The astronauts of the International Space Station's Expedition 39 crew focused Tuesday on preparations for a contingency spacewalk to be conducted in the next week or so. Meanwhile station program managers and SpaceX officials are working toward scheduling dates for the next launch attempt for Dragon cargo spacecraft and that contingency spacewalk, while managing all this around the planned undocking and redocking of a Russian cargo craft next week.

Monday's launch attempt of the SpaceX Dragon cargo craft, loaded with nearly 5,000 pounds of supplies and experiment hardware, was scrubbed due to a helium leak on the Falcon 9 rocket's first stage. The next launch opportunity would be Friday at 3:25 p.m. EDT if the issue can be resolved.

Flight Engineers Rick Mastracchio and Steve Swanson, both NASA astronauts, spent their day preparing for the 2 0.5-hour spacewalk they will conduct to replace a failed multiplexer demultiplexer (MDM) backup computer relay system in the S0 truss that assists in providing insight into truss systems, the operation of the external cooling loops, the operations of the Solar Alpha Rotary joints and the Mobile Transporter rail car.

Swanson performed a full end-to-end checkout of his spacesuit with its new fan separator pump installed. During the checkout, a cranky CO2 sensor was discovered and is now being investigated.

Mastracchio meanwhile gathered and configured the tools he and Swanson will use during the excursion.

Swanson and Mastracchio also participated in an on-board training session for the spacesuit "life jacket," known as the Simplified Aid For EVA Rescue, or SAFER. Should a spacewalker become untethered during a spacewalk and begin floating away from the station, the small nitrogen-jet thrusters of SAFER would propel the astronaut back to safety.

The two NASA astronauts joined up with Commander Koichi Wakata of the Japan Aerospace Exploration Agency for an audio conference with ground team that is finalizing procedures for the spacewalk.

Wakata assisted with the spacewalk preparations Tuesday as he completed a flush of the water inside the cooling loops of the spacesuits and the airlock.

The Japanese commander also found time for station science with another session of the Hybrid Training experiment. This Japan Aerospace Exploration Agency study takes a look the health benefits of applying electric stimulation to a muscle opposing the voluntary contraction of an active muscle. In addition to providing a backup to the traditional exercise devices aboard the station, Hybrid Training may be useful in keeping astronauts fit as they travel beyond low Earth orbit aboard smaller spacecraft.

Later, Wakata helped the ground team scout out locations aboard the station for the upcoming Petri Plants payload, which will use ambient light to sprout seedlings.

Wakata rounded out his day recharging batteries for the spacewalk.

On the Russian side of the complex, Flight Engineer Oleg Artemyev performed another session of the Relaxation experiment, which studies chemical luminescent reactions in the Earth's atmosphere.

Flight Engineer Mikhail Tyurin downloaded micro-accelerometer data from the Identification experiment to provide insight into the station's dynamic loads. The veteran cosmonaut also participated in the Seiner ocean-observation study, documenting color blooms in the oceans' waters for the fishing industry.

Flight Engineer Alexander Skvorstov continued unloading cargo from the ISS Progress 55 cargo ship, which delivered nearly three tons of cargo to the orbital laboratory when it launched and docked with the station's Pirs docking compartment on April 9.

A second Russian cargo ship currently at the station, ISS Progress 53, is set to undock from the Zvezda service module on April 23 at 4:54 a.m. to test its Kurs automated rendezvous equipment. The vehicle will redock with Zvezda on April 25 at 8:16 a.m. Progress 53 delivered 2.9 tons of food, fuel and supplies to the station on Nov. 29 following a four-day journey that included a "flyby" of the station to test a new lighter, revamped Kurs system.