Monday's launch attempt of the SpaceX Dragon cargo craft, loaded with nearly 5,000 pounds of supplies for the International Space Station's Expedition 39 crew, was scrubbed due to a helium leak on the Falcon 9 first stage. The next launch opportunity would be Friday, April 18 at 3:25 p.m. EDT if the issue can be resolved.

The launch of the third SpaceX Dragon commercial resupply services mission from Cape Canaveral Air Force Station in Florida will send the vehicle on course to rendezvous with the station several days later. Commander Koichi Wakata and Flight Engineer Rick Mastracchio will capture the space freighter using the Canadarm2 robotic arm to set it up for its berthing to the Earth-facing port of the Harmony module.

Mission managers and SpaceX had agreed Sunday to proceed with Monday's launch attempt despite the loss Friday of a 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.

The International Space Station Mission Management Team met throughout the weekend to assess the work of the teams of engineers analyzing the problem and determined that the station has enough redundancy to permit the SpaceX Dragon launch to proceed. A contingency spacewalk is now scheduled for no earlier than April 22 to replace the failed backup computer relay unit in the S0 truss.

On Sunday, the Mobile Transporter was moved to a worksite to provide spacewalking astronauts the access they will need to replace the MDM during the contingency spacewalk. The Solar Alpha Rotary joints will be placed at the correct angle following the launch of the Dragon cargo vehicle in advance of its arrival.

Aboard the station Monday, Mastracchio and Flight Engineer Steve Swanson spent most of the day inside the Quest airlock working on the replacement of a fan pump separator in one of the two spacesuits they will wear during the planned 2 0.5 hour contingency spacewalk. This will be the 179th spacewalk in support of station assembly and maintenance.

Mastracchio and Swanson also reviewed training materials for the Dragon spacecraft and studied the plan for unloading the cargo. Wakata conducted 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

The Japanese commander also began the first RaDI-N radiation detection session for Expedition 39. After Flight Engineer Mikhail Tyurin initialized the detectors, Wakata deployed the neutron monitors in the Columbus module.

Results from RaDI-N will help researchers accurately measure the risk assessment of neutron radiation in space and reduce the astronauts' exposure to radiation on future missions.

Wakata rounded out his day flushing the cooling loops of the U.S. spacesuits. On the Russian side of the complex, Flight Engineer Oleg Artemyev conducted the Relaxation experiment, which studies chemical luminescent reactions in the Earth's atmosphere.

Flight Engineer Alexander Skvortsov 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 on April 9. Tyurin participated in a routine hearing assessment and performed routine maintenance on the life-support system in the Zvezda service module.