Six proposals, ranging from a visit to the Asteroid Belt to amazingly sensitive gyroscopes, will undergo close examination during the next six months, as the European Space Agency's science advisors move towards the selection of flexi-missions for launch between 2005 and 2009. Science working groups and the Space Science Advisory Committee have chosen them from 49 ideas submitted in response to a call for proposals last October.
ESA's science programme introduced flexi-missions in 1997, to achieve greater flexibility. They replace the medium-scale projects, of which Huygens (Titan lander) and Integral (gamma-ray astronomy) are current examples. The aim is to have two flexi-missions for the price of one medium mission. Mars Express, already under construction for launch in 2003, is the first flexi-mission, or F1. Now under consideration are F2 and F3, each with a cost to ESA of no more than 176 million euros at 1999 prices.
The frontrunner in the astronomy field for one of these slots is European participation with NASA in the Next Generation Space Telescope, successor to the NASA-ESA Hubble Space Telescope. Although a formal decision will not be taken until later this year, much European effort has already gone into preparing for this NGST project, due for launch in 2008. That intensifies the competition for the other slot.
An embarrassment of riches - of ideas
Multinational teams of scientists from Europe's universities and research institutes are backing each of the proposals selected for assessment, half of which concern the Solar System and the Earth's space environment.
STORMS is a scheme to use three spacecraft to investigate a source of big trouble for technological systems, after solar eruptions. The "ring current" of energetic charged particles circulates around the equator at altitudes of several times the Earth's radius, and when its intensity varies during solar storms it causes magnetic perturbations at the Earth's surface. Three identical spacecraft, orbiting out to 50,000 kilometres and equally spaced around the equator, could clear up several remaining mysteries of the ring current -- and also provide real-time monitoring of magnetic storms.
SOLAR ORBITER would fly on an extended orbit taking it at intervals to within about 30 million kilometres of the Sun -- much closer than the innermost planet, Mercury. At its closest approach the spacecraft would round the Sun at roughly the same rate as the Sun itself rotates, so that it should seem to hover over one region. Besides giving unprecedented close-up views of the solar surface and atmosphere, the orbiter would directly sense the related behaviour of the solar wind and energetic particles in the Sun's vicinity. With the passage of time the orbit would slant at an increasing angle to the Sun's equator.
MASTER would adapt the Mars Express spacecraft for a flyby of Mars and especially a flyby of large asteroids in the Main Belt beyond Mars. Like Mars Express, MASTER would be able to drop a lander on the Red Planet, but instead of going into orbit around Mars it would use the planet's gravity to assist it onwards to the Asteroid Belt. There it would examine one or more asteroids with instruments developed for ESA's Rosetta comet mission and SMART-1 lunar mission. The proposers offer alternative scenarios and target asteroids for launches in 2005, 2007 and 2009.
Two proposals concern fundamental physics, which is a new theme in ESA's science programme. There will be a short-term selection between these two proposals, so that only one will go for the full assessment study. In both cases they would use techniques studied by ESA for other possible fundamental physics projects (STEP, MiniSTEP and LISA) to create a force-free environment for experiments. Effects of atmospheric resistance or the pressure of sunlight on the spacecraft are cancelled by automatically controlled thrusters.
HYPER would test new kinds of atomic gyroscopes and motion sensors of unprecedented precision, exploiting the quantum effect that makes even whole atoms behave as if they were waves instead of particles. Such sensors promise to be as revolutionary as atomic clocks in timekeeping. An atomic gyro, operating in space with a technique called ultra-cold atom interferometry, could in theory be 100 billion times more sensitive than existing optical gyros that use light instead of atoms.
CASIMIR would probe the fundamental nature of empty space. Quantum theory implies that even a perfect vacuum is not really empty but seethes with short-lived particles and forces. Half a century ago, the Dutch physicist Hendrik Casimir predicted that this hidden nature of the vacuum should reveal itself by a novel force between two metal plates. The proposal is to measure the Casimir force between superconducting surfaces a hundredth of a millimetre apart, a million times more accurately than has been done on the ground.
Finally, one of the proposed projects is astronomical. EDDINGTON would take up a station far from the Earth and use a 1-metre telescope with a wide field of view to examine stars for oscillations and passing planets. Oscillations due to sound waves have already revealed many features of the Sun's interior, allowing astrophysicists to check their theories about how stars work, in the nearest case. Now astronomers are beginning to use the same method in other stars, and EDDINGTON would apply it to 50,000 stars of many different kinds. It would also check 700,000 stars for the presence of planets, revealed by a dip in the brightness of a star when a planet passes in front of it.
In addition to the above mentioned proposals, SSAC recommended to study three proposals for accommodation on the International Space Station (ISS):
- EUSO, study of the cosmic neutrinos and extremely high energy cosmic rays,
- LOBSTER, an imaging all-sky X-Ray monitor; MOSS, studying the physics of superconducting ultra-stable microwave oscillators.
"As is always the case, these exciting proposals give us an embarrassment of riches," comments Roger Bonnet, ESA's director of science. "That's thanks to the vigour and imagination of Europe's space science community".
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