Talk of the "International Workshop on Paolo Farinella (1953-2000): the Scientists, the man", Pisa, 14-16 June 2010

1. Asteroid Lightcurve Studies –
Then and Now
Alan W. Harris
Space Science Institute
Paolo Farinella
Memorial Symposium
Pisa, Italy
June 14-16, 2010

2. The Divine Dipsomania
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 scientist
cannot 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.

3. Growth in lightcurve data over time
4000
Number of reliable asteroid rotation periods
3000
2000
1000
Paolo left us here
0
1975 1980 1985 1990 1995 2000 2005 2010 2015
Date

4. Rotation Period vs. Diameter, 1979, 157 Asteroids
0.01
Paolo entered the
0.1 picture here
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

5. Rotation Period vs. Diameter, 1982, 226 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

6. Rotation Period vs. Diameter, 1985, 290 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

7. Rotation Period vs. Diameter, 1986, 399 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

8. Rotation Period vs. Diameter, 1987, 418 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

9. Rotation Period vs. Diameter, 1989, 439 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

10. Rotation Period vs. Diameter, 1990, 463 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

11. Rotation Period vs. Diameter, 1991, 503 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

12. Rotation Period vs. Diameter, 1993, 554 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

13. Rotation Period vs. Diameter, 1994, 565 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

14. Rotation Period vs. Diameter, 1996, 704 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

15. Rotation Period vs. Diameter, 1997, 766 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

16. Rotation Period vs. Diameter, 2000, 871 Asteroids
0.01
Paolo left us here, but had already noted:
- dip in spin rate ~50-100 km diameter
0.1
- rubble pile spin barrier
Rotation Period, hours
- excess of slow rotators
1 - binaries? (not yet)
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

17. Rotation Period vs. Diameter, 2001, 987 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

18. Rotation Period vs. Diameter, 2003, 1428 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

19. Rotation Period vs. Diameter, 2004, 1621 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

20. Rotation Period vs. Diameter, 2005, 1906 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

21. Rotation Period vs. Diameter, 2007, 2291 Asteroids
0.01
0.1
Rotation period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

22. Rotation Period vs. Diameter, 2008, 2940 Asteroids
0.01
0.1
Rotation period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

23. Rotation Period vs. Diameter, 2010, 3643 Asteroids
0.01
0.1
Rotation period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

24. Rotation Period vs. Diameter, 1979, 157 Asteroids
0.01
0.1
Rotation Period, hours
1
10
100
1000
0.01 0.1 1 10 100 1000
Diameter, km

25. 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?

26. Shapes from Asteroid Lightcurves - Then
Inspired 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.

27. Shapes and Poles - Now
Shape 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

28. In the same year, Paolo and his
Spin Rate Distribution - Then 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.
Rotation Period vs. Diameter, 1982, 226 Asteroids
1
Rotation Period, hours
10
100
1000
1 10 100 1000
Diameter, km
This is the data set that was available to reach these conclusions.

29. Spin Rate Distribution - Now
Excess of slow rotators
Large asteroid spin
Spin barrier
rate distribution is well
fit by a Maxwellian
distribution
Uniform in between
Spin rate normalized to mean spin rate
Harris & Pravec (2006) Proc. IAU Symp. 229, 439-447.
Rotation Period vs. Diameter, 2005, 1906 Asteroids
0.01 Pravec, et al. (2008) Icarus 197, 497-504.
0.1
Understanding the “breaking mechanism”
Rotation Period, hours
1 leading to slow rotation has been a long time in
coming, but is now rather certainly understood
10
as due to the YORP effect:
100
df 17 cy/day/m.y. a = heliocentric dist., AU

1000
0.01 0.1 1 10 100 1000
dt a2D2 D = diameter, km
Diameter, km

30. Rubble-pile Structure - Then
Again inspired by
conversations at the
l’Astronomia 49, November 1985, pp. 20-25. 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)
Uncle Scrooge #29, 1960.

31. Rubble-pile Structure - Now
Rotation Period vs. Diameter, 2010, 3643 Asteroids
0.01
YO YORP spins asteroids
R
P
sp
up as well as down,
0.1 in
-u
p
thus smaller asteroids
can spin both faster
Rotation period, hours
1 and slower, resulting
Rubble pile spin barrier
in the “flat”
distribution of spins.
10
This reveals the “spin
barrier”, essentially
wn
proving the “rubble-
o
100
in-d
pile” structure of
P sp
asteroids D > 0.2 km
YOR
1000
or so.
0.01 0.1 1 10 100 1000
Diameter, km

32. Binary Asteroids - Then
Score card (struck out):
Asteroid Binary?
15 Eunomia Nope
39 Laetitia Nope
43 Ariadne Nope
44 Nysa Nope
61 Danae Nope
63 Ausonia Nope
82 Alkmene Nope
192 Nausikaa Nope
216 Kliopatra Yes, but not by lightcurves
624 Hektor Yes, but not by lightcurves

33. Binary Asteroids - Now
Paolo 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.

34. Asynchronous Binary Asteroids
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.
Warner, et al. (2009) Minor Planet Bul. 36, 89-90.

35. Asteroid Lightcurve Studies - Then
Rotation Period vs. Diameter, 1979, 157 Asteroids
Monolithic 0 Binaries
0.1 Super-fast 0 Tumblers
Rotators
Rotation Period, hours
1 Rubble pile spin barrier
10
100
1.0
By 100
4.5 My
1000 By
0.01 0.1 1 10 100 1000
Diameter, km

36. Asteroid Lightcurve Studies - Now
Rotation Period vs. Diameter, 2010, 3643 Asteroids
Monolithic 131 Binaries
Super-fast 48 Tumblers
0.1
Rotators
Rotation Period, hours
1
Rubble pile spin barrier
10
100
1.0
By 100
4.5 My
1000 By
0.01 0.1 1 10 100 1000
Diameter, km

37. Asteroid pairs: a divorce made in Heaven
In 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.

38. Prograde/retrograde Yarkovsky drift
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 Hungaria family, showing Yarkovsky drift,
outside of the center of greater for smaller asteroids (larger H).
the collisional family.

39. Observed secular change in rotation rate!
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.
Taylor, et al., Science 316, 274-277 (2007).

40. Occultation – Shape Models
Occultation 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.
9 Metis 19 Fortuna 135 Hertha
Timerson, et al., Minor Planet Bul. 36, 98-100 (2009). Occultation observations by IOTA, shape models by Durech.