Parker Solar Probe in integrated testing, launch on track for July 2018

by Chris Gebhardt

Engineers at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, have attached the Parker Solar Probe’s solar shadow-shield for final, integrated vehicle testing ahead of launch.  The probe, which will be the first to “touch the Sun” is being readied for an anticipated 31 July 2018 launch aboard a United Launch Alliance Delta IV Heavy rocket from the Cape Canaveral Air Force Station, FL – which will start a 7 year mission to study the Sun.

Parker Solar Probe:

Now less than one year away from launch, the Parker Solar Probe began as an idea in the Outer Planet/Solar Probe program of NASA in the 1990s.

The original mission concept, the Solar Orbiter, was canceled in 2003 as part of the George W. Bush Administration’s restructuring of NASA to focus more on research and development and address management shortcomings in the wake of the 1 February 2003 breakup of the Space Shuttle Columbia that claimed the lives of all seven astronauts aboard.

Six years later, the mission concept was resurrected as a “new mission start” in 2009 with an aim to launch a new solar probe in 2015.

By 2012, as the mission moved into its design phase, the launch was pushed to 2018.

Originally called the Solar Probe Plus, the mission was renamed earlier this year on 31 May 2017, and in so doing NASA radically departed from of its previous mission naming practices.

All prior missions named for people were done so after their deaths in honor of their accomplishments and contributions to science.

Breaking with this tradition, NASA renamed the Solar Probe Plus the Parker Solar Probe after Eugene Parker – making Parker the first living person to have a NASA spacecraft named after him.

A pioneering astrophysicist, Eugene Parker is best known for developing the theory of supersonic solar wind and correctly predicting the shape of the Heliospheric current sheet (or Parker spiral shape) of the solar magnetic field in the outer solar system.

Furthermore, in 1987, Eugene Parker proposed that the solar corona was heated by a myriad of tiny nanoflares – solar flare-like brightenings that occur across the entirety of the Sun’s surface.

Unlike other solar telescopes and missions, the Parker Solar Probe (PSP) will venture where no probe has gone before – into the Sun’s corona.

Mission planning calls for the probe to approach the Sun to within 6 million km (3.7 million miles) or just 0.04 AU – 8.5 solar radii – of the corona.

The mission will make three passes of the Sun at this distance between December 2024 and June 2025 after a prolonged period of orbit reduction maneuvers.

PSP will launch atop a Delta IV Heavy rocket – all components of which are in final processing at the Horizontal Integration Facility at SLC-37 – in a launch window extending from 31 July to 19 August 2018.

Assuming a 31 July launch, the probe will make the first of its seven (7) flybys of Venus on 28 September 2018.

The approach to Venus is calculated to place the PSP into an elliptical, 150 day orbit of the Sun.

The second Venus flyby on 21 December 2019 will further shorten PSP’s orbital period to 130 days.

Over the course of the next five years, an additional five flybys of Venus will occur – the last of which will place PSP into an 6 million km x 109.3 million km (3.7 million mile x 67.9 million mile) orbit with a period of just 88-89 days (roughly the same orbital period of Mercury).

That seventh and final Venus flyby will occur on 2 November 2024 – leading to the first close-approach perihelion of PSP of just 3.7 million miles the Sun’s “surface” and well into the upper corona on 19 December 2024.

A second close perihelion will occur on 18 March 2025 followed by the third and final scheduled dip into the corona on 14 June 2025.

Special heat shield and cooling system:

Diving that close to the Sun, PSP will, according to NASA, “explore what is arguably the last and most important region of the solar system to be visited by a spacecraft and will finally answer top-priority science goals of the last five decades.”

Overall, the mission objectives including: Determining the structure and dynamics of the magnetic fields at the sources of solar wind, tracing the flow of energy that heats the corona and accelerates the solar wind, determining what mechanisms accelerate and transport energetic particles, and exploring dusty plasma near the Sun and its influence on solar wind and energetic particle formation.

In order to survive the intense environment of the outer corona, an area in which the probe will experience solar intensity 520 times greater than Earth does, a specialized heat shield and cooling system were designed to protect the spacecraft and scientific instruments.

The heat shield (or solar shadow-shield), which was installed for integrated vehicle testing last week at the Johns Hopkins Applied Physics Lab (APL), is made of reinforced carbon-carbon composite.

Reinforced carbon-carbon is most widely and infamously known for its use on the Space Shuttle, as the nose cap and Wing Leading Edge elements of the Thermal Protection System on the five Orbiters – though it was initially developed for the nose cones of intercontinental ballistic missiles and is currently used in the brake systems for Formula One racing cars.

For the PSP, reinforced carbon-carbon will serve as the solar shadow-shield, which will block direct radiation from the Sun for the probe’s instrumentation and experiment packages.

After integrated testing with the solar shadow-shield is complete, the structure will be removed and will not be reinstalled until shortly before spacecraft encapsulation for launch next year.

Moreover, the mission’s proximity to the Sun also necessitated the development and use of a revolutionary cooling system to ensure the probe’s solar arrays continue to operate at peak efficiency in the extremely hostile conditions of the corona.

The arrays are designed with an upward bend at their outer edges.  These edges will stick out beyond the solar shadow-shield during coronal passes to provide the PSP with enough power for the spacecraft’s systems.

“Our solar arrays are going to operate in an extreme environment that other missions have never operated in before,” said Mary Kae Lockwood, spacecraft system engineer for Parker Solar Probe at APL.

While the surface of the solar shadow-shield will reach temperatures in excess of 2,500° F, the specially designed cooling system for the solar arrays will keep the arrays at a temperature of just 320° F or below.

This will be the first-of-its-kind actively cooled solar array system and was developed by the APL in partnership with United Technologies Aerospace Systems (which manufactured the cooling system) and SolAero Technologies (which produced the solar arrays).

The cooling system itself is composed of a heated accumulator tank that will hold water (the coolant) during launch, two-speed pumps, and four radiators made of titanium tubes and aluminum fins just two hundredths of an inch thick.

Water was chosen as the coolant because of the temperature range the system will encounter throughout the mission.

“For the temperature range we required, and for the mass constraints, water was the solution,” said Lockwood.

During and immediately after launch, the solar arrays and cooling system radiators will undergo wide temperature swings from 60° F (15° C) inside the payload fairing to -85° F to -220° F (-65° C to -140° C) once exposed to space before they can be warmed by the Sun.

A pre-heated coolant tank will keep the coolant water from freezing.

“One of the biggest challenges in testing this is those transitions from very cold to very hot in a short period of time,” Lockwood said.  “But those tests, and other tests to show how the system works when under a fully-heated TPS, correlated quite well to our models.”

Testing and modeling showed the team that they needed to increase the thermal blanketing on the first two radiators that will be activated after launch in order to balance maximizing their capacity at the end of the mission with further reducing the risk of water freezing early in the mission.

Of the numerous scientific results expected from the mission, one very important one for life on Earth and out technological dependence will be the increased ability to forecast major space-weather events – like Coronal Mass Ejections (significant releases of plasma and magnetic fields from the corona) and solar flares (the ejection of clouds of electrons, ions, atoms along with the electromagnetic waves through the corona).

(Images: NASA, Johns Hopkins University Applied Physics Laboratory)

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