By Dr David Whitehouse BBC News Online science editor

Two US astronomers have been looking for a suspected belt of asteroids close to the Sun by making observations from the back seat of an F-18 jet.

This is the most comprehensive, constraining search yet conducted for these objects

Dr Alan Stern, planetary scientist

Dan Durda and Alan Stern, from the Southwest Research Institute (SwRI) in Boulder, Colorado, are looking for the Vulcanoids, a ring of debris lying between Mercury and our star.

First postulated over a century ago, the Vulcanoids are thought to range in size from one to 25 kilometres. Finding them would change our understanding of the innermost region of our Solar System.

If they do exist, it is possible they could still contain fragments of the earliest materials that formed next to our star when it was newborn.

"Most comprehensive search"

Durda and Stern are flying at a height of 15 kilometres (49,000 feet) to get the observing conditions that will best enable them to prove the belt's existence.

There is a short period of opportunity to catch the space rocks

Some theories suggest that a small number of kilometre-sized and larger Vulcanoids could have survived in the inner Solar System, inside the orbit of the planet Mercury, until now.

Named after the Roman god of fire, these asteroids would be exceedingly difficult to observe from the ground because of the Sun's glare and the distortion caused by the Earth's atmosphere.

"Our Vulcanoids search programme, conducted from an altitude of 49,000 feet over the Mojave Desert, gave us a view of the twilight sky near the Sun that is far darker and clearer than can be obtained from the ground," says Dr Durda.

Shuttle camera

"This is the most comprehensive, constraining search yet conducted for these objects," adds Dr Stern, director of the SwRI Space Studies Department.

The camera was designed with the shuttle in mind

Astronomers have conducted ground-based searches for the Vulcanoids before, during total solar eclipses, and during the twilight period after sunset just before the Vulcanoids themselves would set.

But to date, the asteroids have not been seen. Observations have only placed upper limits on how many might exist.

The camera used in the latest search was originally conceived for the space shuttle. It is trained on the region of space close to the Sun after the star has dipped below the Earth's horizon. The camera grabs twilight images at a rate of 60 frames a second.

The researchers are currently analysing their data. They hope to know whether the Vulcanoids exist in a month or two.