Comet 46/Wirtanen is relatively small, approximately .75 miles in diameter. It has a short orbit period of 5.4 years and is considered to be in the Jupiter family of comets because it's orbit does not extend beyond the planet. It is also considered to be a "hyperactive" comet in that it contains more ice (and therefore emits more water) than expected. It appears bright in the sky because it is the closest it has been to the Earth in centuries (i.e., approximately 7.5 million miles).