You might have noticed a recurring headline in space exploration: space probe XYZ has successfully been launched from Earth in May 2024 and should reach its destination Planet ABC in 2036. These numbers are not exaggerated: 12 years is exactly the time it took for Voyager 2 to reach Neptune, while the European-spacecraft Juice that launched last year still has 7 years of travel to Jupiter. Almost half a century has passed between the launch and development of these two spacecrafts, yet there seems to be no improvement in their travelling speeds. Has our technology stagnated? Is the taxpayer’s money going straight to the drain? Why does it take so long to reach planets?

The TRUE Distance to Planets

The first thing to keep in mind is that planets within our Solar System really are that far away. Their distances to the Sun are often stated in Astronomical Units, where 1 AU is the distance Earth-Sun which is about 150 million kilometers.

To have an idea of scale, this is more than 3700x the circumference of the Earth. If your mind works better with durations than with distances: the fastest passenger plane (Airbus A380) would take over 14 and a half years to reach the Sun travelling at its top speed. Yes, 14 years just to reach the Sun.

solar system distances to scale

Figure 1: Solar System Distances To Scale, planet sizes are relative to each other not absolute. (Aeddub~commonswiki, 2006)

Planets such as Jupiter and Neptune mentioned earlier are on average 5 AU and 30 AU distant from Earth respectively, so you would need to multiply this duration by 5x or 30x if you were to reach them using that plane (70-420yrs). An observant reader might have noticed that the distance to Neptune is 6x as large as the one to Jupiter, yet the voyage time difference is only 1.33x as big.

So, distances do play a key role in the time it takes to reach planets, but there is obviously more than meets the eye.

Space Travel 101

We can agree that the example of travelling by plane in space is rather an exercise in futility and only necessary for distance representation.

When spacecrafts are launched from Earth their trajectory takes a different shape based on their velocity. If a spacecraft’s speed is more than 11.2 km/s, the trajectory ends up being hyperbolic and the spacecraft escapes Earth’s gravitational field; if the speed is lower, the spacecraft stays in an elliptical path around the Earth (orbit). The middle ground when the speed is exactly 11.2 km/s is called a parabolic trajectory.

We thus require speeds higher than 11.2 km/s to reach other planets.

diagram showing spacecraft trajectories around earth for different speeds

Figure 2: Velocity and Trajectory Shape (Durant, 2024).

If we only take into account the Earth’s and the destination planet’s position to set a simple Earth-Planet hyperbolic trajectory, we would have to launch the spacecraft at velocities which are far beyond our current launcher capabilities. Figure 2 shows the more hyperbolic the trajectory, the bigger the required spacecraft velocity, so reaching those far-away planets would effectively be impossible as it requires a very hyperbolic trajectory and thus a very high speed.

Yet we have successfully launched spacecraft to other planets since the early 1970s, so how do engineers pull it off? The answer is a simple trick derived from orbital mechanics: the gravity assist manoeuvre.

Planet-hopping using Gravity

Since we cannot solely rely on launch velocity, spacecrafts have to gain speed on their journey. The best way to do so is by using planetary bodies in their way.

When a spacecraft approaches the planet, it is pulled inwards and thus its speed is increased: the planet accelerates the spacecraft via its massive gravitational force. Looking from the Sun’s perspective, the spacecraft is then within the planet’s gravitational influence before being released at higher speed once the flyby is over.

gif gravity assist in sun frame

Figure 3: Spacecraft (blue) gravity assist around planet (black) from the Sun’s perspective (Rachelz9999, 2017).

The released spacecraft will thus have gained speed from the planet’s movement around the Sun.

This type of manoeuvre is extremely difficult to set up and realise: if the spacecraft does not go fast enough, it will simply be pulled towards the planet and crash on it; if it goes too fast, it will escape the planet gravitational influence in the wrong direction and be lost in the void of space.

Nonetheless, gravity assists are very effective: Voyager 2 approached Neptune in 1989 at a speed of 20km/s, a value doubled from its 1979 Jupiter flyby thanks to the successful consecutive gravity assists around Jupiter, Saturn and Uranus. You can indeed use gravity assists as much as possible (as long as there is a planet in the way) to reach the desired velocity and direction, effectively planet-hopping until destination. It allowed Voyager 2 to completely escape the Sun’s gravitational field and escape the Solar System entirely in 2018.

Planetary Alignment and Implications

While this technique is cost and fuel efficient as the spacecraft needs less of a velocity push in the beginning, it takes a long time to reach every single planet needed for the gravity assists; and it takes a longer time still to get an adequate planetary alignment for reaching them.

The planetary alignment that Voyager 2 benefited from to visit all gas giants is recurring only every 175 years! It was effectively a stroke of luck that humans had the will, budget and technological capabilities of conceiving and launching a spacecraft at a time when such a favorable alignment occurred. It also explains why Voyager 2 was able to get to Neptune in such a short amount of time: the 3 gravity assists it used to get there were the most optimal ones in almost 200 years.

gif voyager2 trajectory gravity assists

Figure 4: Voyager 2’s Trajectory around the Sun with Gravity Assists (Phoenix7777, 2018).

On the other hand, the current planetary alignment requires Juice to perform 5 gravity assists (1 around the Moon, 1 around Venus, 3 around Earth) to reach its destination Jupiter.

Animation of JUICE around Sun gravity assists

Figure 5: JUICE’s Trajectory around the Sun with Gravity Assists (Phoenix7777, 2022).

It thus explains why it only took Voyager 2 2 years to reach Jupiter, instead of 8 for Juice: our technology may have much improved, but the planetary alignments are less favorable today! Another major difference between the two missions: Voyager 2 was only supposed to flyby Jupiter whereas Juice will enter its orbit to stay. This adds a speed condition that further restricts Juice’s trajectory which translates into a longer travel time.

A Compromised Technique

While distances in space are huge and difficult to get a sense for, they are only one part of the equation when looking at spacecraft travel time.

Spacecrafts must take the most optimal path to reach the velocities necessary to arrive at their destination planet, which involve undertaking gravity assist maneuvers, often around multiple planets. These maneuvers are heavily dependent on planet positions in space and while there exist favorable alignments allowing quick travel, most of the time engineers have no choice but charting a long and lengthy trajectory.

Our technology has indeed much improved since the 1970s in instruments aboard a spacecraft (precise cameras, navigations systems, high power efficiency…) yet we are still bound by the same laws of physics as we always were and are thus highly dependent on planetary positions to travel within our Solar System and beyond.

Image Sources and References

Wikipedia contributors (2024, April 8). Voyager 2. Retrieved April 2024, from Wikipedia: https://en.wikipedia.org/wiki/Voyager_2

Wikipedia contributors (2024, April 1). Jupiter Icy Moons Explorer. Retrieved April 2024, from Wikipedia: https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer

Aeddub~commonswiki (2006, July 20). Solar system distances. Retrieved May 2024, from Wikimedia Commons: https://commons.wikimedia.org/wiki/File:Solar_system_distances.JPG

Durant, F. C. (2024, January 9). Spaceflight. Retrieved April 2024, from Encyclopedia Britannica: https://www.britannica.com/science/spaceflight

Wikipedia contributors, (2024, January 2024). Gravity assist. Retrieved April 2024, from Wikipedia: https://en.wikipedia.org/wiki/Gravity_assist

Rachelz9999. (2017, January 1). Gravity assist. Retrieved May 2024, from Wikipedia: https://en.wikipedia.org/wiki/Gravity_assist#/media/File:GravAssis.gif

Phoenix7777. (2018, July 7). Gravity assist. Retrieved April 2024, from Wikipedia: https://en.wikipedia.org/wiki/Gravity_assist#/media/File:Animation_of_Voyager_2_trajectory.gif

Phoenix7777. (2022, April 19). Animation of JUICE around Sun. Retrieved April 2024, from Wikipedia: https://en.wikipedia.org/wiki/Jupiter_Icy_Moons_Explorer#/media/File:Animation_of_JUICE_around_Sun.gif

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