Exactly how many of these threatening objects are lurking unseen in the depths of the Solar System no one knows, but scientists estimate that events such as Tunguska occur on average once every 200 years.

Larger objects arrive less frequently but pack a much greater punch.

How can we find out more about these Near Earth Objects (NEOs) and possibly find a way of preventing them from sending the human race the way of the dinosaurs? One way is to send spacecraft to study them at close range.

At the UK/Ireland National Astronomy Meeting in Dublin, Simon Green (Open University) described SIMONE (Smallsat Intercept Missions to Objects Near Earth), a UK-led proposal to launch a fleet of low-cost microsatellites that will individually rendezvous with different types of Earth-grazing asteroids.

This would be the first interplanetary microsatellite mission.

With the first spacecraft costing less than 50 million Euro and additional satellites costing 30 million Euro each, a flotilla of five could be launched as a piggyback payload on an Ariane 5 rocket for the normal budget of one spacecraft.

Each 120 kg microsatellite would be despatched to a different target, using onboard solar electric propulsion driven by lightweight, high power solar arrays, a technology in which the UK is a world leader.

After rendezvous with the asteroid, five state-of-the art experiments would map its surface in great detail, in addition to determining its mass, density and composition.

"SIMONE would greatly improve our knowledge and understanding of the diverse NEO population," said Dr. Green. "The data would be crucial for the development of effective methods to deflect different types of objects that might impact the Earth in the future."

Assuming a launch by Ariane 5 in 2008 and arrivals 2010-11, the provisional target list includes:

• 1989 UQ -- a C-type Aten asteroid between 560 and 760 m in diameter
• 2001 CC21 -- an X-type Apollo asteroid with diameter between 390 and 1100 m
• 1996 FG3 -- a 1100 m diameter C-type Apollo asteroid with a large moon
• 1982 DB (Nereus) -- an X-type Apollo asteroid with diameter between 470 and 1330 m
• 1999 YB -- an S-type Apollo asteroid with diameter of about 640 m

SIMONE+, a larger spacecraft built using the same design principles could provide a low-cost option for longer range interplanetary missions including Mars.

The mission design and feasibility study for SIMONE was recently completed under contract to ESA as part of a Near Earth Objects Space Mission Preparation contract. The mission responds directly to one of the key recommendations of the UK Task Force Report on NEOs, issued in September 2000.

The SIMONE mission study team is led by QinetiQ (UK) in partnership with the Planetary and Space Sciences Research Institute (PSSRI) of the Open University (UK), SciSys (UK), Politecnico di Milano (Italy) and Telespazio (Italy).

A typical SIMONE payload would include:

• A Multispectral Imaging System, a camera to measure the asteroid's size, shape, topography, morphology and albedo.
• A Radio Science Investigation to determine the asteroid's mass from its gravitational influence on the spacecraft trajectory.
• An X-ray Spectrometer to determine its elemental composition.
• An Infrared Spectrometer for mapping minerals and surface variations.
• A Laser Altimeter to measure the surface topography and range from the spacecraft.

The explosion of the 1908 Tunguska object released energy equivalent to a modern nuclear warhead, about 10 megatons or 500 times the destructive power of the Hiroshima atomic bomb.

Related Links
Simone at ESA
The Planetary and Space Sciences Research Institute
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