NASA and its partners are on track to launch astronauts from Florida's Space Coast to the International Space Station as soon as 2017, thanks to critical progress made in 2015. Through partnerships with NASA's Commercial Crew Program, Boeing and SpaceX are developing a new generation of American rockets and spacecraft to open low-Earth orbit like never before.

Commercial crew missions will increase the number of astronauts aboard the space station from six to seven, doubling the amount of crew time to conduct scientific research off the Earth, for the Earth, while advancing the journey to Mars. This year, both companies began to lock in their designs, paving the way for flight tests that will begin as early as 2016. Here are the top 15 ways commercial crew made progress toward certification and missions during 2015:

1: NASA Named First Four Astronauts to Train with Boeing and SpaceX - NASA selected veteran astronauts Bob Behnken, Eric Boe, Doug Hurley and Suni Williams to work closely with Boeing and SpaceX through final system development and flight test training. The experienced test pilots boast considerable spaceflight credentials ranging from space shuttle missions to long-duration stays on the space station. Following their selection in July, the four astronauts spent the rest of the year getting a close look at designs and hardware.

2: NASA Ordered Post-Certification Missions from Boeing and SpaceX - With Boeing and SpaceX in the final development of their systems, NASA ordered its first crew rotation missions to the station from Boeing for its Crew Space Transportation, CST-100 Starliner and SpaceX for its Crew Dragon. Their systems will be certified by NASA before making any operational flights to the station. Flight tests, with and without crew, are part of each company's certification objectives.

3: Crew Dragon Completed Pad Abort Test - The morning of May 6 opened with fire and smoke as a SpaceX Crew Dragon capsule roared into the sky above Florida's Space Coast. The spacecraft's launch abort system is designed to push astronauts to safety in the unlikely case of an emergency at the launch pad. The Crew Dragon flew more than a mile above Space Launch Complex 40 at Cape Canaveral Air Force Station, powered by eight SuperDraco engines before deploying its parachutes and splashing down just offshore in the Atlantic Ocean.

4: Boeing Began Building Structural Test Article of CST-100 Starliner - The first Starliner that emerges from Boeing's manufacturing facility at NASA's Kennedy Space Center in Florida won't fly in space, but it will be built to the same demanding specs as one that would. Engineers and technicians began building the spacecraft, known as a structural test article, in 2015 and it will be complete early in 2016. The test article will include all the systems, wiring and thrusters that a Starliner will require for flight. It will be subjected to months of testing in specialized chambers that simulate vibration, electromagnetic resistance and temperature changes. All this is done to prove the spacecraft design and manufacturing processes are ready to build operational Starliners.

5: SpaceX Modified Historic Launch Pad 39A for Crew Dragon Missions - This is the place where Saturn V rockets and space shuttles began their breathtaking missions of exploration. SpaceX has been upgrading the launch pad to meet the needs of a new generation of spacecraft and rockets. The company built a horizontal processing hangar at the base of the pad and installed rails leading up the 42-foot-tall incline to the launch pad. The company also assembled a transporter-erector capable of standing a Falcon Heavy rocket on the pad above the flame trench for launch.

6: Boeing Unveiled C3PF, Complete with Wrap - Boeing unveiled the Commercial Crew and Cargo Processing Facility, or C3PF for short, during a ceremony in July showcasing the work performed to convert the former space shuttle hangar to an assembly floor for the Starliner. Boeing is using the former engine shop and low bay areas to manufacture the structural test article, while the high bay renovations near completion. The exterior of the high bay got a ground-to-roof mural of a Starliner in orbit above Florida's Space Coast. The mural wrap was installed during nearly two weeks of careful work.

7: United Launch Alliance Completed Crew Access Tower Column at SLC-41 - United Launch Alliance modified its launch pad used historically for expendable rockets and robotic spacecraft with the addition of a 200-foot-tall tower. The tower will give the launch complex everything it needs to safely get astronauts on board their Starliner spacecraft and ready for liftoff. The groundbreaking to final construction of the tower's main column was completed in a matter of months with the pad maintaining its operational status for Atlas V launches. The corridors of the tower are wider than usual so astronauts can maneuver through the areas without snagging their suits or running into obstacles.

8: SpaceX Revealed Interior of Operational Crew Dragon - Slick and modern were some of the terms brought to mind when SpaceX updated the crew compartment of the Crew Dragon spacecraft. The company's imagery showed seating reclined for launch and a slim instrument panel that does away with the huge boards of dials and switches of earlier spacecraft in favor or touchscreens and multi-purpose displays. The spacecraft can carry up to seven people into orbit, but is slated to carry up to four astronauts at a time plus time-critical cargo for NASA missions to the space station.

9: Astronauts Reconfigured Station for Commercial Crew Spacecraft - Astronauts donned their spacewalking spacesuits and headed outside the station to install cables and fittings that will enable ports on the station to be used by visiting vehicles, including the Starliner and the Crew Dragon. Two of the ports will later be fitted with International Docking Adapters, or IDAs, that will work with sensors on board new commercial crew spacecraft to enable autonomous rendezvous and docking maneuvers. Engineers in the Space Station Processing Facility at NASA's Kennedy Space Center in Florida also tested the sophisticated mechanisms that will allow commercial crew spacecraft to communicate with the IDAs at the station.

10: SpaceX Conducted Propulsive Landing Tests - SpaceX is building its Crew Dragon capsule to return to Earth and land as safely and precisely as a helicopter by using thrusters instead of parachutes. The company's engineers began testing the propulsion system at facilities in Texas in late 2015. Early flights for NASA with astronauts aboard will come home under parachutes for water landings, but successful testing now could clear the way to later flights ending on a landing pad where the astronauts will step out on solid ground and the spacecraft will be processed for another flight.

11: Docking System Testing Increased in Realism - The docking system SpaceX will use to autonomously steer its Crew Dragon spacecraft to the space station and then dock safely was evaluated during several runs in the Six Degrees of Freedom Dynamic Test System at NASA's Johnson Space Center in Houston. The test stand is big enough to hold the docking components of spacecraft and run them through full-scale simulations of a spaceflight to perfect sensors, hardware and software in the system to make sure everything will run correctly during a flight.

12: Boeing Built Trainers for CST-100 Starliner - A suite of cloud-based simulation and training devices that astronauts and flight controllers will use to train on for CST-100 Starliner flights is nearing completion at Boeing's St. Louis facility. The machines will provide realistic rehearsals for all scenarios that may arise between launch to recovery of the space capsule. Eric Boe and Bob Behnken - two of the four astronauts training to fly flight tests to the station - focused on the systems used for learning to manipulate switches and display panels during a visit to the facility in December.

13: Flight Software Demonstration by Boeing - Boeing engineers conducted a full demonstration of the CST-100 Starliner's flight software during sessions in Houston that simulated a mission by the spacecraft. A cockpit simulator of the CST-100 was used in the demonstration, which also included a Boeing ground control team and NASA's International Space Station controllers.

14: SpaceX Tested Avionics for Crew Dragon - The flight computers and systems that will handle the work of flying the Crew Dragon spacecraft from launch to orbit and back to Earth were tested extensively by SpaceX during evaluations that ensured the computers and networks are up to the responsibility of controlling and operating a spacecraft carrying people.

15: NASA Began Recruiting Astronauts Who Will Fly in Commercial Crew Spacecraft - In anticipation of returning human spaceflight launches to American soil, and in preparation for the agency's journey to Mars, NASA is now accepting applications for the next class of astronaut candidates. With more human spacecraft in development in the United States today than at any other time in history, future astronauts will launch once again from the Space Coast of Florida on American-made commercial spacecraft - Boeing's CST-100 Starliner and the SpaceX Crew Dragon. These spacecraft will allow NASA to add a seventh crew member to each station mission, effectively doubling the amount of time astronauts will be able to devote to research in space, expanding scientific knowledge and demonstrating new technologies.