Astrodynamical Space Test of Relativity using Optical Devices I (ASTROD I)—a class-M fundamental physics mission proposal for cosmic vision 2015–2025: 2010 Update

ASTROD I is a planned interplanetary space mission with multiple goals. The primary aims are: to test General Relativity with an improvement in sensitivity of over 3 orders of magnitude, improving our understanding of gravity and aiding the development of a new quantum gravity theory; to measure key solar system parameters with increased accuracy, advancing solar physics and our knowledge of the solar system; and to measure the time rate of change of the gravitational constant with an order of magnitude improvement and the anomalous Pioneer acceleration, thereby probing dark matter and dark energy gravitationally. It is envisaged as the first in a series of ASTROD missions. ASTROD I will consist of one spacecraft carrying a telescope, four lasers, two event timers and a clock. Two-way, two-wavelength laser pulse ranging will be used between the spacecraft in a solar orbit and deep space laser stations on Earth, to achieve the ASTROD I goals.For this mission, accurate pulse timing with an ultra-stable clock, and a drag-free spacecraft with reliable inertial sensor are required. T2L2 has demonstrated the required accurate pulse timing; rubidium clock on board Galileo has mostly demonstrated the required clock stability; the accelerometer on board GOCE has paved the way for achieving the reliable inertial sensor; the demonstration of LISA Pathfinder will provide an excellent platform for the implementation of the ASTROD I drag-free spacecraft. These European activities comprise the pillars for building up the mission and make the technologies needed ready. A second mission, ASTROD or ASTROD-GW (depending on the results of ASTROD I), is envisaged as a three-spacecraft mission which, in the case of ASTROD, would test General Relativity to one part per billion, enable detection of solar g-modes, measure the solar Lense-Thirring effect to 10 parts per million, and probe gravitational waves at frequencies below the LISA bandwidth, or in the case of ASTROD-GW, would be dedicated to probe gravitational waves at frequencies below the LISA bandwidth to 100?nHz and to detect solar g-mode oscillations. In the third phase (Super-ASTROD), larger orbits could be implemented to map the outer solar system and to probe primordial gravitational-waves at frequencies below the ASTROD bandwidth. This paper on ASTROD I is based on our 2010 proposal submitted for the ESA call for class-M mission proposals, and is a sequel and an update to our previous paper (Appouchaux et al., Exp Astron 23:491?C527, 2009; designated as Paper I) which was based on our last proposal submitted for the 2007 ESA call. In this paper, we present our orbit selection with one Venus swing-by together with orbit simulation. In Paper I, our orbit choice is with two Venus swing-bys. The present choice takes shorter time (about 250?days) to reach the opposite side of the Sun. We also present a preliminary design of the optical bench, and elaborate on the solar physics goals with the radiation monitor payload. We discuss telescope size, trade-offs of drag-free sensitivities, thermal issues and present an outlook.     

Satellite aided capture

A two body, patched conic analysis is presented for a planetary capture mode in which a gravity assist by an existing natural satellite of the planet aids in the capture. An analytical condition sufficient for capture is developed and applied for the following planet/satellite systems: Earth/Moon, Jupiter/Ganymede, Jupiter/Callisto, Saturn/Titan and Neptune/Triton. Co-planar, circular planetary orbits are assumed. Three sources of bodies to be captured are considered: spacecraft launched from Earth, bodies entering the solar system from interstellar space, and bodies already in orbit around the Sun. Results show that the Neptune/Triton system has the most capability for satellite aided capture of those studied. It can easily capture bodies entering the Solar System from interstellar space. Its ability to capture spacecraft launched from Earth is marginal and can only be decided with better definition of physical properties. None of the other systems can capture bodies from these two sources, but all can capture bodies already in orbit around the Sun under appropriate conditions.     

Source regions and timescales for the delivery of water to the Earth

Abstract— In the primordial solar system, the most plausible sources of the water accreted by the Earth were in the outer asteroid belt, in the giant planet regions, and in the Kuiper Belt. We investigate the implications on the origin of Earth's water of dynamical models of primordial evolution of solar system bodies and check them with respect to chemical constraints. We find that it is plausible that the Earth accreted water all along its formation, from the early phases when the solar nebula was still present to the late stages of gas‐free sweepup of scattered planetesimals. Asteroids and the comets from the Jupiter‐Saturn region were the first water deliverers, when the Earth was less than half its present mass. The bulk of the water presently on Earth was carried by a few planetary embryos, originally formed in the outer asteroid belt and accreted by the Earth at the final stage of its formation. Finally, a late veneer, accounting for at most 10% of the present water mass, occurred due to comets from the Uranus‐Neptune region and from the Kuiper Belt. The net result of accretion from these several reservoirs is that the water on Earth had essentially the D/H ratio typical of the water condensed in the outer asteroid belt. This is in agreement with the observation that the D/H ratio in the oceans is very close to the mean value of the D/H ratio of the water inclusions in carbonaceous chondrites.     

Finding KBO Flyby Targets for New Horizons

Development of the New Horizons mission to Pluto and the Kuiper Belt is now fully funded by NASA (Stern and Spencer, this volume). If all goes well, New Horizons will be launched in January 2006, followed by a Jupiter gravity assist in 2007, with Pluto arrival expected in either 2015 or 2016, depending on the launch vehicle chosen. A backup launch date of early 2007, without a Jupiter flyby, would give a Pluto arrival in 2019 or 2020. In either case, a flyby of at least one Kuiper Belt object (KBO) is planned following the Pluto encounter, sometime before the spacecraft reaches a heliocentric distance of 50 AU, in 2021 or 2023 for the 2006 launch, and 2027 or 2029 for the 2007 launch. However, none of the almost 1000 currently-known KBOs will pass close enough to the spacecraft trajectory to be targeted by New Horizons, so the KBO flyby depends on finding a suitable target among the estimated 500,000 KBOs larger than 40 km in diameter. This paper discusses the issues involved in finding one or more KBO targets for New Horizons. The New Horizons team plans its own searches for mission KBOs but will welcome other U.S, or international team who wish to become involved in exchange for mission participation at the KBO.     

Planetary Perturbers in Debris Disks

Neptune dominates the dynamics of the Kuiper Belt. By examining images of debris disks around other stars, we may be able to infer what kinds of planets shape the outer edges of other planetary systems. The last few years have seen a burst of progress in the modeling of azimuthal structures in debris disks created by planetary perturbers; new models incorporate planets on substantially eccentric orbits. I review this recent progress in debris disk dynamics and discuss the Kuiper Belt as a key example.     

The 2010 European Venus Explorer (EVE) mission proposal

The European Venus Explorer (EVE) mission described in this paper was proposed in December 2010 to ESA as an ‘M-class’ mission under the Cosmic Vision programme. It consists of a single balloon platform floating in the middle of the main convective cloud layer of Venus at an altitude of 55?km, where temperatures and pressures are benign (～25°C and ～0.5 bar). The balloon float lifetime would be at least 10 Earth days, long enough to guarantee at least one full circumnavigation of the planet. This offers an ideal platform for the two main science goals of the mission: study of the current climate through detailed characterization of cloud-level atmosphere, and investigation of the formation and evolution of Venus, through careful measurement of noble gas isotopic abundances. These investigations would provide key data for comparative planetology of terrestrial planets in our solar system and beyond.     

Odyssey: a solar system mission

The Solar System Odyssey mission uses modern-day high-precision experimental techniques to test the laws of fundamental physics which determine dynamics in the solar system. It could lead to major discoveries by using demonstrated technologies and could be flown within the Cosmic Vision time frame. The mission proposes to perform a set of precision gravitation experiments from the vicinity of Earth to the outer Solar System. Its scientific objectives can be summarized as follows: (1) test of the gravity force law in the Solar System up to and beyond the orbit of Saturn; (2) precise investigation of navigation anomalies at the fly-bys; (3) measurement of Eddington’s parameter at occultations; (4) mapping of gravity field in the outer solar system and study of the Kuiper belt. To this aim, the Odyssey mission is built up on a main spacecraft, designed to fly up to 13 AU, with the following components: (a) a high-precision accelerometer, with bias-rejection system, measuring the deviation of the trajectory from the geodesics, that is also giving gravitational forces; (b) Ka-band transponders, as for Cassini, for a precise range and Doppler measurement up to 13 AU, with additional VLBI equipment; (c) optional laser equipment, which would allow one to improve the range and Doppler measurement, resulting in particular in an improved measurement (with respect to Cassini) of the Eddington’s parameter. In this baseline concept, the main spacecraft is designed to operate beyond the Saturn orbit, up to 13 AU. It experiences multiple planetary fly-bys at Earth, Mars or Venus, and Jupiter. The cruise and fly-by phases allow the mission to achieve its baseline scientific objectives [(1) to (3) in the above list]. In addition to this baseline concept, the Odyssey mission proposes the release of the Enigma radio-beacon at Saturn, allowing one to extend the deep space gravity test up to at least 50 AU, while achieving the scientific objective of a mapping of gravity field in the outer Solar System [(4) in the above list].      

New Challenges to Trajectory Design by the Use of Electric Propulsion and Other New Means of Wandering in the Solar System

The design of spacecraft trajectories is a crucial part of a space mission design. Often the mission goal is tightly related to the spacecraft trajectory. A geostationary orbit is indeed mandatory for a stationary equatorial position. Visiting a solar system planet implies that a proper trajectory is used to bring the spacecraft from Earth to the vicinity of the planet. The first planetary missions were based on conventional trajectories obtained with chemical engine rockets. The manoeuvres could be considered 'impulsive' and clear limitations to the possible missions were set by the energy required to reach certain orbits. The gravity-assist trajectories opened a new way of wandering through the solar system, by exploiting the gravitational field of some planets. The advent of other propulsion techniques, as electric or ion propulsion and solar sail, opened a new dimension to the planetary trajectory, while at the same time posing new challenges. These 'low thrust' propulsion techniques cannot be considered 'impulsive' anymore and require for their study mathematical techniques which are substantially different from before. The optimisation of such trajectories is also a new field of flight dynamics, which involves complex treatments especially in multi-revolution cases as in a lunar transfer trajectory. One advantage of these trajectories is that they allow to explore regions of space where different bodies gravitationally compete with each other. We can exploit therefore these gravitational perturbations to save fuel or reduce time of flight. The SMART-1 spacecraft, first European mission to the Moon, will test for the first time all these techniques. The paper is a summary report on various activities conducted by the project team in these areas.     

Planet X — The current status

Neptune and Pluto were discovered because of predictions derived from the differences between the observations and ephemerides of Uranus, but Pluto wasn't the predicted planet and the discrepancies still exist. This continuing existence of systematic differences between the observations and ephemerides of Uranus and Neptune has led to predictions of a Planet X. The demise of the dinosaurs and the existence of comets have been cited as additional evidence for another celestial object.Therefore, possible bodies have been hypothesized in the outer part of the solar system, or out beyond the solar system, including a binary companion, Nemesis. The theory of relativity and the incompleteness of the law of gravity have also been suggested as explanations for the outer planet discrepancies. Predictions of the possible locations of planet X have been made, with rather large uncertainties, and selected searches of some regions have yielded nothing. IRAS and Pioneer observations exist as additional sources of useful observational data.     

Astrodynamical Space Test of Relativity Using Optical Devices I (ASTROD I)—A class-M fundamental physics mission proposal for Cosmic Vision 2015–2025

ASTROD I is a planned interplanetary space mission with multiple goals. The primary aims are: to test general relativity with an improvement in sensitivity of over three orders of magnitude, improving our understanding of gravity and aiding the development of a new quantum gravity theory; to measure key solar system parameters with increased accuracy, advancing solar physics and our knowledge of the solar system; and to measure the time rate of change of the gravitational constant with an order of magnitude improvement and the anomalous Pioneer acceleration, thereby probing dark matter and dark energy gravitationally. It is an international project, with major contributions from Europe and China and is envisaged as the first in a series of ASTROD missions. ASTROD I will consist of one spacecraft carrying a telescope, four lasers, two event timers and a clock. Two-way, two-wavelength laser pulse ranging will be used between the spacecraft in a solar orbit and deep space laser stations on Earth, to achieve the ASTROD I goals. A second mission, ASTROD (ASTROD II) is envisaged as a three-spacecraft mission which would test General Relativity to 1 ppb, enable detection of solar g-modes, measure the solar Lense–Thirring effect to 10 ppm, and probe gravitational waves at frequencies below the LISA bandwidth. In the third phase (ASTROD III or Super-ASTROD), larger orbits could be implemented to map the outer solar system and to probe primordial gravitational-waves at frequencies below the ASTROD II bandwidth.

Dust Disks around Main Sequence Stars

Many nearby main sequence stars are surrounded by cool dust radiating strongly in the far-IR. The characteristics of some of the prototype systems will be discussed. The dust is understood to represent debris related to planet formation, and the known disk systems may be dense analogs of the Kuiper Belt structure in our outer solar system. ISO observing programs plan to address questions regarding how common planetary material is around normal stars and how its amount and location depend on system age. These questions are central to an understanding of the place of the "Vega/β Pic" disks in stellar evolution. Preliminary results from the first year of ISO operation will be reported. This revised version was published online in August 2006 with corrections to the Cover Date.     

Inner Solar System dynamical analogs of plutinos

By studying orbits of asteroids potentially in 3:2 exterior mean motion resonance with Earth, Venus, and Mars, we have found plutino analogs. We identify at least 27 objects in the inner Solar System dynamically protected from encounter through this resonance. These are four objects associated with Venus, six with Earth, and seventeen with Mars. Bodies in the 3:2 exterior resonance (including those in the plutino resonance associated with Neptune) orbit the Sun twice for every three orbits of the associated planet, in such a way that with sufficiently low libration amplitude close approaches to the planet are impossible. As many as 15% of Kuiper Belt objects share the 3:2 resonance, but are poorly observed. One of several resonance sweeping mechanisms during planetary migration is likely needed to explain the origin and properties of 3:2 resonant Kuiper Belt objects. Such a mechanism likely did not operate in the inner Solar System. We suggest that scattering by the next planet out allows entry to, and exit from, 3:2 resonance for objects associated with Venus or Earth. 3:2 resonators of Mars, on the other hand, do not cross the paths of other planets, and have a long lifetime. There may exist some objects trapped in the 3:2 Mars resonance which are primordial, with our tests on the most promising objects known to date indicating lifetimes of at least tens of millions of years. Identifying 3:2 resonant systems in the inner Solar System permits this resonance to be studied on shorter timescales and with better determined orbits than has been possible to date, and introduces new mechanisms for entry into the resonant configuration.     

New Horizons: The First Reconnaissance Mission to Bodies in the Kuiper Belt

NASA has long been planning a mission of exploration to Pluto-Charon and the Kuiper Belt (e.g., Terrile et al., 1997). In 2001 NASA selected such a mission (NASA, 2001), called New Horizons, for design and development. New Horizons is now funded and planning a launch in January 2006. The mission plans to carry 8 scientific sensors and make flybys of Pluto-Charon and one or more KBOs. Statistical Monte Carlo simulations indicate that New Horizons has sufficient fuel to reach one or more KBOs with diameters exceeding 35 km. If launched as planned in 2006, the mission will use a Jovian gravity assist, arriving at Pluto-Charon in 2015 or 2016; if launched in its backup window in 2007, a Jovian gravity assist is not feasible and arrival will be later – 2019. Below we briefly summarize the New Horizons mission, concentrating on its role in Kuiper Belt exploration.     

The future of Stardust science

Recent observations indicate that >99% of the small bodies in the solar system reside in its outer reaches—in the Kuiper Belt and Oort Cloud. Kuiper Belt bodies are probably the best‐preserved representatives of the icy planetesimals that dominated the bulk of the solid mass in the early solar system. They likely contain preserved materials inherited from the protosolar cloud, held in cryogenic storage since the formation of the solar system. Despite their importance, they are relatively underrepresented in our extraterrestrial sample collections by many orders of magnitude (~10¹³ by mass) as compared with the asteroids, represented by meteorites, which are composed of materials that have generally been strongly altered by thermal and aqueous processes. We have only begun to scratch the surface in understanding Kuiper Belt objects, but it is already clear that the very limited samples of them that we have in our laboratories hold the promise of dramatically expanding our understanding of the formation of the solar system. Stardust returned the first samples from a known small solar system body, the Jupiter‐family comet 81P/Wild 2, and, in a separate collector, the first solid samples from the local interstellar medium. The first decade of Stardust research resulted in more than 142 peer‐reviewed publications, including 15 papers in Science. Analyses of these amazing samples continue to yield unexpected discoveries and to raise new questions about the history of the early solar system. We identify nine high‐priority scientific objectives for future Stardust analyses that address important unsolved problems in planetary science.     

Mass of the Kuiper belt

The Kuiper belt includes tens of thousand of large bodies and millions of smaller objects. The main part of the belt objects is located in the annular zone between 39.4 and 47.8 au from the Sun; the boundaries correspond to the average distances for orbital resonances 3:2 and 2:1 with the motion of Neptune. One-dimensional, two-dimensional, and discrete rings to model the total gravitational attraction of numerous belt objects are considered. The discrete rotating model most correctly reflects the real interaction of bodies in the Solar system. The masses of the model rings were determined within EPM2017—the new version of ephemerides of planets and the Moon at IAA RAS—by fitting spacecraft ranging observations. The total mass of the Kuiper belt was calculated as the sum of the masses of the 31 largest trans-Neptunian objects directly included in the simultaneous integration and the estimated mass of the model of the discrete ring of TNO. The total mass is \((1.97 \pm 0.35)\times 10^{-2} \ m_{\oplus }\). The gravitational influence of the Kuiper belt on Jupiter, Saturn, Uranus, and Neptune exceeds at times the attraction of the hypothetical 9th planet with a mass of \(\sim 10 \ m_{\oplus }\) at the distances assumed for it. It is necessary to take into account the gravitational influence of the Kuiper belt when processing observations and only then to investigate residual discrepancies to discover a possible influence of a distant large planet.     

Project Pan-STARRS and the Outer Solar System

Pan-STARRS, a funded project to repeatedly survey the entire visiblesky to faint limiting magnitudes (m_R～ 24), will have asubstantial impact on the study of the Kuiper Belt and outer solarsystem. We briefly review the Pan-STARRS design philosophy and sketchsome of the planetary science areas in which we expect this facility tomake its mark. Pan-STARRS will find ～20,000 Kuiper Belt Objectswithin the first year of operation and will obtain accurate astrometryfor all of them on a weekly or faster cycle. We expect that it willrevolutionise our knowledge of the contents and dynamical structure ofthe outer solar system.     

The dynamical stability of a Kuiper Belt-like region

The dynamics of the Kuiper Belt region between 33 and 63 au is investigated just taking into account the gravitational influence of Neptune. Indeed the aim is to analyse the information which can be drawn from the actual exoplanetary systems, where typically physical and orbital data of just one or two planets are available. Under this perspective we start our investigation using the simplest three-body model (with Sun and Neptune as primaries), adding at a later stage the eccentricity of Neptune and the inclinations of the orbital planes to evaluate their effects on the Kuiper Belt dynamics. Afterwards we remove the assumption that the orbit of Neptune is Keplerian by adding the effect of Uranus through the Lagrange–Laplace solution or through a suitable resonant normal form. Finally, different values of the mass ratios of the primary to the host star are considered in order to perform a preliminary analysis of the behaviour of exoplanetary systems. In all cases, the stability is investigated by means of classical tools borrowed from dynamical system theory, like Poincaré mappings and Lyapunov exponents.     

Definitive orbit of minor planet (29) Amphitrite from 91 oppositions 1825--1985 and a new determination of the mass of Jupiter

Minor planet (29) Amphitrite will serve as a target for the first asteroid flyby with NASA spacecraft Galileo on December 6, 1986. It also represents a first priority object for solar system observations with the ESA astrometry satellite Hipparcos which is scheduled to be launched in 1988. In order to meet the high astrometric accuracy requirements, a definitive orbit based on 1,577 observations from 91 apparitions covering the time span 1825 to 1985 is evaluated. (29) Amphitrite moves near to the 3:1 resonance to Jupiter and seems particularly suited for a new determination of the mass of the Jupiter system. A most probable mass value of 1/(1047.369±0.029) solar masses is obtained and the relaibility of this result is discussed.     

The solar radiation incident at the top of the atmospheres of Uranus and Neptune

The latitudinal and seasonal variation of the direct solar radiation incident at the top of the atmosphere of Uranus and Neptune has been recalculated by use of updated values for the period of axial rotation and the oblateness. Values for the solar radiation are given in Watt per square meter instead of the unit used in earlier papers (calories per square centimeter per planetary day). The solar radiation averaged over a season and a year as a function of planetocentric latitude has also been reviewed. In addition, attention is made to the ratio of the solar radiation incident on an oblate planet to that incident on a spherical planet.