     

Possible collisions of P/Halley and P/Machholz 1 with Jupiter and the terrestrial planets

The perturbed motion of comet Halley and comet Mackholz 1 1986 VIII was investigated within a time interval of about 20 millennia. The minimal distance of 0.043 AU between P/Halley and Venus may occur on April 4, 4868 AD. The distance of 0.036 AU between P/Halley and Jupiter will take place on April 1, 6616 AD. The orbit of P/Machholz 1 crosses the orbits of Mercury and Venus eight times, that of the Earth six or eight times, and the orbit of Mars four times per a period of advance of the argument of perihelion. A distance of about 0.06 AU between P/Machholz 1 and the Earth may take place in August 2576 AD and 5751 AD and in February 4770 AD. The minimal comet-Earth distance of 0.035 AU occurs on September 14, 5971 AD. The closest encounter between P/Machholz 1 and Jupiter at the distance of 0.098 AU may be in May 4499 AD. These results may be considered as a forecast of possible collisions.     

Possible collisions of P/Halley and P/Machholz 1 with Jupiter and the terrestrial planets

The perturbed motion of comet Halley and comet Mackholz 1 1986 VIII was investigated within a time interval of about 20 millennia. The minimal distance of 0.043 AU between P/Halley and Venus may occur on April 4, 4868 AD. The distance of 0.036 AU between P/Halley and Jupiter will take place on April 1, 6616 AD.The orbit of P/Machholz 1 crosses the orbits of Mercury and Venus eight times, that of the Earth six or eight times, and the orbit of Mars four times per a period of advance of the argument of perihelion. A distance of about 0.06 AU between P/Machholz 1 and the Earth may take place in August 2576 AD and 5751 AD and in February 4770 AD. The minimal comet-Earth distance of 0.035 AU occurs on September 14, 5971 AD. The closest encounter between P/Machholz 1 and Jupiter at the distance of 0.098 AU may be in May 4499 AD. These results may be considered as a forecast of possible collisions.     

A review of penetrometers for subsurface access on comets and asteroids

Abstract— The characterization of comet and asteroid interiors will eventually require in situ exploration with drills, penetrators/penetrometers, hypervelocity impactors, excavators or other devices. Because they offer desirable scientific capabilities and relative mechanical simplicity, penetrators and penetrometers, which use only axial force to push beneath the surface, are a good choice for near‐term missions. Penetrometers are instruments, generally deployed from a larger vehicle, that measure subsurface mechanical properties and may also contain additional scientific instruments. There are three basic types: “fast” penetrometers are released from above and plunge into the surface. Static and dynamic (collectively referred to as “slow”) penetrometers use, respectively, a constant slow penetration speed and periodic hammering impulses. The low gravity environment of asteroids and comets presents a key challenge to instrument deployment and also greatly affects the mechanical properties of surface materials, and in turn penetrometer performance. The Rosetta mission, currently en route to comet 67P/Churyumov‐Gerasimenko, will be the next mission to try both fast and slow, dynamic penetrometry, when it arrives in 2014. We present some new concepts of static penetrometers for small body exploration that are adapted to the low gravity environment. The low gravity environment also presents challenges for the testing of penetrometers on Earth and a number of previous solutions are described and new methods suggested. In the next generation of missions to study comets and asteroids, penetrometers could provide important data on their mechanical, seismic, thermal, electromagnetic, and chemical characteristics, as well as sample collection.     

Quantum physics exploring gravity in the outer solar system: the SAGAS project

We summarise the scientific and technological aspects of the Search for Anomalous Gravitation using Atomic Sensors (SAGAS) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015–2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements and technologies.     

A new orbit for comet C/1858 L1 (Donati)

A new orbit for comet C/1858 L1 (Donati), based on 1036 observations in α and 971 in σ made between 7 June 1858 and 5 March 1859, is calculated using iteratively reweighted least squares. Residuals were weighted by the Welsch weighting function. The orbit represents a high eccentricity ellipse, e = 0.996265, with large semi‐major axis, a = 154.8612 AU, and long period, P = 1927.22 yr. The residuals are relatively random, a 10.7% chance of being random, but with a slight indication of possible nongravitational forces influencing the motion. The comet will not return until the year 3759, when it will pass 0.8442 AU from the Earth. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Space missions to small bodies: asteroids and cometary nuclei

The knowledge of the physical and dynamical properties, distribution, formation, and evolution of small bodies is fundamental to understand how planet formation occurred and, even more importantly, if and how these objects have played a role in the apparition of life on Earth. In the last century, asteroids began to no longer appear as starlike points of light in our telescopes, but to be resolved worlds with distinctly measurable sizes, shapes, and surface morphologies. Only in the last 25 years, the exploration of small bodies by spacecraft has begun and revealed objects widely diverse in formation region, evolution and properties (e.g. shape, albedo density, gravity, regolith size distribution, and porosity). In this paper we will provide a chronological analysis of comet nuclei and asteroids as revealed by space missions. The real breakthrough began with the ESA Giotto mission in 1986 to the comet Halley, while the latest JAXA Hayabusa mission was devoted to hover above the small asteroid Itokawa with a touch-and-go for a sample return of asteroidal regolith. Comet and asteroid science stands at the threshold of a new exceptional era, with many new missions to be devoted to these widely diverse and still poorly known small bodies.     

SMART-1 mission description and development status

SMART-1 is the first of the Small Missions for Advanced Research in Technology of the ESA Horizons 2000 scientific programme. The SMART-1 mission is dedicated to testing of new technologies for future cornerstone missions, using Solar-Electric Primary Propulsion (SEPP) in Deep Space. The chosen mission planetary target is the Moon. The target orbit will be polar with the pericentre close to the South-Pole. The pericentre altitude lies between 300 and 2000 km, while the apocentre will extend to about 10,000 km. During the cruise phase, before reaching the Moon, the spacecraft thrusting profile allows extended periods for cruise science. The SMART-1 spacecraft will be launched in the spring of 2003 as an auxiliary passenger on an Ariane 5 and placed into a Geostationary Transfer Orbit (GTO). The expected launch mass is about 370 kg, including 19 kg of payload. The selected type of SEPP is a Hall-effect thruster called PPS-1350. The thruster is used to spiral out of the GTO and for all orbit maneuvers including lunar capture and descent. The trajectory has been optimised by inserting coast arcs and the presence of the Moon's gravitational field is exploited in multiple weak gravity assists.The Development Phase started in October 1999 and is expected to be concluded by a Flight Acceptance Review in January 2003. The short development time for this high technology spacecraft requires a concerted effort by industry, science institutes and ESA centres. This paper describes the mission and the project development status both from a technical and programmatic standpoint.     

P/2010 TO20 LINEAR-Grauer: A comet in transition from Centaurs to the Jupiter family

The object P/2010 TO20 LINEAR-Grauer, discovered at a heliocentric distance of over 5 AU, and at first classified as a Trojan, is now believed to be a comet. This paper reports special observations of the object that have allowed a significant refinement of its orbit and investigation of its dynamic evolution. It is shown that P/2010 TO20 LINEAR-Grauer is not a Trojan yet demonstrates unusual dynamic features. In particular, the object moves in a temporary satellite orbit relative to Jupiter over the observation interval. The comet has been in the Hill sphere for about two years and has made one revolution around the planet. The jovicentric distance function has two minima, and the smallest distance is 0.075 AU. Our estimates show that, with a probability of 0.76, the comet is likely to move in a Jupiter family orbit with a perihelion distance of less than 2.5 AU. The average time for such a transition is around forty thousand years.     

Comet Tempel-Tuttle and the Leonid meteors

The distribution of dust surrounding periodic comet Tempel-Tuttle has been mapped by analyzing the associated Leonid meteor shower data over the 902–1969 interval. The majority of dust ejected from the parent comet evolves to a position lagging the comet and outside the comet's orbit. The outgassing and dust ejection required to explain the parent comet's deviation from pure gravitational motion would preferentially place dust in a position leading the comet and inside the comet's orbit. Hence it appears that radiation pressure and planetary perturbations, rather than ejection processes, control the dynamic evolution of the Leonid particles. Significant Leonid meteor showers are possible roughly 2500 days before or after the parent comet reaches perihelion but only if the comet passes closer than 0.025 AU inside or 0.010 AU outside the Earth's orbit. Although the conditions in 1998–1999 are optimum for a significant Leonid meteor shower, the event is not certain because the dust particle distribution near the comet is far from uniform. As a by-product of this study, the orbit of comet Tempel-Tuttle has been redetermined for the 1366–1966 observed interval.     

Non-LTE radiative transfer for sub-millimeter water lines in Comet 67P/Churyumov-Gerasimenko

The European Space Agency (ESA) Rosetta spacecraft (Schulz, R., Alexander, C., Boehnhardt, H., Glassmeier, K.H. (Eds.) [2009]. “ROSETTA - ESA”) will encounter Comet 67P/Churyumov-Gerasimenko in 2014 and spend the next 18 months in the vicinity of the comet, permitting very high spatial and spectral resolution observations of the coma and nucleus. During this time, the heliocentric distance of the comet will change from ∼3.5 AU to ∼1.3 AU, accompanied by an increasing temperature of the nucleus and the development of the coma. The Microwave Instrument for the Rosetta Orbiter (MIRO) will observe the ground-state rotational transition (1₁₀-1₀₁) of H₂¹⁶O at 556.936 GHz, the two isotopologues H₂¹⁷O and H₂¹⁸O and other molecular transitions in the coma during this time (Gulkis, S. et al., [2007]. MIRO: Microwave Instrument for Rosetta Orbiter. Space Sci. Rev. 128, 561-597).The aim of this study is to simulate the water line spectra that could be obtained with the MIRO instrument and to understand how the observed line spectra with various viewing geometries can be used to study the physical conditions of the coma and the water excitation processes throughout the coma. We applied an accelerated Monte Carlo method to compute the excitations of the seven lowest rotational levels (1₀₁, 1₁₀, 2₁₂, 2₂₁, 3₀₃, 3₁₂, and 3₂₁) of ortho-water using a comet model with spherically symmetric water outgassing, density, temperature and expansion velocity at three different heliocentric distances 1.3 AU, 2.5 AU, and 3.5 AU. Mechanisms for the water excitation include water-water collisions, water-electron collisions, and infrared pumping by solar radiation.Synthetic line spectra are calculated at various observational locations and directions using the MIRO instrument parameters. We show that observations at varying viewing distances from the nucleus and directions have the potential to give diagnostic information on the continuum temperature and water outgassing rates at the surface of the nucleus, and the gas density, expansion velocity, and temperature of the coma as a function of distance from the nucleus. The gas expansion velocity and temperature affect the spectral line width and frequency shift of the line from the rest frequency, while the gas density (which is directly related to the outgassing rate) and the line excitation temperature determine the antenna temperature of the absorption and emission signal in the line profile.     

The legacy of Venus Express: highlights from the first European planetary mission to Venus

The ESA/Venus Express mission spent more than 8 years in orbit around Venus to extensively study its atmosphere, ionosphere and plasma environment and unveil new aspects of its surface. Extensive reviews of the work of Venus Express are underway, to cover in-depth studies of the new face of Venus revealed by Venus Express and ground-based concurrent observations. This paper intends to give a summarized and wide overview of some of the outstanding results in all the science areas studied by the mission. This paper will first review the main aspects of the mission and its instrumental payload. Then, a selection of results will be reviewed from the outermost layers interacting with the Solar wind, down to the surface of Venus. As Venus Express is already considered by space agencies as a pathfinder for the future of Venus exploration, perspectives for future missions will be given, which will have to study Venus not only from orbital view, but also down to the surface to solve the many remaining mysteries of the sister planet of the Earth.     

Finite difference time domain simulation of radar wave propagation through comet nuclei dielectric models

Abstract— The 90 MHz radar‐wave experiment, Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT), on board the Rosetta mission (ESA, 2004) is expected to probe the nucleus of the comet 67P/Churyumov‐Gerasimenko (67P/C‐G) to reveal information on its physical properties, chemical composition, and internal structure. This investigation assesses the potential to recognize lithological structure in the comet nucleus with a radar experiment such as CONSERT. Radar simulations at 90 MHz were performed with a finite difference time domain (FDTD) method. The amplitude and losses of the transmitted and reflected electric field components of an incident radar pulse were evaluated as a function of time. Seven different dielectric models of sections of a hypothetical comet nucleus were used, representative of existing theories of comet nuclei. Values of dielectric constant assigned to these models are based on mixing laws for a porous mixture of ice and meteoritic dust, employing laboratory measured values of relative permittivity for mainly chondritic meteorites. Our results confirm that structural differences such as layers or inclusions are discernable from transmitted and reflected radar signals at 90 MHz, the central frequency of the CONSERT instrument. Such simulations help to constrain the ambiguities that might exist in future radar data associated with the nature of the comet nuclei, whether conglomerate or layered in nature.     

P/2008 T1 (Boattini): Another comet under transition from Saturn to Jupiter

The newly discovered periodic comet P/2008 T1 (Boattini) is found to have experienced a recent capture into its present orbit, following a close approach to Saturn in 1995 to within 0.17 AU. This orbital change transferred the comet into an orbit tangent to that of Jupiter, which lead to an even closer passage within 0.02 AU with that planet in 2003 decoupling it from the influence of Saturn (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Venus express: Highlights of the nominal mission

Venus Express is the first European (ESA) mission to the planet Venus. Its main science goal is to carry out a global survey of the atmosphere, the plasma environment, and the surface of Venus from orbit. The payload consists of seven experiments. It includes a powerful suite of remote sensing imagers and spectrometers, instruments for in-situ investigation of the circumplanetary plasma and magnetic field, and a radio science experiment. The spacecraft, based on the Mars Express bus modified for the conditions at Venus, provides a versatile platform for nadir and limb observations as well as solar, stellar, and radio occultations. In April 2006 Venus Express was inserted in an elliptical polar orbit around Venus, with a pericentre height of ～250 km and apocentre distance of ～66000 km and an orbital period of 24 hours. The nominal mission lasted from June 4, 2006 till October 2, 2007, which corresponds to about two Venus sidereal days. Here we present an overview of the main results of the nominal mission, based on a set of papers recently published in Nature, Icarus, Planetary and Space Science, and Geophysical Research Letters.