Massimino is one of seven astronauts on STS-125, the fifth and final servicing mission to the Hubble Space Telescope (HST). The seven astronauts departed Kennedy Space Center on May 11th aboard the Space Shuttle Atlantis and plan to return to earth on May 22nd after eleven days in orbit.

As a mission specialist aboard STS-125, Massimino is performing two space walks that will provide repairs and upgrades to the twenty-year-old HST. The crew is carrying several new instruments to the HST including a ultraviolet spectrograph, a new thermal blanket, and a set of gyroscopes and batteries that will allow the Hubble to remain operational until at least 2014 when its successor, the James Webb Space Telescope, is due for launch.

"Mass," was the first to "tweet" from space. He has been chronicling the events of STS-125 by posting to his Twitter feed from orbit. For instance, an entry dated Monday, May 18, reads: "Just flew over the US, Baja to Miami in about 10 minutes! Beautiful Day!!" He can be followed at Astro_Mike on Twitter. Massimino's first flight into space was in 2002 when he flew aboard the Space Shuttle Columbia on a similar eleven-day mission to administer repairs the HST,

Massimino originally joined the Georgia Tech community in 1995 as an assistant professor in the Stewart School of Industrial and Systems Engineering, but left the faculty when he was selected as an astronaut candidate by NASA one year later. Since 1998, Massimino has enjoyed an adjunct assistant professor position in the Stewart School of ISyE, and he is a Member Emeritus of the ISyE Advisory Board. A former systems engineer with IBM, Massimino received his Ph.D. in mechanical engineering from MIT in 1992.