NASA’s Parker Solar Probe (PSP) is edging closer to its long-awaited launch, scheduled no sooner than (NET) 3:33 a.m. EDT (19:33 GMT) on Saturday, August 11. The spacecraft will launch from Cape Canaveral Air Force Station’s Space Launch Complex 37B (SLC-37B) atop a United Launch Alliance (ULA) Delta IV Heavy rocket on its mission to study the Sun’s corona and the solar wind.
The sedan-sized probe will fly eight times closer to the surface of the Sun than any other previous spacecraft.
This close proximity study of the Sun is expected to yield data that can shed light on many mysteries about the corona and the solar wind that astrophysicists have been trying to solve for decades.
“There are a whole bunch of questions about the Sun’s atmosphere and the solar wind — this million mile an hour ionized gas that flows out from the Sun in all directions in space all the time,” David McComas, one of the Parker Solar Probe principal instrument investigators, told Spaceflight Insider. “We don’t know how it’s accelerated up to those speeds. We don’t know how the energy from the Sun gets pumped into the lower corona to heat the corona and cause the solar wind. Even though we’ve tried to figure these things out for all these years, until we get the local in situ data, it is just not possible to answer those really important questions.”
A solar probe project has been talked about between NASA and the astrophysics community for decades. After more than 50 years, maturing technologies, scientific priorities, and budget availability, finally intersected to make this mission possible.
In partnership with NASA and other industry and university partners, a science and engineering team from Johns Hopkins University Applied Physics Laboratory (APL) designed and developed the spacecraft and will be managing the mission. If everything goes as planned PSP is expected to last through 2025 and possibly beyond.
The spacecraft’s systems and instruments were built, integrated, and tested at APL over a nearly three year period from late 2014 to 2017. In the fall of 2017, the spacecraft was sent to the Goddard Spaceflight Center for a series of checkouts, including thermal vacuum testing. Those tests were completed at the end of March of this year (2018).
At the same time, technicians at ULA’s Horizontal Integration Facility near SLC-37B mated the Delta Cryogenic Second Stage to the center booster of the three already-mated Common Booster Cores that comprise the first stage of the Delta IV Heavy rocket.
The spacecraft arrived at the Astrotech Space Systems facility located in Titusville, Florida on April 3. It then underwent final testing and preparations before integration with the launch vehicle. Astrotech’s many tasks in the lead-up to launch included installing the spacecraft’s power-generating solar arrays. These arrays are designed to withstand the extreme temperatures of the Sun’s coronal environment, and are equipped with a unique water cooling system to keep them operational in the intense heat.
While the spacecraft was getting prepped for its mission, the launch vehicle was also getting ready for flight. On April 16 ULA engineers rolled the assembled Delta IV Heavy rocket from the facility hanger to the SLC-37B launch mounts, where it was erected on the pad the following day.
The Delta IV Heavy and all its systems were put through an extensive series of tests and launch rehearsals throughout May, June, and July. The complexities of the three-core configuration of the rocket, combined with the narrow launch window for PSP’s interplanetary trajectory, which includes a number of flybys of Venus, necessitated the extraordinary level of preparation for PSP’s launch.
These preparations included two of what ULA calls Wet Dress Rehearsals, which involve a complete fueling of the Delta IV Heavy under the same conditions expected for launch day. A simulated countdown progresses through every stage of a real countdown, with a number of anomalies thrown into the simulation was part of this process. The ULA launch teams conducted two of these rehearsals, one on June 27 and the second on July 6.
Also on June 27, the spacecraft’s 8-feet-wide (2.4 meters) 4.5-inch-thick (11.4 cm) carbon composite heat shield was installed. PSP will make its observations from as close as 3.83 million miles (6.16 million km) from the surface of the Sun, which will place it in an environment with temperatures near 2,500 degrees Fahrenheit (1377 degrees Celsius). But with the heat shield acting as a shade at one end of the spacecraft, all of the probe’s apparatus and instruments in the shaded area behind the shield will experience heat barely higher than normal room temperatures. Only a pair of sensors and the very tips of the solar arrays will peak out from behind the heat shield during these heat-intense phases of the mission.
The Sun’s intense heat is not the only stress or hazard the spacecraft may encounter on this mission.
“If there is any dust orbiting the Sun in the opposite direction of the planets, we are looking at maybe a 400 km per second impact,” Justin Kasper, a Parker Solar Probe primary instrument investigator, told Spaceflight Insider. “That is so fast that a speck of dust like a grain of sand could blow a hole the size of a quarter through a steel plate at those speeds. So you do the best you can to make things light to go into space. But you also try to armor it so it will survive.”
On July 13, a minor leak in the purge ground support tubing for the spacecraft’s Northrop Grumman Star 48BV upper stage motor was detected at the Astrotech processing facility. The processing team required some extra time to address the issue, but the work did not alter the schedule of launch events.
Late on July 18, NASA announced the targeted launch date of August 4 was delayed an additional two days – pushing the flight to August 6. Technicians at Astrotech needed additional time to address an issue they has encountered while encapsulating the Parker Solar Probe inside the Delta IV Heavy’s payload fairing. The time was needed to evaluate the condition of one of the cable clamps in the fairing.
Then on July 24, the launch date slipped again, this time to August 11.
“During final inspections following the encapsulation of the spacecraft, a small strip of foam was found inside the fairing and additional time is needed for inspection,” NASA announced via an agency-issued statement.
Parker Solar Probe’s initial launch window was from July 31 through August 19. The window is centered around a correct interplanetary orbital alignment with Venus, in order to use the planet’s gravity for seven flybys that will reduce the spacecraft’s orbit into closer and closer passes of the Sun.
The slip to August 11 already cut the window in half. Further analyses of the mission’s orbital parameters, however, indicated that the window could be extended until August 23 if needed. If the launch slips to these extra days on the schedule, it would still see the spacecraft placed on the correct trajectory to fulfill its mission.
If technical problems and/or weather conditions should conspire to prevent the launch before the close of the window on August 23, NASA, APL scientists and the ULA launch team will have to wait until May of 2019. This delay is needed to allow for another Earth-Venus orbital alignment to open the next launch window opportunity.
The spacecraft, attached to its solid-fueled 48BV upper stage motor, and encapsulated in its 63-feet-tall (19-meter) payload fairing, left Astrotech late on July 30 for its short, but slow, overnight journey to the launch pad at Canaveral’s SLC-37B launch site.
A heavy-lift crane hoisted the assembly atop the Delta IV Heavy, and technicians completed its attachment to the massive launch vehicle during the morning hours of July 31.
At a media event later that day, the mission’s lead scientist Nicky Fox said, “I am very happy to say that Solar Probe is in the fairing…on top of the Delta IV Heavy rocket. It was hoisted up this morning. I think it’s fair to say that Parker Solar Probe is ‘Go’ for the Sun.”
ULA and PSP teams have conducted an integrated systems check to verify proper electrical connection between the rocket and the payload, and are conducting further countdown rehearsals with the launch team. Technicians have entered the payload shroud to remove special covers that were put in place to protect sensitive scientific instruments right up until launch.
“A few days ago our solar probe cup instrument scientists went up to the pad with some technicians,” Kasper said. “There is a removable hatch on the side of the payload fairing, and they opened it up and removed the cover from the instrument. And then they replaced that removable hatch with a plate that was sealed in place. I think that was it. No one will ever see the spacecraft again.”
He went on to note that at this point forward the mission’s success rides on the shoulders of the launch team.
“For us, its in the hands of ULA,” Kasper added. “They are doing exercises and rehearsals for launch. Our last activity will be late in the evening on Friday. We will get in touch with the instruments remotely one last time, and make sure they are still ok. That’s when we give our Go for launch. From there, we will be sitting there Saturday drinking coffee at 3 a.m. and watching the launch.”
Scientists hope to learn from the Parker Solar Probe will provide a deeper understanding about what is happening in the solar corona. If everything goes as they hope, it could likely lead to significant leaps in our understanding about a number of other astrophysical phenomena.
“For the broader astronomical community, the same processes are happening when matter falls onto a black hole,” Kasper said. “If you use the Chandra Space Telescope and you look at a supernova shock, and you try to figure out how well the shock heats things by looking at the oxygen or the iron, we know these different elements are heated at different temperatures. So hopefully what we learn from going directly into one of these wild astrophysical plasmas near the Sun is going to inform our understanding of black holes, and accretion disks for young forming planets. Hopefully it will be very broadly applicable.”
Parker Solar Probe is named after Professor Eugene Parker, who originally theorized about the existence of a solar wind in the mid 1950s. His theory was initially scoffed at by much of the astrophysics community. Those who ridiculed him would eat their words a short time later. He was right and they were wrong. His models were later verified in the 1960s, and the significance of his work cannot be understated. His efforts were recognized by a historic honor. The Parker Solar Probe is the first NASA spacecraft to be named after a living person.
