Kelly et al. (2009) contend that the impact was caused by a comet because of the sightings of noctilucent clouds following the impact, a phenomenon caused by massive amounts of water vapour in the upper atmosphere. They compared the noctilucent cloud phenomenon to the exhaust plume from NASA's Endeavour Space Shuttle. A team of Russian researchers led by Edward Drobyshevski in 2009 suggested that the near-Earth asteroid 2005 NB 56 may be a possible candidate for the Tunguska object's parent body as the asteroid made a close approach of 0.06945 AU (27 LD) from Earth on 27 June 1908, three days before the Tunguska impact. The team suspected that 2005 NB 56 's orbit likely fits with the Tunguska object's modelled orbit, even with the effects of weak non-gravitational forces. In 2013, analysis of fragments from the Tunguska site by a joint US-European team was consistent with an iron meteorite.