Observing Asteroids with CCD: Measuring Some Characteristics with “Backyard” Instruments

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

Astrometry (from backyard observations) Characteristics of Asteroids

Astrometry (from backyard observations) Photometry (from backyard observations) Characteristics of Asteroids

Astrometry (from backyard observations) Photometry (from backyard observations) Spectroscopy, Satellite Imaging, Radar (from professional observations) Characteristics of Asteroids

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

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

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 Characteristics of Asteroids

Orbital elements Apollo NEO 88254 (2001 FM129) Potentially Hazardous Asteroid (PHA) Family Discovered March 2001 by the Linear Survey Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape 64 exp, unfiltered, 8-sec each, 12-inch SCT, mag 14, March 2010, IHO Characteristics of Asteroids

Orbital elements Apollo NEO 88254 (2001 FM129) Potentially Hazardous Asteroid (PHA) Family Discovered March 2001 by the Linear Survey Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter Elongation Spin axis Shape Graphic from JPL’s Small-Body Database Browser Characteristics of Asteroids

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

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

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

Orbital elements Family Census Data From the Minor Planet Center (September 2011) Family Rotation period Atiras 1 Atens 672 Amplitude Apollos 3,770 Abs mag (H) Amors 3,022 Hungarias 10,102 Phase slope (G) Phocaeas 11,632 Taxonomy Mars-Crossers 7,184 Main-Belt 527,886 Albedo Hildas 3,308 Diameter Jupiter Trojans 5,073 Centaurs 227 Elongation Plutinos 241 Spin axis Classical TNOs 882 Shape Other TNOs 35 Scattered-Disk Objects 169 Characteristics of Asteroids

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

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

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. Characteristics of Asteroids

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. Characteristics of Asteroids

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. Characteristics of Asteroids

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. Characteristics of Asteroids

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. Characteristics of Asteroids

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) Characteristics of Asteroids

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

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

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

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

Orbital elements Family Rotation period Amplitude Abs mag (H) Phase slope (G) Taxonomy Albedo Diameter

Elongation Differential standard V-band photometry: Astrometrica Spin axis H-G utility: MPO Canopus (Credits R. Baker, F. Pilcher, V. Benishek, 2010) Shape Characteristics of Asteroids

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

Orbital elements The ECAS and SMASSII spectroscopic surveys Family provide the data used by astronomers to assign Rotation period classes. Amplitude • Eight-Color Asteroid Survey (ECAS) was Abs mag (H) conducted in 1984. Most asteroids were Phase slope (G) found to fit into 3 broad categories: Taxonomy carbon, stone and metallic. 14 classes were Albedo established. Diameter • The Small Main-Belt Asteroid Spectroscopic Elongation Survey (SMASSII) made high resolution Spin axis observations in 2002. Total of 22 classes. Shape Characteristics of Asteroids

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

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

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

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

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

Astrometry (from backyard observations) Photometry (from backyard observations) Spectroscopy, Satellite Imaging, Radar (from professional observations) Modeling (backyard & professional collaborations) Organizations Association of Lunar and Planetary Observers. http://alpo-astronomy.org/ British Astronomical Association, Asteroid and Remote Planets Section. http://britastro.org/asteroids/table%20of%20contents.htm JPL Small-Body Database Browser. http://ssd.jpl.nasa.gov/sbdb.cgi Minor Planet Center. http://www.minorplanetcenter.net/ Books Asteroids and How to Observe Them. Roger Dymock. Springer-Verlag. http://www.springerlink.com/ Lightcurve Photometry and Analysis. Brian D. Warner. Springer-Verlag. http://www.springerlink.com/ Software Astrometrica. http://www.astrometrica.at/ MPO Canopus. Bdw Publishing. http://www.minorplanetobserver.com/ References: Baker, R., Pilcher, F., Benishek, V. (2011). “Photometric Observations and Analysis of 1082 Pirola.” Minor Planet Bulletin 38-2, 111-114. http://www.minorplanet.info/mpbdownloads.html Bus, S. J., Binzel, R. P. (2002). “Phase II of the small main-belt asteroid spectroscopic survey: A feature-based taxonomy.” Icarus 158, 146-177. http://fkometes.pagesperso-orange.fr/documents%20pdf/bus_spectrum0_sdarticle.pdf Dymock, R., Miles, R. (2009). “A method for determining the V magnitude of asteroids from CCD images.” Journal of the British Astronomical Association 119, 149-156. http://britastro.org/asteroids/JBAA%20119%20149-156%20Dymock1.pdf Harris, A.W. (1989). “The H-G Asteroid Magnitude System: Mean Slope parameters.” Lunar Planetary Science XX, 375-376. http://www.lpi.usra.edu/meetings/lpsc1989/pdf/1192.pdf Tedesco, E.F., Noah, P.V., Noah, M., and Price, S.D. (2002). “The Supplemental IRAS Minor Planet Survey.” The Astronomical Journal 123, 1056-1085. http://iopscience.iop.org/1538-3881/123/2/1056/pdf/1538-3881_123_2_1056.pdf Warner, B.D. (2007). “Initial Results from a Dedicated H-G Project.” The Minor Planet Bulletin 34-4, 113-119. http://www.minorplanet.info/mpbdownloads.html Questions/Discussion