Traveling the vast distances of space isn’t cheap for a spacecraft. It costs time, fuel and money. Fortunately, nature offers free help along the way which mission designers are happy to take. Enter gravity assists.

In these maneuvers, a spacecraft exchanges momentum in a close encounter with a planet to gain or reduce velocity, as required by the mission. To save cost and time or even make up for lack of technological abilities, gravity assists have been used in numerous interplanetary missions to propel spacecraft towards their destinations. This includes using Jupiter’s gravity to slingshot the twin Voyager spacecraft out of the solar system!

This article takes a look at some of the most insane and lesser talked about gravity assist maneuvers in past space missions. To get more familiar with the ingenious technique of gravity assists, read my explainer article with plenty of real mission examples.

Studying the Sun from above

In 1990, NASA and ESA launched a joint mission called Ulysses to get the first ever look at our Sun’s poles. For that, it would have to go out of the orbital plane of the solar system, the plane in which planets go around the Sun.

Artist’s impression of the Ulysses spacecraft looking over at the Sun’s poles. Credit: NASA JPL

The only problem? No rocket in existence was, or is, capable of giving the required high inclination orbit. The solution lay in the solar system’s most massive planet — Jupiter.

Much like how a close encounter with a planet can change a spacecraft’s velocity, as in the case of Voyagers, it can also alter the spacecraft’s inclination. In 1992, Ulysses passed closely over Jupiter’s north pole and the planet’s intense gravity bent the spacecraft’s trajectory southward. This put Ulysses in a solar orbit that would take it past the Sun’s north and south poles at an inclination of 80° w.r.t the orbital plane of the planets.

Visual showing the orbital plane of the planets of the solar system in orange. The Ulysses spacecraft achieved a first-of-a-kind polar orbit around the Sun. Credit: tes teach

Note that Ulysses didn’t literally “see” the Sun’s poles because it didn’t have an optical camera but it had other instruments which took various scientific measurements. Ulysses turned out to be an incredibly successful and long lasting mission spanning 18 years. Among various revelations about the solar poles, a standout discovery was that the Sun’s south pole is colder than its north pole!

Ulysses also gave us first direct measurements of interstellar dust particles and of interstellar helium atoms in the solar system. The spacecraft also unexpectedly found itself in gaseous tails of comets every now and then, achieving bonus science points.

The “Billion Euro gamble” of the comet chasing probe

ESA’s Rosetta spacecraft — the first to orbit a comet — extensively studied and monitored the elegant two-lobed comet named 67P.

Early activity of comet 67P as imaged by the Rosetta spacecraft between January 31 to March 25, 2015. Credit: ESA

However, getting to comet 67P in the first place was an ordeal. To rendezvous with the comet, Rosetta had to match its velocity. This was achieved using gravity assists around multiple solar system objects over 10 years.

Rosetta’s 10-year journey through the solar system involved multiple flybys, including one around Mars. Credit: ESA

One of those gravity assists involved a low-altitude Mars flyby in 2007. It was a risky maneuver as the spacecraft passed by Mars by just 250 km. During the maneuver, Rosetta would be in Mars’ shadow, having to rely on the limited battery supply instead of solar panels. The risk was that the batteries weren’t designed for the task. The spacecraft was put in standby mode and communications were turned off to save power.

Ultimately, the Mars flyby proved successful and the spacecraft flew onward to its elusive target. This sweat-breaking maneuver came to be popularly known as “The Billion Euro gamble”. But hey, we did get a pretty picture of Mars and Rosetta.

Image of Rosetta spacecraft above Mars as taken by the Philae lander camera onboard Rosetta. Credit: ESA

Using the Sun to sail

NASA’s MESSENGER spacecraft was the first ever to orbit Mercury. It was not without its challenges.

Since Mercury lies fairly close to the Sun, any spacecraft on a direct trajectory to the planet will be accelerated by the Sun. MESSENGER would thus reach Mercury with too high a velocity to achieve orbit without use of excessive fuel, which is limited onboard. The spacecraft cannot use aerobraking either since Mercury lacks a significant atmosphere, like Venus or Earth have.

Gravity assists were again a savior. MESSENGER performed multiple reverse gravity assists – one around Earth, two around Venus and three around Mercury – to slow itself down and ultimately allow orbital capture by Mercury. But it was critical to have high precision in all the flybys.

Trajectory of MESSENGER spacecraft from launch to orbit insertion around Mercury. Credit: APL

Flying too close could crash the spacecraft into a planet and flying too high would mean excess use of fuel to achieve the goal. Either way, getting off target could affect the mission in critical ways.

The solution engineers came up with was to use the Sun’s radiation pressure. Much like how wind can help a boat navigate, solar radiation can help a spacecraft maneuver.

Solar radiation at Earth is not intense enough to exert enough pressure and move a spacecraft in a significant way. However, Mercury is much closer. Since radiation pressure increases by the inverse square law, it is about seven times more powerful at Mercury than Earth. This made solar sailing a viable option for MESSENGER.

When passing by Mercury in all the flybys, engineers sent commands to MESSENGER to align its solar panels in a manner that the radiation pressure can slow the spacecraft down. By solar sailing at the right angles, the MESSENGER team eliminated use of fuel in all the Mercury flybys without sacrificing accuracy. Reserved fuel also meant increased mission lifetime and more of the fantastic science at Mercury!

Artist’s impression of NASA’s MESSENGER spacecraft orbiting Mercury. Its solar panels were titled for navigating using solar radiation pressure. Credit: NASA

This was the first time a spacecraft had successfully used solar sailing as a propulsion-free trajectory control method for planetary flybys. For those interested in the nerdy details, the MESSENGER team published a paper on the maneuvers.

MESSENGER image from the a Mercury flyby showing lava-flooded craters and large expanses of smooth volcanic plains on the planet. Credit: NASA

To wrap up, space mission designers have used ingenious gravity assist maneuvers to see over the Sun, chase a comet and solar sail! While there are many more missions with crazy trajectories, the ones highlighted here are some of the less popular ones. Do you know of more such intriguing orbital maneuvers? Comment below!