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Space travel began as a struggle against gravity, a bid to overcome this fundamental law of the universe. Scientists kept building bigger and faster rockets so they could overcome the pull of the Earth and voyage into the vast unknown. But it soon became apparent that gravity is one of cosmic exploration’s greatest accomplices. For more than 50 years, this elemental force has been helping slingshot spacecraft around the solar system and extending the ambitions of space missions.
On January 8 this year, the BepiColombo probe executed its sixth and final “gravitational slingshot” around the planet Mercury, bringing the probe to within 100 miles of the surface. It was the lowest planetary fly-by ever, allowing the probe’s cameras to photograph the mysterious planet in detail never before possible, revealing vast lava plains and craters in permanent shadow near its north pole that may be among the coldest places in the solar system. The force of that slingshot will now propel the two orbiters on the probe toward their final stable orbits around the planet.
The BepiColombo probe is named after the late Italian mathematician and engineer Guiseppe “Bepi” Colombo, who first proposed using gravity as a slingshot for spacecraft in 1974. At the time, a robotic NASA probe named the Mariner 10 was set to explore both Venus and Mercury, the first time a space probe would explore two planets in one mission. Colombo calculated how a single slingshot around Venus could bend the rocket’s trajectory, enabling it to fly past Mercury multiple times while using hardly any rocket thruster fuel. The rocket ultimately made three such flybys, the nearest only 200 miles above the surface, providing our first up-close looks at Mercury.
Gravity assists are sometimes likened to skipping a stone across the surface of a pond.
The gravitational slingshot, also known as a gravity assist maneuver, allows a space probe to radically change its speed and direction. Navigators here on Earth can work out exactly where the probe needs to be and when to take advantage of a planet’s gravitational pull. If a space probe approaches a planet from behind as the planet travels along its orbital path, for example, the planet’s gravity will grab hold and accelerate it; but a probe approaching in the opposite direction will lose relative speed. Gravity obeys strict laws and the exact locations of the planets are well known, and so these distant navigators can calculate precisely when to fire the probe’s rocket thrusters, in short bursts known as “burns,” to guide it through the gravitational slingshot and ensure it is thrown off into space on the next leg of its long journey—a maneuver sometimes likened to skipping a stone across the surface of a pond.
Gravity assists performed at low altitudes above the planet—a few hundred miles, say—can impart much greater changes to a probe’s trajectory than gravity assists at extreme distances, sometimes hundreds of thousands of miles away. And because the planets orbit the sun in a constant and predictable celestial dance, space navigators can arrange a whole series of slingshots around different orbiting bodies to carefully orient a probe’s trajectory.
Exploiting gravity like this is now a regular feature of space travel, and has been used by many space probe missions—including Pioneer 11, the Voyagers, and the New Horizons probe to Pluto—to preserve valuable rocket thruster fuel as they explore the solar system. In fact, only a few recent space missions have not relied on gravity assists—mostly ones that aim to measure aspects of the sun itself, which needed no such assistance, or ones that travel to the Lagrange point of gravitational stability between the sun and the Earth, a location about a million miles away that serves as a low-maintenance hangout for many robotic solar observatories.
When a space probe relies on a gravity slingshot rather than on rocket thrusters, it can carry far less fuel to achieve the same changes in trajectory; and carrying less fuel means a probe can be larger and schlepp more scientific instruments. The two orbiters on the BepiColombo probe, for example, have a combined weight of a little more than four tons, which makes it one of the heaviest probes ever sent to space.
Spacecraft engineer Ignacio Clerigo, the European Space Agency operations manager for the BepiColombo mission, says BepiColombo’s path through space is one of the most complicated ever attempted, and testifies to the importance of gravitational slingshots for making it possible. Precise calculations for the trajectory had to be quickly redone last year, when the probe’s thrusters failed to produce full power; but BepiColombo has successfully completed two slingshots around Mercury since then, he says.
Although its target is Mercury in the inner solar system, an average of just 35 million miles from the sun, BepiColombo’s voyage has been at least as complicated as a space probe voyage to an outer planet, like Jupiter, almost 500 million miles away—and maybe more so. Mercury’s accelerated orbit around the sun means critical events occur much more quickly: The probe’s last two slingshots around Mercury, for example, occurred within just a few weeks of each other, while space probe slingshots around outer planets typically happen less than once a year, he says.
Physicist Roberto Peron, an expert in gravitation physics at Italy’s National Institute of Astrophysics, explains that slingshots can significantly change both a spacecraft’s direction and speed—something otherwise only possible with fuel-hungry thrusters. But relying on gravity slingshots takes a lot of time, he says: BepiColombo, for example, launched to Mercury six years ago and it still has almost two years of interplanetary journey to go. The probe could have used a direct transfer orbit, with rocket thrusters, to cover the same distance in less than two months.
Nevertheless, Peron expects gravitational slingshots to continue to be widely used in the years to come. Once the greatest enemy of space travel, gravity has become a cherished friend. 
Lead images from ESA/JAXA BepiColombo mission. Left to right; Mercury’s shadowy north pole revealed by M-CAM 1, Mercury’s sunlit north viewed by M-CAM 1, Lava and debris brighten Mercury’s surface.
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