Buzz Aldrin, the second man to walk on the Moon, has had to cancel a planned visit to Cork.
The astronaut was due to launch the International Space University's (ISU) 30th Space Studies Program.
But a statement from the organisers say he has had to pull out of the event on medical advice.
Dr Aldrin (87) was also set to deliver a lecture in the Cork Institute of Technology (CIT) on June 27th.
The organisers say they are planning to proceed with the lecture with an alternative option.
In a communication to ISU President Walter Peeters and CIT President Brendan Murphy, Dr Aldrin expressed profound regret at having to cancel his visit.
He said he was very much looking forward to visiting Ireland.
Prof Peeters and Dr Murphy of CIT have wished Dr Aldrin a speedy recovery.