Parker Solar Probe ready to unlock mysteries of the Sun’s corona

A mission nearly 60 years in the making is ready to launch on a historic flight to become the first spacecraft to “touch the surface of the Sun”.  NASA’s Parker Solar Probe, named after Dr. Eugene Parker, will unlock many of the mysteries still held by our solar system’s star. The probe is set to launch atop at United Launch Alliance Delta IV Heavy rocket on Saturday, 11 August in a 65-minute launch window that opens at 03:33 EDT (0733 UTC) from SLC-37B at Cape Canaveral Air Force Station, Florida.

Parker Solar Probe:

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 Space Shuttle Columbia accident.

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 on 31 May 2017 in honor of Dr. Eugene Parker.  In so doing, NASA radically departed from its previous mission naming practices. All prior missions named after people were done so after their deaths in honor of their accomplishments and contributions to science.

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

The mission patch for the Parker Solar Probe flagship science mission to “touch the Sun.” (Credit: NASA)

A pioneering astrophysicist, Dr. 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, Dr. 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 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.

During its mission, Parker Solar will seek to answer three very important questions about the Sun:

  1. Why and how is the solar wind accelerated to supersonic speeds inside the corona?
  2. What is the mechanism that heats and accelerates particles in the corona?
  3. What is accelerating some particles, very few, to near the speed of light (creating highly energetic particles)?

Answering that third question holds potentially great significance for our lives here on Earth and our quest to move beyond Earth and out into the solar system because these highly energetic particles are highly charged and can penetrate walls of spacecraft and be harmful for astronauts – like giving them a constant x-ray.  

These highly energetic particles can also wreak havoc with our electronics on Earth, in orbit, and in space.  Therefore, part of Parker Solar’s mission is to help us better understand how the particles are accelerated/created in the corona – which in turn will help us better predict their occurrence and create improved plans for how to protect our technology and astronauts.

Special heat shield and cooling system:

Diving that close to the Sun, Parker Solar Probe 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.”

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 in September 2017 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 Parker Solar Probe, 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 and will keep temperatures behind the shield at a comfortable 85°F (29.4℃) while temperatures on the Sun-facing side of the shield will soar to 2,500°F (1,377℃) during closest approaches.

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.

An artist’s depiction of the Parker Solar Probe as it dives toward the Sun for one of its close flybys into the corona. (Credit: NASA/APL)

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 Parker Solar Probe 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 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.

Parker Solar Probe undergoing pre-flight checkouts. (Credit: NASA/APL)

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 through -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.”

Moreover, this 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 reducing the risk of the water freezing early in the mission.

Getting Parker Solar Probe to the Sun – Calling the Delta IV Heavy:

One might think that getting to the Sun is easier than getting to the outer planets and the farthest reaches of our solar system.  But each actually include unique challenges that are put on full display with Parker Solar Probe.

Artist’s depiction of NASA’s Parker Solar Probe as it heads toward a grazing encounter with the Sun. (Credit: NASA)

The challenge of getting to the Sun is the reverse of getting to the outer planets.  When trying to reach the outer planets and reaches of the solar system, you seek to increase your velocity as you move through the solar system via gravitational assists – mainly from Jupiter.

But Parker Solar Probe seeks to do the exact opposite, slowing itself down and giving energy (speed) to the inner planets – in this case, Venus – as it performs numerous flybys of the second rock from the Sun.

So the questions then arise: if Parker Solar Probe needs to “go slow” to reach the Sun, why launch it at such a high velocity?  And why is a high velocity bad for Parker Solar Probe when it’s going to become the fastest human-made object ever.

The answer to the first question is the same as with all things space exploration: physics.  A specific amount of energy (speed) is needed to escape Earth’s gravitational force. And that’s what the main part of the Delta IV Heavy has to do.

But the Probe also has to overcome the speed at which Earth is moving around the Sun in its Orbit.

A ULA Delta IV-Heavy rocket launches at night from Cape Canaveral Air Force Station’s SLC-37B. (Credit: U.S Air Force)

Here, the specific mission parameters that call for Parker Solar Probe to make 24 close flybys of the Sun require a very specific trajectory and orbit.  So to get to that orbit when Earth is moving around the Sun (and taking Parker with it), Parker Solar Probe actually has to start slowing down (relative to the Sun) during the powered phase of launch.

This sounds contradictory to what we generally think of for launches, but part of the job of the third stage, in this case, is to start that slowing down process.  

During launch, the third stage’s velocity will increase because the speed relative to Earth is increasing.  But, in fact, the velocity relative to the Sun is slowing down. This slow down will allow the Sun’s gravity to begin pulling Parker Solar inward toward Venus.

Parker Solar Probe will then execute seven gravitational assist flybys of Venus so that it can perform progressively closer and closer flybys of the Sun’s surface.  These progressively closer orbits are achieved by the probe’s interactions with Venus, which slow Parker Solar down (the slower you go, the closer you get to the Sun’s surface due to the Sun’s gravitational forces) and gives some of its energy to Venus in the process.

The answer to the second question, why is such a high launch velocity bad for Parker Solar Probe’s operational mission, has to do with the parameters of the mission.  Parker Solar is designed to perform multiple, close flybys of the Sun. If the probe were not to encounter Venus and fly directly toward the Sun at its full launch velocity, it would continuously gain speed as it approached the Sun and be flung off into a highly elliptical orbit that would not permit it to perform its 24 flybys within the spacecraft’s available lifetime.

Parker Solar Probe’s trajectory over the course of its planned 7-year mission. (NASA/APL)

In short, it’s complicated.  We have to launch Parker Solar at a high enough velocity to escape Earth’s gravitational field while simultaneously slowing the probe down so it doesn’t get flung out into an orbit that takes too long to complete for its scientific objectives – an event that would violate the mission’s entire purpose.

So to do this, Parker Solar, while quite small and lightweight (weighing in at only 1,510 lbs, or 685 kg), needs a heavy-hitter launch vehicle.  Enter the United Launch Alliance Delta IV Heavy.

The mighty and majestic beast of the United Launch Alliance rocket family, the Delta IV Heavy will be tasked with sending Parker Solar on its merry way to the Sun.  This will be the 10th flight of Delta IV Heavy as well as this rocket variant’s first mission to deliver an extremely important scientific payload to space.

It will also be the lightest-weight known payload lifted to space by Delta IV Heavy.  Of the non-classified Delta IV Heavy missions to date, the lightest-weight payload was Defense Support Program (DSP) -23 at 5,200kg.

Assembly of this Delta IV Heavy rocket began in July and August 2017 with the arrival of the three Common Booster Cores that form the first stage of the Delta IV Heavy configuration.  The Delta IV cores were all assembled in Decatur, Alabama, just west of Huntsville.

United Launch Alliance takes the integrated three Common Booster Cores and Second Stage of the Delta IV Heavy for Parker Solar out to the launch pad in April. (Credit: NASA)

After mating the three Common Booster Cores together, technicians inside the Horizontal Integration Facility at SLC-37B mated the Delta Cryogenic Second Stage (a modified version of which will serve as the SLS Block 1 rocket’s second stage) to the top of the three boosters in March 2018.

Immediately thereafter, the Parker Solar Probe itself arrived in Titusville, Florida, at the Astrotech processing center on 3 April – where its final sequence of processing activities and checkouts for launch began.

For the rocket, after a month of integrated checkouts in the integration facility, United Launch Alliance engineers rolled the assembled Delta IV Heavy the short way from its hanger to the launch mounts at SLC-37B on 16 April and erected the rocket on the pad the following day.

A series of three Wet Dress Rehearsals were undertaken by the United Launch Alliance team for this particular Delta IV Heavy rocket in an attempt to ferret out any ground and vehicle issues that required attention and fixing prior to Saturday morning’s scheduled launch.

The number of Wet Dress Rehearsals (WDR) conducted for this mission was unusual – with two scheduled ahead of time and planned for because this is the first Delta IV rocket East Coast flight with the new common avionics suite.

However, the first WDR was scrubbed due to lightning and the second resulted in issues that only permitted fueling of the three Common Booster Cores and not the second stage, so a third WDR was then scheduled.

The third WDR was completed successfully, and a Mission Dress Rehearsal earlier this week and a final Flight Readiness Review all cleared the rocket and payload for launch.

Presently, Parker Solar Probe and the Delta IV Heavy are slated to launch on Saturday, 11 August at 03:33 EDT (0733 EDT).  That time is the opening of a 65-minute launch window for Saturday.

After liftoff, Delta IV Heavy will pitch downrange and head due east over the Atlantic Ocean.  Shortly after liftoff, the center core of the three Common Booster Cores (CBCs) of the first stage will throttle back to conserve propellant as the two side CBCs provide the brunt of the force lifting the rocket out of the dense lower atmosphere.

At T+3 minutes 56 seconds into the flight, the two side cores will separate, and the center core will power back up to full thrust, burning until T+5 minutes 36 seconds – after which the center core will separate.

The Delta Cryogenic Second Stage (DCSS) will then ignite for the first of its two burns.   The first of these burns will end at T+10 minutes 37 seconds – at which point Parker Solar will be in its initial parking orbit.

The DCSS will reignite for a second burn at T+22 minutes 25 seconds.  This burn will last for about 14 minutes.

The Star 48 third stage fires to send the Parker Solar Probe into its correct orbit toward Venus. (Credit: NASA)

After the second DCSS engine cutoff, the 2nd and 3rd stages will separate – with the Northrop Grumman-built third stage, the STAR 48BV.  This is a solid propellant stage that will produce 17,490 lbs of thrust for just 84 seconds. But in that 84 seconds, the third stage will impart two-thirds of the total velocity of the launch phase.

(Of note, this is not the only part of the Delta IV Heavy built by Northrop Grumman.  The first stage engine nozzles, pressurization tanks, payload fairing, and most of the white areas on the rocket are all built by Northrop Grumman.)

Once that small duration burn is complete, Parker Solar Probe will separate from the third stage and be on its inward dive toward Venus.

Assuming launch on 11 August, Parker Solar will encounter Venus for the first of seven flybys on 2 October 2018.  It will then perform its first close flyby of the Sun – perihelion – on 5 November 2018.

Overall, Parker Solar Probe has a launch window that extends to 23 August 2018 due to the need to intercept Venus.  If, for some reason, the mission has not launched by then, launch will have to wait until May 2019 for the next Earth-Venus alignment.

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