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Old September 25th 03, 05:34 PM
Ron Baalke
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Default Hubble Uncovers Smallest Moons Yet Seen Around Uranus

FOR RELEASE: 11:00 a.m. (EDT) September 25, 2003

Donna Weaver
Space Telescope Science Institute, Baltimore, MD
(Phone: 410-338-4493; E-mail: )

Kathleen Burton
NASA Ames Research Center, Moffett Field, CA
(Phone: 650-604-1731; E-mail:

Mark Showalter
Stanford University and NASA Ames Research Center
(Phone: 650-604-3382; E-mail:

Jack Lissauer
NASA Ames Research Center
(Phone: 650-604-2293; E-mail:



Astronomers have discovered two of the
smallest moons yet found around Uranus. The
new moons, uncovered by NASA's Hubble
Space Telescope, are about 8 to 10 miles
across (12 to 16 km) - about the size of San

The two moons are so faint they eluded
detection by the Voyager 2 spacecraft, which
discovered 10 small satellites when it flew by the gas giant
planet in 1986. The newly detected moons are orbiting even
closer to the planet than the five major Uranian satellites, which
are several hundred miles wide. The two new satellites are the
first inner moons of Uranus discovered from an Earth-based
telescope in more than 50 years. The International Astronomical
Union (IAU) will announce the finding today. The Hubble
telescope observations also helped astronomers confirm the
discovery of another tiny moon that had originally been spotted
in Voyager pictures.

"It's a testament to how much our Earth-based instruments
have improved in 20 plus years that we can now see such faint
objects 1.7 billion miles (2.8 billion km) away," says Mark
Showalter, a senior research associate at Stanford University in
Stanford, Calif., who works at the NASA Ames Research
Center, in Moffett Field, Calif. Showalter and Jack Lissauer, a
research scientist at the NASA Ames Research Center, used
Hubble's Advanced Camera for Surveys (ACS) to make the
discovery. The images were taken Aug. 25, 2003.

The newly discovered moons are temporarily designated as
S/2003 U 1 and S/2003 U 2 until the IAU formally approves their
discovery. S/2003 U 1 is the larger of the two moons, measuring
10 miles (16 km) across. The Hubble telescope spotted this
moon orbiting between the moons Puck, the largest satellite
found by Voyager, and Miranda, the innermost of the five
largest Uranian satellites. Astronomers previously thought this
region was empty space. S/2003 U 1 is 60,600 miles (97,700
km) away from Uranus, whirling around the giant planet in 22
hours and 9 minutes.

The smallest Uranian moon yet found, S/2003 U 2, is 8 miles (12
km) wide. Its orbital path is just 200 to 450 miles (300 to 700
km) from the moon Belinda. S/2003 U 2 is 46,400 miles (74,800
km) away from Uranus and circles the planet in 14 hours and 50
minutes. The tiny moon is part of a densely crowded field of 11
other moons, all discovered from pictures taken by the Voyager

"The inner swarm of 13 satellites is unlike any other system of
planetary moons," says co-investigator Jack Lissauer. "The
larger moons must be gravitationally perturbing the smaller
moons. The region is so crowded that these moons could be
gravitationally unstable. So, we are trying to understand how
the moons can coexist with each other."

One idea is that some of the moons are young and formed
through collisions with wayward comets. For example, the
Hubble telescope spotted two small moons orbiting very close
to the moon Belinda. One of them is the newly detected moon,
S/2003 U 2, which is traveling inside Belinda's orbit. The other,
designated S/1986 U 10, was found in 1999 by astronomer Erich
Karkoschka of the University of Arizona, who uncovered the
satellite in Voyager pictures. But the finding required
confirmation by an Earth-based telescope. This is the first time
this moon has been seen since Voyager snapped a picture of it.
S/1986 U 10 is 750 miles (1,200 km) away from Belinda.

"Not all of Uranus's satellites formed over 4 billion years ago
when the planet formed," Lissauer says. "The two small moons
orbiting close to Belinda, for example, probably were once part
of Belinda. They broke off when a comet smashed into Belinda."

The astronomers hope to refine the orbits of the newly
discovered moons with further observations. "The orbits will
show how the moons interact with one another, perhaps
showing how such a crowded system of satellites can be
stabilized," Showalter explains. "This could provide further
insight into how the moon system formed. Refining their orbits
also could reveal whether these moons have any special role in
confining or 'shepherding' Uranus's 10 narrow rings."

Astronomers stretched the limit of Hubble's ACS to find the
tiny satellites. "These moons are 40 million times fainter than
Uranus," Showalter says. "The moons are at 25th magnitude
and Uranus is at sixth magnitude. They are blacker than
asphalt, if their composition is like the other small, inner moons.
So they don't reflect much light. Even with the sensitivity and
high resolution of Hubble's ACS, we had to overexpose the
images of Uranus to pinpoint the moons."

The newly detected moons, when approved by the IAU, will
bring the Uranian satellite total to 24. Uranus ranks third in the
number of IAU-certified moons behind Jupiter (38) and Saturn
(30). Excluding the outer moons that travel in elongated orbits
and are probably captured asteroids, Uranus holds the record for
the most satellites with 18 in its inner system. All of them have
nearly circular orbits. Saturn is second with 17.

Electronic images, movies, and additional information are available at:

The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA), for
NASA, under contract with the Goddard Space Flight Center, Greenbelt,
MD. The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency (ESA).