One might be “bright enough to be seen in the day” and the other is projected to become “one of the brightest in history” and possibly “outshine the Moon“.
The first comet, named C/2011 L4 (PANSTARRS), is expected to make its arrival in March 2013. According to astronomers, it will be “potentially visible to the naked eye low in the western horizon just after sunset” and also has the “potential that it may be visible during the day.”
Richard Wainscoat, an astronomer at the University of Hawaii who co-discovered the comet, claims there is no danger of collision with Earth but admits “since we don’t have a lot of data on it, we really don’t know the orbit well enough right now.”
The second comet, the one which could possibly outshine the Moon, was only just discovered this September and is called 2012 S1 (ISON). Astronomers predict “current orbital predictions indicate the comet will look brightest to us in the weeks just after its closest approach to the sun, on November 28, 2013.”
Comet ISON may be visible for the last two months of the year as well as early 2014 and is the more likely of the two to be a spectacular event:
The comet is already remarkably bright, given how far it is from the sun, astronomer Raminder Singh Samra said. What’s more, 2012 S1 seems to be following the path of the Great Comet of 1680, considered one of the most spectacular ever seen from Earth.
‘If it lives up to expectations, this comet may be one of the brightest in history,’ said Samra, of the H.R. MacMillan Space Centre in Vancouver, Canada. (Source)
This comet is considered to be larger than most at 3 kilometers wide (roughly 2 miles) adding to the anticipation of its arrival. It, too, may be visible during the day according to astronomers.
The last Great Comet highly visible from Earth, Hale-Bopp, arrived 16 years ago in 1997.
Astronomers are cautious not to make wild predictions about approaching comets, always adding the caveat that they never really know how comets will react as they get closer to the sun. Yet, it does seem significant that 2013 may have two Great Comets.