WATER ON MOON MAY CLOUD PLANS FOR LUNAR ASTRONOMY
RAS
The recent discovery of water on the Moon may have an impact on future
plans for Moon-based astronomy. Space scientists from the Chinese
Academy of Sciences have calculated that the scattering caused by
molecules vaporised in sunlight could distort certain observations
from telescopes mounted on the Moon. The research has particular
implications for the Chinese lunar lander, Chang'E-3, which is planned
to be launched in 2013. An ultraviolet astronomical telescope will be
installed on the lander, which will operate on the sunlit surface of
the Moon, powered by solar panels. At certain ultraviolet
wavelengths, hydroxyl molecules cause a particular kind of scattering
where photons are absorbed and rapidly re-emitted.
The Moon's potential as a site for astronomical observatories has been
discussed since the era of the Space Race. Moon-based telescopes
could have several advantages over telescopes on Earth, including a
cloudless sky and low seismic activity. The far side of the Moon
could be an ideal site for radio astronomy, being permanently shielded
from interference from the Earth. Radio observations would not be
affected by plausible hydroxyl levels.
PHOBOS MAY HAVE FORMED FROM IMPACT DEBRIS
RAS
The origin of the Martian satellites Phobos and Deimos is a
long-standing puzzle. It has been proposed that both moons may be
asteroids that were captured by Mars' gravity. Spectra of Phobos at
visible and near-infrared wavelengths have been considered to be
analogous to those of carbonaceous-chondrite meteorites, commonly
associated with asteroids dominant in the middle part of the asteroid
belt, but recent thermal-infrared observations show poor agreement
with any class of chondritic meteorite. They instead argue in favour
of in-situ formation, by re-accretion of rocky debris lofted into
orbit round Mars by a large impact. Two quasi-independent
compositional analyses of thermal-infrared spectra, from the Mars
Express and Mars Global Surveyor missions, yield very similar
conclusions. The re-accretion picture is strengthened by the
measurements of Phobos's high porosity from the radio-science
experiment on Mars Express.
Astronomers detected for the first time a type of mineral called
phyllosilicates on the surface of Phobos, particularly in the areas
northeast of Stickney, its largest impact crater. That is intriguing,
as it implies the interaction of silicate materials with liquid water
on the parent body prior to incorporation into Phobos. Alternatively
phyllosilicates may have formed in situ, but that would mean that
Phobos required sufficient internal heating to enable liquid water to
remain stable. More detailed mapping, in-situ measurements from a
lander, or sample return would ideally help to settle the issue.
Other observations appear to match the types of minerals identified on
the surface of Mars. Thus, the derived composition on Phobos appears
more closely related to Mars than to objects elsewhere in the Solar
System. The asteroid-capture proposals also have difficulty in
explaining the current near-circular and near-equatorial orbits of
both Martian moons.
The radio-science team has used the Doppler shifts in the frequency of
the spacecraft's transmissions to determine the perturbations of the
craft's trajectory by the gravitational attraction of Phobos, and
hence the satellite's mass. The resulting estimate of Phobos' density
is 1.86 ± 0.02 g/cm3 -- significantly lower than the density of
meteoritic material associated with asteroids. It is held to imply a
sponge-like structure, with voids making up 25-45% in Phobos's
interior. High porosity seems anyway to be required in order to
absorb the energy of the large impact that generated Stickney crater
without destroying the body. A highly porous interior of Phobos
supports the idea of formation by re-accretion of rocky material in
orbit round Mars. In that process, modelling suggests that the
largest blocks re-accrete first because of their larger mass, forming
a core with large boulders. Then the smaller debris re-accrete but do
not fill the gaps left between the large blocks because of the low
self-gravity of the small body in formation. Finally, a relatively
smooth surface of finely-divided material masks the voids inside the
body.
14 NEW TRANS-NEPTUNIAN OBJECTS FOUND
Harvard-Smithsonian CfA
Beyond the orbit of Neptune are countless icy rocks known as trans-
Neptunian objects (TNOs). Four of the biggest, including Pluto, are
classified as dwarf planets. The region also has many comets, such as
Halley's Comet. TNOs are small and receive little sunlight, so they
are faint and difficult to detect. Now, astronomers searching the
data archives of the Hubble telescope have added 14 new ones to the
catalogue, and their method promises to turn up many more. As TNOs
slowly orbit the Sun, they move against the starry background,
appearing as streaks in time-exposure photographs. The team developed
software to analyze hundreds of Hubble images looking for such
streaks. After promising candidates were flagged, the scientists
themselves examined the images to try to confirm or refute each
discovery.
Most TNOs are located near the ecliptic, so the team searched within
5° of the ecliptic to increase the chance of success. They found 14
objects, including one binary pair. All were very faint, most being
between magnitudes 25 and 27. According to estimates based on their
distances and magnitudes, the new-found TNOs range from 40 to 100
kilometres across. The initial study examined only one-third of a
square degree of the sky, so hundreds more TNOs may well be
discoverable in the rest of the Hubble archives.
DATA CLIPPERS SET SAIL TO ENHANCE FUTURE PLANETARY MISSIONS
RAS
A new golden age of sailing may be about to begin -- in space. Future
missions to explore the outer planets could employ fleets of
'data clippers', manoeuvrable spacecraft equipped with solar sails, to
ship vast quantities of scientific data back to the Earth. The
technology could be ready in time to support mid-term missions to the
moons of Jupiter and Saturn. Space-rated flash memories will soon be
able to store the huge quantities of data needed for the global
mapping of planetary bodies at high resolution. But a full
high-resolution map of, say, Europa or Titan would take several
decades to download from orbiters of current designs. Downloading
data is a major design driver for interplanetary missions, and
scientists believe that data clippers would be an efficient way of
overcoming the bottleneck. They have carried out a preliminary
assessment for a data-clipper mission. The concept is for a clipper
to fly close to a planetary orbiter, upload its data and then pass
close to the Earth, at which point terabytes of data could be
downloaded to a ground station. A fleet of data clippers cruising
around the Solar System could provide support for an entire suite of
planetary missions.
Scientists have looked at the challenges of a data-clipper mission and
think that it could be ready for a launch in the late 2020s. Recent
advances in technology mean that spacecraft propelled by solar sails,
which use radiation pressure from photons emitted by the Sun, or
electric sails, which harness the momentum of the solar wind, can now
be envisaged for mid-term missions. The Japanese Space Agency, JAXA,
is currently testing a solar-sail mission, IKAROS. Using the Sun as a
propulsion source has the considerable advantage of requiring no
propellant on board. As long as the hardware does not age too much
and the spacecraft is manoeuvrable, the duration of the mission can be
very long. The use of data clippers could lead to a valuable
down-sizing of exploration missions and lower ground-operation costs.
The orbiting spacecraft would still download some samples of their
data directly to Earth to enable real-time discoveries and interactive
mission operations. But the bulk of the data is less urgent and is
often processed by scientists much later, so data clippers could
provide an economical delivery service from the outer Solar System,
over and over again.
Topic: September Astronomy Bulletin (Read 620 times)