N.19 harris asteroids-lightcurve-studies-then-and-now

Alan W. HarrisSpace Science Institute

Paolo Farinella

Memorial Symposium

Pisa, Italy

June 14-16, 2010

Asteroid Lightcurve Studies –Then and Now

The reward of the young scientist is the emotional thrill of being the first person in the history of the world to see something or to understand something. Nothing can compare with that experience,it engenders what Thomas Huxley called the Divine Dipsomania. The reward of the old scientist is the sense of having seen a vague sketch grow into a masterly landscape. Not a finished picture, of course; a picture that is still growing in scope and detail, with the application of new techniques and new skills. The old scientistcannot claim that the masterpiece is his own work. He may have roughed out part of the design, laid on a few strokes, but he has learned to accept the discoveries of others with the same delight that he experienced his own when he was young.

- Cecilia Payne-Gaposchkin, in her acceptance speech for the Henry Norris Russell Prize of the American Astronomical Society, 1977.

The Divine Dipsomania

Growth in lightcurve data over time

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Rotation Period vs. Diameter, 1979, 157 Asteroids

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Paolo left us here, but had already noted:

- dip in spin rate ~50-100 km diameter

- rubble pile spin barrier

- excess of slow rotators

- binaries? (not yet)

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Even with the meager data set available back then, Paulo recognized that asteroid lightcurves could be key to understanding the structure and evolution of asteroids:

• Shapes: equilibrium figures?• Spin rate distribution: collisional history?• Excess of slow rotators?• Rubble-pile Structure?• Binary asteroids?

Shapes from Asteroid Lightcurves - ThenInspired by the 1979 meeting Asteroids in Tucson in 1979, Paolo and his colleagues investigated whether asteroid shapes are equilibrium figures. Although the answer to this question appears to be “no”, it did encourage a program of “photometric geodesy” by PSI colleagues in Tucson, and eventually to the development of rigorous lightcurve inversion techniques that allow fairly detailed determination of asteroid shapes, pole orientations, and sidereal spin rates.

Shapes and Poles - NowShape and pole studies have finally come of age, with reliable inversion techniques.

These results show aligned spin axes due to YORP alteration.

Photometric data by Steve Slivan

Shape and pole models by Mikko Kaasalainen

Shape and pole of Ida by Galileo Mission

Spin Rate Distribution - Then In the same year, Paolo and his colleagues published a study of asteroid spin rates, in which they demonstrated an excess of slow rotators, too much to be the “tail of the distribution” and likely due to some breaking mechanism (yes, but YORP, not tides). They also speculated on the possible collisional formation of binary asteroids.

This is the data set that was available to reach these conclusions.

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Spin Rate Distribution - Now

Spin rate normalized to mean spin rate

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Large asteroid spin rate distribution is well

fit by a Maxwelliandistribution

Pravec, et al. (2008) Icarus 197, 497-504.

Harris & Pravec (2006) Proc. IAU Symp. 229, 439-447.

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Understanding the “breaking mechanism” leading to slow rotation has been a long time in coming, but is now rather certainly understood as due to the YORP effect:

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Rubble-pile Structure - Then

l’Astronomia 49, November 1985, pp. 20-25.

Uncle Scrooge #29, 1960.

Again inspired by conversations at the 1979 Asteroids meeting, Paolo and his colleagues inferred the likely “rubble-pile” structure of small asteroids. Always one to give proper credit where due, Paolo attributed the discovery to Donald Duck (Paperino), as drawn in a Walt Disney comic in 1960 by cartoonist Carl Barks (1901-2000). The asteroid (2730) Barks is named in his honor (name suggested by Peter Thomas)

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YORP spins asteroids up as well as down, thus smaller asteroids can spin both faster and slower, resulting in the “flat” distribution of spins. This reveals the “spin barrier”, essentially proving the “rubble-pile” structure of asteroids D > 0.2 km or so.

Binary Asteroids - Then

Yes, but not by lightcurves624 Hektor

Yes, but not by lightcurves216 Kliopatra

Nope192 Nausikaa

Nope82 Alkmene

Nope63 Ausonia

Nope61 Danae

Nope44 Nysa

Nope43 Ariadne

Nope39 Laetitia

Nope15 Eunomia

Binary?Asteroid

Score card (struck out):

Binary Asteroids - NowPaolo and his colleagues had a good idea, but were ahead of their time. We now have discovered dozens of binaries from their lightcurves, both synchronous binaries (like Pluto-Charon), and asynchronous binaries, where one or both components are not spin-synchronized to the orbit period. Below is the synchronous binary (90) Antiope.

Keck AO system on May 31, 2005 Lightcurves from May/June 2005, SAAO

Descamps, et al., Icarus 187 (2007) 482–499.

Asynchronous Binary Asteroids

Warner, et al. (2009) Minor Planet Bul. 36, 89-90.

A kind of binary not anticipated before they were found are partially or fully asynchronous, where the primary is not synchronized to the satellite orbit period, and the secondary may or may not be synchronized. These are clearly evolved systems, but likely not by tides – more likely by YORP. Indeed, their formation is probably driven by YORP spin-up to fission.

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Asteroid Lightcurve Studies - Then

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Asteroid Lightcurve Studies - Now

Asteroid pairs: a divorce made in HeavenIn the past couple years, a number of pairs of asteroids have been found in heliocentric orbits so similar that they must have originated from a single body or bound (binary) pair – very recently, in some cases less than 100,000 years. We (Pravec et al.) have investigated the spin statistics of mainly the larger components, and found strong evidence that these are the result of prompt ejection of binary components.

Prograde/retrograde Yarkovsky drift

Hungaria family, showing Yarkovsky drift, greater for smaller asteroids (larger H).

Yarkovsky effect (radiation pressure) causes prograde rotators to drift outward and retrograde rotators to drift inward, inversely proportional to size. (434) Hungaria has been found to rotate in a prograde sense, and is indeed somewhat outside of the center of the collisional family.

Observed secular change in rotation rate!

Taylor, et al., Science 316, 274-277 (2007).

The asteroid (54509) YORP is in a near-one year period orbit, allowing repeated observations every year for five years. Linking the observations year after year reveals a clear spin-up, due to YORP effect.

Occultation – Shape ModelsOccultation observations are the only direct, model-independent measure of the absolute dimensions of an asteroid. However, in order to relate an instantaneous 2-dimensional profile to a 3-dimensional figure, one needs a shape model, which now can be obtained by lightcurve inversion.

Timerson, et al., Minor Planet Bul. 36, 98-100 (2009). Occultation observations by IOTA, shape models by Durech.

9 Metis 19 Fortuna 135 Hertha

Education

N.19 harris asteroids-lightcurve-studies-then-and-now