Dr Gentry Lee, who worked on the Mars Viking mission, has spent decades designing probes that land on other worlds.
Speaking recently at the American Association for the Advancement of Science (AAAS) conference in Phoenix, Dr. Lee expressed his opinion that alien life is still out there for us to find.
"[It has] just got to be there somewhere," he said.
"We are going to find life of some kind somewhere else. The odds are overwhelming."
That said, he didn't think that intelligent aliens had ever visited the Earth.
"There exists nothing today that says any alien or any alien machine has ever landed on the planet Earth," he said. "If you believe otherwise, you are being misled."
On the subject of finding life on other worlds, Dr. Lee stated that scientists would need to be ready to find forms of extraterrestrial life that are very different to anything we see here on our own world.
The Earth, he argues, may actually not be all that interesting to an alien scientist.
"Extraterrestrial biologists would come to the Earth and would go back and report: 'Not a terribly interesting planet. All life is the same.'"
"'All of it reproduces in the same way using the same major [DNA] molecule.'"
You can watch part of the conference featuring Dr. Lee in the video below.