2005 AMOS Technical Conference

Synthetic Lightcurve Signatures of Unresolved Objects: A Comparison

with Observations

Eileen V. Ryan, William H. Ryan, Ruth Juarez (NM Tech/Magdalena Ridge Observatory),

Carlos Martinez (UNM), Lou Blackwell (AFRL)

2005 AMOS Technical Conference

Asteroid/Satellite Photometry

• Asteroids/satellites shine from reflected sunlight-- prop. to cross-sectional area at small solar phase angles (0 is sun behind observer).

• As the object rotates, the amount of light varies.

• By measuring the light variation over time, we derive a rotation rate and constrain orientation & shape.

• Triaxial ellipsoid model works for large number of asteroids (Drummond et al. 1988; Michalowski et al. 1990; etc.)

• Objective: Extend triaxial model to more complex shapes for irregular asteroids & satellites.

Asteroid Ida(Rotation Images, Galileo Spacecraft)

2005 AMOS Technical Conference

“Typical” Asteroid Lightcurve

Temporal Visible/IR asteroid data.

Ryan et al. 2004

Ryan et al. 2004

Doubly periodic behavior– two minima & maxima per cycle.

2005 AMOS Technical Conference

Model Lightcurve

Asteroid Kleopatra

Origin for doubly periodic behavior– reproduced even by irregular shapes.

2005 AMOS Technical Conference

Triaxial Ellipsoid Model (Aspect=)

Lightcurve amplitude isdependent on viewing geometry (Aspect=): Maximum at Aspect= 90 and minimum at Aspect= 0 (looking along the rotation axis).

Ellipsoid’s axial dimensions: (a, b, c)

2005 AMOS Technical Conference

Amplitude Decreases with Aspect Angle

2005 AMOS Technical Conference

Triaxial Ellipsoid Model (Aspect= ):Observed Amplitude and Magnitude

H(0) = absolute magnitude

2005 AMOS Technical Conference

Aspect Angle Determined by Observer Geometry

Observing targets at various viewing geometries hopefully will allow us to cover a broad range of aspect angles to properly fit the model parameters.

2005 AMOS Technical Conference

Generalized Inverse Problem

This method allows you to fit A and H as a function of rotation axis position and the object’s shape parameters.

2005 AMOS Technical Conference

Inverse Problem Solution

2005 AMOS Technical Conference

Asteroid 16 Psyche Test Case: Resulting Fit

Parameters of fit:

Triaxial ellipsoid model has historically been an effective approach for basic shape studies.

2005 AMOS Technical Conference

More Complex Lightcurve: 3155 Lee

Ryan et al. 2000

NASA-funded work (Vesta Family): This is what prompted us to consider the crossover to satellite studies– same solution for complex objects is needed.

2005 AMOS Technical Conference

IDL Demo for Lee (possibility):

2005 AMOS Technical Conference

3782 Celle: 5 January 2003

Ryan et al. 2004

2005 AMOS Technical Conference

January 2003 (Residuals): Binary

Secondary/Primary diameters = 0.43 0.01Primary Diameter ~ 6 km (H ~ 12.5)Primary Rotational Period = 3.84 hoursOrbital Period = 36.57 0.03 hours

(Ryan, Ryan & Martinez 2004)

Mean density = 2.2 0.4 g/cm3 (Basalt ~ 2.9 g/cm3)

2005 AMOS Technical Conference

Challenges Extending this work to Artificial Satellites:

• Complex shapes and surface properties

• Coupled rotational and solar phase angles, especially for LEO’s

• Observational Challenges – Trailed images for rapid rotators (low signal)

– Tracking challenges for LEO’s

2005 AMOS Technical Conference

Current Telescope: 0.35-meter Meade

• Used for Satellite characterization studies.

• Also used for student training & ground- based observations of Deep Impact.

Before After (0.5 magnitude brightening)

Deep Impact

2005 AMOS Technical Conference

Meade 0.35-meter Data: NORAD 08840

Ryan et al. 2005

Rocket body in geosynchronous transfer orbit:

Tracking at sidereal rates and letting the rocket body trail.

2005 AMOS Technical Conference

Composite Lightcurve

Ryan et al. 2005

Peaks are very sharp--not diffuse scattering surface like asteroid lightcurves; a more mirror-like surface.

2005 AMOS Technical Conference

Work in Progress

• Continued observations of different objects (extend to LEO’s).

• Continued development of Direct Model (different surface characteristics, complex shapes, etc.).

• Collaborations to develop generalized inverse model.

• Bigger telescope!!! (next slide)

2005 AMOS Technical Conference

Future Facility: MRO 2.4-meter Telescope

Scale Model

First Light: September 2006See poster:The Magdalena Ridge Observatory 2.4-meter Telescope (Gordon Pentland, EOS Technologies)