“NASA’s Parker Solar Probe Sets Its Sights on a Historic Christmas Eve Encounter!”

Certain regions in the solar system will remain unexplored by humanity. The surface of Venus, characterized by its dense atmosphere and intense pressure, is virtually unreachable. The more distant worlds, such as Pluto, are too far away to be viable for anything other than robotic exploration at present. Additionally, the sun, our luminous sphere of hydrogen and helium, is exceedingly hot and chaotic, making it unsafe for astronauts to approach closely. Instead, one daring robotic adventurer, the Parker Solar Probe, has been executing a series of remarkable maneuvers toward our star, getting closer than any spacecraft ever has to unveil its mysteries. Now, it is ready to undertake its final, closest orbits, entering the solar atmosphere like never before.

“This is a significant moment,” states Yanping Guo, a mission designer at the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Maryland. “Over 60 years of space exploration, the sun has presented itself as the most challenging destination to achieve.”

On Christmas Eve, December 24, Parker will pass merely 6.1 million kilometers above the sun’s surface, or 9.86 solar radii from the sun’s core, ten times closer than the orbit of Mercury, marking the first of three extremely close flybys. It will achieve this at an extraordinary velocity of 690,000 km per hour, surpassing the speed of any spacecraft in history (albeit still reaching just 0.064 percent of light speed). During this flyby, Parker will move rapidly enough to traverse from London to Paris in under two seconds; its velocity will be so extreme that relativistic phenomena like time dilation and frame dragging may be observed by the spacecraft’s instruments.

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The spacecraft will navigate through the sun’s atmosphere, its corona, where some of the most pressing inquiries about our star remain unanswered, such as why the corona is significantly hotter than the solar surface and what propels the solar wind. While other missions have observed the sun before, Parker is the only one to have come this close. “There is no previous example,” claims Thomas Zurbuchen, former associate administrator for the Science Mission Directorate at NASA. “This is genuinely an exploratory mission.”

Zurbuchen was the individual who named the spacecraft after the late U.S. solar physicist Eugene Parker, who predicted the existence of the solar wind in the 1950s. The project commenced in 2018, culminating decades of research on how to “touch” the sun. Approaching our star is surprisingly challenging because you need to “reduce the orbital speed of Earth,” explains Ralph McNutt, a leading scientist in the space department at JHUAPL. Scientists had long believed the optimal approach was to travel to Jupiter and use the gas giant’s gravitational force to loop in toward the sun. This strategy would place you very near, merely four solar radii away, but it would also be incredibly arduous and protracted, yielding perhaps only one or two close encounters with the sun over an orbital period of nearly five years.

In 2007, Guo suggested that multiple flybys of Venus could be utilized to maneuver a spacecraft into a similar, albeit slightly more distant, position while allowing for dozens of encounters over several years with an orbital period of just under three months. “The requirement was to be sufficiently close to sample particles within the solar corona,” Guo mentions. “I discovered that utilizing seven Venus flybys would suffice.” The final of these flybys took place on November 6, with the spacecraft gliding 387 km above Venus’s surface. Since then, it has been on a course that will take it closer to the sun than any of its previous 21 encounters over the last six years; its most recent solar flyby in September was roughly a million km, or 10.4 solar radii, farther out.

Parker possesses four instruments designed to analyze the sun. This includes a camera to capture images of our star and its surroundings, a device for measuring electric and magnetic fields, and two others to examine solar particles and the plasma inundating the spacecraft. Throughout its mission so far, Parker has contributed to the discoveries that magnetic fields on the sun’s surface might be responsible for transferring heat into the corona and has identified more particles emanating from the sun than expected. These findings are largely due to Parker’s successful infiltration and traversal through the sun’s corona, which first occurred in April 2021.

These upcoming flybys will allow the spacecraft to delve deeper into the corona, although it will not exceed a supersonic limit near four solar radii, where the solar wind is presumed to accelerate to the speed of sound. Even from Parker’s comparatively distant position, approximately 10 solar radii out, researchers are keen to gain further insights into the solar wind, specifically what accounts for the distinction between its “fast” (up to 800 km per second) and “slow” (as low as 300 km per second) categories. “We hypothesize that fast wind originates from coronal holes, while slow wind may come from the edges of these holes,” remarks Steph Yardley, a solar scientist at Northumbria University in England. “However, this is still under discussion.” This process may also enhance our understanding of space weather phenomena on Earth as the solar wind approaches our planet. “The closer we venture into that particle source region responsible for space weather, the more knowledge we acquire,” affirms Joseph Westlake, director of the heliophysics division at NASA headquarters in Washington, D.C.

There’s also the hope that Parker may fly past the sun during an eruption. Currently, the sun is in its so-called solar maximum, a period of heightened activity in our star’s 11-year cycle. This increases the likelihood of coincident eruptions impacting the spacecraft, a scenario that has occurred at least once previously. “We’re optimistic about gathering more of these events very close to the sun,” states Nour Rawafi, an astrophysicist and project scientist for Parker at JHUAPL, “as we need to comprehend how occurrences like flares and coronal mass ejections accelerate particles to relativistic speeds.” Parker may also pass through a dust-free zone, a theorized area near the sun where debris drifting inward from around the solar system is “vaporized,” according to John Wirzburger, systems engineer for Parker at JHUAPL. “We’ve been detecting hints of that as we move closer and closer.”

The action this time commences on December 20, when the spacecraft reaches 0.25 times the Earth-sun distance, roughly 37 million km or 53 solar radii from the sun. Here, in preparation for its descent, the spacecraft will

first transmit a brief beacon signal to Earth, affirming its good condition. To endure the close flyby and the extreme circumstances that ensue, Parker must conceal most of its apparatus behind a carbon-composite thermal shield. This thermal shield is so efficient that, even with temperatures soaring to around 1,000 degrees Celsius, the spacecraft’s instruments situated behind remain at “essentially room temperature,” Wirzburger notes.

The arrangement of the spacecraft and its thermal shield, coupled with the aim to optimize data collection, means that the spacecraft cannot communicate with Earth during this passage. It operates entirely independently, adjusting its position very slightly to follow the sun’s movement, ensuring the thermal shield is accurately oriented toward the star to create a conical shadow that thoroughly envelops its valuable instruments. The sole other segment of Parker that will catch sunlight during the flyby will be a narrow strip of solar panels, integrated into the spacecraft’s sides, to harness energy from our star’s vast brilliance.

The whole interaction with the sun will span approximately a week, with the spacecraft reaching its nearest point on Christmas Eve at around 6:40 A.M. EST. If you could withstand the conditions and escape immediate blindness from the sun’s overwhelming brightness—500 times more intense than perceived from Earth—our star would appear in your line of sight as an enormous disk 22 times the size of the full moon in our planet’s sky. “It would occupy a substantial portion of the space in front of you,” Rawafi explains. Parker’s camera, directed to the side, will monitor for traces of particles streaming through the surrounding corona, while its other instruments compile their critical data. However, what precisely they will observe remains uncertain. “We don’t really know,” Zurbuchen indicates.

The team will not ascertain the success of the flyby until December 27, when the spacecraft reaches 35 solar radii on its return journey and transmits another beacon signal back home to declare its survival. The team will then gear up to receive the spacecraft’s invaluable data beginning on New Year’s Day, which will trickle in over the following weeks and months. In March, Parker will repeat the process as it passes near the sun again, followed by another, final close approach in June. The gravitational influence of distant Jupiter will lead to these two subsequent flybys being technically marginally closer to the sun—approximately 100 km nearer each, a negligible rounding error in the multimillion-kilometer distance separating Parker from our star. Practically speaking, the Christmas Eve flyby will be the nearest Parker gets to the sun.

The completion of these close encounters will also signal the conclusion of the spacecraft’s primary mission, but it may potentially be extended. Parker could, for example, remain in its current orbit to observe the impacts of the declining solar maximum. “It would be incredible to monitor this decline,” Westlake mentions, as many significant solar phenomena are anticipated to take place during this timeframe. The spacecraft could also start to transition into a more inclined orbit with its remaining fuel, maneuvering itself out of the ecliptic plane where most planets revolve to gain a slightly different perspective of the sun and glimpse its polar regions, albeit at the significant cost of flying a bit farther away. “We wish to maintain close proximity to the sun,” Rawafi states.

Regardless of the mission’s outcome, the data Parker gathers will be analyzed for many years. “This is the closest humanity has ever approached a star,” Westlake remarks—a benchmark unlikely to be surpassed in the foreseeable future.