Why do deep space probes circle before reaching their destination?

According to foreign media reports, the BepiColombo Mercury probe jointly developed by the European Space Agency and Japan will fly over Mercury again in June 2023. The probe, which was launched in October 2018, will not finally enter orbit around Mercury until December 2025. For more than seven years, BepiColombo has to pass by Earth, Venus and Mercury from time to time. So why do deep space probes have to keep passing by other planets to reach their destination instead of flying directly to the target?

Schematic diagram of the BepiColombe Mercury probe

Weak power, running in circles

Mercury's average orbital radius is about 57.9 million kilometers, while Earth's orbital radius is about 150 million kilometers. However, BepiColombo cannot fly directly from Earth to Mercury, but has to fly around the solar system many times. According to the plan, since its launch in October 2018, the probe will fly over Earth once, Venus twice, and Mercury six times before entering orbit around Mercury.

Schematic diagram of the flight trajectory of the BepiColombo Mercury probe

As we all know, the shortest distance between two places is in a straight line, so BepiColombo has to spend a lot of time circling.

Both the Earth and Mercury revolve around the Sun. Flying from the Earth to Mercury is similar to a satellite changing its orbit from a high Earth orbit to a low Earth orbit, which is a deceleration process. The speed of a planet's revolution is much faster than the speed of a satellite orbiting the Earth, so it is more difficult to lower the orbit. If BepiColombo is directly launched into an elliptical orbit around the Sun with a perihelion of 57.9 million kilometers and an aphelion of 150 million kilometers, it will not only need to launch the probe into a hyperbolic orbit that escapes the Earth's gravity, but also provide the probe with a residual speed of more than 7 kilometers per second.

BepiColombo was launched using an Ariane 5ECA rocket. After escaping the Earth's gravitational field, the remaining speed was only 3.475 km/s, far below the required speed of 7 km/s. If the probe's engine is used to change its orbit, a large amount of fuel will be consumed. Therefore, the most practical way is to use the gravity of a large planet to help the probe change its orbit.

Gravity assists the probe to change its orbit. Although the relative speed between the probe and the planet does not change, the direction of the probe's speed changes, creating an effect just like the probe being bounced off the planet.

Depending on the orbit design, after the probe is ejected by the planet, its speed relative to the sun increases or decreases, thus achieving the goal of changing the orbit without fuel. The effect of gravity-assisted probe orbit change is to exchange time and space for speed. In the final analysis, the probe's orbit change by circling with the help of the planet's gravity is a helpless choice caused by insufficient power.

Gravitational orbit change has high value

Looking back at the history of deep space exploration, we can find that gravity-assisted probe trajectory changes play an irreplaceable role in many missions.

Generally speaking, probes departing from Earth to the Moon, as well as nearby Venus and Mars, do not need gravity-assisted orbit change, but more distant celestial bodies such as Jupiter may need gravity to accelerate. For example, the US Juno probe performed an Earth flyby and accelerated to more than 3.9 kilometers per second before heading to Jupiter.

If the 3.9 km/s acceleration brought by the Earth's gravity was missing, and only the probe's engine was used to provide the same acceleration, the Juno probe's dry weight would be reduced to a fraction of its current level, and the detection payload it could carry would be very small.

From the United States' "Mariner 10" to "MESSENGER" and then to "BepiColombo", humanity's only three Mercury probes have all used gravity-assisted trajectory changes, which is the best option given the limitations of rocket and probe propulsion systems.

For Mercury or Jupiter probes, although gravity-assisted orbit change has huge benefits, it does not mean that they cannot fly directly to their destination. The American Voyager probe flew directly to Jupiter after launch, and the later New Horizons probe did the same.

Although humans do not have a probe that can fly directly into Mercury's orbit so far, the Parker Solar Probe launched by the United States, after entering orbit, has its perihelion inside the orbit of Mercury.

For some other deep space probes, gravity-assisted orbit change is an indispensable option. For example, the famous Ulysses solar probe was launched and, with the help of Jupiter's gravity, its orbital plane changed to an orbit with an angle of 80.2 degrees to the ecliptic plane, allowing humans to see the sun's north and south poles for the first time.

New impetus brings new opportunities

Using the gravity of a large planet to assist in changing orbits is currently the preferred option for humans to carry out deep space exploration with high acceleration requirements. Compared with existing chemical energy propulsion systems, planets orbit the sun very quickly, and the speed increment required for changing orbits is very large, and the acceleration requirement for changing the orbital inclination is also high. Therefore, gravity-assisted probe orbit change is the best realistic way.

In order to better enable the probe to reach its destination quickly, humans have developed and proposed a number of advanced high-specific impulse engine concepts, providing an opportunity to get rid of the gravity-assisted trajectory change of "trading time for speed".

The most practical new propulsion system is the high specific impulse electric propulsion engine. At present, Hall electric propulsion or ion electric propulsion can achieve a high specific impulse of 3-4000 seconds. Although the thrust of the existing electric propulsion engine is still very small, it has shown a bright application prospect in actual use.

The United States' Dawn asteroid probe uses three NEXT electric thrust engines, which provide the probe with a total speed increase of more than 10 kilometers per second during its 11-year lifespan. Such acceleration capability is beyond the reach of probes using traditional engines.

Schematic diagram of the Dawn asteroid probe

However, Dawn also passed the gravity acceleration of Mars and headed to the asteroid belt at a faster speed. In the future, with larger electric propulsion engines, the need for gravity-assisted orbit change for deep space probes will be greatly reduced.

At present, the United States, Russia and other countries are developing high-thrust electric propulsion engines with thrust of several Newtons. Some American companies are also developing a new concept of variable specific impulse magnetoplasma rocket engine (VASIMR).

The United States once had a bold idea of ​​using high-power electric propulsion engines to send astronauts to land on Jupiter's moons. Thanks to the high specific impulse and high thrust of the electric propulsion engines, there is no need to rely on the gravity of the Earth or Mars to change orbits when designing the orbit, allowing the probe to fly to Jupiter faster.

The VASIMR engine not only has a specific impulse of 3,000-50,000 seconds, but also has an easier way to amplify its thrust. If a 200,000-kilowatt super VASIMR engine is used, astronauts can reach Mars in as little as 39 days.

If the VASIMR engine becomes practical in the future, solar polar probes like Ulysses, or even interstellar probes flying outside the solar system, will not need to rely on the gravity of large planets to change their orbits, but can rely on "brute force" to fly directly to their destinations. (Author: Zhang Xuesong)