The asteroids were spotted orbiting and eclipsing one another between Mars and Jupiter. Loretta Kuo

An extremely rare pair of asteroids has been discovered orbiting and eclipsing one another between Mars and Jupiter.

The asteroid, 3905 Doppler, was found to be an eclipsing binary asteroid by students at the University of Maryland.

Fewer than 100 asteroids of this type have ever been identified in the main asteroid belt between the two planets, astronomy teacher Melissa Hayes-Gehrke said.

3905 Doppler was first discovered in 1984 but has been paid little attention since.

The students discovered the asteroid was not what it first appeared to be in October last year after picking it to study due to its position in the sky.

They were observing and photographing the asteroid in Spain and were looking for changes in its intensity of reflected light and turn the images into a lightcurve – a graph of a celestial object's brightness over time.

Asymmetrical objects like asteroids produce peaks and valleys as the reflected light given off varies. They noticed something was wrong when they used a computer program to chart the lightcurve, which showed the light fading to almost nothing.

Alec Bartek, a senior physics major, said: "It was incredibly frustrating. For some reason our light curve didn't look right."

Hayes-Gehrke then suggested that 3905 Doppler was actually two asteroids orbiting one another. When one of them blocked the telescope's view it produced an asteroid eclipse and a dip in the lightcurve.

Later observations by Italian astronomer Lorenzo Franco later confirmed the existence of a binary asteroid.

Brady Bent, an economics student, said: "Even then I was not fully aware of how special the discovery was. I thought it just meant we would have to do more work. As we continued to analyse our data, other professors in the Astronomy Department came over to view our work. At this point I understood just how rare our find was."

Hayes-Gehrke said the two asteroids are potato-shaped with impact craters from hitting other space debris. The smaller of the two is around three quarters the length of the larger one.

University of Maryland Astronomy Professor Drake Deming said: "This is a fantastic discovery. A binary asteroid with such an unusual lightcurve is pretty rare. It provides an unprecedented opportunity to learn about the physical properties and orbital evolution of these objects."