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Bering - The first deep space mission to map asteroidal diversity, origin and transportation

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Bering – The first deep space mission to map asteroidal diversity, origin and transportation Anja C. Andersen1 , Ren´ Michelsen2 , Henning Haack3 , and John L. Jørgensen4 e NORDITA, Blegdamsvej 17, 2100 Copenhagen, Denmark, E-mail: anja@nordita.dk Astro. Obs., NBIfAFG, Juliane Maries Vej 30, 2100 Copenhagen Denmark, E-mail: rene@astro.ku.dk 3 Geological Museum, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark, E-mail: hh@savik.geomus.ku.dk 4 Ørsted*DTU, MIS, Building 327, Tech. Uni. of Denmark, 280
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    a  r   X   i  v  :  a  s   t  r  o  -  p   h   /   0   3   1   0   4   2   8  v   1   1   5   O  c   t   2   0   0   3 Bering – The first deep space mission to mapasteroidal diversity, srcin and transportation Anja C.Andersen 1 , Ren´e Michelsen 2 , Henning Haack 3 , and John L.Jørgensen 41 NORDITA, Blegdamsvej 17, 2100 Copenhagen, Denmark, E-mail: anja@nordita.dk  2 Astro. Obs., NBIfAFG, Juliane Maries Vej 30, 2100 Copenhagen Denmark, E-mail: rene@astro.ku.dk  3 Geological Museum, Øster Voldgade 5-7, 1350 Copenhagen K, Denmark, E-mail: hh@savik.geomus.ku.dk  4 Ørsted*DTU, MIS, Building 327, Tech. Uni. of Denmark, 2800 Lyngby, Denmark, E-mail: jlj@oersted.dtu.dk  Abstract Asteroids are remnants of the material from which the So-lar System formed. Fragments of asteroids, in the form of meteorites, include samples of the first solid matter to formin our Solar System 4.5 mia years ago. Spectroscopic stud-ies of asteroids show that they, like the meteorites, rangefrom very primitive objects to highly evolved small Earth-like planets that di ff  erentiated into core mantle and crust.The asteroid belt displays systematic variations in abun-dance of asteroid types from the more evolved types in theinner belt to the more primitive objects in the outer reachesof the belt thus bridging the gap between the inner evolvedapart of the Solar System and the outer primitive part of the Solar System. High-speed collisions between asteroidsare gradually resulting in their break-up. The size distribu-tion of kilometer-sized asteroids implies that the presentlyun-detected population of sub-kilometer asteroids far out-number the known larger objects. Sub-kilometer asteroidsare expected to provide unique insight into the evolutionof the asteroid belt and to the meteorite-asteroid connec-tion. We propose a space mission to detect and character-ize sub-kilometerasteroids between Jupiter and Venus. Themission is named Bering after the famous navigatorand ex-plorer Vitus Bering. A key feature of the mission is an ad-vanced payload package, providingfull on board autonomyof both object detection and tracking, which is required inorder to study fast moving objects in deep space. The au-tonomy has the added advantage of reducing the cost of running the mission to a minimum, thus enabling scienceto focus on the main objectives. 1. Introduction Our presentunderstandingof asteroidsandtheir orbitsis al-most entirely based on surveys of main-belt asteroids withdiameters larger than 10 km. Ground based telescopescannot detect smaller objects except within the immediatevicinity of Earth and no spacecraft has, so far, detected anypreviously unknown asteroids. Despite the fact that sev-eral spacecrafts to date, statistically, must have passed bysmaller asteroids, the technology employed in these ves-sels has not held the capability of detecting these objects.Therefore such encounters have gone unnoticed by. Re-cent development in the autonomy of space-borne image- Figure 1. A fragment of the carbonaceous chondrite Al-lende that fell in Mexico in 1969. The meteorite is com- posed of dark fine grained dust, mm-sized spherical inclu-sions (chondrules) and white inclusions know as calcium-aluminum-rich inclusions (CAIs). The CAIs formed 4567  My ago and are the oldest known solids formed in the Solar System. Carbonaceous chondrites probably srcinate fromC-type asteroids which are common in the outer main-belt. and computer-technologyhas changedthis, so that it is nowpossible to detect, classify and observe during an encounterwith a small asteroid.The sub-kilometer objects between Jupiter and Venus, inparticular the Near-Earth Asteroids (NEAs), are expectedto fill the gap between the meteorites that we have studiedin very great detail in the laboratory and their large parentasteroids in the main belt that may be studied with Earth-based telescopes. The meteorites have been knocked o ff  theirparentasteroidsthroughimpacts. Theseimpactsdeliv-ered fragments in a large range of sizes. Streams of smallasteroids are connected to parent asteroids via dynamicalmechanisms responsible for the transfer of material to theinner Solar System [1].Meteorites are highly diverse geological samples of aster-oids, the Moon and Mars. They range from very primitivesamples of the first solids to form in the Solar System (Fig.1) to highly evolved samples of di ff  erentiated planetary ob- jects. The latter include iron meteorites from asteroid metal  cores resembling the core of the Earth and basaltic mete-orites from the surfaces of asteroids that had an active vol-canic activity more than 4 billion years ago. Studies of me-teorites provide detailed information about the chronologi-cal, geochemicalandgeologicalevolutionofthe earlySolarSystem. But unlike geological samples from the Earth, me-teorites are deliveredwithoutanyinformationaboutthe set-ting of the sampling site. Small asteroids, which representsfragments of asteroid collisions in the recent past, probablyhave fresh surfaces with minimal regolith cover and withminimal exposure to cosmic rays, hence with a surface thatis more representativeofthe interior. Fragmentsin the formof meteorites may therefore more easily be linked to smallasteroids than large asteroids with highly evolved surfaceproperties.Thesmall asteroidsarethereforevitalforourunderstandingof mass transportation in the inner Solar System, as well asfor providing a firm basis for the dynamical and physicalrelation between meteorites, NEAs and the asteroid mainbelt. For a thorough discussion on asteroid research see thebook by Bottke et al. [2].The Bering mission will consist of two fully autonomousspacecrafts which detects the asteroids, determine their or-bital parameters, light curve and spectral characteristics.The two spacecrafts will be identical and fly in a loose for-mation. The spacing between the two probes make it possi-ble to determinethe orbitalparametersof theasteroid. Eachprobe will be able to provide autonomous detection, track-ing, mapping and ephemeris estimation of asteroids. Theautonomous instrumentation also include automatic linkupwith Earth and inter spacecraft communication. The au-tonomous operations of the instruments are centered on theAdvanced Stellar Compass [3].The Bering mission will consist of two fully autonomousspacecrafts which detects the asteroids, determine their or-bital parameters, light curve and spectral characteristics. Alaser ranger will be used to keep track of the relative po-sitions of the two spacecraft. Simultaneous observationsfrom both spacecraft will allow us to accurately determinethe distance to detected objects and thus make it possible todetermine the orbital parameters of objects that are quicklypassing out of view. The autonomous instrumentation alsoinclude automatic linkup with Earth and inter spacecraftcommunication. The autonomous operations of the instru-ments are centered on the Advanced Stellar Compass, cf.[3], [4] and [5]. 2. Asteroids and meteorites We have very little information on the abundance and char-acteristics of objects smaller than about 1 km except forthose that have been observed in the immediate vicinity of Earth. The power law distribution of asteroid sizes suggeststhat objects smaller than 1 km are veryabundant,see Fig.2.Within the asteroid belt we have no informationabout theseobjects, since they cannot be observed from Earth and nospacecraft have been actively looking for them.The orbits of these small objects can be perturbed by phys-ical processes in the asteroid main belt, such as collisions.Since any fragmentation process tends to generate powerlaw size distributions of the fragments we should not besurprised to see that the size distribution of asteroids follow Figure 2. Nine di  ff  erent estimates of the main-belt asteroid size distribution. The small size distribution is obtained byextrapolating the observed large size trends. Figure taken from [6]. a power law distribution. There is, however, reason to be-lievethat the verysmallest asteroidsare less abundantin themain belt than a simple extrapolation of the data from thelarger asteroids would suggest. Smaller asteroids are moreeasily influenced by the Yarkovskye ff  ect 1 [7] and may thusbe removed from the belt on a shorter time scale than thelarger asteroids. Still smaller fragments may be removedasa consequence of the Poynting-Robertson e ff  ect 2 . A directmeasurement of the size distribution would allow us to gainevidence of these physical mechanisms, and how they mayhave influenced the development of the main belt.Meteorites are fragments of approximately 150 di ff  erentmain belt asteroids. Since meteorites are well studied rep-resentatives ofthe abundantlow mass tail of the objects thatimpactonEarth, a betterunderstandingoftheir srcinin theasteroid belt and their subsequent orbital evolution will al-low us to better understand the transfer of objects from themain belt to the NEA population. Detailed studies of mete-orites allow us to determine age constraints for the disrup-tions of their parent asteroids and the subsequenttransfer of fragments to the inner Solar System.Asteroid photometry shows that asteroids are very diversein terms of surface mineralogy. The variation is equiva-lent to the variation observed among meteorites although anear exact match between the spectrum of an asteroid anda group of meteorites is extremely di ffi cult to find. Possi-ble reasons for the di ff  erences between reflectance spectraof asteroids and meteorites include fine grained dust coveron asteroids, micro meteorite impacts on asteroids and ex-posure to cosmic radiation. Changes of the asteroids re-flectance spectrum due to these poorly characterized pro-cesses are referredto as space weathering. Only in one casehas it been possible to establish a reasonably good case for 1 The e ff  ect of its rotation on the path of a small object orbiting the Sun.Rotation causes a temperature variation, so thermal energy is re-radiatedanisotropically. 2 The e ff  ect of solar radiation on small objects orbiting the Sun, whichcauses them to spiral slowly in. The object absorb solar energy that isstreaming out radially, but re-radiate energy equally in all directions. As aconsequence there is a reduction in the kinetic energy, and thus in orbitalvelocity, which has the e ff  ect of reducing the size of the orbit.  Figure 3. Variations in the distance to Earth as a functionof time of the Near-Earth Asteroid 2002NY31. Epochof the figure is June 10, 2003 = JD 2452800.5. a specific asteroid-meteorite relationship. The unique spec-trum of the basaltic surface of 4 Vesta makes it the primecandidate for about 400 igneous meteorites known as theHED meteorites. There is considerable interest in estab-lishing links between other groups of meteorites and theirparent asteroids.Unlike the larger asteroids studied from space so far, smallobjects are expected to have young surfaces, and their sur-faces are therefore representative of their interior. A long-standing debate has been the relationship between the sili-caceous (S-type) asteroids and the ordinarychondritemete-orites. Di ff  erences in spectral characteristics have been at-tributed to a poorly constrained space weathering process.Since small asteroids probably have smaller life times andless gravityweshouldexpectthemtohaveyoungersurfacesthat have been exposed to space weathering for a shorterperiod of time. Also the lower gravity should reduce thebuild-up of a regolith cover on their surfaces that may hidegeologicunits underneath. Both ofthese e ff  ects will makeacomparison with spectral characteristics of meteorites andother materials easier. Data on the orbital distribution of objects with spectral characteristics similar to a group of meteorites may provide new constraints on the meteorite-asteroid relationship. 3. Detection of asteroids A number of things distinguish the brightness of an aster-oid from that of a distant star. Where the emitted radiationfrom a star is due to internal nuclear processes, the bright-ness of an asteroid entirely depends on reflected sunlight interms of the illuminated area, as well as the albedo. Thisimplies a dependency on the distances asteroid-observerand asteroid-Sun as well as the phase angle. In total 5 pa-rameters are needed to describe the brightness variations,of which 3 parameters have an explicit time dependency.Thus, even for a constant distance between an observer andthe asteroid, the brightness will vary due to the changingdistance to the Sun, quite a di ff  erent situation from observ-ing remote stars. In addition, asteroids are objects movingwith velocities of the same order of magnitude as the Earth,with distances to the Sun at the same order of magnitude Figure 4. Variations in the V-magnitude (m V  ) as a functionof time of the Near-Earth Asteroid 2002 NY31 as seen from Earth. EpochofthefigureisJune10,2003 =  JD2452800.5.The absolute magnitude of this object is H  = 17 . 3  , corre-sponding to a diameter of around 1 km. as the distance Sun-Earth. This introduces brightness vari-ations, which are not present for distant self-luminous ob- jects. Examples of these variations are illustrated in Figs.3and4.Due to the eccentricity of  e = 0 . 55 of 2002 NY31, the dis-tance to the Earth shows a very large variation over time,in terms of repeated periodic variations. This behavior di-rectly influence the V  -magnitude ( m V  ) as seen from Earth.It is seen (Fig.4) that the brightness peaks seems to fall outfrom a faint background magnitude, and that only withinconstrained regions is the object observable from a tele-scope with a given magnitude limit.These variations are dependent on the mentioned physicaland geometric parameters, so the smaller the objects, themore restricted are the favorable periods of observability.An example 3 of this can be found in Figs.5and6 The synthetic object in these figures has an absolute mag-nitude of  H  = 32 . 7, corresponding to a diameter of around1 m. It is seen, that upon a close approach to the Earth, themagnitude decreases drastically, from a background levelabove m V  = 30 to a sudden brightness of around m V  = 15.It is also noticed that the brightness peak is very sharp, infact the object magnitude is below m V  = 20 for merely 4.8hours. For a telescope with a given magnitude limit, theseobjects are only observable during the occurrence of such abrightness peak, unless the telescope is able to reach veryfaint magnitudes i.e. m V  = 30. In addition, the brightnesspeakmust appearwhile the objectis withinthe fieldof viewof the observer. In practice, a survey telescope must con-stantly monitorthe wholesky in ordernotto miss the objectdue to the short time of visibility.An obstacle, compared to the traditional way of makingground based observations, is that during one night, eachfield of sky will typically only be imaged one time as a se-ries of three or more short exposures, and the reduction of  3 The synthetic object has the orbital parameters a = 1 . 00583363, e = 0 . 04361874, i = 23 . 00978088, Ω = 29 . 24312401, ω = 89 . 19306946,  M  = 67 . 975616464, epoch = JD 2452000 . 5, H  = 32 . 70, slope parameter G = 0 . 32.  Figure 5. Distance to Earth of a synthetic NEA over 400days. The resolutionis 0.02days = 28.8minutes. The figurewas obtained using modifications to the SWIFT integrator [8], see also [9] for more details. the images will be done during the following day or days.Due to the transient nature of the brightness peak of thesmall asteroids, the image reduction would however haveto be done real time, in order to detect the object imme-diately, and initiate follow-up observations for a verifica-tion. A ground based fast-response survey has been pro-posed [10], however with a per-night data reduction it isstill remote from a real time solution.There is an additional obstacle that must be overcome. Forsub-meter objects, the brightness peaks only occurs uponvery close approach to the observer. This means, that theangularvelocityrelativetotheobserverbecomesverylarge,for the shown example it is of the order of a few hundred”  /  sec. A ground based observer, using a survey telescope,will typically need to make an optimization of the exposuretime involving the pixel size, the seeing, the astrometric ac-curacy and the angular velocity of the objects of interest.Typically, a survey using a large telescope going to faintmagnitudes, will make use of a series of 60s–120s expo-sures. Due to the large radial velocity, the signal from theobject will be smeared out on the detector (trailing loss). Figure 6. Variations in the V-magnitude (m V  ) of a synthetic NEA as seen from Earth. See Fig.5and text for details. In fact, in order to reach a resolution around 1” for thefast-moving objects, a typical survey telescope would berestricted to exposure times around a hundredth of a sec-ond, presumably posing heavy demands regarding the sizeof the telescope.Outside the brightness peak, the object is moving with anangular velocityof a few ”  /  min, so evenfor a telescope ableto reach m V  = 30, the exposure time would be constrainedto 60–120s.In summary, a survey telescope trying to detect these smallasteroids must be capable of an all-sky monitoring, able toperformrealtimedataanalysis,andabletohandlefastmov-ing objects. The alternative would be a telescope able toreach m V  = 30, or beyond, with exposure times per imageframe limited to a few minutes. For the latter option, ourconjecture is that such a telescope is currently not techno-logically feasible. For the first option, we propose Beringas a solution to meet these requirements in terms of the au-tonomy and the application of the Advanced Stellar Com-pass. Inadditionto thesimpleobjectdetection,discussedinthe above, Bering also provides the possibility of perform-ing spectroscopy  /  photometry of the feasible objects, thusadding another dimension of performancein comparison toground based observations. 4. Mission profile requirements For the reasons outlined above, the main goal of the pro-posed Bering mission is to detect and characterize a sizableamount of sub-kilometer objects from space. This will bethe first systematic survey of sub-kilometer objects in theSolar System. The numbers detected need to be su ffi cientlyhigh that the distribution of small objects with similar spec-tral characteristics and thereforepotentially identical parentasteroid may be established throughout the main belt.Due to the transient visibility variations, the probes mustdetect the objects and guide the science instruments in afully automated process.The two spacecrafts will each carry an Advanced StellarCompass (ASC) systemwith 7 cameraheads, a foldingmir-ror based multi-spectral telescope imager, magnetometersand a laser-ranger. These instruments will make it possi-ble to obtain orbital parameters, light curves, rotation state,surface composition, and in some cases albedo, size, highquality images, mass and magnetic properties, cf.[12].Inorderto determinethedistributionanddynamicsofsmallobjects and their links to the NEA population we need todetect objects in the main asteroid belt as well as insidethe Earths orbit. The objects within the mean motion res-onances in the asteroid belt that are either already NEAsor are becoming NEAs within a short time frame, gener-ally have aphelion within the main belt and spend most of their time outside the Earth orbit. We therefore propose amission profile that would give us data on the distributionof small objects all the way from 0.7 AU to 3.5 AUs, seeFig.7.Such an orbit can be achieved with a single unpow-ered gravity assist maneuver from Venus whereas the ∆ Vrequired to reach Venus could be delivered directly by thelauncher, which could be of the Soyuz class.For each detected object we proposeto automaticallydeter-mine:
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