The Starbase One Astronomy & Space Collection/STARBASE_ONE.ISO/hst/satrpc3l.txt

Text File | 1996-01-12 | 4KB | 75 lines

FOR RELEASE:  June 5, 1995

PHOTO FILE NO.:  PRC95-25B

HUBBLE VIEWS SATURN RING-PLANE CROSSING

This sequence of images from NASA's Hubble Space Telescope
documents a rare astronomical alignment -- Saturn's magnificent
ring system turned edge-on.  This occurs when the Earth
passes through Saturn's ring plane, as it does approximately
every 15 years.

These pictures were taken with Hubble's Wide Field Planetary
Camera 2 on 22 May 1995, when Saturn was at a distance of  919
million miles (1.5 billion kilometers) from Earth.  At Saturn,
Hubble can see details as small as 450 miles (725 km) across.  In
each image, the dark band across Saturn is the ring shadow cast
by the Sun which is still 2.7 degrees above Saturn's ring plane.
The box around the western portion of the rings (to the right of
Saturn) in each image indicates the area in which the faint light
from the rings has been multiplied through image processing  (by a
factor of 25) to make the rings more visible.

[Top]  -
This image was taken while the Earth was above the lit face of
the rings.  The moons Tethys and Dione are visible to the east
(left) of Saturn; Janus is the bright spot near the center of the
ring portion in the box, and Pandora is faintly visible just
inside the left edge of this box.  Saturn's atmosphere shows
remarkable detail:  multiple banding in both the northern and
southern hemispheres, wispy structure at the north edge of the
equatorial zone, and a bright area above the ring shadow that  is
caused by sunlight scattered off the rings onto the atmosphere.
There is evidence of a faint polar haze over the north pole of
Saturn and a fainter haze over the south.

[Center] -
This image was taken close to the time of ring-plane crossing.
The rings are 75% fainter than in the top image, though they do
not disappear completely because the vertical face of the rings
still reflects sunlight when the rings are edge-on.  Rhea is visible
to the east of Saturn, Enceladus is the bright satellite in the rings
to the west, and Janus is the fainter blip to its right.  Pandora is just
to the left of Enceladus, but is not visible because Enceladus is too
bright. An oval-shaped atmospheric feature has just rotated into view
(near the eastern limb, at the northern edge of the equatorial zone),
and appears to be a local circulation pattern that is not penetrated by
the bright clouds that are deflected around it.

[Bottom] -
This image was taken approximately 96 minutes (one Hubble orbit)
after the center image.  The rings are 10% brighter than they
were in that image.  Rhea is visible just off the eastern limb of
Saturn, and casts a shadow on the south face of Saturn. During
this exposure, the Earth and Sun were on opposite sides of
Saturn's ring plane (they remain in this configuration until
10 August 1995).  The atmospheric circulation pattern has rotated
to just past the center of the planet's disk, and is followed by
more wispy structure in the bright band of clouds, reminiscent of
the structure seen during the Saturn storm observed in 1990.

These images will be used to determine the time of ring-plane
crossing and the thickness of the main rings and to search for
as yet undiscovered satellites.  Knowledge of the exact time of
ring-plane crossing will lead to an improved determination of the
rate at which Saturn "wobbles" about its axis (polar precession).

Technical Notes
Each of these images is a 7-second exposure at 8922 Angstroms in
a methane absorption band.  North is up and east is to the left.

Credit:  Amanda S. Bosh (Lowell Observatory), Andrew S. Rivkin
(Univ. of Arizona/LPL), the HST High Speed Photometer Instrument
Team (R.C. Bless, PI), and NASA.