(CNN) -- NASA's plans to fly a fifth and final space shuttle mission to service the Hubble Space Telescope have suffered another set-back.
Atlantis was prepared to launch in September before an onboard computer failed, delaying the mission.
Hubble managers say ground testing of a critical replacement computer that they hope to install on the orbiting telescope is taking longer than previously expected. And that means an additional flight delay.
"Delivery in April to support a May launch, I think is a fair thing to say," said Hubble program manager Preston Burch. "Right now I think we have a very good chance of meeting a readiness date in that time frame."
The additional delay is just the latest dip in an emotional rollercoaster ride for the Hubble team over the past few months.
In late September, astronauts were mere weeks away from launching to Hubble aboard the space shuttle Atlantis, with spacewalks planned to make the telescope more powerful than ever and extend its expected lifespan an additional five years.
That flight had to be postponed when the onboard computer that downlinks scientific data to the ground suddenly failed on September 27th. While that problem has been corrected using a back-up system, NASA managers have decided the computer needs to be completely replaced in order to keep a fully redundant back-up capability available.
A spare computer was built prior to Hubble's launch in 1990, and has been warehoused at the Goddard Space Flight Center in Maryland ever since. Initially, engineers had hoped that spare could be quickly tested, certified "flight ready," and shipped to the Kennedy Space Center in Florida to be packed aboard Atlantis in time for a February launch.
But a closer evaluation of the spare shows more than six months of work will be needed to get it ready to fly.
Among other issues, it was partially disassembled other the years so that parts could be used for other systems. It has been largely put back together, but now programming anomalies are cropping up.
And once engineers get it in good working order, it will have to go a battery of environmental tests to make sure it is ready to hold up in the harsh conditions of space. But the engineers are confident that they will eventually overcome all the obstacles.
"We've gotten a lot smarter about the condition of this unit over the last four weeks, said Burch. "We don't want to take any chances in bringing a box up there that isn't going to be 100% working to the absolute best that it can. So we want to take some extra time and make sure that we subject this to a very rigorous test program and we don't want to leave any stones unturned on the way to the launch pad."