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Daring gravity assist maneuvers of past space missions

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With great science comes great engineering

Gravity assists are a powerful technique which can help both extend the duration of a space mission and reduce its overall cost. This article will have a look at some of the most insane gravity assist techniques used in past space missions. If you’re not familiar with the concept of gravity assists, here is a simple explainer.

Studying the Sun from above

1990 saw the launch of joint NASA-ESA mission called Ulysses to study the poles of the Sun for the first time. To study the solar poles, the spacecraft would have to leave the orbital plane of the Solar System and achieve a high inclination orbit.

Artist’s impression of the Ulysses spacecraft. Source: NASA JPL

The only problem? No rocket in existence was (and is) capable of doing that. The solution lay in the largest planet of the Solar System — mighty Jupiter.

Stealing from a planet’s high momentum had been an efficient and established way to achieve high velocities required to achieve mission targets. The Voyager mission had previously utilized this technique to leave the Solar System.

Much like how a change in velocity can be achieved by doing a gravity assist around a planet, a change in inclination is also possible. In 1992, Ulysses performed a gravity assist flyby around Jupiter whose intense gravity bent the spacecraft’s trajectory southward. This put the spacecraft in a final orbit around the Sun 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. The Ulysses spacecraft had to achieve a ‘first of its kind’ polar orbit around the Sun. Source: tes teach

The Ulysses spacecraft went on to have a incredibly successful and long lasting 18-year mission. Among various revelations about the solar poles, the one that stood out most was the discovery that the south pole of the Sun is colder than its north pole!

Artist’s impression of Ulysses passing through the tail of comet Hyakutake in May 1996. Source: NASA JPL

Apart from studying the solar poles, lots of interesting science came out of the mission like first direct measurements of interstellar dust particles and interstellar helium atoms in the Solar System. Every now and then, Ulysses also unexpectedly found itself in tails of comets, achieving bonus science points.

The “Billion Euro gamble” of the comet chasing probe

The Rosetta spacecraft — first to orbit a comet — studied the comet 67P rigorously. Rosetta monitored the comet as it approached its closest point to the Sun and became active. Have a look at the elegant two-lobe shaped comet.

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

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Getting to the comet in the first place however was an ordeal. To rendezvous with the comet, Rosetta had to match the comet’s velocity. This was achieved using gravity assists around multiple objects in the inner Solar System.

Rosetta’s 10-year journey through the Solar System involved multiple flybys, including one around Mars. Source: ESA

As a part of the mission, Rosetta had to do a low-altitude flyby of Mars in 2007 to correct its trajectory. It was a risky maneuver as the estimated altitude of the flyby was merely 250 km. During the maneuver, Rosetta would be in Mars’ shadow and it would not be possible to use solar panels. It would be a time where the spacecraft would face a critical power shortage.

The spacecraft was put in standby mode and communications were turned off to save power. The batteries had to power the spacecraft during the maneuver but the risk was that the batteries were originally not designed for the task.

Ultimately, the Mars maneuver was successful and the spacecraft flew onward to its elusive target. This sweat-breaking maneuver came to be popularly known as “The Billion Euro gamble”. And we got a pretty Mars+Rosetta picture.

Stunning image of Rosetta spacecraft above Mars taken by the Philae lander camera onboard Rosetta. Source: ESA
Side note: The Mission Control Systems Expert at TeamIndus — Mohini Parameswaran — has been a part of the Rosetta team. She worked with ISRO and ESOC (European Space Operations Center) on numerous space missions and we are privileged to have her work with us on our mission to the Moon.

Using radiation pressure from the Sun to perform a flyby

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

Artist’s impression of MESSENGER orbiting Mercury. Source: Wikipedia

Since Mercury lies deep in the Sun’s gravitational well, a direct trajectory of the spacecraft from Earth to Mercury will be constantly accelerated by the Sun. The MESSENGER spacecraft would thus reach Mercury with a velocity that is too high to achieve orbit without excessive use of fuel, which is limited onboard. Mercury also lacks a significant atmosphere like Venus/Earth so aerobraking cannot be used to slow down the spacecraft either.

Gravity assists were thus extensively used to slow down MESSENGER enough to enable orbital capture by Mercury. The gravity assist flybys included one using Earth, two using Venus and three using Mercury to slow down the spacecraft.

A key element of MESSENGER’s successful trajectory was achieving the proper gravity assist from each planetary flyby. It was critical to have high precision in each flyby otherwise orbit insertion could fail or would require trajectory changes that increased mission complexity, cost and risk.

The three Mercury flybys would take the spacecraft to ~200 kilometers of the planet. Flying too low could crash the spacecraft into the planet and flying too high would make the spacecraft consume excess fuel. Either way, getting off target could affect the mission in critical ways.

The solution? Use radiation pressure from the Sun to navigate the spacecraft a.k.a. solar sailing!

Artist’s impression of the Japanese spacecraft IKAROS. It is accelerated using the Sun’s radiation pressure. Source: Wikipedia

Solar Radiation Pressure (SRP) around Earth is not intense enough to navigate a spacecraft in a significant way. However, Mercury lies much closer to the Sun and the radiation pressure increases by the square of the distance closer to the Sun. So the SRP around Mercury is ~10x more than around Earth. This makes using SRP to navigate MESSENGER a viable option.

MESSENGER’s solar panels were tilted in a manner that the SRP can hit the spacecraft and slow it down just enough to achieve accurately targeted Mercury flybys. By using solar sailing, the MESSENGER team was able to eliminate use of fuel in all the Mercury flybys without sacrificing accuracy. It also thus increased the mission lifetime!

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

This was the first time a spacecraft had successfully used solar sailing as a propulsion-free trajectory control method for planetary flybys. If we lived in the Star Trek era, the MESSENGER team would be the coolest guys to hang out with.

For those interested in the nerdy details, the team has published a paper on how the flyby targeting using SRP was achieved:

SolarSailingAPL2011.Final.doc - OShaughnessy.LCPMC.2011.pdf

Conclusion

These crazy maneuvers show the ingenuity of human science, engineering and creativity. While there are many more missions with insane orbits (NASA’s Juno comes to mind), the ones highlighted here are some of the less popular ones with respect to their maneuvers. Do you know of more such intriguing orbital maneuvers? Comment below and let us know!

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