University of Michigan astronomy professor Mario Mateo was a little jealous when he learned a group of Ypsilanti bus drivers and a teacher saw an apparent fireball Thursday morning.

"Shoot. I wish I'd seen it!'' he said. "They can be very, very bright and are quite impressive.''

This bolide - or fireball - may have been nothing more than a rock up to about a foot in diameter, Mateo said.

It certainly impressed Josephine Watson and Shirley Lucas.

The bus drivers were in the parking lot at the end of Railroad Street in Ypsilanti at about 6:50 a.m. when they spotted a line of fire in the sky that tracked roughly west-to-east.

"Then poof, it disappeared,'' Lucas said.

"We were shocked. We had never seen anything like that,'' Watson said.

When Watson arrived at Estabrook Elementary School, she said a teacher reported seeing the phenomenon, too.

Mateo said some fireballs he's seen were short-lived, while others lasted many seconds and traversed tens of degrees in the sky.

He described one as: "A beautiful, slow bolide that just was getting brighter and brighter. When it got to the point that it was considerably brighter than Venus, it just exploded, as if a fiery hand opened up and then rapidly disappeared from view.''

Mateo said about 100 tons of extraterrestrial matter fall on Earth every day - about the capacity of a railroad car - and most is just space dust, with the occasional pebble or stones that become bolides.

But most fireballs occur over oceans and uninhabited regions, and many are masked by daylight, he noted.

And despite their name, fireballs aren't actually on fire. The rocks move so fast that friction with the upper atmosphere, even where there's little air, heats them until they glow, Mateo said.