Presented to: Chagrin Valley Astronomical Society September 3, 2011

Observing Asteroids with CCD: Measuring some characteristics with “backyard” instruments
Presented to: Chagrin Valley Astronomical Society September 3, 2011 By Ron Baker, CVAS member

Introduction: CCD observations of asteroids by “backyard” astronomers can yield many characteristics including: Orbital elements Rotational period Absolute magnitude Light scattering properties of the surface Orientation of spin axis Size & shape All with telescopes in the 8 to 14 inch range and commonly available CCD cameras.

Main-belt asteroid 581 Tauntonia
FOV 12 X 8 arcmin, 120-sec exposure, mag 14.8V, 12-inch SCT, February 2010, IHO

Reference stars from the CMC-14 catalog selected by Astrometrica.
Astrometry & Photometry Measuring the position and brightness of 581 Tauntonia Reference stars from the CMC-14 catalog selected by Astrometrica.

Characteristics of Asteroids
Orbital elements Family Rotational period Amplitude Absolute magnitude (H) Phase slope parameter (G) Taxonomic class Albedo Diameter Equitorial elongation Spin axis Shape

Orbital elements Family Rotational period Amplitude Absolute magnitude (H) Phase slope parameter (G) Taxonomic class Albedo Diameter Equitorial elongation Spin axis Shape Astrometry (from backyard observations)

Orbital elements Family Rotational period Amplitude Absolute magnitude (H) Phase slope parameter (G) Taxonomic class Albedo Diameter Equitorial elongation Spin axis Shape Astrometry (from backyard observations) Photometry (from backyard observations)

Orbital elements Family Rotational period Amplitude Absolute magnitude (H) Phase slope parameter (G) Taxonomic class Albedo Diameter Equitorial elongation Spin axis Shape Astrometry (from backyard observations) Photometry (from backyard observations) Spectroscopy, Satellite Imaging, Radar (from professional observations)

Orbital elements Family Rotational period Amplitude Absolute magnitude (H) Phase slope parameter (G) Taxonomic class Albedo Diameter Equitorial elongation Spin axis Shape Astrometry (from backyard observations) Photometry (from backyard observations) Spectroscopy, Satellite Imaging, Radar (from professional observations) Modeling (backyard and professional collaborations)

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Minor Planet Center (MPC) Operates with authority of IAU. MPC accepts observations from specially designated observatories. Official observatory code for IHO: [H75] Large scale surveys do most of the discovering now. Examples include Catalina Sky Survey, Linear & Wise. Follow up observations on newly discovered objects are critical.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Minor Planet Center (MPC) A set of 3 or more observations separated in time are needed to determine an initial set of orbital elements. Follow-up observations of new objects are needed several weeks after discovery to confirm the object’s existence and to improve the set of orbital elements. The MPC publishes lists of new objects needing confirmation. Some turn out to be Near Earth Objects (NEO) which cross the Earth’s orbit and are potentially hazardous, others are much more distant and pose no threat.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape 1998 KN3, Stack of 48 images, 60-sec each, 12-inch SCT, March 2010, IHO

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Apollo NEO (2001 FM129) Potentially Hazardous Asteroid (PHA) Discovered March 2001 by the Linear Survey 64 exp, unfiltered, 8-sec each, 12-inch SCT, mag 14, March 2010, IHO

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Apollo NEO (2001 FM129) Potentially Hazardous Asteroid (PHA) Discovered March 2001 by the Linear Survey Graphic from JPL’s Small-Body Database Browser

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Long Period Comet C2010 F1 (Boattini) Discovered February 2010 by the Catalina Sky Survey 24 exp, unfiltered, 120-sec each, 12-inch SCT, mag 19, March 2010, IHO

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Long Period Comet C2010 F1 (Boattini) Discovered February 2010 by the Catalina Sky Survey Graphic from JPL’s Small-Body Database Browser

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Major asteroid families Main Belt asteroids Semi-major axis 2-3 AU (between Mars & Jupiter). More than 500,000 known to exist. Near-Earth objects Cross Earth orbit Apollo, Aten, Amor Nearly 7,500 are known at present time. Trojans Occupies an orbit similar to a planet. Gravitationally locked before/after a planet. Most are associated with Jupiter, but also Neptune and Mars. The first Earth Trojan was recently discovered. Trans-Neptunian and Kuiper Belt objects Many sub-classes, semi-major axis 30 AU to 500+ AU

Family Census Data From the Minor Planet Center (September 2011) Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Atiras 1 Atens 672 Apollos 3,770 Amors 3,022 Hungarias 10,102 Phocaeas 11,632 Mars-Crossers ,184 Main-Belt 527,886 Hildas 3,308 Jupiter Trojans 5,073 Centaurs 227 Plutinos 241 Classical TNOs 882 Other TNOs 35 Scattered-Disk Objects 169

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Rotational period is the time required for the asteroid to make one complete rotation about its own axis in relation to the Earth. Asteroids have irregular shapes. During rotation, the asteroid presents a varying surface area to our line of sight. The period and amplitude is determined from a lightcurve, which is built by plotting observations obtained in a time series of images. There is usually no need to place the instrumental magnitudes on a standard magnitude system. When observations are made on succeeding nights, the lightcurve plot must be phased. Phased plots are usually bi-modal.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape A collaborative observing campaign was formed in the fall of 2010 to study asteroid 1082 Pirola. The results are used here to describe how photometry can be used to determine the rotational period and absolute magnitude of an asteroid. Details about this project can be found in the Minor Planet Bulletin article listed in the references. The rotational period and some other characteristics were unknown before the study. Lightcurves and data points for use in constructing a phase curve were obtained during the several months before and after opposition. The lightcurve data were archived for future spin axis and shape modeling.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape First time series: duration 6 hours.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape 2 time series separated by 24 days.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape With trial and error, the data is phased to 1 rotational period using the Fourier routine built into MPO Canopus.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Complete coverage from 3 sessions, increasing precision.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape A session late in the apparition shows the amplitude has increased. But the rotational period remains consistent.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Differential photometry & period analysis: MPO Canopus. (Credits R. Baker, F. Pilcher, V. Benishek, 2010)

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape H-G parameters were originally defined by the International Astronomical Union in 1985. Absolute magnitude (H) is a calculated value based on observed magnitudes. It represents the magnitude of an asteroid when 1 AU from both the Earth and the Sun, and at 0 degree phase angle (actually impossible orientation geometrically). Phase slope parameter (G) describes how an asteroid brightens due strictly to change in phase angle, especially near opposition. Estimated magnitude of an asteroid can be predicted using these parameters.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Phase Angle Asteroids move through phases (similar to the Moon or Venus). Asteroid phases are defined by an angle formed by 2 lines. The first line runs from the asteroid through the Earth, the second from the asteroid through the Sun. The phase angle approaches 0 degrees when the asteroid is at opposition. Most asteroids do not cross the ecliptic right at opposition. So the minimum phase angle is usually at least a few degrees (and often much more) above or below the ecliptic.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Changing Brightness Due to rotation: The amplitude of a typical lightcurve is between 0.1 and 0.5 magnitude. Most asteroids have a rotational period greater than 4 hours but less than 1 day. Due to changing distance: The distance from the Earth and also the Sun influence the asteroid’s apparent brightness. These changes occur slowly over several weeks and months. Due to changing phase: Changes slowly over several months. The change is linear at phase angles greater than roughly 7 degrees. But a surge in brightness is often observed at smaller phase angles. (Opposition effect.)

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Phase Curve The effects on brightness due to rotation and change in distance must be removed from the observations. Result is reduced magnitude. The phase curve plot yields an estimated measurement of absolute magnitude (H) and the phase slope parameter (G). H & G studies require standard magnitudes from observations in the V-band covering a wide range of phase angles. For best results the minimum phase angle should be < 1 degree.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Differential standard V-band photometry: Astrometrica H-G utility: MPO Canopus (Credits R. Baker, F. Pilcher, V. Benishek, 2010)

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Spectroscopy: Asteroids are placed in taxonomic classes on the basis of their spectral classification. Reflectance spectrum is a plot showing varying levels of reflectivity dependent on wave length. Horizontal axis is the wavelength in microns. Vertical axis is the reflectance or albedo. This is the ratio of incident sunlight reflected back from the surface. Spectral features such as absorption bands reveal the presence of specific minerals. s

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape The ECAS and SMASSII spectroscopic surveys provide the data used by astronomers to assign classes. Eight-Color Asteroid Survey (ECAS) was conducted in Most asteroids were found to fit into 3 broad categories: carbon, stone and metallic. 14 classes were established. The Small Main-Belt Asteroid Spectroscopic Survey (SMASSII) made high resolution observations in Total of 22 classes. s

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Satellite Observations The diameter of some asteroids have been measured directly by orbiting satellites. The observations are made in the Infrared band of the spectrum. Infrared Astronomical Satellite (IRAS) The Supplemental IRAS Minor Planet Survey (SIMPS) is based on IRAS, and contains data relative to asteroids. Wide-Field Infrared Survey Explorer (WISE) Research is currently underway on the vast amount of data produce by this survey.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Radar The Arecibo Observatory and Goldstone Solar System Radar are used to measure many asteroid characteristics, including the diameter of larger objects.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape A mathematical relation exists between diameter, albedo and absolute magnitude (H). It is possible for a large asteroid with a small albedo can have the same absolute magnitude (H) as a smaller asteroid with a larger albedo. When an asteroid’s diameter has been measured directly, albedo can be calculated using the observed absolute magnitude (H). Diameter can also be estimated by using the observed absolute magnitude (H) and an assumed value for albedo.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape The equitorial elongation of an asteroid can be estimated from the amplitude of the lightcurve. The amplitude normally decreases as the phase angle decreases, then increases again after opposition. Observations recorded near 0 degrees phase angle should be used for the calculation.

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape The spin axis is referenced to the ecliptic, and defined by the ecliptic longitude & latitude. An asteroid’s shape can be estimated with special modeling software. Mathematically intensive. Requires significant amounts of lightcurve data preferably at many phase angles. Data from several apparitions are needed. Duration of these studies is long-term. Suitable for collaborations between professional and amateur astronomers.

Characteristics of Asteroids (Summary)
Orbital elements Family Rotational period Amplitude Absolute magnitude (H) Phase slope parameter (G) Taxonomic class Albedo Diameter Equitorial elongation Spin axis Shape Astrometry (from backyard observations) Photometry (from backyard observations) Spectroscopy, Satellite Imaging, Radar (from professional observations) Modeling (backyard & professional collaborations)

Organizations Books Software
Association of Lunar and Planetary Observers. British Astronomical Association, Asteroid and Remote Planets Section. JPL Small-Body Database Browser. Minor Planet Center. Books Asteroids and How to Observe Them. Roger Dymock. Springer-Verlag. Lightcurve Photometry and Analysis. Brian D. Warner. Springer-Verlag. Software Astrometrica. MPO Canopus. Bdw Publishing.

References: Baker, R., Pilcher, F., Benishek, V. (2011). “Photometric Observations and Analysis of 1082 Pirola.” Minor Planet Bulletin 38, Bus, S. J., Binzel, R. P. (2002). “Phase II of the small main-belt asteroid spectroscopic survey: A feature-based taxonomy.” Icarus 158, Dymock, R., Miles, R. (2009). “A method for determining the V magnitude of asteroids from CCD images.” Journal of the British Astronomical Association 119, Harris, A.W. (1989). “The H-G Asteroid Magnitude System: Mean Slope parameters.” Lunar Planetary Science XX, Tedesco, E.F., Noah, P.V., Noah, M., and Price, S.D. (2002). “The Supplemental IRAS Minor Planet Survey.” The Astronomical Journal 123, Warner, B.D. (2007). “Initial Results from a Dedicated H-G Project.” The Minor Planet Bulletin 34,

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Presented to: Chagrin Valley Astronomical Society September 3, 2011

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