Outgoing space station commander Scott Kelly and crewmates Alexander Kaleri and Oleg Skripochka geared up for return to Earth aboard their patched up Soyuz TMA-01M spacecraft early Wednesday to wrap up a 159-day tour of duty.

Credit: Energia

Launched to the International Space Station last Oct. 7 from the Baikonur Cosmodrome in Kazakhstan, Kelly, Soyuz commander Kaleri and flight engineer Skripochka planned to bid farewell to their station crewmates -- Expedition 27 commander Dmitry Kondratyev, Italian astronaut Paolo Nespoli and Catherine "Cady" Coleman -- closing the hatch to the TMA-01M spacecraft around 9 p.m. EDT Tuesday (GMT-4).

Undocking from the Poisk compartment atop the station's Zvezda command module is expected at 12:27 a.m. If all goes well, Kaleri will oversee a four-minute 17-second rocket firing starting at 3:03:17 a.m., slowing the ship by 258 mph and setting up a landing in Kazakhstan.

The three modules making up the Soyuz TMA-01M spacecraft should separate around 3:28 a.m., just before atmospheric entry, and the central crew module carrying Kaleri, Skripochka and Kelly will line up for a fiery descent to a parachute- and rocket-assisted touchdown around 3:53 a.m. near Arkalyk, Kazakhstan.

Forecasters predicted snow, temperatures in the 20s and wind gusts of 25 knots or more.

The returning Soyuz TMA-01M is the first of a new breed of Russian crew ferry craft featuring upgraded, lightweight digital electronics and navigation gear. During launch, the capsule's Neptune computer instrumentation experienced problems and Kaleri installed replacement rate measurement units called "ammeters" Feb. 2 to ensure normal operation during the descent to Earth.

Shortly after undocking, and before leaving the immediate vicinity of the space station, Kaleri plans to test the new equipment to make sure it is working properly, using it to maintain a specific orientation and verifying the operation of hand controllers to manually fire the spacecraft's thrusters.

If the test goes smoothly, the crew will have the full suite of Soyuz re-entry modes available for descent to Kazakhstan: two automatic modes and two manual modes.

"If we can't do an automated entry for any reason (based on miss distance, not the ammeter test) the crew can then do a manual entry," according to a NASA space station status report. "If the rate gyros in the ammeter test are not operational then we will go into a ballistic descent. If there are issues with two of three ammeters then we will go into the reserve ballistic mode."

In a normal descent, the orientation of the Soyuz is controlled to provide more lift, allowing the spacecraft to fly farther down range and subjecting the crew to less extreme braking forces. In a ballistic entry, lift is not adjusted, the capsule is spun up for stability and it rifles back to Earth on a steeper trajectory, subjecting the crew to more severe deceleration.

Back-to-back Soyuz re-entries on Oct. 21, 2007, and April 19, 2008, ended with ballistic descents because of module separation problems. All re-entries since then have followed the normal, less-stressful trajectory.

But as with all Soyuz descents, Russian recovery forces will be deployed to quickly assist the crew whether the Soyuz TMA-01 spacecraft follows the planned trajectory or if it lands short due to a ballistic descent.

Kelly has had a challenging stay in orbit, hearing from afar about an attack on his twin brother Mark Kelly's wife, Rep. Gabrielle Giffords, who was shot in the head in Tucson in January.

Offering a "moment of silence" Jan. 10 to remember the victims of the shooting spree, Scott Kelly said "we have a unique vantage point here aboard the International Space Station."

"As I look out the window, I see a very beautiful planet that seems very inviting and peaceful," he said. "Unfortunately, it is not. These days, we're constantly reminded of the unspeakable acts of violence and damage we can inflict upon one another, not just with our actions but also with our irresponsible words. We are better than this. We must do better."

During a brief change-of-command ceremony aboard the station Monday, Kelly said "we'll miss this place, but we look forward to getting back to Earth."

"It's a real honor and a privilege for me to have been in command of this really miraculous facility for oh, several months now," he said. "If you could see this place in person, it really is, in my opinion, one of the most amazing engineering achievements that people have accomplished.

"If we can build something like this, we can meet any challenge and we certainly have challenges back on Earth, and we recognize that. Our hearts go out to our partners in Japan that have suffered greatly. We really feel for them and know they will recover from this. The Japanese people are very, very resilient."

Kondratyev, Nespoli and Coleman will have the space station to themselves until early to mid April when three fresh crew members -- Alexander Samokutyaev, Andrey Borisenko and Ronald Garan -- arrive aboard the Soyuz TMA-21 spacecraft. Launch originally was planned for March 29, but the flight was delayed to replace suspect components in the craft's communications gear.

With the retirement of the space shuttle after two final missions, NASA will rely on Soyuz spacecraft to ferry U.S., European, Canadian and Japanese astronauts to and from the International Space Station until new commercial rockets and spacecraft are designed, built and tested.

NASA officials Monday announced an extension to the agency's existing contract with the Russian Federal Space Agency covering crew transportation, rescue and related services from 2014 through June 2016. The contract is valued at $753 million and covers launch-through-landing support for 12 astronauts. That works out to $62.75 million per seat.

Here is an updated timeline of major re-entry events (in EDT and mission elapsed time):

DATE/EDT......DDD...HH...MM...SS...EVENT

3/15
09:44:00 PM...159...02...33...05...Sunrise at landing site
11:10:00 PM...159...03...59...05...US to Russian motion control handover
11:33:00 PM...159...04...22...05...ISS maneuver to undocking attitude

3/16
12:00:54 AM...159...04...49...59...Sunrise
12:23:00 AM...159...05...12...05...ISS to free drift
12:23:16 AM...159...05...12...21...Daily Orbit 15 Russian ground station AOS
12:24:00 AM...159...05...13...05...Undock Command
12:27:00 AM...159...05...16...05...Physical separation/Hooks open
12:27:30 AM...159...05...16...35...Crew activativation of 
12:29:00 AM...159...05...18...05...Manual Soyuz separatoin burn #1
12:29:00 AM...159...05...18...05...ISS maneuver to test attitude LVLH
12:30:15 AM...159...05...19...20...Stationkeeping for test (range: 50 meters)
12:31:00 AM...159...05...20...05...Avionics test start
12:36:00 AM...159...05...25...05...Manual Soyuz separation burn #2
12:37:00 AM...159...05...26...05...Crew avionics activation (range: 100 meters)
12:37:30 AM...159...05...26...35...Test/ammeters start (range: 120 meters)
12:46:52 AM...159...05...35...57...Daily Orbit 15 Russian ground station LOS
12:50:00 AM...159...05...39...05...ISS maneuver to duty attitude
01:02:57 AM...159...05...52...02...Sunset
01:41:00 AM...159...06...30...05...Russian to US motion control handover
03:03:17 AM...159...07...52...22...Soyuz deorbit burn start (115.2 m/s)
03:04:11 AM...159...07...53...16...Sunrise
03:07:34 AM...159...07...56...39...Deorbit burn complete
03:27:57 AM...159...08...17...02...Separation of modules (140.0 km altitude)
03:30:50 AM...159...08...19...55...Atmospheric entry (102.3 km altitude)
03:32:36 AM...159...08...21...41...Entry guidance start (80.5 km altitude)
03:37:27 AM...159...08...26...32...Maximum G-load (33.7 km altitude)
03:39:23 AM...159...08...28...28...Command to open parachute (10.6 km altitude)
03:53:43 AM...159...08...42...48...Landing