1,630 research outputs found
Asteroid detection at millimetric wavelengths with the Planck survey
The Planck mission, originally devised for cosmological studies, offers the
opportunity to observe Solar System objects at millimetric and submillimetric
wavelengths. We concentrate in this paper on the asteroids of the Main Belt. We
intend to estimate the number of asteroids that can can be detected during the
mission and to evaluate the strength of their signal. We have rescaled the
instrument sensitivities, calculated by the LFI and HFI teams for sources fixed
in the sky, introducing some degradation factors to properly account for moving
objects. In this way a detection threshold is derived for asteroidal detection
that is related to the diameter of the asteroid and its geocentric distance. We
have developed a numerical code that models the detection of asteroids in the
LFI and HFI channels during the mission. This code perfoprms a detailed
integration of the orbits of the asteroids in the timespan of the mission and
identifies those bodies that fall in the beams of Planck and their signal
stenght. According to our simulations, a total of 397 objects will be observed
by Planck and an asteroidal body will be detected in some beam in 30% of the
total sky scan--circles. A significant fraction (in the range from ~50 to 100
objects) of the 397 asteroids will be observed with a high S/N ratio. Flux
measurements of a large sample of asteroids in the submillimeter and millimeter
range are relevant since they allow to analyze the thermal emission and its
relation to the surface and regolith properties. Furthermore, it will be
possible to check on a wider base the two standard thermal models, based on a
nonrotating or rapidly rotating sphere. Our method can also be used to separate
Solar System sources from cosmological sources in the survey. This work is
based on Planck LFI activities.Comment: Contact person [email protected]. Accepted for pubblication in
New Astronomy (2002). 1 figure in .eps format. Needs elsart.cls style +
harvard.st
Multicolor Photometry of the Uranus Irregular Satellites Sycorax and Caliban
We report on accurate BVRI photometry for the two Uranus irregular satellites
Sycorax and Caliban. We derive colours, showing that
Sycorax is bluer than Caliban. Our data allows us to detect a significant
variability in the Caliban's light-curve, which suggests an estimated period of
about 3 hours. Despite it is the brighter of the two bodies, Sycorax does not
display a strong statistically significant variability. However our data seem
to suggest a period of about 4 hoursComment: 17 pages, 2 eps figures, in press in Astronomical Journa
Constitutional Law - Standing - Separation of Powers
The Supreme Court of the United States refused to rule on whether the Line Item Veto Act of 1996 violated the Constitutional separation of powers by impermissibly delegating legislative powers to the President because the Congressmen who brought the suit did not have standing to maintain the action.
Raines v. Byrd, 117 S. Ct. 2312 (1997)
1999 Quadrantids and the lunar Na atmosphere
Enhancements of the Na emission and temperature from the lunar atmosphere
were reported during the Leonids meteor showers of 1995, 1997 and 1998. Here we
report a search for similar enhancement during the 1999 Quadrantids, which have
the highest mass flux of any of the major streams. No enhancements were
detected. We suggest that different chemical-physical properties of the Leonid
and Quadrantid streams may be responsible for the difference.Comment: 5 pages, 1 figure, accepted for publication in MNRA
An example of dynamic QoS negotiation
The traffic generated by multimedia applications presents a high degree of burstiness that can be hardly described by a static set of traffic parameters. The following paper presents a dynamic QoS negotiation scheme applied to a video streaming application. In applications that uses RSVP, the dynamic and efficient usage of the resources can be reached with the introduction of the renegotiable variable bit rate (RVBR) service, which is based on the renegotiation of the traffic specification. In this paper we describe and discuss the RVBR service and how it applies to resource reservation for Internet traffic with RSVP. For that we propose an architecture design that we evaluate by accomplishing a prototype implementation, whose performance are measured with real MPEG2 video traces. The results we obtained indicate that renegotiation is an efficient mechanism to accommodate traffic fluctuations over the burst time-scale, and that RVBR service can be easily implemented, to this aim, in real applications, using available technology
Fluids mobilization in Arabia Terra, Mars: depth of pressurized reservoir from mounds self-similar clustering
Arabia Terra is a region of Mars where signs of past-water occurrence are
recorded in several landforms. Broad and local scale geomorphological,
compositional and hydrological analyses point towards pervasive fluid
circulation through time. In this work we focus on mound fields located in the
interior of three casters larger than 40 km (Firsoff, Kotido and unnamed crater
20 km to the east) and showing strong morphological and textural resemblance to
terrestrial mud volcanoes and spring-related features. We infer that these
landforms likely testify the presence of a pressurized fluid reservoir at depth
and past fluid upwelling. We have performed morphometric analyses to
characterize the mound morphologies and consequently retrieve an accurate
automated mapping of the mounds within the craters for spatial distribution and
fractal clustering analysis. The outcome of the fractal clustering yields
information about the possible extent of the percolating fracture network at
depth below the craters. We have been able to constrain the depth of the
pressurized fluid reservoir between ~2.5 and 3.2 km of depth and hence, we
propose that mounds and mounds alignments are most likely associated to the
presence of fissure ridges and fluid outflow. Their process of formation is
genetically linked to the formation of large intra-crater bulges previously
interpreted as large scale spring deposits. The overburden removal caused by
the impact crater formation is the inferred triggering mechanism for fluid
pressurization and upwelling, that through time led to the formation of the
intra-crater bulges and, after compaction and sealing, to the widespread mound
fields in their surroundings
An example of traffic-accomodating application
The traffic generated by multimedia applications presents a great amount of burstiness, which can hardly be described by a static set of traffic parameters. The dynamic and efficient usage of the resources is one of the fundamental aspects of multimedia networks: the traffic specification should first reflect the real traffic demand, but optimise, at the same time, the resources requested. This paper presents an example of application able to accommodate its traffic to managing QoS dynamically. The paper is focused on the technique used to implement the Dynamic Reallocation Scheme (RVBR) taking into account problems deriving from delay during the reallocation phase
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