Parker Solar Probe changed the game even before its launch |

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The Parker solar probe was launched on August 12 in Cape Canaveral

ABOVE THE VIDEO: It is surprisingly difficult to go to the sun.

(NASA) – On October 3, 2018, Parker Solar Probe performed the first significant celestial maneuver of its seven-year mission. As the orbits of the spacecraft and Venus converged to the same point, Parker Solar Probe slipped past the planet, allowing the gravity of Venus – relatively small by celestial standards – to twist and change speed.

This maneuver, called gravitational assistance, reduced Parker's speed against the Sun by 10%, or 7,000 miles an hour, bringing the nearest point of its orbit, called perihelion, closer to 4 million miles. of the star.

Achieved six more times over the seven years of the mission, these gravity aids will allow the Parker Solar Probe to approach a record of 3.83 million kilometers from the surface as close as possible. of the Sun, about one-seventh of the current record holder Helios 2., which crossed 27 million miles from the Sun in 1976.

Even before its closest approach, Parker Solar Probe is expected to exceed this record and become the closest human fabrication object to the Sun in late October 2018.

A long-time dream

Scientists and engineers have been concerned with solar collectors since the end of the 1950s, when a new theory and the first satellite measurements of the Sun's constant output of materials, called the solar wind, indicated a never-before-seen complexity. But if you had asked someone before 2007 – long after the serious planning of such a mission had begun – Venus would not have been considered the key to the mission puzzle.

For more than three decades, during which various committees and teams worked on different concepts for the solar probe mission, it was widely recognized that the only way to dive into the solar atmosphere was to send the spacecraft to Jupiter. first.

"No one thought that using gravimetric aids from Venus would be possible because the gravimetric help that a planetary body can provide is proportional to body mass, and the mass of Venus is so much smaller – only 0.3% of Jupiter's, "said Yanping Guo, mission designer. and responsible for navigation for the Parker Solar Probe mission of the Applied Physics Laboratory at Johns Hopkins University, Laurel, Maryland.

"You compare the gravimetric support that Venus can provide to what Jupiter can provide, and you need to do repeated flyovers to get the same change. Then you will have a very long mission. "

Approaching the Sun is more difficult than one might think. Any spacecraft launched from Earth begins to travel at a lateral speed of 67,000 km / h on our planet, a speed it must neutralize before it can get closer to the Sun.

Gravitational help is one of the most powerful tools in an orbit designer's toolbox to solve this problem: instead of using a valuable and expensive fuel To change direction or speed (or both), gravitational assistance allows you to exploit the natural appeal of a planet, over time. as the only cost.

Most deep-space missions that use planetary gravimetric aids use them to gain speed – like OSIRIS-Rex, which used Earth's gravity to propel itself to the asteroid Bennu – or to change direction – like Voyager 2, which was performing gravimetric assistance after its final planetary flight. to Neptune to go to his moon, Triton.

The idea of ​​gravitational assistance by solar probe was a little different. In the original orbital planes, Jupiter's gravitational assistance was primarily aimed at slowing the speed of the spacecraft and projecting it upwards from the almost flat plane containing all the known planets of the solar system, called ecliptic . plane. This would place the solar probe on a trajectory that allows for a vision that is rare and better than ever before in the polar regions of the Sun, which are difficult to visualize, but which are scientifically important because they produce part of the solar wind at high speed from the Sun.

Almost all of our solar observatories evolved into the ecliptic plan, with the exception of Ulysses, who used gravimetric assistance from Jupiter to make polar passes more than 200 million kilometers from the Sun.

But sending a spaceship to Jupiter and bringing it back into the inner solar system is difficult. First, no matter how you plan your trip, it's a long mission, with a minimum of almost half a decade between two significant events. Most of the time would be spent navigating the depths.

Second, so far from the sun, you have to be creative with power. Near Jupiter, sunlight is about 25 times weaker than what we live on Earth. The only options are huge solar panels to get the most out of daylight, or another source of energy, such as nuclear power. Large solar panels, however, pose a problem for a solar sensor because they should be protected during solar encounters to avoid overheating.

The size of a solar panel needed to propel the spacecraft near Jupiter is too big to be able to be effectively arrayed near the Sun. It would therefore be necessary to dump it on perihelion – and this limits you to a single solar passage, once you have lost your source of power.

With nuclear energy – a radioisotope thermoelectric generator, or RTG, the same source used for deep space missions such as Cassini and New Horizons -, a gravimetric assistance from Jupiter is a viable option.

Changing the paradigm of the mission

But the design of the mission was soon to change. David McComas, chairman of the definition committee, remembers a call from Andy Dantzler, then project manager of the Solar Probe mission at APL. Dantzler died in 2011 at 49 years old. the Delta IV Heavy rocket carrying Parker Solar Probe in space was dedicated to him.

"Andy asked if it was possible for the committee to go on a mission in which you stay in the ecliptic plane, but you have a lot of passes near the Sun and slowly reduce perihelion," said McComas, who is now the principal investigator of the integrated mission. Science Investigation of the Sun, or ISʘIS, suite and professor of astrophysical sciences at Princeton University, New Jersey.

It was an entirely new paradigm for the mission. One of the distinctive signs of the original plan passed over the poles of the Sun, the source of the Sun's fast solar wind, but a region of relative mystery for scientists. Plus, staying in the ecliptic plane would almost certainly mean getting farther away from the Sun than expected.

"If you negotiate a perihelion distance, you have to trade it for something that will give you the opportunity to supplement the science in another way," McComas said.

Subsequently, two developments reinforced the choice to make these modifications to orbit and to create the Parker Solar Probe mission that we know today.

The first was a new research published in 2009 by Thomas Zurbuchen – then a scientist at the University of Michigan and now associate director of the Science Mission Directorate at NASA's headquarters in Washington. This research has shown that the solar wind that can be measured from the ecliptic plane actually came from various sources. It was not only the slower solar wind known to be more common near the equator of the Sun, but also the high-speed solar wind that often came closer to the Sun's poles. By sampling the solar wind of the ecliptic for several years, scientists were able to learn more about this fast solar wind in an unexpected way.

The second evolution is the change that made this sampling possible: the design of the current Parker Solar Probe trajectory.

"At first I had no idea if I could find a solution," said Guo, the mission trajectory designer. "Everyone thought that Jupiter was the only practical way to get closer to the Sun, within a radius of 10 solar rays."

In 2007, she proposed five alternatives to keep the spacecraft close to the ecliptic and do not need to travel to Jupiter. These trajectory options combined gravity aids from the Earth and Venus to gradually bring the spacecraft closer to the Sun over the years. One of them fulfilled all the requirements for the Solar Probe mission – a total mission duration of less than 10 years, with a final approach closer to framing less than 10 solar beams (equivalent to 4.3 million miles).

This was chosen as the trajectory of the current mission, now called Parker Solar Probe after Dr. Eugene Parker, comprising seven Venus gravity aids that move the spiral orbit closer and closer to the Sun. during the seven years of his mission.

The main obstacle to overcome for a trajectory with such repeated gravitational aids is the phasing. Of course, Venus is constantly moving around the Sun. So, every time the spaceship passes the planet and turns around our star, Venus is in a completely different place. But Guo's design solves this problem by offering many launching possibilities. This trajectory design carries the spacecraft on 24 orbits around the Sun. Venus's seven gravitational aids occur at different locations in the orbit of the spacecraft, to explain the phasing problem – some, such as that of Oct. 3, occur when the spacecraft heads toward the Sun. , while others intervene when Parker Solar Probe flies. of the sun.

This orbit is decidedly different from the original concept of gravitational assistance to a single Jupiter. Rather than two passes over the Sun's poles, less than 1.23 million miles from the surface, this version of the mission offers 24 passes around the Sun near its equator, less than 3.83 million miles of the Sun's surface.

Even though Parker's solar probe is not as close to the Sun, this version of the trajectory has spent more than 900 hours in this critical inner region of the solar corona, within a radius of 20 solar rays (about 8.65 millions of miles). In comparison, previous designs using Jupiter's gravimetric aids predicted less than 100 hours in this region.

"This was the safest, cheapest and best scientific mission with all the samples we received," McComas said. "The Sun is not a stable object, but a variable, which would allow us to do better scientific work."

This mission change has also solved the problem of feeding. Instead of requiring a RTG or solar panels of unmanageable size, Parker Solar Probe is powered by a pair of articulated solar panels that are slowly lured into the heat shield while the probe is getting closer to the Sun. At the closest approach, only a small area remains exposed to generate the necessary energy for the spacecraft, cooled by the mission's first solar panel cooling system.

But while this solved a major problem, rethinking the mission in this way also required a complete rethinking of the spacecraft itself.

"The entire design of the spacecraft has changed dramatically," said Nicola Fox, a former research scientist with the mission at APL. Fox is now the director of the heliophysics division at NASA headquarters. "With the anterior trajectory, the heat shield was the spaceship. It was like a cone with a pointed end facing the sun, because when you make such a fast polar orbit, it's hard to keep a shield properly oriented. "

"We do not go that far with the new trajectory, so we could adopt a simpler form for the heat shield because it is possible to keep the heat shield oriented between the probe and the sun at all times. Everything looks really different. "

The mission team thanks Andy Dantzler for guiding him through this fundamental shift in mission design that led to the mission we have today.

"When Andy called to ask if the definition team would do it, I really did not know the answer," McComas said. "While our definition team was working in the scientific field, I became convinced that it was not just an equivalent mission, but a better scientific mission, because we had a lot more time near the Sun and many more samples at different times. "

The first overview

During the gravitational assistance of 3 October, the Parker solar probe approached approximately 1,500 km from the surface of Venus and reached its nearest point at approximately 4:45 (EDT).

Venus is an interesting case for heliophysicists, who study not only the Sun, but also its effects on the planets. Unlike the Earth, Venus does not have an internal magnetic field. Instead, a weak magnetic field is induced on the surface by the constant barrage of charged particles of solar energy that circulate on the planet and interact with its very dense atmosphere.

This first overview provided a unique opportunity for calibration, as Parker's solar probe flew through the lower end of Venus's magnetic field, called a magnetotail. Three of Parker Solar Probe's four instrument suites – SWEAP, ISIS, and FIELDS – collected data during the flight of particles and fields in this region.

Although the data still comes back to Earth, the scientific team hopes to analyze it before focusing on Parker Solar Probe's next great celestial encounter: its first approach to the Sun. The first solar meeting of Parker Solar Probe will take place from October 31st to November 11th. The closest approach will take place on November 5th at a distance of 15 million miles from the Sun. The scientific data from this meeting will begin to be brought back to Earth in early December.

Parker Solar Probe is part of NASA's Living with a Star program, which explores aspects of the Sun-Earth system that directly affect life and society. The Living with a Star program is managed by the agency's Goddard Space Flight Center, located in the Greenbelt of Maryland, on behalf of NASA's Science Mission Directorate in Washington. APL designed, built and operates the spacecraft.

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