The fifth and final Hubble Space Telescope servicing mission was put on indefinite hold today after a critical component in the telescope's communications system failed and brought to a screeching halt all scientific observations. It now appears that Atlantis mission to the telescope will be delayed until mid-February, at the earliest.
The delay will give NASA time to ready replacement hardware and train the seven-person STS-125 crew for the additional task of repairing the science data formatter in a mission already packed with tasks to be conducted during five back-to-back spacewalks.Hubble's latest troubles began around 8 o'clock Saturday evening. Side A of the control unit/science data formatter (CU/SDF-A) began showing signs or erroneous behavior, which prompted the telescope's flight computer to shut it down and place the payload computer and science instruments into "safe mode." An initial attempt to reset the data formatter was unsuccessful. The loss of channel A leaves only the redundant B channel operational. Engineers will work over the next several days to switch the telescope to the B side of the data formatter. Until that happens, Hubble will remain unable to conduct any scientific observations. Resorting to the backup channel is the only option for restoring Hubble to an operational capability. "Our only option at this point is to switch over to science data formatter B, which is the redundant channel," said Hubble Space Telescope Program Manager Preston Burch. "Unfortunately, switching to that side will require the switch over of the spacecraft data management system to the B side as well."However, the electronics on the B side haven't been powered up since Hubble was launched 18 years ago. It's a complex process that involves switching the data formatter and five other data management modules to their B sides as well. They were last activated during prelaunch ground tests in the 1980's and early 1990. Engineers are confident that the backup systems can be powered up and activated and will operate as well as the primary systems, which has performed perfectly since 1990.However, simply transitioning to the backup system presents long-term risks for Hubble. Losing the data formatter's A channel means that, even when the telescope is transitioned to the B side, it will have lost redundancy in the data formatter. Any failure of the B channel would essentially kill Hubble's use as a scientific observatory for good.Had the fifth servicing mission not been approved, that would have been NASA's only choice, and perhaps Hubble's fate. There isn't room in the shuttle manifest for a sixth servicing mission before the shuttle's retirement in 2010. However, agency managers decided to take advantage of the upcoming mission to repair the faulty formatter. "If we go that route, just go to side B, we would be left with a system that has several single point failures and that would pose a risk to the mission," said NASA Associate Administrator for Space Science Ed Weiler. "By going ahead and accepting a delay of perhaps several months, we can actually get our SIC&DH (science instruments command and data handling) spare unit tested and ready to go. And if we can put that in there sometime in the winter, we would now have an observatory that was again doubly redundant.""If we're going to spend the money and take all the risk involved in a shuttle mission, we want to be sure we leave Hubble as healthy as we possibly can and potentially lasting for five or 10 more years," he added.In fact, it may be a fortunate stroke of good luck that the failure happened when it did rather than two weeks later."Think about if this failure had occurred two weeks after the servicing mission," said Weiler. "We'd have several single point failure, we could have lost the mission in six, 12, 18 months. So in some sense, if this had to happen it couldn't have happened at a better time."Repairing the science data formatter should be fairly straightforward and take up only a couple hours of EVA time. A spare control unit/science data formatter is at Goddard Spaceflight Center in Maryland. It consists of a 135 pound command and data handling system mounted in a box measuring 9.5 x 21.5 x 32.5 inches. The replacement unit would be bolted to the door of electronics bay 10 with 10 electrically isolated bolts and an electrical connector. Mission managers are hoping the repair task can be inserted into the current mission timeline without having to drop any other tasks. The leading candidate is EVA #5. Repairing Hubble's Advanced Camera for Surveys (ACS), which will be repaired during EVA's 3 and 5. If the astronauts can complete the work in the first of those two spacewalks, that would free up time on the fifth spacewalk. Right now, it looks like the earliest the STS-125 mission can launch is mid-February. A decision will be made in the coming days, but it's likely the Hubble mission will be targeted for February (if the hardware is ready and training complete in order to support the additional repair tasks). A February launch would allow for space shuttle Discovery, scheduled for launch on a space station construction mission February 12, to serve as the rescue shuttle for Atlantis. The space station has a busy schedule early next year which constrains NASA's flexibility with space shuttle Discovery's STS-119 mission in February. Two Soyuz missions are scheduled to launch to the orbiting complex in the first half of 2009 that shuttle managers must work around. Additionally temperature and power constraints due to the station's orbit that time of year also limit when shuttles may visit. NASA could also opt to delay the Hubble mission until May, when Endeavour would be available as the rescue vehicle."There is a time starting on March 13 and ending May 28 where we have two Soyuz launches and a significant beta cutout whereas the Hubble mission is not constrained by any of that," said space shuttle Program Manager John Shannon during today's media teleconference. "So we will consider that once we know more about the Hubble need date. If we could put the Hubble in and fly STS-119 before that Soyuz, that would be good. If it looks like Hubble needs a little more time than that, then it's very possible we could fly 119 first, let Hubble fly sometime in that Soyuz and beta cutout."With the delay to STS-125, Endeavour, currently sitting on pad B, has been relieved of launch on need duty at least until next May, so processing will move forward for its November 16 space station logistics and repair mission. The payload will be removed from Atlantis cargo bay this coming weekend and returned to its handling facility. Atlantis will then be moved off pad 39-A and returned to the Vehicle Assembly Building sometime in the third week of October.With pad A vacant. Endeavour will be rolled around from pad B, where it has been prepped for readiness in case an emergency rescue mission for Atlantis' crew was needed, and placed on pad A for final processing and launch in November. Launch Complex 39-B is currently undergoing renovations to support project Constellation and the Ares I rocket and can no longer adequately support shuttle launches. So, Endeavour was rolled out to pad B for prelaunch processing for the rescue mission, STS-400, while Atlantis was prepped on pad A, but it also must be moved to pad A for launch.