On Oct. 31, 2018, Parker Solar Probe began its first of 24 solar encounters. This period — which lasts until Nov. 11 — is the time during which the spacecraft is within 0.25 astronomical units, or 23.2 million miles, of the Sun’s center. Parker Solar Probe’s four suites of science instruments are on and collecting data throughout this phase, giving scientists their closest-yet look at this dynamic region of the Sun’s outer atmosphere.
This solar encounter encompasses the first perihelion of the mission, the point at which Parker Solar Probe is closest to the Sun. Perihelion is expected at about 10:28 p.m. EST on Nov. 5. The spacecraft will come within 15 million miles of the Sun’s surface and clock in at a top speed of 213,200 miles per hour relative to the Sun — setting new records for both closest solar approach and top heliocentric speed by a spacecraft. At perihelion, Parker Solar Probe will fly through material at about 3.6 million degrees Fahrenheit — but because material in this region is so tenuous, it doesn’t influence the temperature of the spacecraft. However, the Sun’s intense radiation heats the Sun-facing side of the spacecraft’s heat shield, called the Thermal Protection System, to about 820 F.
For several days around the Nov. 5 perihelion, Parker Solar Probe will be completely out of contact with Earth because of interference from the Sun’s overwhelming radio emissions.
Parker Solar Probe employs a host of autonomous systems to keep the spacecraft safe without guidance from Earth — including automatic retraction of the solar panels to regulate their temperature, attitude control using solar limb sensors that ensures all of the instruments remain in the heat shield’s shadow, and a sophisticated guidance and control system that keeps the spacecraft pointed correctly. This autonomy is key not only during no-contact phases around the 24 planned perihelia but also throughout the mission, when the round-trip light time — the time it takes for radio signals to go back and forth between Earth and Parker Solar Probe — can be up to 31 minutes.
During the solar encounter phase, Parker Solar Probe’s four instrument suites measure the properties of material directly within the Sun’s outer atmosphere. These observations, gathered closer to the Sun than ever before, will help scientists begin to answer outstanding questions about the Sun’s fundamental physics — including how particles and solar material are accelerated out into space at such high speeds and why the Sun’s atmosphere, the corona, is so much hotter than the surface below.
Because of the spacecraft’s distance from Earth and position relative to the Sun, it will be several weeks after the end of the solar encounter before Parker Solar Probe begins transmitting this science data back to Earth.
At about 10:54 p.m. EDT, Parker Solar Probe surpassed 153,454 miles per hour — as calculated by the mission team — making it the fastest-ever human-made object relative to the Sun. This breaks the record set by the German-American Helios 2 mission in April 1976.
Parker Solar Probe will repeatedly break its own records, achieving a top speed of about 430,000 miles per hour in 2024.
On Sept. 25, 2018, Parker Solar Probe captured a view of Earth as it sped toward the first Venus gravity assist of the mission. Earth is the bright, round object visible in the right side of the image.
Two days after Parker Solar Probe flew past Venus toward its rendezvous with the Sun, the spacecraft had drawn close enough to our star that its power-generating solar array wings began to tilt themselves inward – a task directed by the spacecraft itself, based on the rising temperatures – away from the Sun and behind the spacecraft’s heat shield. This is the first time that autonomous, closed-loop solar array angle control based on temperature has taken place on a spacecraft.
This solar array movement, controlled by software within the spacecraft’s main processor, began on Oct. 5, soon after Parker Solar Probe’s distance from the Sun dropped below about 65 million miles.
On Oct. 3, Parker Solar Probe successfully completed its flyby of Venus at a distance of about 1,500 miles during the first Venus gravity assist of the mission. These gravity assists will help the spacecraft tighten its orbit closer and closer to the Sun over the course of the mission.
Detailed data from the flyby will be assessed over the next few days. This data allows the flight operations team to prepare for the remaining six Venus gravity assists which will occur over the course of the seven-year mission.
We like to call Parker Solar Probe the coolest, hottest, fastest mission under the Sun — and fall 2018 will prove why. Here are a few mission milestones to look forward to over the coming months.
Oct. 3, 2018 (about 4:45 a.m. EDT) — Parker Solar Probe performs its first Venus gravity assist. This maneuver — to be repeated six more times over the lifetime of the mission — will change Parker Solar Probe’s trajectory to take the spacecraft closer to the Sun.
Oct. 29, 2018 — Parker Solar Probe is expected to come within 27 million miles of the Sun. This is the record currently held by Helios 2, set in 1976.
Oct. 30, 2018 — Parker Solar Probe is expected to surpass a heliocentric speed of 153,454 miles per hour. This is the record for fastest spacecraft measured relative to the Sun, set by Helios 2 in 1976.
These speed and distance estimates could change after Parker Solar Probe performs its Venus gravity assist on Oct. 3.
Oct. 31 – Nov. 11, 2018 — Parker Solar Probe performs its first solar encounter. Throughout this period, the spacecraft will gather valuable science data. It will not be in contact with Earth because of the Sun’s interference and the orientation needed to keep the spacecraft’s heat shield between it and the Sun. The spacecraft is expected to reach its closest approach on Nov. 5. Like the distance and speed records, this estimate could change after the Venus gravity assist.
December 2018 — Parker Solar Probe will downlink the science data gathered during its first solar encounter.
Just over a month into its mission, Parker Solar Probe has returned first-light data from each of its four instrument suites. These early observations – while not yet examples of the key science observations Parker Solar Probe will take closer to the Sun – show that each of the instruments is working well. The instruments work in tandem to measure the Sun’s electric and magnetic fields, particles from the Sun and the solar wind, and capture images of the environment around the spacecraft.
“All instruments returned data that not only serves for calibration, but also captures glimpses of what we expect them to measure near the Sun to solve the mysteries of the solar atmosphere, the corona,” said Nour Raouafi, Parker Solar Probe project scientist at the Johns Hopkins University Applied Physics Lab in Laurel, Maryland.
The mission’s first close approach to the Sun will be in November 2018, but even now, the instruments are able to gather measurements of what’s happening in the solar wind closer to Earth. Let’s take a look at what they’ve seen so far.
WISPR (Wide-field Imager for Solar Probe)
As the only imager on Parker Solar Probe, WISPR will provide the clearest-yet glimpse of the solar wind from within the Sun’s corona. Comprising two telescopes, WISPR sits behind the heat shield between two antennae from the FIELDS instrument suite. The telescopes were covered by a protective door during launch to keep them safe.
WISPR was turned on in early September 2018 and took closed-door test images for calibration. On Sept. 9, WISPR’s door was opened, allowing the instrument to take the first images during its journey to the Sun.
Russ Howard, WISPR principal investigator from the Naval Research Laboratory, studied the images to determine the instrument was pointing as expected, using celestial landmarks as a guide.
“There is a very distinctive cluster of stars on the overlap of the two images. The brightest is the star Antares-alpha, which is in the constellation Scorpius and is about 90 degrees from the Sun,” said Howard.
The Sun, not visible in the image, is far off to the right of the image’s right edge. The planet Jupiter is visible in the image captured by WISPR’s inner telescope — it’s the bright object slightly right of center in the right-hand panel of the image.
“The left side of the photo shows a beautiful image of the Milky Way, looking at the galactic center,” said Howard.
The exposure time – i.e. the length of time that light was gathered for this image, an interval which can be shortened or lengthened to make the image darker or brighter – is on the lower end, and there’s a reason: “We intentionally wanted to be on the low side in case there was something very bright when we first turned on, but it is primarily because we are looking so far from the Sun,” explains Howard.
As the spacecraft approaches the Sun, its orientation will change, and so will WISPR’s images. With each solar orbit, WISPR will capture images of the structures flowing out from the corona. While measurements have been made before by other instruments at a distance of 1 AU – or approximately 93 million miles – WISPR will get much closer, about 95% of the way to the Sun from Earth, dramatically increasing the ability to see what’s occurring in that region with a much finer scale than ever before and providing a more pristine picture of the solar corona.
ISʘIS (Integrated Science Investigation of the Sun)
ISʘIS (pronounced “ee-sis” and including the symbol for the Sun in its acronym) measures high-energy particles associated solar activity like flares and coronal mass ejections. (The mission’s other particle instrument suite, SWEAP, focuses on low-energy particles that make up the solar wind.) ISʘIS’ two Energetic Particle Instruments cover a range of energies for these activity-driven particles: EPI-Lo focuses on the lower end of the energy spectrum, while EPI-Hi measures the more energetic particles. Both instruments have gathered data under low voltage, making sure their detectors work as expected. As Parker Solar Probe approaches the Sun, they will be fully powered on to measure particles within the Sun’s corona.
EPI-Lo’s initial data, on the left, shows background cosmic rays, particles that were energized and came rocketing into our solar system from elsewhere in the galaxy. As EPI-Lo’s high voltage is turned on and Parker Solar Probe gets closer to the Sun, the particles measured will shift toward solar energetic particles, which are accelerated in bursts and come streaming out from the Sun and corona.
On the right, data from EPI-Hi shows detections of both hydrogen and helium particles from its lower-energy telescopes. Nearer to the Sun, scientists expect to see many more of these particles — along with heavier elements — as well as some particles with much higher energies, especially during solar energetic particle events.
“The ISʘIS team is delighted with instrument turn-on so far,” said David McComas, Professor of Astrophysical Sciences at Princeton University and principal investigator of the ISʘIS instrument suite. “There are a few more steps to go, but so far everything looks great!”
The FIELDS instrument suite aboard Parker Solar Probe captures the scale and shape of electric and magnetic fields in the Sun’s atmosphere. These are key measurements to understanding why the Sun’s corona is hundreds of times hotter than its surface below.
FIELDS’ sensors include four two-meter electric field antennas — mounted at the front of the spacecraft, extending beyond the heat shield and exposed to the full brunt of the solar environment — as well as three magnetometers and a fifth, shorter electric field antenna mounted on a boom that extends from the back of the spacecraft.
The data above, gathered during the boom deployment shortly after the spacecraft’s launch in August, shows how the magnetic field changes as the boom swung away from Parker Solar Probe. The early data is the magnetic field of the spacecraft itself, and the instruments measured a sharp drop in the magnetic field as the boom extended away from the spacecraft. Post-deployment, the instruments are measuring the magnetic field in the solar wind — illustrating the very reason such sensors need to be held out far from the spacecraft.
In early September, the four electric field antennas on the front of the spacecraft were successfully deployed — and almost immediately observed the signatures of a solar flare.
“During its commissioning time, FIELDS measured its first radio burst from a solar flare,” said principal investigator Stuart Bale, of the Space Sciences Laboratory at the University of California, Berkeley. Such bursts of radio waves can be detected during solar flares — enormous eruptions of energy and light — and are associated with the energetic electrons that flares release. This radio burst was captured by the FIELDS electric field antennas, shown above with measurements from NASA’s Wind spacecraft (on the top) for comparison.
“FIELDS is one of the most comprehensive fields and waves suites ever flown in space, and it is performing beautifully,” said Bale.
SWEAP (Solar Wind Electrons Alphas and Protons)
The SWEAP suite includes three instruments: Two Solar Probe Analyzers measure electrons and ions in the solar wind, while the Solar Probe Cup sticks out from behind Parker Solar Probe’s heat shield to measure the solar wind directly as it streams off the Sun. After opening covers, turning on high voltages and running internal diagnostics, all three instruments caught glimpses of the solar wind itself.
Because of Parker Solar Probe’s position and orientation, the science team expected that Solar Probe Cup would mostly measure background noise at first, without picking up the solar wind. But just after the instrument was powered on, a sudden, intense gust of solar wind blew into the cup, visible in the data as the red streak. As the spacecraft approaches the Sun, such observations will be Solar Probe Cup’s bread and butter — and will hopefully reveal new information about the processes that heat and accelerate the solar wind.
The two Solar Probe Analyzers (SPAN) also caught early peeks of the solar wind. During commissioning, the team turned the spacecraft so that SPAN-A — one of the two SPAN instruments — was exposed to the solar wind directly. It captured about 20 minutes’ worth of data (right), including measurements of solar wind ions (top) and electrons (bottom). While SPAN-A and its sister instrument, SPAN-B, will measure solar wind electrons throughout the mission, the spacecraft’s orientation now means that SPAN-A will likely go several more years before it captures such ion measurements again. This is because solar wind electrons can be measured from any direction, as their low mass and high temperature make their motion much more random, while the much heavier solar wind ions follow a relatively direct path out from the Sun.
“SWEAP’s solar wind and corona plasma instrument performance has been very promising,” said Justin Kasper, principal investigator of the SWEAP instrument suite at University of Michigan. “Our preliminary results just after turn-on suggest we have a set of highly sensitive instruments that will allow us to do amazing science close to the Sun.”
On Sept. 13, Parker Solar Probe’s first-of-its-kind water-cooled Solar Array Cooling System (or SACS) was made fully operational. The SACS will protect Parker Solar Probe’s solar arrays — responsible for powering the spacecraft — from the intense heat of the Sun.
Though the solar arrays rely on the Sun’s energy to create electrical power for the spacecraft, they’re also very sensitive to overheating, and Parker Solar Probe is the first scientific mission to use a water-cooled solar array thermal management system. Water flows through mini-channels embedded in the solar arrays to absorb heat, then flows into four radiators to release that heat into space. This keeps the solar panels cool while near the Sun, allowing them to efficiently generate power for the spacecraft.
Though the Sun-facing side of Parker Solar Probe’s heat shield will reach temperatures as high as 2,500 degrees Fahrenheit when the spacecraft is close to the Sun, the SACS will keep the solar arrays — partially exposed to the Sun’s direct radiation — at less than 302 degrees.
As planned, the cooling system came partially online shortly after launch on Aug. 12. Roughly one hour after Parker Solar Probe’s 3:31 a.m. EDT launch, the spacecraft autonomously released the launch locks on its two solar arrays and deployed the panels. The spacecraft then released approximately two-thirds of a gallon of deionized water from a heated tank into two of four large radiators, mounted just below the spacecraft’s heat shield.
Then on Sept. 13, at around 11 p.m. EDT — when the spacecraft had reached a distance of about 84 million miles (135 million kilometers) from the Sun — the remaining one-third of a gallon of water was released, activating the last two radiators and making the SACS fully operational. These events were controlled by the mission operations team at the Johns Hopkins Applied Physics Lab in Laurel, Maryland.
“There are a number of technological breakthroughs on Parker Solar Probe that make the mission possible,” said APL’s Andy Driesman, project manager for mission. “The Solar Array Cooling System is really the heart and circulatory system of the spacecraft. Without it, the solar arrays would not survive the heat from the Sun, and we would not be able to operate the instruments that will explore the Sun’s corona and the systems that protect the spacecraft from the intense solar environment.”
As of 12 p.m. EDT on Sept. 14, Parker Solar Probe was 21 million miles (34 million km) from Earth, traveling at about 51,000 miles per hour (82,000 kph). Track the spacecraft’s progress online.
Parker Solar Probe continues to bring its instruments and secondary systems online — slightly ahead of schedule — as it speeds away from Earth.
On Friday, Aug. 31, flight controllers at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland performed a second planned Trajectory Correction Maneuver (TCM-2), a thruster burn which lasted for 35.2 seconds. This maneuver, which was executed with a high degree of precision, adjusted the direction of the spacecraft to position it for its Venus flyby on Oct. 3, when it will use Venus’ gravity to shed speed and draw its orbit closer to the Sun in preparation for its first solar approach.
On Sept. 2, four two-meter electric field antennas, part of the FIELDS instrument suite, were deployed. These antennas (as well as a fifth, which is mounted on the long boom at the other end of Parker Solar Probe) need to be extended away from the spacecraft to accurately measure the electric fields of the corona. These four antennas are not protected by Parker Solar Probe’s Thermal Protection System, or heat shield, so they are made of niobium C-103, a high-temperature alloy that can withstand the intense solar heat.
Just a few hours after the FIELDS antennas were deployed, the Solar Wind Electrons Alphas and Protons (SWEAP) investigation team successfully opened the covers of two instruments, the Solar Probe Analyzer (SPAN) instruments. The SPAN instruments are used to measure the solar wind when it is coming in at an angle relative to the spacecraft.
Before opening the SPAN instrument doors, the team ramped up high voltages on the Solar Probe Cup (SPC) instrument, also part of SWEAP. Solar Probe Cup measures the thermal solar wind plasma flowing radially from the Sun — requiring this instrument to be mounted outside the heat shield and pointed directly at the Sun. Like the FIELDS antennas, Solar Probe Cup’s heat shield is constructed of niobium C-103.
Other systems and instruments have completed checkouts as well. The spacecraft’s high gain antenna — which will be used to send high-rate science data to Earth — has been moved through its full range of motion.
EPI-Lo and EPI-Hi, the two Energetic Particle Instruments that make up the IS☉IS suite (pronounced “ee-sis” and short for Integrated Science Investigation of the Sun), have been turned on and have completed low voltage checks.
The Wide-field Imager for Solar Probe (WISPR) instrument has been turned on and has taken closed-door test images to calibrate the systems and imagers.
“The spacecraft continues to perform as designed, and thanks to the team’s careful planning and execution, we’re commissioning instruments slightly ahead of schedule,” said APL’s Andy Driesman, Parker Solar Probe project manager.
“The science team is excited to begin the investigation phase of the mission,” said Nour Raouafi of APL, Parker Solar Probe project scientist. “We’re looking forward to seeing this initial science data and getting our first look at what we know will be many discoveries that Parker Solar Probe will make.”
As of 12 p.m. EDT on Sept. 4, Parker Solar Probe was more than 15 million miles from Earth, travelling at about 44,700 miles per hour (72,000 kilometers per hour).
Editor’s note: The original version of this post misstated the length of the TCM-2 thruster burn. This version has been updated with the correct figure.