WASHINGTON — The four identical spacecraft constituting NASA’s Magnetospheric Multiscale (MMS) mission are undergoing final preshipment tests at the agency’s Goddard Space Flight Center in Greenbelt, Maryland, and are expected to reach Cape Canaveral Air Force Station, Florida, in November in preparation for a March launch.

The four formation-flying spacecraft, built for a combined $875 million, will ship in pairs by truck to the Cape. The first pair is scheduled to ship Oct. 27 and the second pair Nov. 12, Gary Davis, Goddard-based spacecraft systems engineer for MMS, said Oct. 6. Launch aboard a United Launch Alliance Atlas 5 is scheduled for March 12,  according to ULA spokeswoman Jessica Rye.

In the meantime, the MMS craft are undergoing a final battery of tests at Goddard to ensure the identically instrumented satellites are properly balanced. As of Oct. 6, two of the four MMS spacecraft had completed these tests, Davis said. MMS observatories will spin at three rotations per minute during on-orbit science operations, so engineers at Goddard are subjecting them to spin-table tests in Greenbelt before shipping them out for integration with Atlas 5.

During its two-year primary mission, MMS, the fourth in NASA’s Solar Terrestrial Probes program, will study how the magnetic fields of Earth and the sun interact. The mission has dealt with the occasional bump in the road since development started in 2009.

In spring 2013, NASA discovered component defects during routine tests of MMS’s Fast Plasma Instrument suites, construction of which was overseen under a roughly $225 million fixed-price contract awarded in 2004 to the Southwest Research Institute in San Antonio. The failures prompted NASA to replace some of the components, electrical devices known as optocouplers.

MMS was also hindered by the two-week partial government shutdown in October 2013. Engineers were locked out of Goddard while MMS development was in full swing, setting the mission on course to overrun the $850 million development cost NASA committed to in 2009 by more than $25 million, the U.S. Government Accountability Office wrote in an April report.

Just prior to the partial shutdown last October, MMS was forced to let the core science module for the $8.8 billion James Webb Space Telescope — a long-delayed astrophysics telescope whose every developmental hiccup earns NASA a dressing-down from Congress — cut in line for thermal vacuum chamber tests at Goddard. As a result, the four MMS spacecraft were bounced to the U.S. Naval Research Laboratory here for thermal vacuum tests, which finished in July, Davis said.

MMS is one of the three big heliophysics missions NASA will have to launch before the agency can begin to refocus its solar physics program to include more smaller missions, as scientists serving on the National Research Council’s solar and space physics subcommittee recommended in a 10-year planning document published in 2012 called “Solar and Space Physics: A Science for a Technological Society.”

The other two missions are Solar Probe Plus, a $1.5 billion observatory set to launch in 2018, and Solar Orbiter, a roughly $1 billion collaboration led by the European Space Agency to which NASA is contributing a pair of instruments and an Atlas 5 launcher. ESA is targeting a 2017 launch for Solar Orbiter.

Dan Leone is a SpaceNews staff writer, covering NASA, NOAA and a growing number of entrepreneurial space companies. He earned a bachelor’s degree in public communications from the American University in Washington.