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European
Space Agency study paves the way for a better understanding of space
weather
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Northern lights – example of space weather
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15
June 2009
Earth observation satellites in low orbit are continually buffeted
by the wisps of atmosphere that remain. Predicting how much air drag
a satellite can encounter is critical to the design, cost and
operation of a mission – an ESA study shows how.
Earth's atmosphere is often portrayed as a fragile, finite, thin
layer of gas blanketing the planet. However, since the number of
atmospheric particles decrease exponentially with altitude, there is
no real boundary between the atmosphere and outer space. Even though
the density of the air at satellite level is at least a billion
times lower than at sea level, the speed at which satellites move in
orbit is so high they can still experience drag.
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CHAMP accelerometer-derived densities
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Just
like sailors and airline pilots have to account for the weather in
the lower layers of the atmosphere – the troposphere and
stratosphere, satellite operators have to be able to predict the
weather in the upper regions of the atmosphere. Understanding air
density is crucial when designing a mission, for example, air
density and wind affect how much fuel is consumed and the lifetime
of a satellite mission. In addition, knowing what the weather is
like in space is important for planning manoeuvres, predicting
re-entry and assessing the risk of collision.
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Atmospheric density changes with altitude
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The
thermosphere starts at an altitude of around 90 km. This atmospheric
layer is strongly influenced by ultraviolet radiation and charged
particles originating from the Sun. It is also influenced by the
magnetic and electric fields that surround Earth. The weather in the
thermosphere is completely unlike that experienced on the ground.
Winds can reach speeds of hundreds of metres per second and air
density can vary by orders of magnitude, depending on the activity
of the Sun.
The results of a recent study commissioned by ESA's General Studies
Programme have shown that accelerometers, carried on current and
future Earth observation satellites, can act as space weather
observatories. These instruments measure acceleration relative to
'free fall' of an object in a near-circular orbit and can also
provide valuable data to improve air density models. In contrast,
ESA's gravity mission GOCE, which was launched into a very low orbit
in March, has accelerometers that measure the drag the spacecraft is
experiencing and ion thrusters for drag compensation.
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Atmospheric temperature changes with altitude
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The
18-month study was carried out by an international team led by the
Delft University of Technology in the Netherlands and investigated
accelerometer data from the German mission CHAMP and US-German
mission GRACE. It also simulated data for ESA's Earth Explorer Swarm
mission, which is due for launch in 2010. Swarm aims to improve our
understanding of Earth's magnetic field including near-Earth current
systems and their coupling with thermospheric density and winds.
Eelco Doornbos from the Delft University of Technology
explained, "The study involved the cooperation of European experts
in upper-atmospheric physics, satellite aerodynamics, orbital
mechanics, modelling and data processing. We have investigated the
most accurate way possible of deriving density and wind speeds from
the accelerometer data, compared the results with existing models,
created improvements to these models and have made recommendations
for future satellite missions.”
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CHAMP in motion as a result of atmospheric particles
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Heiner Klinkrad of ESA’s Space Debris Office summarised the merits
of the present study, “The modelling of aerodynamic forces requires
a good knowledge of all key parameters: the total air density, the
effective aerodynamic cross-section, the velocity relative to a
dynamic atmosphere, and the surface-molecule interaction parameters,
of which the drag coefficient is the most important. The current
study uses principles that were already developed at the beginning
of space flight. The current accelerometer data, however, provide
drag information that is several orders of magnitude better than in
those days – both in absolute magnitudes and spatial/temporal
resolution. Likewise, the molecule-surface interaction and the
effective aerodynamic cross-section are known much better today, so
we are able to extract very reliable information on the total
density and aerodynamic velocity. I believe that the current study
has significantly advanced research in this area.”
The results of this important study have shown that air density and
wind models can be significantly improved. This can help to reduce
some of the current uncertainties involved in the complex tasks of
planning, developing and operating satellites in low-Earth orbit.
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Grids Step-up to
a Set of New Records: Scale
Testing for the Experiment
Program ’09 (STEP’09)
Geneva, 1 July 2009.
Preparations are under way for
the restart of the Large Hadron
Collider (LHC) the world's most
powerful particle accelerator.
One of the most important
systems needed to support the
experiments that will utilise
this great machine is the global
computing grid: the worldwide
LHC Computing Grid (WLCG). After
months of preparation and two
intensive weeks of 24 x 7
operation the LHC experiments
are celebrating the achievement
of a new set of goals aimed at
demonstrating full readiness for
the LHC data taking run expected
to start later this year. Whilst
there have been several
large-scale data-processing
tests in recent years, this was
the first production
demonstration involving all of
the key elements from data
taking through to analysis.
Records of all sorts were
established: data taking
throughput, data import and
export rates between the various
Grid sites, as well as huge
numbers of analysis, simulation
and reprocessing jobs – ATLAS
alone running close to 1M
analysis jobs and achieving
6GB/s, of “Grid traffic”, the
equivalent of a DVD worth of
data a second, sustained over
long periods. This result is
particularly timely as it
coincides with the transition of
Grids into long-term sustainable
e-infrastructures, clearly of
fundamental importance to
projects of the lifetime of the
LHC. With the restart of the LHC
only months away, one can expect
a large increase in the number
of Grid users: from several
hundred unique users today to
several thousand when data
taking and analysis commences.
This can only happen through
significant streamlining of
operations and the
simplification of end-users’
interaction with the Grid.
STEP’09 included massive-scale
testing of end-user analysis
scenarios, including
“community-support”
infrastructures, whereby the
community is trained and enabled
to be largely self-supporting,
backed by a core of Grid and
application experts.
WLCG combines the IT power of
more than 140 computer centres,
the result of collaboration
between 33 countries.
Sergio Bertolucci, director of
research and computing at CERN*
said: “The 4 LHC experiments
–
ATLAS,
CMS,
ALICE and
LHCb – have demonstrated
their ability to manage their
nominal data rates concurrently.
For the first time all aspects
of the experiments’ computing
were exercised simultaneously:
simulation, data processing and
analysis. This gives them the
confidence that they will be
able to efficiently analyze the
first data from the LHC later
this year.”
Bob Jones, director of the EGEE
project remarked “such a
significant achievement is also
a valuable testament to the
state of maturity of the EGEE
infrastructure and its ability
to interoperate with major Grid
infrastructures in other parts
of the world. Ensuring that this
level of service continues
uninterrupted as we transition
from EGEE to EGI is clearly
essential to our users,
including flagship communities
such as High Energy Physics.”
"This is another significant
step to demonstrating shared
infrastructures can be used by
multiple high throughput science
communities simultaneously,"
said Ruth Pordes, executive
director of the Open Science
Grid consortium. "ATLAS and
CMS are not only proving the
usability of OSG, but
contributing to maturing
national distributed facilities
in the US for other sciences."
David Britton, the GridPP
Project Leader reported: “In
the UK, STEP09 ran very smoothly
at the majority of sites, which
allowed the focus to be on
understanding the performance
and tuning the infrastructure.
The RAL Tier-1 performed
exceedingly well with only a
single out-of-hours call out
over the two week period.
Valuable information was
obtained on the performance of
tape-drives under realistic
workflows; the OPN network was
tested by laying down additional
UDP traffic on top of the STEP09
data; and the fairshare system
was successfully tuned to
balance the load between
experiments.”
Gonzalo Merino, manager of the
Tier1 centre in Barcelona wrote:
"The Spanish WLCG sites met
the STEP09 targets. It has been
a very valuable exercise since
many of the experiment workflows
have been tested simultaneously
at unprecedented scale, well
above the nominal values for LHC
data taking. The Tier-1 at PIC
has provided a very stable and
reliable service at record
breaking levels: exchanging up
to 80 Terabytes per day with
other WLCG sites and processing
data at more than 2 GBytes per
second. This gives us confidence
that the Spanish WLCG sites are
ready for data taking."
David Foster, head of the LHC
Optical Private Network activity
concluded "The LHC Optical
Private Network transporting
data between the sites has
proven its capability both in
terms of performance and
resiliency during STEP'09. New
capabilities emerging in the
40Gbps and 100Gbps range should
enable us to keep up with the
anticipated data distribution
needs of the LHC experiments."
About the Large Hadron Collider
The LHC, located at CERN near
Geneva, Switzerland, is the
world’s largest particle
accelerator. For thousands of
physicists, analysing LHC data
using the LHC Computing Grid
will be like sifting for digital
gold. Their search is predicted
to unearth evidence of new
fundamental particles that will
provide clues to the ultimate
nature of matter and the origins
of our Universe.
About grid computing
Grid computing connects
computers distributed over a
wide geographic area. Just as
the World Wide Web enables
access to information, computing
grids enable access to computing
resources. These resources
include data storage capacity,
processing power, sensors,
visualisation tools and more.
Grids can combine the resources
of thousands of different
computers to create a massively
powerful computing resource,
accessible from the comfort of a
personal computer and useful for
multiple applications in
science, business and beyond. |
