Launch of NASA’s Parker Solar Probe rescheduled for Aug. 6

Parker Solar Probe sits inside a clean room at the Astrotech spacecraft processing facility in Titusville, Florida. Credit: NASA/Johns Hopkins APL/Ed Whitman

The launch of NASA’s Parker Solar Probe, on the verge of kicking off a seven-year mission culminating in passages through the sun’s atmosphere, has been delayed to Aug. 6 to resolve a technical snag encountered during encapsulation of the spacecraft inside the nose shroud of its United Launch Alliance Delta 4-Heavy rocket.

The two-day delay from the previous target launch date of Aug. 4 was announced by NASA late Wednesday, as ground crews inside the Astrotech spacecraft processing facility in Titusville, Florida, encapsulated Parker Solar Probe inside the payload fairing of its Delta 4-Heavy launch vehicle.

“Additional time was needed to evaluate the configuration of a cable clamp on the payload fairing,” NASA said. “Teams have modified the configuration and encapsulation operations have continued.”

Liftoff is planned for approximately 4:08 a.m. EDT (0808 GMT) Aug. 6.

The spacecraft and its Star 48 upper stage motor, which will provide an extra push to the probe after its launch from Cape Canaveral, are scheduled to head for the Complex 37 launch pad in the coming days for lifting atop the Delta 4-Heavy, the most powerful rocket in ULA’s fleet.

NASA said technicians at the Astrotech processing facility have repaired a leak discovered last week in purge ground support tubing on the Star 48 rocket motor.

The launch was originally scheduled for July 31, the opening of a 19-day interplanetary launch period governed by the positions of Earth and Venus. Parker Solar Probe will depart Earth on a trajectory to fly by Venus in September, the first of seven gravity assist maneuvers with Earth’s sister planet to crank the spacecraft’s orbit closer to the sun.

Parker Solar Probe will gradually approach the sun, moving closer after each flyby of Venus before reaching a perihelion — or closest point to the sun — at a distance of around 3.8 million miles (about 6.2 million kilometers) in late 2024. That’s well inside the orbit of Mercury, and closer to the sun than the U.S.-German Helios 2 mission reached during its record-setting mission in 1976.

The mission is named for Eugene Parker, the scientist who predicted in 1958 the influence of the solar wind, a stream of plasma that travels outward through the solar system.

Technicians installed the heat shield on Parker Solar Probe on June 27 inside the Astrotech processing facility in Titusville, Florida. Credit: NASA/Glenn Benson

Fitted with a custom carbon composite heat shield, Parker Solar Probe will fly through the corona, a super-heated envelope of plasma surrounding the sun where temperatures soar to millions of degrees. The temperature at the surface of the sun is hundreds of times cooler, but still a blistering 10,000 degrees Fahrenheit (6,000 degrees Celsius).

“That just doesn’t make sense,” said Nicola Fox, Parker Solar Probe’s project scientist at the Johns Hopkins University Applied Physics Laboratory, which built the spacecraft and leads the science team. “You have a heat source, and it gets hotter as you move away. It’s like walking away from a campfire and suddenly getting hotter. It breaks the laws of nature. It breaks the laws of physics.”

Scientists believe the million-mile-per-hour solar wind is generated inside the corona.

Key questions Parker Solar Probe was designed to address include charting the flow of heat and energy that accelerates the solar wind, collecting data that could help scientists forecast solar storms that might affect Earth.

Since arriving in Florida from the Johns Hopkins University Applied Physics Laboratory in April, Parker Solar Probe has been tested and had its heat shield installed. Ground crews have filled the craft with hydrazine fuel, and mounted it on its Star 48 rocket motor, built by Northrop Grumman Innovation Systems, formerly known as Orbital ATK.

Even ULA’s Delta 4-Heavy rocket, one of the most powerful launchers in the world, is unable to give Parker Solar Probe enough speed to get started on its lengthy, circuitous route through the inner solar system. Engineers devised the unique combination of the two-stage Delta 4-Heavy and the Star 48 upper stage motor to meet the requirements for Parker’s high-speed departure from Earth.

Engineers crafted a similar arrangement for the launch of NASA’s New Horizons spacecraft on an Atlas 5 rocket in 2006, adding an extra solid-fueled upper stage to give the probe enough velocity to make a speedy nine-year trip to Pluto.

Officials pushed back Parker’s launch from July 31 to Aug. 4 in order to conduct additional software testing of spacecraft systems.

Tim Dunn, a launch director at NASA’s Launch Services Program at the Kennedy Space Center, said last month that Parker Solar Probe engines wanted to complete further modeling and testing of temperature sensors on the spacecraft. The probe’s platinum resistance thermometers provide data feedback to the craft’s solar arrays and cooling system during its passage through the scorching solar corona.

“Right before they shipped (to Florida), they were having some temp sensor hardware issues on the spacecraft,” Dunn said. “So what they wanted to do is they wanted to further evaluate where they were with that system and do some more modeling, additional testing and then they needed to take that through the layers of the independent review.”

ULA’s launch team put the Delta 4-Heavy rocket through a pair of fueling tests, or wet dress rehearsals, earlier this month on its launch pad at Cape Canaveral.

Parker Solar Probe’s interplanetary launch window closes Aug. 19, but ULA and NASA engineers have plotted launch trajectories for four additional days through Aug. 23, Dunn said.

“However, there is still discussion from the spacecraft (team) as to whether they want to choose to accept that,” Dunn said. “The deeper we get into our window and we haven’t launched, they may change their decision-making process.”

If Parker misses its launch period in August, the next opportunity to send the mission toward the sun will open in May 2019.

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