Minor Planets

1
Minor Planets

• Small interplanetary bodies of non-cometary
  appearance

2
Discovery, Numbering

• First objects
• (1) Ceres, (2) Pallas, (3) Juno, (4) Vesta
• Minor planet numbers
• (100) in 1868 (1000) in 1923 (10000) in 1999
  (100000) in 2005
• Current numbering extends to 129436
• Techniques
• Photography, wide-field imaging, CCD
• Detection, identification, linkage

3
Orbital Classification

• Main Belt (Ceres 1801)
• Near-Earth Objects (Eros 1898)
• Trojans (Achilles 1906)
• Cometary (Hidalgo 1920), including
• Damocloids in Halley-type orbits
• Centaurs (Chiron 1977)
• Transneptunians (Pluto 1930 1992 QB1 1992)

4
Population sizes

• The absolute magnitude measures the product of
  albedo and geometrical cross-section, pvR2
• As a rough guideline,
• R1 km ? H17
• R10 km ? H12
• R100 km ? H7

5
Main Belt Resonances

• Kirkwood gaps
• (3/1, 5/2, 7/3, 2/1)
• Hildas (3/2)
• Thule (4/3)
• Empty zones along resonant surfaces
• Isolation of groups
• (Hungaria, Phocaea)

6
Collisional Families

• Hirayama families (Hirayama 1918)
• Eos, Koronis, Themis

7
Asteroid diameters (1)

• Radiometric method
• visual brightness
• lower pv ? higher temperature measure thermal
  flux in the infrared

8
Asteroid diameters (2)

• Polarimetric method
• empirical correlation between k tan? and pv
• observe the polarization phase curve

9
Asteroid diameters (3)

• Stellar occultations
• timing of the occultation from different
  places along the track

10
Asteroid diameters (4)

• Direct imaging
• Asteroids imaged so far Gaspra, IdaDactyl,
  Mathilde, Eros, Itokawa

Gaspra
11
Close-up images of asteroids
Mathilde
Ida Dactyl
Eros
Itokawa
12
Asteroid collisions (1)

• Distance traveled in time T VT
• Volume cut out by the target area pR2VT
• Number of collisions N pR2VTn
• Average time between collisions

13
Asteroid collisions (2)

• Typical relative velocity V ? 5 km/s
• Estimate the Main Belt volume by a donut at 2.5
  AU from the Sun with a cross-sectional radius
  0.55 AU ? volume 5?1025 km3
• Total number of asteroids larger than a given
  radius r
• r gt 1 km -- 5?105
• r gt 10 km -- 2?103
• r gt 100 km -- 35

14
Asteroid collisions (3)

• E.g., for an asteroid with R100 km
• t 2?107 yr (r gt 1 km)
• t 5?109 yr (r gt 10 km)
• t 3?1011 yr (r gt 100 km)
• Catastrophic fragmentation limit

15
Asteroid satellites (1)

• Ida-Dactyl (flyby imaging) was the first
• 1994 AW1 the first via light curve
• 2000 DP107 the first via radar

16
Asteroid satellites (2)

• NEAs 25 cases 16
• Main Belt 23 cases 10 (10-40 km)
• Trojans 1 case
• TNOs 21 cases 11
• Formation scenarios
• NEAs -- tidal splitting / rubble-pile spin-up
• MBTNO -- asymmetric collisions
• orbital semimajor axes a few of Hill radius

17
Asteroid spin rates

• Very rapid spins, i.e., periods shorter than 1
  hour, only exist for very small asteroids
• Interpretation all the larger asteroids have a
  rubble-pile structure that does not survive too
  fast spin

18
Asteroid taxonomic types (1)

• Based on spectroscopy or spectrophotometry
• Examples
• S bright, reddish
• C dark, grey
• D dark, red
• M metallic(?)
• V Vesta-like

19
Asteroid taxonomic types (2)

• S and C correspond, respectively, to ordinary and
  carbonaceous chondrites
• The S-types posed a problem until the Eros
  close-up studies showed the analogy with ordinary
  chondrites (problem caused by space weathering
  effects)
• M-types may represent remains of partially or
  totally differentiated asteroids

20
Inner Planet Crossers

• Mars Crossers (1.3 lt q lt ?1.6 AU)
• Amors (1.017 lt q lt 1.3 AU)
• Apollos (q lt 1.017 AU a gt 1 AU)
• Atens (a lt 1 AU Q gt 0.983 AU)
• Inner Earth Objects (Q lt 0.983 AU)

21
Near-Earth Asteroids (1)

• Spaceguard survey for NEAs discover 90 of
  km-sized objects before 2008
• About 70 of these have now been discovered
• The most difficult ones largely remain

22
Near-Earth Asteroids (2)

• Relative cross-section of the Earth 1?10-9
• Increased by gravitational attraction
• With 1000 km-sized NEAs, we expect one or two
  impacts each million years

23
Trojans

• L4 cloud
• (preceding Jupiter)
• L5 cloud
• (trailing Jupiter)
• There may be a larger population of Trojans
  in the L4 group the reason for such a difference
  is not known

24
Minor planets in comet-like orbits

• Hidalgo (q1.95 AU Q9.5 AU i43?)
• Don Quijote (q1.2 AU Q7 AU i31?)
• Damocles (q1.58 AU Q22 AU i62?)
• 30 in Jupiter Family orbits
• 10 in Halley-type or long-period orbits
• Likely, predominantly cometary origin

25
Trans-Neptunian Objects (1)

• Main categories
• Plutinos (2/3 resonance with Neptune)
• Classical Kuiper Belt (practically isolated from
  Neptune)
• Scattered Disk (coupled to Neptune)
• Extended TNO population (e.g., Sedna)

26
Trans-Neptunian Objects (2)

• The classical Kuiper Belt has a cold and a
  hot component, in terms of the inclination
  distribution
• This may correlate with a difference in surface
  properties or composition