Researchers from Khalifa University are investigating ways to reduce the cost of sending spacecraft to the Solar System’s outer planets. They use deep-space electric propulsion to decrease the cost of entering Saturn’s orbit, which will pave the way for new opportunities to explore Saturn and other outer planets with significantly less fuel.
Arguably one of the most captivating of our solar system’s planets, Saturn is a massive ball made mostly of hydrogen and helium, adorned with a system of icy rings and surrounded by more than 60 known moons. It is home to some of the most fascinating landscapes in the solar system, many of which we are still exploring, and is a rich source of scientific discovery and mystery.
To investigate the outer planets, spacecraft need to get there. The spacecraft must travel through interplanetary space to its target planet, and then decelerate relative to that planet using an orbit insertion rocket or some other means.
Dr. Elena Fantino, Assistant Professor of Aerospace Engineering at Khalifa University, and Dr. Robert Flores, Research Scientist, have investigated deep-space electric propulsion in a paper for the Journal of Guidance, Control and Dynamics to dramatically reduce the excess speed of a spacecraft arriving at Saturn. Their interplanetary trajectory includes a gravity assist at Jupiter, combined with low-thrust manoeuvres to allow the spacecraft to slow down enough to be ‘captured’ by Saturn’s gravitational pull.
“The giant planets have a special place in our quest for learning about the origins of our planetary system and our search for life, and robotic missions are essential tools for this scientific goal,” explained Dr. Fantino.
This work uses deep-space electric propulsion to decrease the cost of entering Saturn’s orbit, which will pave the way to new opportunities to explore Saturn and the other outer planets with significantly reduced amounts of propellant.
Planning for any mission to the outer planets needs careful consideration of mass and cost. Four spacecraft have visited Saturn so far, with Pioneer 11, Voyager 1 and Voyager 2, each providing valuable flyby insights on the planet. In 2004, the Cassini mission arrived in orbit and studied Saturn from its orbit for 13 years before it was plunged into the planet’s atmosphere in 2017. Studies are now underway to launch the Titan Saturn System Mission as a joint ESA-NASA project.
“Missions to the outer planets have been prioritized by both NASA and ESA, including orbiter missions to Uranus and Neptune,” said Dr. Fantino. “In this case, the amount of propellant required to decelerate and be captured by the planets’ gravity on arrival is very large, and the support of techniques like aerobraking and aerocapture is being explored.”
The Cassini mission travelled to Saturn using a VVEJGA (Venus-Venus-Earth-Jupiter Gravity Assist) trajectory: it executed two consecutive gravity assists with Venus, one with Earth, one with Jupiter and used midcourse manoeuvres.
A gravity assist involves a spacecraft’s approach carefully timed so that it passes by the planet in its orbit around the sun. A gravity assist at Jupiter has a spacecraft come into Jupiter’s gravitational influence, fall towards Jupiter, and then change its speed assisted by the motion of the gravitating planet as it pulls on the spacecraft. This is also known as a gravitational slingshot and is used to reduce expense and save on propellant.
The Cassini spacecraft entered Saturn’s orbit with approximately 800kg of liquid propellant. Bringing Cassini into an orbit close enough for observations consumed another 314kg, while deep space maneuvers and course corrections before orbit insertion required another 1000kg of propellant.
“Clearly, the impact of these operations on the size and cost of the mission was considerable,” explained Dr. Fantino. “One alternative to reducing the cost of exploring the giant planets is to use an electrodynamic tether, which can produce a significant thrust to assist in orbit insertion.”
Electrodynamic tethers (ET) are long conducting wires which can operate on electromagnetic principles as generators, by converting their kinetic energy to electrical energy, or as motors, converting electrical energy to kinetic energy. Electric potential is generated across a conductive tether by its motion through a planet’s magnetic field.
The ET concept paves the way towards missions to explore Saturn and its moons with spacecraft masses below one ton—for comparison, the Cassini launch mass was 5600kg, or just over five tons.
“We also explored the possibility of using the electric propulsion system to reduce the high cost of a direct Earth-to-Jupiter transfer,” added Dr. Fantino. “This is an unusual choice. Schemes involving Earth and Venus gravity assists are much more common because they help to reduce the hyperbolic excess speed of the spacecraft. We selected a direct transfer from Earth to Jupiter for its simplicity but our findings are applicable to any VVEJGA (Venus, Earth, Jupiter, Gravity Assist) trajectory.”
While the use of gravity assists reduces fuel consumption, orbit insertion required a huge amount of fuel for Cassini. If the spacecraft is not slowed on its approach to its target planet, the hyperbolic excess velocity will carry the spacecraft beyond its target as its speed will be too great for the gravitational pull of the planet to bring it into orbit. It will simply speed past. Shedding excess velocity is typically achieved by an orbit insertion burn, which requires fuel.
Dr. Fantino’s method substantially decreases the hyperbolic excess speed of a spacecraft approaching Saturn to facilitate gravitational capture. The interplanetary trajectory includes a gravity assist at Jupiter, combined with low-thrust manoeuvres. While the trajectory between Jupiter and Saturn requires a long transfer time of eight years, the reduced excess velocity at Saturn means a significantly decreased insertion impulse needed to achieve the same initial orbit as the Cassini mission.
“The reduced impulse opens the door for more efficient braking methods, such as electrodynamic tethers, or even direct capture by means of a Titan flyby,” explained Dr. Fantino. “The reduced excess velocity comes at the cost of a long Jupiter-to-Saturn transfer time because a moderate eccentricity trajectory tangent to Saturn’s orbit is required.”
Titan is the largest moon of Saturn. It is massive enough to deviate the path of a spacecraft and even to convert its trajectory relative to Saturn from hyperbolic to elliptical. In other words, a gravity assist with Titan may be sufficient to accomplish orbit insertion and no further adjustment burn would be required. Arriving at Saturn with a low hyperbolic excess speed (like in this work) increases the effectiveness of a flyby with Titan. In other words, if the spacecraft approaches Saturn at a low speed and then passes by Titan, the deviation caused by the moon is bigger.
The strategy designed by Dr. Fantino can be applied to new missions to Saturn, such as those proposed by NASA. These missions can benefit from reduced excess hyperbolic velocity, which would enable a new, inexpensive and more flexible category of missions to Saturn. The results can even be applied further afield to missions to the outermost planets of Uranus and Neptune.
News and Features Writer
29 March 2020