Probing the Sun: Parker Solar Probe's record-breaking speed
Launched into space in 2018, the Parker Solar Probe is increasingly tightening its orbit around the Sun. The most significant moment of the mission will occur on December 24, 2024. The probe will approach its closest distance to the Sun and accelerate to 193 kilometres per second.
The earliest vehicles that allowed humans to travel faster than walking were horse-drawn carriages and boats powered by human muscle or wind. With the Industrial Revolution, engine-powered vehicles began to emerge. This eventually led to the development of airplanes, and a few decades later, rockets, enabling us to achieve very high speeds.
To send a probe into orbit, the vehicle must accelerate to about 8 kilometres per second. To fly to the Moon, it requires about 11 kilometres per second. Without gravitational assist, reaching Mars requires an acceleration to approximately 11 kilometres per second. The fastest vehicle launched from Earth was the New Horizons probe (which visited Pluto); after its launch in 2006, it achieved a speed of almost 18 kilometres per second, although it used the gravitational pull of Jupiter along the way.
The world of earthly speeds and a record near the Sun
On Earth, we achieve significantly lower speeds. One kilometre per second is equivalent to about 3,600 kilometres per hour. That's more than the maximum speed of most fighter jets. Passenger planes typically travel at speeds of around 1,000 kilometres per hour. On highways, the speed limit is 140 kilometres per hour, and the cruising speed of high-speed trains is about 350 kilometres per hour. In contrast, walking speed is only about 6 kilometres per hour.
Rockets remain the fastest vehicles, as overcoming Earth's gravity requires high speeds. However, our technological capabilities, specifically achieving rapid high thrust through combustion, and economic constraints impose limitations similar to those seen at the dawn of space flight. In some cases, we continue to rely on a powerful ally—nature, specifically gravity.
On December 24, 2024 something unprecedented will happen. The Parker Solar Probe will reach a speed of 193 kilometres per second, which is just over 690,000 kilometres per hour. At that time, it will fly record-breakingly close to the Sun, at a distance of 6.1 million kilometres from the photosphere, which is the layer we see during the day. There are no humans aboard the probe, but if such a speed could be attained in Earth's atmosphere—here we overlook the problem of friction and issues of acceleration or deceleration, not to mention the deadly stresses on a passenger—we could cover the distance between Warsaw and New York in about 35 seconds. At this speed, we could also circle our planet more than 17 times in an hour.
Why was the Parker Solar Probe built?
Life on Earth depends on our daytime star. Understanding its behaviour, enjoyed by northern lights observers, is crucial in the era of digital technology, satellites in Earth orbit, and planned manned interplanetary journeys. One of the most enigmatic elements of the Sun is the corona, its outermost layer.
People can see it with the naked eye during total solar eclipses as a jagged halo. The Parker Solar Probe was launched in 2018, and three years later, it became the first human-made vehicle to enter the Sun's corona and study it on-site. This occurred exactly 60 years after the concept of such a mission was conceived. The probe was named in honour of the man who predicted the existence of the solar wind. Eugene Parker, as he was known, passed away in 2022.
So far, the Parker Solar Probe has orbited the Sun 21 times. It has examined the so-called Alfven surface (the area where the solar wind is generated), showing the irregularity of its shape. It also demonstrated the existence of a zone around the Sun where interplanetary dust is so heated that it sublimates, turning into gas.
The probe also studied the behaviour of the magnetic field inside the corona, revealing the mechanism of magnetic reconnection that drives the solar wind. The orbit passing through various regions of the solar corona enabled it to show the complexity of the paths along which high-energy particles travel before they embark on a journey away from the star. Until now, these were all just theories and hypotheses. Data from the probe has also indicated the likely source of the Geminid meteor shower, whose peak occurred in mid-December.
The fastest vehicle could not return to Earth
During its first orbit around the Sun, the Parker Solar Probe achieved a speed of 95 kilometres per second, breaking the previous speed record of 71 kilometres per second, which belonged to the Helios-B probe from the 1970s. However, our home star cannot be moved from one place to another to be used as a gravitational slingshot for any mission. The record speed is possible due to an orbit that wraps close to the Sun's surface, which is why the probe's orbit has been steadily tightened since its launch. The gravitational assist maneuver from Venus that put the Parker Solar Probe on the closest-ever orbit around the Sun took place on November 6, 2024.
It should also be noted that the probe's speed is not constant—it accelerates at the closest point in its orbit to the Sun (called perihelion) but significantly slows down as it moves away from the star. When the probe reaches the farthest point of its current orbit before turning back toward the Sun to gather speed again, it will travel at a speed of about 11 kilometres per second. Even though the Parker Solar Probe will accelerate to 193 kilometres per second, it does not mean it will then fly off into the far reaches of space like the Voyagers launched from Earth. For that to happen, the probe would have to travel at a speed of about 618 kilometres per second relative to the Sun. Today, even a return to the vicinity of Earth is already impossible, but there is no need for it.
What will happen on December 24 will remain a mystery until Friday, December 27, when communication with the probe will be reestablished, and scientists will download the first data. Then, after about 88 days, in March 2025, the probe will return near the Sun and will do so at least several more times.
Speed is relative
It is important to note that all speeds are given relative to a particular reference point. The direction of movement is also significant. The speeds achieved by probes launched from Earth are usually given relative to our planet.
The speed of the Parker Solar Probe is measured relative to the Sun. If we wanted to determine the resultant speed relative to even larger structures, the result would be surprising. The Solar System orbits the centre of the Milky Way at a speed of 230 kilometres per second, and our Galaxy moves even faster relative to the centre of the Virgo Supercluster.
