As surely as the sun rises in the east and sets in the west, NASA has neither the time nor the money to build and launch the Solar Probe Plus mission by 2015, no matter how much some U.S. lawmakers would like to see that happen.

The $1.2 billion mission entails sending a heavily shielded spacecraft on a seven-year journey to repeatedly dip into the near-sun environment, gathering data on coronal heating and the origins of solar wind. NASA says the mission promises to answer critical heliophysics questions that have confounded scientists for decades. Solar Probe Plus also is expected to make important contributions to the understanding and prediction of the radiation environment future space explorers will encounter when they leave Earth’s orbit.

The National Research Council, in its 2003 decadal survey, “The Sun to the Earth — and Beyond: A Decadal Research Strategy in Solar and Space Physics,” ranked Solar Probe Plus ahead of all other flagship mission candidates in the field of heliophysics.

A series of studies ensued, and by 2008 NASA zeroed in on a mission approach that entails utilizing seven swings past the planet Venus in order to put Solar Probe Plus on a course to fly within a blistering 3.6 million kilometers of the sun’s surface. The planetary alignment necessary for that to happen occurs only every 19 months; if Solar Probe Plus misses its four-week launch window, the program would suffer a costly delay.

NASA only recently selected the five science investigations that will comprise the mission, committing in September to spend $180 million over the next few years on development of the probe’s suite of instruments. All told, NASA plans to spend some $400 million on Solar Probe Plus over the next five years before shifting the program into higher gear to make an August 2018 target launch window.

The agency’s 2011 budget request includes $14 million to transition the Solar Probe Plus project by next summer from preliminary analysis to a 30-month definition phase that will set the mission’s technical requirements, schedule and specifications before the detailed design and development work begins in earnest in 2014.

A NASA spending bill that cleared the Senate Appropriations Committee in July — only to stall on the way to the Senate floor — provides the $14 million the agency is seeking for Solar Probe Plus in 2011, but also directs NASA to “work to achieve a launch no later than 2015.”

Obviously, the lawmakers behind that provision — Sen. Barbara Mikulski (D-Md.), who chairs the Senate Appropriations subcommittee with NASA oversight, comes to mind since Solar Probe Plus will be built in her state — could always earmark additional funds for Solar Probe Plus before the bill reaches the president’s desk. It was only last year, after all, that Congress took NASA’s $4 million request for Solar Probe Plus and multiplied it by 10. But NASA had to draw the additional $36 million from other heliophysics accounts to comply with that direction, and plowed that money into additional Solar Probe Plus studies that the agency probably could have done without.

NASA has budgeted a total of $3.4 billion between 2011 and 2015 for a heliophysics program that includes more than a dozen ongoing space missions, sounding rocket and balloon campaigns, and development of multiple satellites slated to launch over the next five years. Chief among these are the $694 million Radiation Belt Storm Probes mission launching in 2012 and the $1.07 billion Magnetospheric Multiscale Mission featuring four identical satellites to be launched in 2015.

Even assuming Solar Probe Plus could be accelerated by three years and kept within its estimated $1.2 billion total mission cost — and that’s a big assumption — making that happen would require Congress and the White House to substantially boost NASA’s heliophysics five-year funding profile during a time period when the agency’s budget as a whole is not expected to keep pace with inflation.

When one considers that NASA also is under congressional direction to embark on a heavy-lift rocket development program not included in the agency’s current funding profile — an expensive undertaking likely to put even more pressure on other agency efforts — that’s an awfully big wish to be making upon a single star. The best thing lawmakers can do to support the Solar Probe Plus mission is to continue providing the funding necessary to keep the project on the schedule that has been laid out by those who understand the mission best — NASA’s own experts.