ISS Status Report #16 – 30 May 2001

International Space Station engineers are continuing to troubleshoot
problems with the Canadarm2 robotic arm on the complex after an unsuccessful
attempt earlier today to solve a communications glitch with one of the crane
‘s joints through a software modification.

With more time now needed to complete an analysis of the communications
problems between the arm’s shoulder pitch joint in its redundant or backup
mode, and the arm’s backup computer unit, Shuttle and Station program
managers decided to postpone the launch of Atlantis on the STS-104 mission
to deliver the Joint Airlock to the ISS until no earlier than early July.
The next ISS assembly flight after that, STS-105 aboard Discovery, was also
delayed until no earlier than early August to accommodate the Airlock
mission and to allow managers to consider a replacement of the shoulder
joint on the August flight during one of two spacewalks planned by Dan Barry
and Patrick Forrester. No final decision on replacing the joint has yet been
made.

The recently installed Canadarm2 has fully redundant computer control
systems. All of the arm’s joints are functioning perfectly through its
primary channel, but the backup channel has run into a series of recent
problems, including the unexpected activation of a switch for the arm’s
brakes earlier this month in the backup mode of operation during a test of
the arm’s wrist. That problem has not reoccurred in subsequent testing. The
shoulder pitch joint problem cropped up last week during other arm checkouts
when the joint experienced intermittent dropouts in communicating with the
backup computer unit.

It had been hoped that a software patch developed by Canadian engineers who
designed the arm would clear the communications dropouts, but it did not
resolve the problem, resulting in the need for further testing and analysis.
Robotics experts suspect that the problem resides in the shoulder pitch
joint hardware, not the software or the computers associated with its
operation. Expedition Two Flight Engineers Jim Voss and Susan Helms had
hoped to perform a “dry run” of the Airlock installation procedures today,
but that simulation was postponed.

The Canadarm2 is required for the grapple of the Airlock in Atlantis’ cargo
bay and its installation on the starboard docking port of the Unity module
of the ISS. The 12 1/2 ton Airlock will enable future Station spacewalks to be
conducted in either U.S. or Russian spacesuits rather than from the Russian
Zvezda module. The Shuttle robot arm cannot reach the Airlock installation
location.

In the meantime, ISS managers will decide Friday whether to ask Voss and
Expedition Commander Yury Usachev to add the replacement of the arm computer
unit to a previously scheduled “internal” spacewalk in the Zvezda’s
spherical transfer compartment on June 8. That first ISS-based spacewalk is
designed to reposition a docking mechanism in preparation for the arrival of
a Russian docking module later this year. If approved, the computer
replacement task would involve Usachev and Voss venturing outside the
Station in Russian suits for their spacewalk before reentering Zvezda’s
docking compartment for the mechanism repositioning work.

Engineers are also assessing the performance of a motor on one of the two
wings of the P6 solar array truss structure that enables the solar mast to
track the sun as the ISS orbits the Earth. Although there is more than ample
power being generated for all Station systems, the motor is generating
higher than normal electrical currents and may need to be replaced on a
subsequent Shuttle assembly flight.

Meanwhile, Usachev, Voss and Helms completed the unloading of an unmanned
Russian Progress resupply vehicle which arrived at the ISS last week loaded
with 3100 pounds of supplies, food, clothes and spare parts. A spare
computer hard drive brought to the ISS on the Progress was installed in one
of three central Station computers, bringing the outpost’s computers back to
full functionality.

Science investigations continue onboard under the guidance of the Payload
Operations Center at NASA’s Marshall Space Flight Center in Huntsville, AL,
except for the Human Research Facility, which is monitored and controlled
from the Telescience Support Center (TSC) at the Johnson Space Center,
Houston. For details on ISS science, visit the following website:
http://www.scipoc.msfc.nasa.gov

A status briefing on Expedition Two activities will be broadcast on NASA
Television on Thursday, May 31 at 2 p.m. Central time. The briefing will
include multi-center question and answer capability from NASA centers. A
news conference with the Expedition Two crew for U.S. reporters is scheduled
Friday at 8 a.m. Central time and will also be broadcast on NASA Television.

The International Space Station is orbiting at an altitude of around 250
miles (401 km). The next ISS Status Report will be issued as mission events
warrant.