UPDATED July 25 at 5:55 p.m. EDT

GREENBELT, Md. — Solar Probe Plus, a flagship heliophysics mission NASA expects to cost some $1.5 billion to build and launch around July 2018, needs a bigger rocket than United Launch Alliance’s Atlas 5, according to a senior official at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, where the solar observatory is being built.

“The plan we had was to go on an Atlas 5, but the problem is that required us to develop a new, high-performance custom upper stage, and that represented a fairly significant risk for the project,” Michael Ryschkewitsch, head of APL’s Space Sector, said in a July 22 interview here. “Our team made the case to NASA headquarters that the overall risk to the mission would be lowered if we went to a heavy-class launch vehicle. The obvious players right now are Delta 4 Heavy and Falcon Heavy.”

“The decision to allow the consideration of a Heavy-class vehicle for Solar Probe Plus was made by the Science Mission Directorate in the spring of 2014 based on input from the Project and advice from Launch Services,” James Norman, NASA’s director of launch services, said July 25 via email. He declined to comment on why NASA cleared the program to procure a bigger launch vehicle, saying “the specifics associated with the decision are procurement sensitive and competition sensitive data.”

Trading up to a Delta 4 Heavy or Falcon Heavy means the money paid to ATK Missile Products of Beltsville, Maryland, for work on the customized kick-stage motor, known as STAR 48GXV, was a wasted effort for Solar Probe Plus. APL spokesman Michael Buckley said July 23 that ATK was paid $15.7 million for the work, which culminated with a December test firing of the solid-fueled STAR motor.

The flight-proven Delta 4 Heavy is Denver-based ULA’s most powerful launcher, typically used for the largest classified national security satellites. Falcon Heavy, which is still in development at SpaceX, features a first stage composed of three nine-engine Falcon 9 core stages.

Falcon Heavy’s first launch is planned for 2015. SpaceX advertises the rocket as the most powerful launcher since the Saturn 5, able to lift 53 metric tons to low Earth orbit at one-third the cost of a Delta 4 Heavy.

The Delta 4 Heavy, which has flown seven times including its 2004 debut, is capable of lifting roughly 26,000 kilograms to low Earth orbit.

NASA is paying $375 million for the Delta 4 Heavy it ordered in 2011 for this December’s unmanned test launch of the Orion crew capsule.

Although Falcon Heavy has yet to fly, NASA’s Launch Services Program left the door open for SpaceX to compete for the Solar Probe Plus launch contract in a draft request for proposals released July 10. While NASA will consider only rockets certified to loft Category 3 payloads — the internal designation reserved for the most important and risk-averse missions — agency rules allow for Category 3 certification with as few as three launches, NASA spokesman Joshua Buck wrote in a July 24 email.

SpaceX has three Falcon Heavy launches on its manifest between now and 2017: an inaugural demonstration launch in 2015; a shared launch of the National Oceanic and Atmospheric Administration’s Deep Space Climate Observatory and the U.S. Air Force’s Space Test Program-2 experimental spacecraft, also in 2015; and a 2017 launch of a commercial communications satellite for Intelsat of Washington and Luxembourg. SpaceX spokesman John Taylor declined to comment on whether SpaceX would seek certification to launch Solar Probe Plus.

The ATK-built stage that was scrapped along with plans to use ULA’s most-powerful Atlas 5 — the so-called 551 variant that features five solid-rocket boosters and a 5.4-meter payload fairing —  was based on a similar solid-fuel upper stage ATK made for the Atlas 5 that launched the roughly 480-kilogram New Horizons probe toward Pluto in 2006. APL thought Solar Probe Plus could use a similar setup, but the amount of energy required to launch from Earth toward the sun, coupled with Solar Probe Plus’ heftier 658-kilogram launch mass, was more than the New Horizons launch rig could handle, Ryschkewitsch said July 22.

NASA’s Science Mission Directorate approved Solar Probe Plus to proceed into development in March. The orbiter is bound for a nearly seven-year primary mission “to the fires of hell,” Nicky Fox, APL-based Solar Probe Plus project scientist, told SpaceNews here after a Maryland Space Business Roundtable Luncheon attended by employees of APL, NASA Goddard and many local NASA contractors.

At its closest, Solar Probe Plus and its 10 instruments will fly about 6 million kilometers from the sun’s surface. The mission will measure blasts of charged particles, or solar winds, emanating from the star’s corona. Solar winds can disrupt sensitive electronic equipment in space and on Earth’s surface. Sending a dedicated probe to the sun was identified as the top priority in a 10-year heliophysics roadmap, “The Sun to the Earth — and Beyond,” published in 2003 by the National Research Council.

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.