Hayabusa Succeeds in Observing the 'Opposition Surge' of Itokawa

http://www.isas.ac.jp/e/snews/2005/1027_itokawa.shtml

Hayabusa succeeded to observe the "opposition surge" of Itokawa!
Japan Aerospace Exploration Agency (JAXA)
October 27, 2005

How much brighter is the full Moon than the quarter Moon? If we
consider
the ratio of the illuminated area, "Double" seems to be a reasonable
answer. But in reality, the full Moon is more than four times bighter
than the quarter Moon. Why?

The "phase angle" is defined as the angle between the lines connecting
tne Sun-Target and the Target-Observer. Namely, the phase angle for the
quarter Moon is 90 degree, and that for the full Moon is almost 0
degree. It is well known that airless bodies in our Solar System, such
as the Moon, generally show a brightness enhancement when the phase
angle approaches zero. This phenenomenon is called the "opposition
surge".

Do we observe the "opposition surge" on Itokawa? You can find the
answer
in the images below. The phase angle was abour 35 degrees for the left
image and less than 1 degree for the right image. The Hayabusa camera
team confirmed the "oposition surge" through careful inspection of
these
images. More detailed analysis provides important information on the
surface state, such as the difference between bare rocks and sands.

The observation of the "opposition surge" was not a serendipity, but
one
result of an elaborate operation. Hayabusa had to come to the
Sun-Itokawa line at the distance of 3 hundred million kilometers from
the Earth. The position tolerance was approximately the size of
Itokawa,
namely a few hundred meters. The Hayabusa navigation team accomplished
the feat by skillful guiding.

.

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