Two NASA astronauts and an Italian flight engineer are sheltering in the space station's Russian segment as mission control analyzes a possible leak of toxic ammonia coolant aboard the complex.

NASA says there is no confirmation of a leak, but they ordered commander Butch Wilmore and flight engineers Terry Virts and Samantha Cristoforetti into the Russian part of the outpost as a precaution.

The crew is safe, according to reports by NASA and the Russian space agency -- Roscosmos.

Welcome aboard! The newest residents have floated into the International Space Station from their Soyuz capsule for a welcoming ceremony.

Hatch opening occurred at 12:00 a.m. EST (0500 GMT).

The docking probe on the front of Soyuz has retracted, allowing the hooks and latches to close and form a seal between the capsule and station. Pressure and leak checks will be performed over the next orbit before the hatchway is opened for the crew to enter into the station in a couple of hours.

Over the next few minutes, the Soyuz docking probe will retract to allow hooks and latches to bring the spacecraft to a firm seal with the station. Hatches between the two vehicles will be opened around 11:30 p.m. EST (0430 GMT).

Docking occurred over the Pacific Ocean off the coast of Ecuador.

DOCKING! The Soyuz TMA-15M spacecraft has docked to the Rassvet module of the space station, delivering Russian cosmonaut Anton Schkaplerov, European Space Agency astronaut Samantha Cristoforetti and NASA astronaut Terry Virts to the international outpost for their five-and-a-half month space mission.

Standing by for contact and capture momentarily.

Range is now about 30 meters, or 98 feet.

The Soyuz spacecraft is closing in on the Rassvet docking port at 0.2 meters per second, or about one-half mile per hour.

Range is now 75 meters, or 246 feet.

The Soyuz completed the flyaround to align with the docking port. It's now on a computer-controlled final approach to the space station's Rassvet module.

The flyaround has begun. Soyuz is flying itself around the international outpost to get into the approach corridor leading to the Rassvet module's docking port. This flyaround occurs at a range of about 250 meters.

A camera mounted on the Soyuz spacecraft is now returning images of the space station, which is reported to be in the proper attitude for docking in 34 minutes.

No major problems have been reported during the Soyuz rendezvous sequence. The Soyuz TMA-15M spacecraft should have established a communications link with the International Space Station by now, the capsule's Kurs navigation radar should be activated, and the automated rendezvous is underway.

Docking with the space station's Earth-facing Rassvet module is set for one hour from now at 0253 GMT (9:53 p.m. EST).

Russian Soyuz commander Anton Schkaplerov, ESA astronaut Samantha Cristoforetti and NASA astronaut Terry Virts have arrived in orbit following a good launch aboard the Soyuz TMA-15M spacecraft from the Baikonur Cosmodrome.

Solar arrays have been unfurled aboard the spacecraft to generate electricity, and the first major orbit-adjustment maneuver is planned for 4:43 p.m. EST (2143 GMT) to begin raising the capsule's altitude to match that of the space station.

A second "delta velocity" burn is set for 5:26 p.m. EST (2226 GMT), followed by several more firings over the next few hours to set up for rendezvous and docking.

The 7.9-ton capsule's automated rendezvous sequence, guided by its Kurs radar system, will commence at 7:46 p.m. EST (0046 GMT).

The Soyuz should be in position to start a flyaround maneuver at range of about 400 meters, or 1,300 feet, at about 9:30 p.m. EST (0230 GMT) to line up with the docking port on the space station's Earth-facing Rassvet module. Soyuz commander Anton Schkaplerov will be standing by to take over manual flying of the spacecraft if required. Final approach will begin about 11 minutes before docking, which is scheduled for 9:53 p.m. EST (0253 GMT).

The docking should occur 5 hours and 52 minutes after liftoff.

NASA says the Soyuz spacecraft has completed a programmed sequence to deploy the power-generating solar arrays, as well as antennas for navigational and communication systems.

T+plus 9 minutes. CAPSULE SEPARATION! The Soyuz spacecraft is flying free after the upper stage finished its engine firing and then separated away. The capsule is in pursuit of the International Space Station for a planned docking at 9:53 p.m. EST (0253 GMT).

T+plus 8 minutes. About a minute remains in the propulsion by the upper stage. The motor consumes kerosene and liquid oxygen just like the Soyuz rocket's other powerplants.

T+plus 7 minutes. The four-nozzle RD-0110 engine of the upper stage continues to burn to put the spacecraft into orbit.

T+plus 6 minutes. Soyuz's upper stage is firing to propel the spacecraft into a stable orbital perch around Earth on the six-hour, four-orbit trek to catch the International Space Station.

T+plus 5 minutes. The core stage of the Soyuz rocket has shut down and separated at an altitude of approximately 105 miles, leaving the upper stage to complete the job of injecting the Soyuz capsule into orbit.

T+plus 4 minutes. The core RD-108A engine continues to fire on its propellant mixture of kerosene fuel and supercold liquid oxygen.

T+plus 3 minutes. The safety escape tower and launch shroud have been jettisoned from the atop the Soyuz capsule.

T+plus 2 minutes, 10 seconds. The four strap-on boosters clustered around the Soyuz rocket's main stage have burned out and separated. The core engine continues to fire as Soyuz streaks into space at more than 3,300 mph.

T+plus 60 seconds. Good performance one minute into this ascent for the Soyuz rocket and its three-person crew from the Kazakh launch base.

Cosmonaut Samantha Cristoforetti is strapped into the left-hand seat serving as co-pilot, veteran commander Anton Schkaplerov is in the center seat for his role as the Soyuz commander and NASA astronaut Terry Virts is riding in the right-hand seat.

T+plus 30 seconds. Schkaplerov, Cristoforetti and Virts are beginning a five-and-a-half month expedition in space.

The Soyuz rocket has maneuvered on course for a rendezvous with the space station six hours from now. The station currently is flying 260 miles over southern Russia.

LIFTOFF! Liftoff of the Expedition 42 crew en route to the International Space Station for docking in six hours!

T-minus 40 seconds. The first umbilical arm has separated from Soyuz. The second will retract in the next few seconds.

T-minus 1 minute and counting. The Soyuz has been placed on internal power.

T-minus 2 minutes and counting. Rocket propellant tank pressurization is underway. The vehicle's onboard measurement system is activated. Oxidizer and fuel drain and safety valves of the launch vehicle have been closed.

T-minus 4 minutes and counting. The nitrogen purge of the combustion chambers of side and central engine pods of the rocket is being performed in preparation for ignition.

T-minus 5 minutes and counting. Soyuz has switched to onboard control, the ground measurement system and the capsule commander's controls are being activated.

T-minus 6 minutes. The automatic program for final launch operations is being initiated. And the launch key has been inserted in the bunker for liftoff.

Launch is set for 4:01:14 p.m. EST (2101:14 GMT), the moment Earth's rotation carries the Baikonur Cosmodrome under the International Space Station's ground track.

T-minus 9 minutes and counting. The crew has closed its helmet visors.

T-minus 10 minutes. The crew inside the Soyuz capsule are starting recorders to collect data during launch.

T-minus 14 minutes. The Soyuz telemetry systems are being activated. They will relay real-time data back to Earth during today's launch.

T-minus 17 minutes. Now in the launch count, realignment of the Soyuz rocket's trajectory control system and checks of internal batteries should be complete. The Soyuz telemetry system will soon be activated and monitoring of Soyuz's thermal control system also will begin.

T-minus 20 minutes and counting. The three-stage Soyuz rocket will insert the 15,700-pound space capsule into a 143 by 118 mile orbit, inclined 51.6 degrees to the equator, according to NASA.

T-minus 22 minutes and counting. At the time of launch, the International Space Station will be flying 260 miles over northeast Kazakhstan. When the Soyuz TMA-15M spacecraft deploys from the Soyuz rocket's third stage, the space station will be flying about 2,200 miles ahead of the Soyuz.

The space station will fly almost directly overhead the Baikonur Cosmodrome about three minutes before launch.

The capsule will close that distance over the next six hours, with docking to the space station's Rassvet module scheduled for 9:53 p.m. EST (0253 GMT).

T-minus 24 minutes. The crew is completing leak checks of the Sokol launch spacesuits at this point in the countdown.

T-minus 25 minutes and counting. The Soyuz rocket will fly northeast from the Baikonur Cosmodrome, jettisoning its launch escape tower 1 minutes, 54 seconds, after liftoff. Four seconds later, the rocket's four liquid-fueled strap-on boosters will separate as the core RD-108A engine, also known as the Block A second stage, continues firing.

The Soyuz launch shroud will release at T+plus 2 minutes, 37 seconds, followed by ignition of the third stage RD-0110 engine and separation of the Soyuz second stage at T+plus 4 minutes, 45 seconds.

The third stage's RD-0110 engine will fire for four minutes to inject the Soyuz spacecraft into orbit. Shutdown is set for T+plus 8 minutes, 45 seconds, followed by separation of the capsule in orbit three seconds later.

A series of communications and navigation antennas will deploy moments later as the Soyuz begins its six-hour chase of the International Space Station.

Docking is set for 9:53 p.m. EST (0253 GMT).

T-minus 34 minutes and counting. The emergency escape system is being armed. The system would be employed if a major malfunction occurs, propelling the Soyuz capsule off the top of the rocket to safety.

T-minus 40 minutes and counting. The two-piece service structure which enclosed the Soyuz rocket is being retracted to a horizontal position. The towers protected the rocket and provided workers and the Soyuz crew with access to the spacecraft since the rocket arrived at the launch pad at sunrise Sunday.

Several other umbilical arms connecting the rocket to the ground will be retracted at various times later in the countdown.

T-minus 50 minutes and counting. Soyuz TMA-15M commander Anton Schkaplerov, ESA astronaut Samantha Cristoforetti and NASA astronaut Terry Virts were awakened about nine hours ago to begin launch day activities.

They signed the doors at the Cosmonaut Hotel and received religious blessings before boarding a bus that took the three crewmates the 25-mile distance into the cosmodrome. They donned their white Sokol launch and entry suits, met with officials from their respective space agencies and then headed for the pad. Crowds of well-wishers gathered to wave goodbye as the crew reached the rocket. An elevator took the trio up to the capsule-level of the tower to begin climbing aboard the cramped spacecraft.

T-minus 61 minutes. Live streaming video coverage of today's rocket flight to orbit begins now.

T-minus 90 minutes. The Soyuz rocket is fueled, the crew has traveled to the launch pad and the countdown is progressing toward liftoff of the space station's Expedition 42/43 crew from the Baikonur Cosmodrome at 4:01 p.m. EST (2101 GMT).

The hatch to the Soyuz TMA-15M spacecraft is due to be closed in about 10 minutes, followed by the start of leak checks to ensure the capsule has a tight pressure seal.

Live launch coverage and commentary begins in the stream at 3 p.m. EST.

The three-person crew has arrived at Launch Pad No. 31 at the Baikonur Cosmodrome, where they will take an elevator ride to the top of the Soyuz rocket to board the Soyuz TMA-15M spacecraft for liftoff at 2101 GMT (4:01 p.m. EST).

The crew is led by Anton Schkaplerov, a 42-year-old Russian Air Force pilot selected as a cosmonaut candidate in 2003. Samokutyaev logged 165 days aboard the International Space Station as flight engineer on the Expedition 29 and Expedition 30 crews in 2011 and 2012.

Schkaplerov will occupy the capsule's center seat, with rookie European Space Agency astronaut Samantha Cristoforetti and NASA astronaut Terry Virts flanking him in the left and right couches.

Cristoforetti will be the primary flight engineer, assisting Samokutyaev with control duties during the launch and docking, which is set for 0253 GMT (9:53 p.m. EST), less than six hours after liftoff.

Making her first spaceflight, Cristoforetti is a 37-year-old captain in the Italian Air Force with more than 500 hours of flying time in six types of military aircraft, including the AM-X ground attack fighter. Cristoforetti was selected as an ESA astronaut in 2009 and will become the first Italian woman in space.

NASA astronaut Terry Virts, 46, is making his second flight into space after serving as pilot aboard space shuttle Endeavour on the STS-130 mission in 2010. Wilmore spent nearly 14 days in space on Endeavour, and he will take over as commander of the space station's Expedition 43 mission next year.

In clear but frigid weather, Russian engineers hauled a Soyuz rocket to the launch pad Friday, setting the stage for launch Sunday on a six-hour flight to ferry a veteran Russian cosmonaut, a NASA shuttle pilot and a European rookie to the International Space Station, boosting the lab’s crew back to six and kicking off a busy winter of research and assembly work.

Read our full story.

Riding a specialized railroad car before dawn Friday, a Soyuz rocket rolled out of a cavernous assembly building and arrived at its launch pad at the Baikonur Cosmodrome in Kazakhstan for liftoff Sunday with American, Russian and Italian crew members heading for the International Space Station.

The three-person crew, led by Soyuz commander Anton Shkaplerov, will stay on the space station until May, conducting experiments, readying the complex for the arrival of new U.S.-built commercial crew transport vehicles, and overseeing a busy logistics line from Earth.

Shkaplerov will occupy the center seat of the Soyuz TMA-15M capsule for Sunday’s launch. European Space Agency astronaut Samantha Cristoforetti, a native of Milan, Italy, will be in the left seat of the Soyuz serving as the capsule’s flight engineer. Veteran shuttle astronaut Terry Virts, a U.S. Air Force test pilot, will be in the right seat.

Liftoff is set for 2101 GMT (4:01 p.m. EST) Sunday, approximately when the space station flies overhead Baiknour, allowing the spacecraft to complete an expedited automated rendezvous and dock with the outpost less than six hours after launch.

Shkaplerov, Cristoforetti and Virts will join three crew members already living on the space station. Virts is due to take over as commander of the outpost’s crew in March.

The Soyuz launcher departed its assembly building on a railroad transporter before sunrise Friday, arriving a few hours later atop Launch Pad No. 31.

Russian ground teams positioned the rocket vertical on top of the launch pad, then access platforms enclosed the Soyuz to allow technicians to complete the final preflight checklist.

Check out photos of the Soyuz rollout.