Presentation on theme: "RADAR INVESTIGATION OF NEAR-EARTH ASTEROIDS Steve Ostro JPL/Caltech"— Presentation transcript:
1 RADAR INVESTIGATION OF NEAR-EARTH ASTEROIDS Steve Ostro JPL/Caltech http://echo.jpl.nasa.gov Frontiers of Astronomy with the World's Largest Radio Telescope Sep. 13, Washington, DCRadar is the most powerful astronomical technique for investigating NEAs, and I’m going to show you a few examples of what radar, and particularly Arecibo, can do.
2 The only NEA radars on the planet are shown to scale here The only NEA radars on the planet are shown to scale here. Arecibo can see twice as far, while Goldstone can see twice as much sky.Our best investigations exploit the capabilities of both instruments, but Arecibo is definitely the key instrument for this work, which has grown exponentially since the upgrade.
3 NEA Radar Investigations Since 1997 This list of NEA radar experiments since the upgrade is color-coded.Arecibo is responsible for some 85% of the observations, but while making that point,I want to stress that Goldstone’s job is to see what Arecibo cannot see,and the information provided by Goldstone is critical because of the geometric leverage it provides.Red = Arecibo Green = Goldstone Blue = G,A Purple = A,G
4 Radar Measurement Precision Range Radial Velocity(meters) (meters/second)Best radar resolution < ~0.0001Asteroid “size” ~0.01 to 1Asteroid “location” 10,000,000, ,000At our radar wavelengths of 3.5 to 13 cm, radar reflectivity is sensitive to surface bulk density,and radar polarization is sensitive to surface roughness. Overall, the diversity of NEA surface properties is extreme.But I want to focus on radar’s measurement precision, which is the primary source of its power.For range and Doppler, the columns show that our resolution is tiny compared to the dimensions ofa good-sized NEA (the source of radar’s imaging power), and miniscule compared to the typical location,so our positional measurements are powerful for orbit refinement.
5 Radar Images of 1620 Geographos Here’s an example of radar images, taken at quarter-rotation intervals.Pretend you’re looking down the pole: the images show the pole-on silhouette.
6 Radar Images and 3D Model of 4179 Toutatis When we have strong images with good orientational coverage, we can invert the data to get a 3D model.Here are some images of Toutatis and corresponding POS views of the model.The cross hairs are centered on the COM, whose location in each image is estimated as part of the fit,and is powerful for orbit refinement. The arrows are the instantaneous spin vector, also estimated during the fit.This is an example of the strong coupling between physical and dynamical properties.Observations that refine the orbit give us physical information and vice versa.
7 radar model Hayabusa Spacecraft 25143 ItokawaFor Itokawa, our orientational coverage was not good, but nonetheless our modelcaptured some of the actual characteristics of this object, the target of Japan’sHayabusa mission, so far the world’s only spacecraft rendezvous with a PHA.radar model Hayabusa Spacecraftapproach image
8 Radar Images 3D Models 4 km Here is a small sample of some NEA images and a different group of models, shown to scale.Clearly the diversity among NEAs is extreme, and there’s no such thing as a typical NEA.3D Models4 km
9 ItokawaArecibo’s combination of sensitivity and 7.5-meter range resolution is letting us obtaingeologically detailed images. On this 2-km PHA we see several-decameter blocks reminiscent ofstructures seen on the much smaller object Itokawa and the much larger object Eros.Eros
10 Contact Binaries 11066 Sigurd 2002 HK12 2002 NY40 4486 Mithra 3908 Nyx Nearly 10% of radar-imaged NEAs as large as 200 m have contact-binary shapes.This is important from the viewpoint of collision hazard mitigation, because a bomb or akinetic energy impactor might wreak havoc with one lobe but leave the other lobe untouched.Disruption of kilometre-sized asteroids by energetic collisions.Asphaug et al. (1998), Nature 393,Near-Earth Asteroid 2005 CR37:Radar Images of a Candidate Contact Binary.L. Benner et al. (2006), Icarus 182,11066 Sigurd HK NY Mithra Nyx
11 Non-Principal-Axis Rotators: ToutatisRadar-determined NEA rotation periods range from minutes to weeks, and about a dozen NEAshave been found to be NPA rotators. Of these, Toutatis has the best defined spin state because ofradar observations.1999 JM8Mithra
12 Castalia-fixed frame Inertial frame Orbits About 4769 CastaliaRotation state is important because the dynamics of orbits close to an object depend on it, as well as onthe object’s size, shape, density distribution, and spin state.Here you see a 17-hour return orbit (0.2 m/s launch, 0.4 m/s return)about the mile-long contact-binary Castalia in asteroid-fixed and inertial frames.Castalia-fixed frame Inertial frameScheeres et al. (1996), Icarus 121,67-87
13 Toutatis Return Orbits 1.2 days2.9 days168 daysOrbits close to a NPA rotator have an entirely different character.Of course, the orbiting particle could be a piece of rock, a robot, or a person.In general, for NEAs,close trajectories will generally be unusual, and there will be issues of stability.Clearly, we want to know spin state and other critical properties before we try a spacecraft rendezvous.Toutatis-fixed frame Inertial frameScheeres et al. (1998), Icarus 132,
14 Arecibo Radar Imaging of (66391) 1999 KW4 About 1/6 of NEAs as large as 200 m apparently are binary systems.Arecibo is responsible for discovering the binary nature of about half of the 30 known NEA binaries.There are more than 130 known binary asteroids in the solar system (including the main-belt and trans-Neptunian populations).Of these 130, this object, called KW4 for short, is the best characterized.The left frame shows one of the nearly one hundred Arecibo images used in estimation of thephysical properties of the primary and secondary components, called Alpha and Beta, and of the system’s dynamics.On the right is a short movie that shows Alpha’s rotation and Beta’s motion.
15 KW4 Movies are at http://echo.jpl.nasa.gov/~ostro/kw4/index.html Alpha is 1.5 km wide, a density like Itokawa’s, a porosity as porous as the lunar surface,and a rotation so fast that it is on the verge of flying apart. On its equatorial ridge,the total acceleration is close to zero and particles are close to being in orbit.The ridge is at the potential-energy low, so particles free to move will roll there.Alpha is the best-characterized PHA larger than a half-km.Beta is a half-km object, also porous but denser, on average tidally locked so one side faces Alpha,but librating around that orientation (VIDEO BOTTOM).The system is dynamically excited, and the components’ spins and the mutual orbit are constantly changing.This is due to perturbations from close passes by the Sun and probably the Earth.It is probably no more than a miilion years old and could be a lot younger.
16 Direct Detection of the Yarkovsky Effect by Radar Ranging to Asteroid 6489 Golevka S. R. Chesley, S. J. Ostro, D. Vokrouhlicky, D. Capek, J. D. Giorgini, M. C. Nolan,J. L. Margot, A. A. Hine, L. A. M. Benner, and A. B. Chamberlin. Science 302, (2003).
17 Densities of Potentially Hazardous Asteroids Itokawa 1.9 ± 0.13 g/cm3 7% Hayabusa rendezvousKW4 Alpha ± % binary; radar imagingKW4 Beta (+0.82, −0.63) 39% binary; radar imagingGolevka Arecibo rangingand Goldstone imaging2002 CE26 Alpha 0.9 (+0.5, -0.4) 50% binary; Arecibo imaging2000 DP107 Alpha 1.7One of an PHA’s most fundamental properties is its density.Apart from Japan’s Hayabusa spacecraft result for itokawa,radar is responsible for all our PHA density measurements, and the best radardensities are competitive, and enormously cheaper.
18 SUMMARYRadar is our most powerful astronomical source of information about NEA physical properties. Radar is identifying: objects that must be metallic and objects that must be stony featureless spheroids and shapes that are elongated and irregular monolithic pieces of rock and unconsolidated rubble piles small-scale morphology ranging from smoother than the lunar surface to rougher than the rockiest terrain on Earth or Mars objects with geologically interesting features (craters, blocks, and linear structures) rotation periods ranging from minutes to weeks non-principal-axis spin states contact binaries binary systems Arecibo is 20X more powerful than any other existing or planned radar. Arecibo’s exploration of the NEA Frontier has barely begun.Missions that require maneuverability close to NEAs are our destiny, one way of another, not just because of the collision hazard, but also because the enormous population of asteroids in which Earth exists include the energetically most accessible objectsIn the solar system and the most readily available sources of water and complex organics beyond the Earth, not to mention convenient waystations for practice missions leading to a Mars expedition. For such missions, the more we know about physical configuration, including spin state, the better.
19 RADAR INVESTIGATION OF NEAR-EARTH ASTEROIDS Steve Ostro JPL/Caltech http://echo.jpl.nasa.gov Frontiers of Astronomy with the World's Largest Radio Telescope Sep. 13, Washington, DC
21 The Key to Understanding the KW4 System: Simulations that take the model shapes, masses, and average rotations and orbit as initial conditions for integrations using the actual gravitational potentials produced by the shapes and the coupling between the components’ motions: Scheeres et al. 2006, Fahnestock and Scheeres 2007.The brain behind the integrations, done on a JPL supercomputer, is Gene Fahnestock, who is hereand would be happy to answer questions about the system’s dynamics.
22 Significance of the KW4 Investigation best images yet of a binary small body best physical characterization of a km+ PHA most unusual NEA yet observed + novel physical phenomena * Alpha shape * Alpha high porosity ==> rubble pile * Alpha equatorial ridge almost in orbit * Alpha spinning near disruption limit * Beta libration, oscillations in orbit size and shape * coupling of orbital and rotational dynamics is critical * excited: Role of Sun and probably Earth flybys in excitation * young whole new realm of (extremely complex) dynamics major implications for dealing with the collision hazard milestone in understanding exotic processes and properties of NEAs KW4, as unusual as it appears, may be similar to many binary NEAs major lines of inquiry opened up: three-body problem modeling system’s formation understanding dissipation and rigidity in low-g particulate media
23 Radar reveals extreme diversity in NEAs’ surface bulk density and roughnessAt our radar wavelengths of 3.5 to 13 cm, radar reflectivity is sensitive to surface bulk densityand radar polarization is sensitive to surface roughness.High-R, low-SC/OC objects must be metallic.Overall, the diversity of NEA surface properties is extreme.
24 NEA surface roughness depends on compositional class So if optical measurements have not determined the compositional class, radar observations can constrain possibilities, andIf the class is known, so is something about surface structural complexity, which would be important forPlanning sample-return missions.
26 Alpha BetaDimensions: x 1.50 x 1.35 km + 3% x 0.46 x km + 6%Density: g cm ( ) g cm-3Resembles Itokawa ( ) Eros ( )Ida ( )Porosity: 40% to 66%, % to 58%Pole obliquity: º (+4.3º, -3.2º) º assumed (from orbit sol’n)Rotation period: h h assumed (from orbit sol’n);Beta’s average rotation is synchronous with the long axis pointed toward Alpha,but librates around that orientation.Alpha is 1.5 km wide, a density like Itokawa’s, a porosity as porous as the lunar surface,and a rotation so fast that it is on the verge of flying apart. On its equatorial ridge,The total acceleration is close to zero and particles are close to being in orbit.Beta is a half-km object, very porous but denser, on average tidally locked so one side faces Alphabut librating around that orientation.
27 (66391) 1999 KW4 Radar Imaging Data Set These time exposures show the complementarity of Goldstone and Arecibo.Goldstone got better orbital phase coverage but the data are much noisier.
28 data fit model data fit model image image image image You can see the goodness of fit between the data images and the model fit images.The fit for Alpha is excellent, but that for Beta suffers because the object’s spin stateIs not simple rigid body rotation.data fit model data fit modelimage image image image
29 Gravitational Slopes on 6489 Golevka When we have a globally accurate model, we can make reasonable assumptions about bulk density and calculategravitational slope (the angle between a plumb bob and the local surface normal). For Golevka, whose shape suggestsa collision fragment rather than a rubble-pile gravitational aggregate,some slopes are so muchgreater than the typically 35-deg angle of repose for particulate materials that we can infer a lack of regolith there.Such information would be valuable for planning sampling strategy.600 meters
30 Geometry of Delay-Doppler Radar Images RangeRadar images are geometrically different from optical pictures.Here, the three points on this asteroid give echo at thesame time delay and the same doppler frequency, that is, their echo shows up at the same point in the delay-doppler image --a three-to-one mapping. Two-to-one mappings are common, but unless we’re on the equator, the delay-Doppler trajectory ofany surface point is unique, so if we have a many-image sequence that samples enough orientations do least squaresinversion can give us an accurate shape estimate.Doppler Frequency3D Model Radar Image
31 You can go to this web site to get copies of the papers on KW4 and a bunch of accessory information,including pictures and movies. It’s the dynamics that are truly extraordinary.
32 Alpha’s equator has minimum potential energy, widely varying effective gravitational slopes, and nearly vanishing total acceleration.Alpha is an extraordinary object.