Pluto
Discovered in 1930 @ Lowell Observatory in Flagstaff,
Ariz. by Clyde Tombaugh 15 years after calcs. by Lowell based on perturbations of Uranus & Neptune. First called Planet X
Found by coincidence near predicted position - calcs.
in error
Tombaugh used blink comparator to compare pairs of
star fields taken @ different times (few days apart) - image
jumps due to change in position
Problems: were looking in direction of Milky Way
- many stars; asteroids also move w.r.t. stars - false
alarms
Pluto unusual in many respects:
- Highly irregular orbit - 29 1/2 AU - 49 AU
- Inside Neptune's orbit 1978 - 1998. (249 yr. period)
- Orbit inclined 17 deg to ecliptic
- Small and fairly dense, unlike gas giants
- Density= 2 g/cc - rock, frozen ices (water, methane?)
- Thin methane atmosphere
- Has satellite - Charon, disc. 1977; large compared
to Pluto: 2,500 km/1,300 km - double planet
Two additional small satelites recently discovered: http://www.boulder.swri.edu/plutonews/
- Orbital period Charon = 6.39 days= rotational period
Pluto - both locked in synchronous rotation
- Charon's orbit retrograde and inclined 62 deg. w.r.t.
Pluto's orbit - Pluto's obliquity = 118 deg. (180 deg - 62 deg = 118 deg)