Archeops: A stratospheric balloon for in-depth cosmic radiation mapping

On January 29, 2001, Archeops, a stratospheric balloon for in-depth cosmic radiation mapping, made its first successful scientific flight from the Swedish launch site at Esrange near Kiruna. After flying for 7 1/2 hours at an altitude of 31.5 km, the
experiment’s gondola was detached and was then recovered undamaged
in Russia. Physicists from three CNRS* scientific departments
have played a major role in this experiment, which brings together French,
Italian, British and American scientists.

The Archeops experiment aims
to measure the background radiation emitted by the universe about 300,000
years after the Big Bang; its study is vital for the production of more
accurate cosmological data. By that time, the universe had cooled sufficiently
for electrons and protons to combine and produce neutral atoms, and light
began its journey towards us today. Due to the expansion of the universe,
this radiation appears today to have come from a black body at a temperature
of 2.7 Kelvin**. Measuring the spatial fluctuations of
the temperature of this radiation enables us, by comparison with cosmological
models, to determine the different fundamental parameters of the universe
(density, curvature, age) and to understand the formation of observable
clusters and galaxies.

The "new generation" Archeops experiment was carried out using
a telescope equipped with highly sensitive instruments, operating close
to absolute zero to allow very small temperature fluctuations to be measured.
The telescope, suspended under a stratospheric balloon in order to remove
as many atmospheric effects as possible, rotated on its own axis at two
revolutions per minute in order to sweep the sky.

Highly sensitive detectors measured the rise in temperature of an absorbing
web struck by the radiation to characterize the fluctuations being studied:
these bolometers, developed in the USA, are cooled to 100 millikelvin
(mK) using a dilution cryostat produced at the CNRS "Centre de recherche
sur les très basses températures" (CRTBT, Very Low
Temperature Research Center) in Grenoble. The 1.5 meter telescope produces
an angular resolution of about 8 arc minutes: fifty times better than
that measured by COBE, a satellite launched some ten years ago.

An initial technical flight at Trapani, Sicily, in July 1999 mapped 15%
of the heavens at three wavelengths. It demonstrated the feasibility of
using 100 mK bolometers cooled by a dilution cryostat in an air-borne
experiment. Archeops was launched on January 29 from the Esrange base
in Sweden for a complete series of measurements. The flight took place
north of the arctic circle during the polar night to encounter the least
possible disturbance from sunlight, and covered 25% of the sky—much
more than in other experiments. Meteorological conditions limited the
flight to 7 1/2 hours, taking it to the foothills of the Urals. The data
was recovered, routed to Sweden then to France, and an initial check verified
the quality; analysis will require several months’ work by the participating
teams.

CNRS teams were heavily involved in this international project: design
of the cryostat and on-board electronics, calibration tools, on-board
data acquisition and data processing.

Archeops represents an initial project using techniques that will be used
in the Planck satellite. This European Space Agency satellite, due to
be launched in 2007, will enable all of the background radiation emitted
by the universe to be mapped.

Internet sites: http://www.archeops.org
http://journal.archeops.org/KirunaS1/KirunaS1.html

* "Département des Sciences physiques
et mathématiques" (Mathematics and Physical Sciences Department),"Institut
National des Sciences de l’Univers" (INSU, National Institute
for Sciences of the Universe), "Institut National de Physique nucléaire
et de Physique des Particules" (Nuclear and Corpuscular Physics Department),
and physicists from the CEA (Atomic Energy Commissariat). Financial support
for the French contribution to the experiment was provided by CNES (National
Center for Space Studies), CNRS (scientific departments and National Cosmology
Program) and by the Rhône-Alpes Region.

** The Kelvin is the unit of absolute temperature: 0 Kelvin
= -273°C