20180428

Astronomy midterm question: possible IAU classification of re-located Ida?

Astronomy 210 Midterm 2, spring semester 2018
Cuesta College, San Luis Obispo, CA

"Ida and Dactyl in Enhanced Color"
NASA/JPL
jpl.nasa.gov/spaceimages/details.php?id=PIA00069

Ida was the first asteroid discovered to have a moon, Dactyl, orbiting it:
While analyzing images from NASA's Galileo spacecraft, astronomer Ann Harch discovered that the asteroid Ida has a moon: a tiny asteroid subsequently named Dactyl. Ida is 34 miles across its longest axis, and is potato-shaped, as are many other asteroids. Dactyl itself is remarkably spherical, being about 1 mile across.[*]
If Ida is relocated from the asteroid belt to its own orbit around the sun (while still being orbited by Dactyl), discuss how Ida should be classified. Explain using the International Astronomical Union classification scheme.

[*] Matthew Francis, "Moonday: A Bite-Sized Moon," galileospendulum.org/2012/03/05/moonday-a-bite-sized-moon/.

Solution and grading rubric:
  • p:
    Correct. Discusses IAU classification scheme to argue that Ida placed in a new, unshared orbit around the sun would pass qualification I (orbits the sun directly), but would still not pass qualification II (rounded shape), thus categorizing it as solar system debris, despite having its moon Dactyl.
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discussion only tangentially related to the IAU classification scheme.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion unrelated to the IAU classification scheme.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 30674
Exam code: midterm02nVd4
p: 19 students
r: 1 student
t: 1 student
v: 2 students
x: 0 students
y: 1 students
z: 0 students

Section 30676
Exam code: midterm02StWr
p: 30 students
r: 2 students
t: 5 students
v: 5 students
x: 1 student
y: 2 students
z: 0 students

A sample "p" response (from student 2356):

Another sample "p" response (from student 8246):

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