MONTREAL — Great balls of fire have been reported swooping over Eastern Canada and several U.S. states.

Even NASA's on the case.

There are different theories about what was behind the sighting of those fireballs. A NASA spacecraft got a closer look at one of the possible sources today.

The spacecraft flew past Hartley 2 -- taking closeup pictures after the comet made one of its closest passes by Earth this week.

But one expert is skeptical of reports that any fireballs came from Hartley -- which is roughly 1.2 kilometres wide and spews deadly cyanide gas.

Scientist Peter Brown says his meteor group at the University of Western Ontario tracked one of two fireballs while the other was tracked by NASA's Meteoroid Environment Office.