Most comets are small, not that bright, and don't last that long. Although, in less then 24 hours, Comet 17P/Holmes 400,000 times brighter than any other comet before! This big ball of dust and ice could be seen without a telescope for many weeks.It is normal for comets to become brighter when they are revolving close to the Sun. When the comet is near the Sun, the ice on it melts and the Sun's rays reflect its light on the dust part. The Holmes comet was as far as the Earth was to the sun and it never got much heat. When the comet was in orbit, it was not next to the sun. It was five months past the closest location. Scientists explain this for happening because the outer layer disintegrated. Then, the Sun reflected its rays on the halo of dust. This may have been seen five months after its closest point because it was probably soaking up the Sun's rays before. A discovery like this one was discovered in 1892 by an English astronomer. Holmes was a bright comet at one point but then it disappeared. Scientists lost track of it and now it has resurfaced. This whole cycle could happen again but scientists are keeping a close watch on it now. http://www.sciencenewsforkids.org/articles/20071128/Note3.asp
