There's this running debate among astronomers about whether our moon deserves a proper name. After all, it is the way of our solar system. Other moons each have monikers, like Phobos and Deimos, and there is certainly no planet named "planet" nor asteroid named "asteroid." Even worlds beyond our cosmic neighborhood have names, albeit often very boring ones. However, our moon's namelessness does have a silver lining: It forces us to remember that it is, indeed, a moon.
The same can't be said for the sun, whose name makes it easy to forget it's really an incomprehensible, scorching star. But on Christmas Eve this year, we'll be acutely reminded of the sun's cosmic nature thanks to a resilient little spacecraft on a spectacular journey through space: the Parker Solar Probe. On Dec. 24 at 6:53 a.m. ET, this conical explorer will fly dangerously close to none other than our glowing yellow sun.
"In 1969, we landed humans on the moon; on Christmas Eve in 2024, we are going to embrace a star," Nour Raouafi, project scientist of the Parker Solar Probe mission, told Space.com.
On Aug. 12, 2018, NASA launched the Parker Solar Probe toward the sun with the hopes of decoding a bunch of longstanding solar mysteries — perhaps the most perplexing of which concerns the fact that our star's atmosphere is weirdly hotter than its surface. What's heating it up? It seems unintuitive, doesn't it? In the audience on launch day was the late Dr. Eugene N. Parker, who revolutionized our understanding of solar physics before the solar probe began its expedition and indeed gave Parker its name to begin with; this marked the first time a spacecraft's namesake was present for liftoff.
Then, on Dec. 14, 2021, the agency announced that Parker Solar Probe successfully entered the sun's atmosphere, or corona, getting just about 6.5 million miles from the star's surface. This was monumental in itself, but since then, the spacecraft has continued to get closer and closer over 21 orbits around the sun, leveraging Venus' gravity to propel itself while breaking records left and right. For instance, it is officially the fastest-ever human-made object, reaching speeds pushing 394,736 miles per hour (635,266 kilometers per hour).
On Dec. 24 of this year, however, Parker Solar Probe will complete its closest pass to the sun to date, getting within 3.8 million miles (6.1 million kilometers) of the object's surface while zooming at 430,000 mph (690,000 kph). Previous records will be shattered.
"What the Parker Solar Probe is about to do on Christmas Eve of this year is really unparalleled," Raouafi said. "We have been dreaming of this moment for over 16 years."
And, as Raouafi explains, this is likely as close as the probe will be getting.
"Even if technically we can get the spacecraft closer to the sun, we cannot do it because the heat shield is not big enough to protect the spacecraft," he said. "If you get closer, the shadow cone of the heat shield will be narrow, and potentially, parts of the spacecraft will be exposed to direct sunlight — and that's not something we want to do."
In darkness
There's an aspect to Parker Solar Probe's journey that, at first glance, seems like it'd cause a stir among mission members: The spacecraft will be off the grid during the major flyby; we will have no way of contacting it.
Its first proof of life won't arrive before Dec. 27, when a beacon tone will essentially alert the team that Parker Solar Probe survived its trip through the sun's corona and is able to speak. The goods, though (scientific data and images) won't start pouring in until the new year. John Wirzburger, mission systems engineer for Parker Solar Probe, doesn't see this as a major issue. He actually welcomes the silence.
"This has been a great time for people to actually decompress — to take a break — because they know when they come out the other side, we have to get all the science data off and reconfigure the spacecraft," he told Space.com. "So, this encounter is almost like a built-in respite for us."
The dark period of Parker's excursion is of course in part because of the specific trajectory of the probe — it won't be in a great position to relay information back to Earth via NASA's Deep Space Network — but it's also because of the project's scientific goals.
"Because we are so close to the sun, the sun interferes with our communications," Wirzburger said. "But the other thing is, we don't want our communications to interfere with our science collections."
Nevertheless, Raouafi believes Parker Solar Probe's presence will be permeating the room for many scientists, even if through aromas of gingerbread and Scotch tape.
"Trust me, everybody will have in their minds and hearts Parker Solar Probe, and be thinking how that little thing is doing in front of the giant that is our star," he said. "I mean, we have been living with this mission for almost 16 years."
Those thoughts, however, will hopefully echo the Christmas spirit. Like learning to put faith in a child's ability to channel good decisions as they grow up and experience the world, the Parker Solar Probe team's trepidation about sending their robotic explorer so close to the sun has softened into pride over the years.
"Early in the mission, whenever we went close to the sun, I think we were pretty nervous," Raouafi said. "But now we did its 21st orbit, and the system is really doing way better than we thought it would."
No fear; no trembling
The most obvious worst case scenario for Parker Solar Probe in the coming days is probably something like … "Will it explode?" or "Will it just never speak to us again?" Yet neither Raouafi nor Wirzburger are particularly afraid of those outcomes as they say they're very unlikely, especially given the spacecraft's grit and tenacity over the last few years.
"I think the thing that would hurt me the most is not having collected science data through this pass," Wirzburger said, though adding that such pain would only last for the duration of the next orbit: "It's only a three-month hurt from the standpoint of we will jump on it and figure out what needs to be done for orbit 23 in order to remedy that situation."
And in fact, Raouafi is specifically hoping the sun tests Parker Solar Probe's strength as much as possible. The spacecraft launched when our sun was at the beginning of its 11-year solar cycle — a time known as solar minimum, when the sun is at its least active — but it's currently nearing solar maximum, when it's at its most active.
"My hope is the sun will give us one of these big explosions — really humongous explosions — when Parker Solar Probe is so close to it, and let it deal with it," he said. "I'm really hoping for Parker Solar Probe to give us one of the greatest gifts ever."
As to those gifts? Well, the probe has already sprinkled in a few over the last several years. For example, on Sept. 5, 2022, it flew straight through an enormous coronal mass ejection, which you can think of as a blob of plasma that bursts off the sun. This revealed that when one of these events occurs, it literally gives space a deep clean. These ejections appeared to sweep away any and all dust particles in its way, and the mission's crewmembers will be keeping their eyes peeled in case this kind of event is caught on camera again.
"We have other observatories observing the sun at that time; we have data from them. If there is, for example, a big flare or a big coronal mass ejection going toward Parker Solar Probe, we will know about it," Raouafi said. "And you will know about it almost real time, except that we have to wait to learn how Parker Solar Probe dealt with it."
Understanding these events in general is quite important when we consider space weather events; coronal mass ejections, especially when pointed at our planet, can accelerate particles to astonishing speeds. If and when those particles reach us, they can be super hazardous to astronauts in space and pose threats to our power grids.
Fascinatingly, Parker Solar Probe is helping engineers in an indirect way as well; this entire mission is sort of pushing a spacecraft to some of the most extreme limits possible. That means data about the probe's flight patterns and dynamics can inform future missions that may be exposed to intense space environments, like any that intend to explore the regions around Jupiter or Saturn where radiation levels are supremely high.
"We have to actively cool our solar arrays; we pump deionized water through them to keep them cool," Wirzburger said. "Can other missions adopt this? Maybe not for survivability, but for efficiency reasons."
Given all of this stress, you may also think that Parker Solar Probe is pretty worn down (I certainly did) but that couldn't be farther from the truth. Oddly enough, the sun has been acting like a nice car wash for the spacecraft.
"Basically, we take all of our parts and we bake them out to try to remove contaminants and get them nice and clean before we launch them," Wirzburger said. "But we're sending them to one of the best ovens in the solar system, close to the sun."
"We actually believe — especially with the thermal protection system — the shield up top is actually cleaner today than it was when we launched it," he added. "We think the spacecraft probably looks very beautiful and has a very nice white TPS [thermal protection system] on top."
With the exception of some minor dust damage to one of its instruments, and a few micron-size particles that have shot off its body when impacted by classic deep space debris, Parker is in stellar (ha) health.
"The system is basically behaving like we launched it yesterday," Raouafi said. "It's doing so well."
A true star
It isn't just a philosophical wonder that our sun is a star among billions of trillions of others in the universe; it's also a practical concept for astronomers.
"We are using the sun as a laboratory to study other worlds out there," Raouafi said, emphasizing that what Parker Solar Probe finds can remotely explain the intricacies of the billions upon billions of other stars out there in the universe. And indeed, the team hopes that it will continue revealing those intricacies in tandem with the discoveries other scientific fields make about the cosmos for a long, long time.
The probe likely has enough fuel to theoretically allow it to continue working for tens of years — potentially even a hundred years — so long as it remains in orbit around the sun. Per Wirzburger, that fuel supply is enough to maintain the spacecraft's momentum but can't quite allow scientists to alter the spacecraft's trajectory. This means Parker will probably remain a solar explorer, at least for as far as we can see.
"It's not that the spacecraft will be catapulted out and just be thrown out of the solar system or anything like that, or that we're spiraling in," Wirzburger said. "So, for the foreseeable future, we are in our final orbit, and it will maintain that orbit for a long time to come."
"There is a possibility, in the 2032 timeframe, that we potentially could get back to Venus," he speculated. "This is predicated on NASA; on what they would like to do — but the question that comes is, if we can get back to Venus, then what?"
In a sense, when you know it has a finish line, limbo is a nice space to find yourself in because it offers a small sample of reprieve during which your imagination can take charge. Surely, it is the bottomless imaginations of scientists that brought us to this historic moment in the first place.
"When I grew up as a young scientist and I got involved in space," Raouafi said, "I heard about the previous at least one or two attempts of trying to implement a solar probe — and when they all didn't come to fruition, it was really a sour taste.
"For me, the dream was to see this mission happening in my lifetime — but for us to be involved with it so deeply, and also to be leading it, it's more than a dream; we are dreaming with our open eyes."
Update 12/23: The flyby is currently scheduled to occur at 6:53 a.m. ET.
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