Asteroid Flyby Research Articles

Design of optimal impulse transfers from the Sun–Earth libration point to asteroid

The lunar probe, Chang’E-2, is the first one to successfully achieve both the transfer to Sun–Earth libration point orbit and the flyby of near-Earth asteroid Toutatis. This paper, taking the Chang’E-2’s asteroid flyby mission as an example, provides a method to design low-energy transfers from the libration point orbit to an asteroid. The method includes the analysis of transfer families and the design of optimal impulse transfers. Firstly, the one-impulse transfers are constructed by correcting the initial guesses, which are obtained by perturbing in the direction of unstable eigenvector. Secondly, the optimality of one-impulse transfers is analyzed and the optimal impulse transfers are built by using the primer vector theory. After optimization, the transfer families, including the slow and the fast transfers, are refined to be continuous and lower-cost transfers. The method proposed in this paper can be also used for designing transfers from an arbitrary Sun–Earth libration point orbit to a near-Earth asteroid in the Sun–Earth–Moon system.

Modeling method and preliminary model of Asteroid Toutatis from Chang’E-2 optical images

Shape modeling is fundamental to the analysis of dynamic environment and motion around asteroid. Chang’E-2 successfully made a flyby of Asteroid 4179 Toutatis and obtained plenty of high-resolution images during the mission. In this paper, the modeling method and preliminary model of Asteroid Toutatis are discussed. First, the optical images obtained by Chang’E-2 are analyzed. Terrain and silhouette features in images are described. Then, the modeling method based on previous radar model and preliminary information from optical images is proposed. A preliminary polyhedron model of Asteroid Toutatis is established. Finally, the spherical harmonic coefficients of Asteroid Toutatis based on the polyhedron model are obtained. Some parameters of model are analyzed and compared. Although the model proposed in this paper is only a preliminary model, this work offers a valuable reference for future high-resolution models.

The analysis of the Earth co-orbital motion of Chang’e-2 after asteroid flyby

After the Chang’e-2 spacecraft conducted a successful asteroid flyby, where is it flying to in interplanetary space? This question is answered via an introduction to the Chang’e-2’s flight trajectory after asteroid flyby that is termed Earth co-orbital motion. Based on preliminary analysis using the dynamical systems theory, Poincare sections concept, and routine optimization techniques, it is now predicted that, from the point of view of orbital mechanics, Chang’e-2 is capable of returning to the vicinity of the Earth in no more than 20 a and might be recaptured by the Earth if appropriate orbital maneuvers are going to be exerted, either by itself or by another spacecraft that captures Chang’e-2.

The Ginger-shaped Asteroid 4179 Toutatis: New Observations from a Successful Flyby of Chang'e-2

On 13 December 2012, Chang'e-2 conducted a successful flyby of the near-Earth asteroid 4179 Toutatis at a closest distance of 770 ± 120 meters from the asteroid's surface. The highest-resolution image, with a resolution of better than 3 meters, reveals new discoveries on the asteroid, e.g., a giant basin at the big end, a sharply perpendicular silhouette near the neck region, and direct evidence of boulders and regolith, which suggests that Toutatis may bear a rubble-pile structure. Toutatis' maximum physical length and width are (4.75 × 1.95 km) ±10%, respectively, and the direction of the +z axis is estimated to be (250 ± 5°, 63 ± 5°) with respect to the J2000 ecliptic coordinate system. The bifurcated configuration is indicative of a contact binary origin for Toutatis, which is composed of two lobes (head and body). Chang'e-2 observations have significantly improved our understanding of the characteristics, formation, and evolution of asteroids in general.

Trajectory Design for the Transfer from the Lissajous Orbit of Sun-Earth System to Asteroids

This paper investigates the trajectory design issue for the transfer from the Lissajous orbit of CHANGE 2 probe to asteroids. First, an intersection search method, which is a general design method of low-energy transfer trajectories by searching the intersection of unstable and stable manifolds in the circular restricted three-body problem (CRTBP), is applied to produce the zero-cost flyby trajectories for the asteroid flyby missions of CHANGE 2 probe, and the simulation result shows that this method is invalid. Then, for this trajectories design issue, a perturbation method, which consists of a process of searching initial trajectories by applying velocity perturbations in the direction of unstable eigenvectors of the Lissajous orbit and a trajectory correction process with two-level differential correction, is proposed. Finally, the transfer opportunities between the Lissajous orbit of CHANGE 2 and asteroids Toutatis and 2010 JK1 are searched by the perturbation method. The results show that the method proposed in this paper can identify low-energy transfer trajectories for asteroid flyby missions of CHANGE 2. Moreover, this method provides a global understanding of the trend of impulsive maneuvers over the transfer date.

Reconciling main belt asteroid spectral flux density measurements with a self-consistent thermophysical model

Thermophysical models consistent with Earth’s Moon and infrared and submillimeter measurements obtained during the Rosetta fly-by of Asteroid (21) Lutetia have been applied to computations of thermal fluxes from Asteroids (4) Vesta, (1) Ceres, (2) Pallas, and (10) Hygiea. Comparisons are made with Earth- and space-based flux measurements that cover the infrared-to-centimeter wavelength spectrum. The models account for diurnal and seasonal effects as well as viewing geometry at the epochs of the observational data. Model fluxes at submillimeter and longer wavelengths are computed from a radiative transfer integral that takes into account temperature gradients in the subsurface and a plausible range of lunar-like electrical properties. A surface roughness model in the form of hemispherical mini-craters is included in the evaluation of the infrared flux comparisons.Results of the infrared-to-centimeter comparisons confirm that low thermal inertia (I∼5–80J/(Km2s0.5)) material constitutes the upper ∼1cm of the four large asteroids studied. For the non-isothermal models considered, spectral emissivities near 1.0 fit the data of all four asteroids with no significant variations indicated throughout the infrared to microwave spectrum. Enhanced measured infrared flux levels (relative to a smooth surface model) of the four asteroids are constrained by a surface model with up to 50% fractional coverage of hemispherical craters, consistent with both the recent Lutetia result and historical measurements of the Moon’s thermal flux characteristics.

Landing on small bodies: From the Rosetta Lander to MASCOT and beyond

Recent planning for science and exploration missions has emphasized the high interest in the close investigation of small bodies in the Solar System. In particular in-situ observations of asteroids and comets play an important role in this field and will contribute substantially to our understanding of the formation and history of the Solar System.The first dedicated comet Lander is Philae, an element of ESA's Rosetta mission to comet 67/P Churyumov–Gerasimenko. Rosetta was launched in 2004. After more than 7 years of cruise (including three Earth and one Mars swing-by as well as two asteroid flybys) the spacecraft has gone into a deep space hibernation in June 2011. When approaching the target comet in early 2014, Rosetta will be re-activated. The cometary nucleus will be characterized remotely to prepare for Lander delivery, currently foreseen for November 2014.The Rosetta Lander was developed and manufactured, similar to a scientific instrument, by a consortium consisting of international partners. Project management is located at DLR in Cologne/Germany, with co-project managers at CNES (France) and ASI (Italy). The scientific lead is at the Max Planck Institute for Solar System Science (Lindau, Germany) and the Institut d'Astrophysique Spatiale (Paris).Mainly scientific institutes provided the subsystems, instruments and the complete, qualified lander system. Operations are performed in two dedicated centers, the Lander Control Center (LCC) at DLR-MUSC and the Science Operations and Navigation Center (SONC) at CNES. This concept was adopted to reduce overall cost of the project and is foreseen also to be applied for development and operations of future small bodies landers.A mission profiting from experience gained during Philae development and operations is MASCOT, a surface package for the Japanese Hayabusa 2 mission. MASCOT is a small (∼10kg) mobile device, delivered to the surface of asteroid 1999JU3. There it will operate for about 16h. During this time a camera, a magnetometer, a thermal monitor and an IR analytical instrument will provide ground truth and thus will even be able to support the selection of possible sampling sites for the main spacecraft.MASCOT is a flexible design that can be adapted to a wide range of missions and possible target bodies. Also the payload is flexible to some extent (with an overall mass in the 3kg range). For example, the surface package is part of the optional strawman payload for MarcoPolo-R, a European asteroid sample return mission, proposed for ESA Cosmic Vision M-class.

Near-Earth asteroid flyby trajectories from the Sun-Earth L2 for Chang’e-2’s extended flight

Driven by curiosity about possible flight options for the Chang’e-2 spacecraft after it remains at the Sun-Earth L2 point, effective approaches were developed for designing preliminary fuel-optimal near-Earth asteroid flyby trajectories. The approaches include the use of modified unstable manifolds, grid search of the manifolds’ parameters, and a two-impulse maneuver for orbital phase matching and z-axis bias change, and are demonstrated to be effective in asteroid target screening and trajectory optimization. Asteroid flybys are expected to be within a distance of 2 × 107 km from the Earth owing to the constrained Earth-spacecraft communication range. In this case, the spacecraft’s orbital motion is significantly affected by the gravities of both the Sun and the Earth, and therefore, the concept of the “heliocentric oscillating-Kepler orbit” is proposed, because the classical orbital elements of the flyby trajectories referenced in the heliocentric inertial frame oscillate significantly with respect to time. The analysis and results presented in this study show that, among the asteroids whose orbits are the most accurately predicted, “Toutatis”, “2005 NZ6”, or “2010 CL19” might be encountered by Chang’e-2 in late 2012 or 2013 with total impulses less than 100m/s.

Trajectory Analysis and Design for A Jupiter Exploration Mission

The trajectory design for a Jupiter exploration mission is investigated in this paper. The differences between the Jupiter exploration trajectory and the Mars or Venus exploration trajectory are mainly concerned about. Firstly, the selection of the Jupiter-centered orbit is analyzed based on the Galileo Jupiter mission. As for the Earth-Jupiter transfer orbit, the fuel consumption of the direct transfer is too large. So the energy-saving technologies such as the planetary gravity assist should be used for the trajectory to the Jupiter. The different sequences of planetary gravity assists are examined by applying the Particle Swarm Optimization (PSO). According to the searched result, the Venus-Earth-Earth sequence (VEEGA) is the most effective one for the Jupiter mission. During the Jupiter mission, the spacecraft will pass though the main asteroid belt between the orbits of Mars and Jupiter, and may encounter multiple asteroids. Therefore the Jupiter mission is able to combine with the main-belt asteroid flyby mission. The design method of the intermediate asteroid flyby trajectory is also considered. At last, an entire trajectory for the Jupiter mission launched in 2023 is presented.

First-round design of the flight scenario for Chang’e-2’s extended mission: takeoff from lunar orbit

Chang’e-2, Chinese second lunar probe, was inserted into a 100 km altitude low lunar orbit on October 9th, 2010, its purpose is to continuously photograph the lunar surface and possibly chosen landing sites for future lunar missions. The probe will still carry considerable amount of propellant after completing all prescribed tasks in about six months. After the successful launch of Chang’e-2, we began designing the probe’s subsequent flight scenario, considering a total impulse of 1 100 m/s for takeoff from low lunar orbit and a maximum 3×106 km distance for Earth-probe telecommunication. Our first-round effort proposed a preliminary flight scenario that involves consecutive arrivals at the halo orbits around the Earth-Moon L1/L2 and Sun-Earth L1/L2 points, near-Earth asteroid flyby, Earth return, and lunar impact. The designed solution of Chang’e-2’s subsequent flight scenario is a multi-segment flight trajectory that serves as a reference for making the final decision on Chang’e-2’s extended mission, which is a flight to the Sun-Earth L2 point, and a possible scheme of lunar impact via Earth flyby after remaining at the Sun-Earth L2 point was also presented. The proposed flight trajectory, which possesses acceptable solution accuracy for mission analysis, is a novel design that effectively exploits the invariant manifolds in the circular restricted three-body problem and the patched-manifold-conic method.

Interpretation of combined infrared, submillimeter, and millimeter thermal flux data obtained during the Rosetta fly-by of Asteroid (21) Lutetia

The European Space Agency’s Rosetta spacecraft is the first Solar System mission to include instrumentation capable of measuring planetary thermal fluxes at both near-IR (VIRTIS) and submillimeter–millimeter (smm–mm, MIRO) wavelengths. Its primary mission is a 1year reconnaissance of Comet 67P/Churyumov–Gerasimenko beginning in 2014. During a 2010 close fly-by of Asteroid 21 Lutetia, the VIRTIS and MIRO instruments provided complementary data that have been analyzed to produce a consistent model of Lutetia’s surface layer thermal and electrical properties, including a physical model of self-heating. VIRTIS dayside measurements provided highly resolved 1K accuracy surface temperatures that required a low thermal inertia, I<30J/(Km2s0.5). MIRO smm and mm measurements of polar night thermal fluxes produced constraints on Lutetia’s subsurface thermal properties to depths comparable to the seasonal thermal wave, yielding a model of I<20J/(Km2s0.5) in the upper few centimeters, increasing with depth in a manner very similar to that of Earth’s Moon. Subsequent MIRO-based model predictions of the dayside surface temperatures reveal negative offsets of ∼5–30K from the higher VIRTIS-measurements. By adding surface roughness in the form of 50% fractional coverage of hemispherical mini-craters to the MIRO-based thermal model, sufficient self-heating is produced to largely remove the offsets relative to the VIRTIS measurements and also reproduce the thermal limb brightening features (relative to a smooth surface model) seen by VIRTIS. The Lutetia physical property constraints provided by the VIRTIS and MIRO data sets demonstrate the unique diagnostic capabilities of combined infrared and submillimeter/millimeter thermal flux measurements.

Shape reconstruction of irregular bodies with multiple complementary data sources

We discuss inversion methods for shape reconstruction with complementary data sources. The current main sources are photometry, adaptive optics or other images, occultation timings, and interferometry, and the procedure can readily be extended to include range-Doppler radar and thermal infrared data as well. We introduce the octantoid, a generally applicable shape support that can be automatically used for surface types encountered in planetary research, including strongly nonconvex or non-starlike shapes. We present models of Kleopatra and Hermione from multimodal data as examples of this approach. An important concept in this approach is the optimal weighting of the various data modes. We define the maximum compatibility estimate, a multimodal generalization of the maximum likelihood estimate, for this purpose. We also present a specific version of the procedure for asteroid flyby missions, with which one can reconstruct the complete shape of the target by using the flyby-based map of a part of the surface together with other available data. Finally, we show that the relative volume error of a shape solution is usually approximately equal to the relative shape error rather than its multiple. Our algorithms are trivially parallelizable, so running the code on a CUDA-enabled graphics processing unit is some two orders of magnitude faster than the usual single-processor mode.

Rosetta enters hibernation

The International Rosetta Mission was launched on 2nd March 2004 on its 10 years journey to comet 67P/Churyumov–Gerasimenko. Rosetta will reach the comet in 2014, orbit it for about 1.5 years down to distances of a few kilometres and deliver the Lander Philae onto its surface.Following the fly-by of Asteroid (21-)Lutetia in 2010, Rosetta continued its travel towards the planned comet encounter in 2014. In this phase Rosetta became the solar-powered spacecraft that reached the largest Sun distances in history of spaceflight, up to an aphelion at 5.3AU in October 2012. At distances above 4.5AU the spacecraft's solar generator power is not sufficient to keep all spacecraft systems active. Therefore in June 2011 the spacecraft was spun up to provide gyroscopic stabilisation, and most of its on-board units, including those used for attitude control and communications, were switched off. Over this “hibernation” phase of about 2.5 years the spacecraft will keep a minimum of autonomy active to ensure maintenance of safe thermal conditions.After Lutetia fly-by, flight controllers had to tackle two anomalies that had significant impacts on the mission operations. A leak in the reaction control subsystem was confirmed and led to the re-definition of the operational strategy to perform the comet rendezvous manoeuvres planned for 2011 and 2014. Anomalous jumps detected in the estimated friction torque of two of the four reaction wheels used for attitude control forced the rapid adoption of measures to slow down the wheels degradation. This included in-flight re-lubrication activities and changes in the wheels operational speed regime.Once the troubleshooting of the two anomalies was completed, and the related operational scenarios were implemented, the first large (790m/s) comet rendezvous manoeuvre was executed, split into several long burns in January and February 2011. The second burn was unexpectedly interrupted due to the anomalous behaviour of two thrusters, causing attitude off-pointing. Flight controllers modified the thrusters operation parameters in the on-board software and managed to re-start the sequence of burns and successfully complete the manoeuvre. After the manoeuvre, preparation for the critical spin-up and hibernation entry activities, planned for June 2011, began.This paper presents the activities carried out on Rosetta in the final year before hibernation entry. The major anomalies and the related troubleshooting and workaround solutions are detailed. Lessons learned from the operation of the first spacecraft operating with solar power at Jupiter-like distances from the Sun are presented and discussed.

The light curve of asteroid 21 Lutetia measured by VIRTIS-M during the Rosetta fly-by

On 10 July 2010, the Rosetta spacecraft encountered the asteroid 21 Lutetia on its way to the comet 67P/Churyumov-Gerasimenko. This was the second of two planned asteroid fly-bys performed by the probe, the first fly-by being with the much smaller asteroid 2867 Steins in September 2008. The VIRTIS imaging spectrometer (angular resolution 0.250mrad, overall spectral range 0.25–5.1μm) onboard Rosetta acquired data of Lutetia well before the closest approach phase, when the target was spatially unresolved, in order to obtain a light curve of the asteroid in the infrared spectral range extending up to 5μm. Two light curve sequences were obtained, separated by a gap of 3.25h. During the first light curve, the solar phase angle of the asteroid had a constant value of 10.8°, while in the second light curve it decreased from 10.7° to 7.8°.Assuming the latest value derived for the rotational period of Lutetia (8.168270±0.000001h, Carry et al., 2010), each of the light curve sequences covered roughly one rotation of the asteroid. In the overall campaign, VIRTIS collected a total of 16 cubes where Lutetia was captured 165 times, both in the visual and in the infrared range. Given the low signal and the unresolved appearance of the source in the first sequence and at the beginning of the second sequence, several wavelengths turn out to be unsuited to sample the light curve. Nevertheless, in both the VIS and IR ranges we find a similar trend, with two different maxima and minima during one rotational period. In the edge-on view with north pole tilted towards the spacecraft, we find that the near infrared spectrum of Lutetia in its northern hemisphere is essentially flat and grey, with no relevant absorption features in the range 1–3.5μm within the sensitivity of the VIRTIS instrument. We find a marked spectral homogeneity throughout the two explored rotational periods within the 1−σ variability of the data. Consistent with the results obtained at closest approach by Coradini et al. (2011), we confirm the absence of OH features at 1.9, 2.7, and 3μm, and no other absorption features are seen in the near infrared range 1–3.5μm of VIRTIS data at northern latitudes.

Continuum and spectroscopic observations of asteroid (21) Lutetia at millimeter and submillimeter wavelengths with the MIRO instrument on the Rosetta spacecraft

The European Space Agency's Rosetta spacecraft made a close flyby of asteroid (21) Lutetia on July 10, 2010. The spacecraft carries a dual-band radiometer/spectrometer instrument, named MIRO, which operates at 190GHz (1.6mm) and 560GHz (0.5mm). During the flyby, the MIRO instrument measured the temperature of Lutetia in both the northern and southern hemispheres. At the time of the flyby, the northern hemisphere was seasonally sun-lit and warmer than the southern hemisphere. Subsurface (depths from ∼2mm to ∼2cm) temperatures ranged from ∼200K on the northern hemisphere to ∼60K on the southern hemisphere. A lunar-like regolith – very low thermal inertia<20J/(Km2s0.5) in the upper 1–3cm overlaying a layer of rapidly increasing density and thermal conductivity – is required to explain the observations. A spectroscopic search was made for H2O, CO, CH3OH, and NH3 in Lutetia's exosphere but none of the molecules were detected. An upper limit to the water column density was estimated to be <5×1011molecules/cm2 at the time of the flyby.

Astronomers gear up for asteroid fly-by

Astronomers gear up for asteroid fly-by

Optimal Control Problem for Low-Thrust Multiple Asteroid Tour Missions

The use of low-thrust propulsion systems makes very complex missions possible. Among them, multiple asteroid missions have not been the subject of much study. The design of such missions requires visiting a high number of targets. The mathematical problem is of large dimensionality, and to guarantee high accuracy, an efficient solution method should be proposed. This paper presents a possible formulation of the problem.An indirectmethod is used to formulate a multiple asteroid mission problem, which includes specific control constraints necessary for effective control and operations when approaching an asteroid for flyby. No a priori knowledge of the control structure is required. Special care is given to the Jacobian structure formaximum efficiency. The algorithm is implemented with parallelization for computational speed. Overall, the formulation demonstrates robustness in solving highdimensional optimal control problems using an indirect approach, which would mostly be impossible with direct methodswithout significantly impacting the solution accuracy. Interplanetary trajectories are presented as examples of the method. For instance, the method is applied to the problem of the fourth edition of the Global Trajectory Optimisation Competition, and it optimizes a mission scenario with as many as 42 asteroid flybys.

Rosetta visits asteroid (21)Lutetia

The International Rosetta Mission, cornerstone of the European Space Agency Scientific Programme, was launched on 2nd March 2004 to its 10 years journey to comet Churyumov–Gerasimenko. Rosetta will reach the comet in summer 2014, orbit it for about 1.5 years down to distances of a few Kilometres and deliver the Lander Philae onto its surface. After its successful asteroid fly-by in September 2008, Rosetta came back to Earth, for the final gravity acceleration towards its longest heliocentric orbit, up to a distance of 5.3 AU. It is during this phase that Rosetta crossed for the second time the main asteroids belt and performed a close encounter with asteroid (21)Lutetia on the 10th of July 2010 at a distance of ca. 3160 km and a relative velocity of 15 km/s. The payload complement of the spacecraft was activated to perform highly valuable scientific observations. The approach phase to the celestial body required a careful and accurate optical navigation campaign that will prove to be useful also for the comet approach phase. The experience gained with first asteroid flyby in 2008 was fed back into the operations definition and preparation for this highly critical phase; this concerns in particular the operations of the navigation camera for the close-loop autonomous asteroid tracking and of the main scientific camera for high resolution imaging. It was shortly after the flyby that Rosetta became the solar-powered spacecraft to have flown furthest from the Sun (>2.72 AU). This paper presents the activities carried out and planned for the definition, preparation and implementation of the asteroid flyby mission operations, including the test campaign conducted to improve the performance of the spacecraft and payload compared to the previous flyby. The results of the flyby itself are presented, with the operations implemented, the achieved performance and the lessons learned.

Ptolemy operations and results during the Lutetia flyby

The September 5th 2008 Rosetta flyby of asteroid 2867 Šteins raised tantalising questions concerning the possibility of outgassing and extant exospheres surrounding small bodies within the asteroid belt. With this in mind and recognising the size and composition of asteroid 21 Lutetia it was decided to attempt a detection of such possible outgassing using a number of Rosetta instruments, including Ptolemy, a miniature gas chromatograph isotope ratio mass spectrometer aboard the lander Philae. Ptolemy collected 5 sets of mass spectra at hourly intervals during the Lutetia encounter; 2 background measurements either side of close approach and an exosphere detection measurement as the spacecraft passed over the subsolar point. An increase in pressure was detected at the subsolar point but the cause (exosphere or spacecraft) was unresolved with the realisation that any such detection is hindered by outgassing from the spacecraft and instrument itself. Constraints are placed on the properties of Lutetia exosphere, structure and production rates. The experience gained is used to provide suggestions for future missions or spacecraft events aimed at the detection and characterisation of the exospheres of Solar System bodies.

Magnetic field measurements during the ROSETTA flyby at asteroid (21)Lutetia

On July 10, 2010, the ROSETTA spacecraft performed a flyby at asteroid (21)Lutetia at a solar distance of 2.72 AU. The spacecraft–asteroid distance at closest approach was 3120km. The magnetometers onboard ROSETTA were operating but did not detect any conclusive signature of the asteroid. Any magnetic field signature which could possibly be attributed to the asteroid was below 1nT. Consequently an upper limit for the global magnetic properties of asteroid (21)Lutetia could be derived: maximum dipole moment ≤1.0×1012Am2, global maximum magnetization ≤2.1×10−3A/m, specific moment ≤5.9×10−7Am2/kg. Draping of magnetic fields around the nucleus was sought, but evidence for this could not be clearly identified in the data. Plasma simulations taking into account the estimated upper limit of the magnetization and possible outgassing revealed interesting structures very close to the asteroid. The results obtained at Lutetia are contrasted with the results of other asteroid flyby results.