Hubble Uncovers Smallest Moons Yet Seen Around Uranus

http://hubblesite.org/newscenter/archive/2003/29/text

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

CONTACT:
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: )

PRESS RELEASE NO.: STScI-PR03-29

HUBBLE UNCOVERS SMALLEST MOONS YET SEEN AROUND URANUS

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
Francisco.

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
spacecraft.

"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:
http://hubblesite.org/newscenter/2003/29
http://amesnews.arc.nasa.gov/

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).

On 25 Sep 2003 16:34:20 GMT, (Ron Baalke)
wrote:

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.

Something I'm curious about: All the outer planets seem to have
trillions of objects in orbit around them, in a smooth continuum from
moons comparable in size to our own, down to microscopic dust specks.
Yet all the inner planets seem to be completely devoid of orbiting
material, apart from the three moons Earth and Mars possess between
them. Does anyone know the reason for the discrepancy?

--
"Sore wa himitsu desu."
To reply by email, remove
the small snack from address.
http://www.esatclear.ie/~rwallace

In article ,
Russell Wallace wrote:
Something I'm curious about: All the outer planets seem to have
trillions of objects in orbit around them, in a smooth continuum from
moons comparable in size to our own, down to microscopic dust specks.
Yet all the inner planets seem to be completely devoid of orbiting
material, apart from the three moons Earth and Mars possess between
them. Does anyone know the reason for the discrepancy?

The two types of planets form differently (as is obvious anyway, given
their radically different compositions). The major moons of the gas-giant
planets probably formed in place, as part of the formation of the planet,
although there are one or two probable exceptions and the details are not
well understood yet.

A lot of the smaller odds and ends are probably captured asteroids, and
there again the outer planets are favored: their large masses combine
with their greater distance from the Sun to give large gravitational
fields (more properly, areas of influence) with quite fuzzy edges, thus
making it much more likely that a passing object will wander through the
fuzzy area and possibly end up captured.

The formation of the inner planets doesn't seem to leave any room for
parts of them to remain in orbit. And their areas of gravitational
influence are small, with relatively sharp edges, so it's rare for them to
capture anything. (Not impossible, but rare.)

Earth's moon is the result of a huge impact late in Earth's formation, a
relatively unlikely accident.

Mars's moons seem to be captured asteroids, and just how that happened is
deeply mysterious. Their orbits are fairly close and fairly circular,
which is not at all what you'd expect for captured objects. Worse, they
seem to be *outer-belt* asteroids, and how they ended up at Mars is
another puzzle. Again, this is probably the result of some unusual
accident.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

(Russell Wallace) writes:

Something I'm curious about: All the outer planets seem to have
trillions of objects in orbit around them, in a smooth continuum from
moons comparable in size to our own, down to microscopic dust specks.
Yet all the inner planets seem to be completely devoid of orbiting
material, apart from the three moons Earth and Mars possess between
them. Does anyone know the reason for the discrepancy?

Gas-giant planets and "terrestrial" planest are believed to form by
different physical mechanisms. "Terrestrial" planets are believed
to form by collisional accretion of "planetesimals," whereas "gas giant"
planets are believed to form by accretion from a sub-disk embedded within
the main protoplanetary disk that itself resembles a "mini solar system."


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

On 03 Oct 2003 11:49:06 -0500, (Gordon D.
Pusch) wrote:

Gas-giant planets and "terrestrial" planest are believed to form by
different physical mechanisms. "Terrestrial" planets are believed
to form by collisional accretion of "planetesimals," whereas "gas giant"
planets are believed to form by accretion from a sub-disk embedded within
the main protoplanetary disk that itself resembles a "mini solar system."

Could you reference a good site (web, book, or paper) on this for me?
I was under the general impression that it was a similar process, that
it was just a matter of there being greater gasseous material to
accumulate in the region of gas-giant formation.

Trakar writes:

On 03 Oct 2003 11:49:06 -0500, (Gordon D.
Pusch) wrote:

Gas-giant planets and "terrestrial" planest are believed to form by
different physical mechanisms. "Terrestrial" planets are believed
to form by collisional accretion of "planetesimals," whereas "gas giant"
planets are believed to form by accretion from a sub-disk embedded within
the main protoplanetary disk that itself resembles a "mini solar system."

Could you reference a good site (web, book, or paper) on this for me?
I was under the general impression that it was a similar process, that
it was just a matter of there being greater gasseous material to
accumulate in the region of gas-giant formation.

Do a Google search on "gas-giant formation" and you will pull up
approximately 9340 references. You can further specialize your search
by adding the keywords for the two competing gas-giant formation models:
"runaway core accretion" or "disk instability;" the latter "disk" model
is the "mini solar-system" model I referred to.

(The "disk instability" model has been gaining support over the older
"runaway core accretion" model, since the core accretion model does not
appear to be able to form a giant planet fast enough to beat the ignition
and "T-tauri phase" of the system's protostar. The "T tauri" phase produces
Very strong stellar winds that blow away most of the gas remaining in the
protoplanetary disk, terminating gas accretion onto the giant planets.)


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

Gordon D. Pusch wrote:
(Russell Wallace) writes:


Something I'm curious about: All the outer planets seem to have
trillions of objects in orbit around them, in a smooth continuum from
moons comparable in size to our own, down to microscopic dust specks.
Yet all the inner planets seem to be completely devoid of orbiting
material, apart from the three moons Earth and Mars possess between
them. Does anyone know the reason for the discrepancy?


Gas-giant planets and "terrestrial" planest are believed to form by
different physical mechanisms. "Terrestrial" planets are believed
to form by collisional accretion of "planetesimals," whereas "gas giant"
planets are believed to form by accretion from a sub-disk embedded within
the main protoplanetary disk that itself resembles a "mini solar system."


Do Uranus' moons lie in her equatorial plane? (So far my casual googling
is giving me no answer)

Hop
http://clowder.net/hop/index.html

Hop David writes:

Gordon D. Pusch wrote:
(Russell Wallace) writes:

Something I'm curious about: All the outer planets seem to have
trillions of objects in orbit around them, in a smooth continuum from
moons comparable in size to our own, down to microscopic dust specks.
Yet all the inner planets seem to be completely devoid of orbiting
material, apart from the three moons Earth and Mars possess between
them. Does anyone know the reason for the discrepancy?

Gas-giant planets and "terrestrial" planest are believed to form by
different physical mechanisms. "Terrestrial" planets are believed
to form by collisional accretion of "planetesimals," whereas "gas giant"
planets are believed to form by accretion from a sub-disk embedded within
the main protoplanetary disk that itself resembles a "mini solar system."

Do Uranus' moons lie in her equatorial plane?

Yes. Tidal friction tends to damp out an inner moon's orbital inclination
relative to the planetary equator on timescales short compared to the
lifetime of the solar system. High orbital inclination satellites
usually have fairly large orbital radii, and are generally thought
to be relatively recent "captures." (One exception to this is if
their inclinations are being "forced" by some orbital resonance.)


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'

Hop David wrote in message ...

Do Uranus' moons lie in her equatorial plane? (So far my casual googling
is giving me no answer)

Hop
http://clowder.net/hop/index.html

Pretty much all the moons in the solar system are in the host planet's
equatorial plane. There are exceptions, such as the far outer moons of
Jupiter and Saturn's Iapetus and Phoebe. The 2 major exceptions are
Luna and Triton (which is the only major moon with a retrograde
orbit).
There are OTHER sources than Google. Any good astronomy text would
probably give you the answer.

Gordon D. Pusch wrote:
Hop David writes:


Gordon D. Pusch wrote:

(Russell Wallace) writes:


Something I'm curious about: All the outer planets seem to have
trillions of objects in orbit around them, in a smooth continuum from
moons comparable in size to our own, down to microscopic dust specks.
Yet all the inner planets seem to be completely devoid of orbiting
material, apart from the three moons Earth and Mars possess between
them. Does anyone know the reason for the discrepancy?

Gas-giant planets and "terrestrial" planest are believed to form by
different physical mechanisms. "Terrestrial" planets are believed
to form by collisional accretion of "planetesimals," whereas "gas giant"
planets are believed to form by accretion from a sub-disk embedded within
the main protoplanetary disk that itself resembles a "mini solar system."

Do Uranus' moons lie in her equatorial plane?


Yes. Tidal friction tends to damp out an inner moon's orbital inclination
relative to the planetary equator on timescales short compared to the
lifetime of the solar system. High orbital inclination satellites
usually have fairly large orbital radii, and are generally thought
to be relatively recent "captures." (One exception to this is if
their inclinations are being "forced" by some orbital resonance.)

At first I had thought Uranus and her moons must have formed from a
subdisk perpendicular to the solar system. But I guess if a later event
had knocked Uranus' axis off kilter, the moons would've moved into her
equatorial plane over a short time.

Hop
http://clowder.net/hop/index.html