1. Asteroids
SSP 2017

2. What do we know about asteroids, apart from the OD and the fact that we want to avoid this?

3. Asteroids orbit the Sun
There are estimated to be about 150 million objects that are larger than 100 m; only 750,000 of them have been observed (2017).
Most orbit the Sun in circular orbits between Mars and Jupiter

4. Asteroids are comparatively new discoveries

5. But the pace of discovery has increased

6. There are rules about naming them
The Minor Planet Center (MPC) and the International Astronomical Union (IAU) have authority over the naming of asteroids:
• Upon a confirmed discovery, they are given a provisional name like “2002 VJ61” – year of discovery, then the first letter indicates the half of the month of discovery (here, V = November 1 – 15), then the second letter indicates the order of discovery within that time period (if this were just “VJ” then it would have been the 10th asteroid discovered in Nov. 1 – 15, 2002; as it is “VJ61” is the 1535th asteroid discovered).
• After its orbit is reliably determined, the asteroid receives a number (such as “138846”) and the discoverer has 10 years to name it (approved by the IAU, of course).

7. Asteroids may pose a hazard
Proposed by Richard Binzel at MIT in 1995 and adopted in 1999 by the IAU (guess where the conference was?)

8. The understanding of a topic begins by classification
In the same way that you classify chemicals by melting point or appearance or density, you can do the same with asteroids.

9. Asteroids may be classified by:

10. Asteroids may be classified by:
• Orbital characteristics
• Composition
• Origin

11. Asteroids may be classified by:
• Orbital characteristics
• Composition
• Origin

12. A reminder about terms used in solar system astronomy
1 astronomical unit (AU) is the mean distance between the Earth and the Sun.
Orbits are elliptical. The semi-major axis (a) is half the distance along the “long” axis of an orbit.
The eccentricity (e) of an orbit is a measure of how elliptical that orbit is. This value ranges from 0 (circular) to 1 (parabolic).

13. Some more astronomy terms
Body being orbited
Closest point to the body
Furthest point from body
Sun
Perihelion
Aphelion
Earth
Perigee
Apogee

14. Not all NEAs are PHAs
where PHAs are “potentially hazardous asteroids”, which in turn is defined by the Center for Near-Earth Object Studies at JPL as “all asteroids with an Earth Minimum Orbit Intersection Distance (MOID) of 0.05 AU or less and an absolute magnitude (H) of 22.0 or less are considered PHAs.”

15. Why magnitude 22.0 or less?
It’s the result of a calculation that an asteroid of diameter 140 m at a distance of 0.05 AU with an albedo of 0.14 would be that bright. Anything larger or closer would be considered a threat.
Albedo is the proportion of light reflected off an object, expressed as a percentage or a decimal

16. Classifying near-earth asteroids by orbital characteristics
the Amor class – can cross the orbit of Mars but does not cross the orbit of Earth; its perihelion is within 0.05 AU of Earth’s orbit, though
the Aten class – cross the orbit of Earth and have a semi-major axis length < 1 AU
the Apollo class – cross the orbit of Earth and have a semi-major axis length > 1 AU

18. Asteroids may be classified by:
• Orbital characteristics
• Composition
• Origin

19. Differences between asteroid geology and terrestrial geology:
- Plate tectonics is still occurring on Earth
- Erosion is a significant factor
Both factors imply more than one rock type and, therefore, more than one rock origin story.

20. The rock cycle on Earth beautifully illustrates these origin stories…

21. The rock cycle on Earth beautifully illustrates these origin stories…
…but is ultimately irrelevant to asteroids because they are all igneous in origin (frozen from the molten state).

24. The early solar system 4.569 billion years ago
Chondrules (glassy) and calcium-aluminum inclusions (CAIs) begin to crystallize; the age estimate comes from precise measurement of uranium isotope decay products

25. Chondrule (orange) interweaved with pyroxene (gray)

26. Al-26 decays to Mg-26 and enriches the surrounding material with Mg-26; we can detect that, and calculate time of cooling

27. How do we know anything about asteroid composition?
After all, they are quite far away. And, until recently, we haven’t really been able to sample them.

28. How do we know anything about asteroid composition?
After all, they are quite far away. And, until recently, we haven’t really been able to sample them.
But we have sampled their light – that is, the reflected sunlight which we can capture in a telescope.
Spectroscopy – the interaction of light with a material which yields information about the material

29. Different asteroids show different wavelength absorptions in the visible and near-infrared (Bus, Demeo et al., 2008).
Each of these are graphs of wavelength versus reflected light intensity – in other words, spectra of different asteroids.

30. Asteroid classification by spectral type
C-type (“carbonaceous”) albedo 0.05 – % - inner asteroid belt
S-type (“siliceous”) albedo
17% - inner asteroid belt
subtype Q contains mineral olivine and pyroxene
M-type (“metallic”) albedo
8% - middle asteroid belt
P-type and D-type – reddish, outer asteroid belt, albedo 0.02 – 0.06

31. Asteroid compositions are not randomly distributed

32. Wait! How did we take a photomicrograph of a chondrule?!

33. We have actually sampled asteroids (even before probes landed and sampled them):
Meteorites!

34. Sort meteorites by composition
Chondrites % (stony with chondrules)
Achondrites - 7.1% (stony without chondrules
Iron - 5.7% (iron-nickel alloy)
Stony-iron - 1.5% (mixture of chondritic material and iron-nickel)

35. Or more specifically:
Source: Arizona State University, Center for Meteorite Studies

36. Asteroids may be classified by:
• Orbital characteristics
• Composition
• Origin

37. “Differentiated” means there was time enough during the crystallization for a separation by density to occur within the body

38. By “similar”, they mean that the spectra of the asteroid and the meteorite are similar

39. Issues with this correlation
The “finds” of meteorites on Earth do not necessarily correspond to the distribution of compositions in asteroids
Spectroscopy reveals what’s on the surface (most of the time) but not what lies beneath
Landing on, and sampling, the asteroid is certainly a better method

40. Hayabusa (JAXA) lands on asteroid Itokawa in 2005
And retrieves a sample,
which lands in Australia in 2010
Itokawa is an
S-type asteroid!
To avoid contamination problems, the sample was not even opened until 2013 with special handling techniques.

41. So what are your asteroids like?
2002 TX68 size: 722 – 1616 m, Amor, came within 0.02 AU of Mars in 2011
2005 UP156 size: 1030 m diameter, Amor, rotation period 40.5 hrs, has a satellite
1950 LA size: 6600 m diameter, type Sw (stony) asteroid
2000 VJ61 size: 1900 – 4300 m, Apollo
1984 KB size: 1400 m diameter, type K/Sq,S, Apollo, discovered by Carolyn and Eugene Shoemaker
1999 LO28 size: 2716 m, Amor

42. So what do we do if a PHA is predicted to hit us
So what do we do if a PHA is predicted to hit us? 1998 was a good year for this.

43. 1862 Apollo can approach within 0. 025 AU of Earth, and it’s about 1
1862 Apollo can approach within AU of Earth, and it’s about 1.5 km in diameter.

44. Can we “nudge” 1862 Apollo?

46. Barbee and Nuth (Journal of Cosmology, 2009) proposed an asteroid deflection scheme in which we launch an inert asteroid impactor (basically a big rocket) at the asteroid to change its momentum (which is a vector, as you know).
Currently, given the size and mass of a typical PHA, we could impart a few mm/yr deflection in the asteroid’s velocity – so this scheme is not as impractical as it seems.