From within, theologians have debated the implications of alien contact for centuries. And if one already believes in angels, no great leap of faith is required to accept the possibility of other extraterrestrial intelligences.

Since God created the universe, theologians say, he would have created aliens, too. And far from being weakened by contact, Christianity would adapt. Its doctrines would be interpreted anew, the aliens greeted with open -- and not necessarily Bible-bearing -- arms.

"The main question is, 'Would religion survive this contact?'" said NASA chief historian Steven J. Dick, author of The Biological Universe. "Religion hasn't gone away after Copernican theory, after Darwin. They've found ways to adapt, and they'll find a way if this happens, too," Dick says.

The central conundrum posed to Christianity by alien contact would involve the Incarnation -- the arrival of Jesus Christ as God's representative on Earth, his crucifixion and the absolution of humanity's sins through his forgiveness. "It would still be true -- but if there are other races and intelligences, then what is the meaning of this visit to our race at that time?" asked Vatican astronomer Guy Consolmagno, who in 2005 penned the booklet Intelligent Life in the Universe?

Some propose that the Earthly incarnation of Jesus some 2,000 years ago redeemed all intelligent creatures, in all places and -- since a space-faring race is likely older than us -- in all times. Others have suggested that Jesus could take multiple forms.

"Just as Jesus is human like you and I, you would find an alien-specific Jesus," said Pacific Lutheran Theological Seminary professor Ted Peters. But Peters and others also say that aliens may not have fallen into sin, instead existing in a state of grace, neither having nor needing Jesus. In that case, missionaries would have no call to convert them.