Past, Present and Future Missions

The importance of gravity assist manoeuvres is testified by the high number of missions which have made use of this enabling technology in the history of interplanetary space exploration. This section contains an overview of the past and current interplanetary missions that exploited one or more gravity assist. In addition, two very ambitious MGA missions, currently under study, will be presented.

The Mariner 10 probe [9] was the first spacecraft to use the gravitational slingshot effect to reach another planet. In fact, one of the objectives of this NASA mission was to prove the possibility of using gravity assist manoeuvres. The probe swung by Venus on February 5, 1974, on its way to becoming the first spacecraft to explore Mercury.

Pioneer 11 [8] was the second mission of the Pioneer program to investigate Jupiter and the outer solar system (after its sister probe Pioneer 10) and the first to explore Saturn and its main rings. Launched in 1973, Pioneer 11 used Jupiter’s mass in a gravity assist to get the energy to reach Saturn. The spacecraft made a successful swing-by of Saturn and then followed an escape trajectory from the solar system. The interplanetary trajectories of Pioneer 10 and 11 are shown in Fig. 1.1.

After the success of Mariner and Pioneer, NASA decided to continue the interplanetary exploration programme with the Voyager 1 and Voyager 2 probes [19], with the ambitious aim of including MGAs in the same mission. Although they were originally designated to study just Jupiter and Saturn, the two probes were able to continue their mission into the outer solar system. Both probes were injected into a direct transfer to Jupiter in 1977. Voyager 2 was launched first, and encountered Jupiter, Saturn, Uranus and Neptune (sequence EJSUN). Voyager 1 was instead launched on a faster trajectory, which enabled it to reach Jupiter and Saturn sooner at the consequence of not visiting the outer planets. Both spacecraft

are currently on course to eventually exit the solar system, and Voyager 1 is currently the human made object farthest from Earth (see Fig. 1.2).

The two Voyager missions were made possible by a very favourable alignment of the outer planets (Jupiter, Saturn, Uranus and Neptune), which happened from 1970 to 1990: a similar alignment will not occur again until the middle of the 22nd century. This is the reason why the trajectories of the two probes were named

“Grand Tour” [20]. In fact, the main design constraint of a MGA trajectory is to have all the celestial bodies in the right place at the right time. This is known as the

“phasing problem”, and, throughout this work, we will see how to address it and when to neglect it.

Fig. 1.1. Pioneer 10 and 11 trajectories (credit: NASA).

Fig. 1.2. Voyager 1 and Voyager 2 trajectories (credit: NASA).

The Galileo [21] spacecraft was originally designed to be injected into a direct transfer to Jupiter from the space shuttle. New safety protocols introduced as a result of the Challenger accident, in 1986, forced Galileo to use a lower-powered upper stage booster rocket, which could not provide the necessary energy. The trajectory of Galileo was re-designed, including a swing-by of Venus and two swing-bys of the Earth, allowing the probe to reach Jupiter in 1995. After injection, the spacecraft travelled around Jupiter in elongated elliptical orbits, designed for close up fly-bys of Jupiter's largest moons. The successful mission was even extended to perform a number of fly-bys of Europa and Io.

The trajectory of Cassini-Huygens [22-24] is the most complex MGA trajectory designed for a mission to an outer planet. The aim of the mission was to study Saturn and its complicated planetary system of satellites. The mass of the spacecraft at launch was an impressive 5600 kg, of which more than 3000 kg of propellant, needed for deep space manoeuvring and for the final injection around Saturn. Due to the high mass budget of the spacecraft, the launch hyperbolic escape velocity was very low (below 4 km/s). Therefore a complex MGA trajectory was designed: the spacecraft was launched in 1997 and entered into orbit around Saturn in 2004, exploiting two gravity assists of Venus, one of the Earth, and one of Jupiter (sequence EVVEJS, Fig. 1.3). The Cassini probe, once orbiting around Saturn, made also use of a huge number of swing-bys of several moons to modify its orbit in several ways, such to study the Saturn system in the most complete way possible.

Fig. 1.3. Cassini trajectory (credit: ESA/NASA).

The second mission to Mercury, NASA’s MESSENGER [25, 26], launched in 2004, also made use of a swing-by of the Earth and two additional swing-bys of Venus to reach its target. It also performed three swing-bys of Mercury to lower its relative velocity and ease the orbital insertion manoeuvre, which is scheduled for 2011. The complete planetary sequence is then EEVVMeMeMeMe (see Fig. 1.4).

For this mission, the gravity assist manoeuvres were used to both change the in-plane velocity components of the spacecraft (energy change), and to change the

orbital inclination, as the orbit of Mercury has an inclination of about 7 degrees over the ecliptic.

Fig. 1.4. MESSENGER trajectory (from [25]).

The European Space Agency (ESA) Rosetta [27] probe was launched in 2004, with the scope of rendezvousing the comet 67P/Churyumov-Gerasimenko, and releasing a lander, Philae, on it. Similar to Cassini, this mission exploited a rather complicated sequence of gravity assists perform the rendezvous with minimum propellant mass. Rosetta used an Earth swing-by, a Mars swing-by, followed by two more Earth swing-bys. The rendezvous with the comet is scheduled for 2014 (sequence EEMEECo, Fig. 1.5).

A fast trajectory (Fig. 1.6), exploiting only one gravity assist (of Jupiter), was designed, instead, for the NASA New Horizons [28] mission to Pluto and the Kuiper belt. The spacecraft was launched in 2006, and the Jupiter swing-by happened in 2007. Despite the fast hyperbolic heliocentric trajectory, the arrival at Pluto is expected to occur in 2015, and in the following years the probe will cross the Kuiper belt.

The missions described so far performed gravity assist manoeuvres to reach destinations with a small orbital inclination difference with respect to the ecliptic.

The gravity manoeuvres were used to change mainly the energy of the orbit. Other missions, instead, used the gravitational slingshots to change the orbital plane. The space probe Ulysses [29], designed by NASA and ESA to study the solar poles and launched in 1990, exploited a single swing-by of Jupiter, in 1992, to increase the inclination to the ecliptic by 80.2 deg.

Other MGA missions are currently under investigation by the major space agencies.

Fig. 1.5. Rosetta trajectory (credit: http://www.enterprisemission.com).

Fig. 1.6. New Horizons interplanetary trajectory (from [28]).

BepiColombo [30-34] is the ESA cornerstone mission to Mercury, in partnership with JAXA (Japan Aerospace Exploration Agency). Several options have been studied in the last 10 years. All of them include one or more swing-bys of the Earth, Venus and Mercury. Some include also one or more swing-bys of the Moon. An example showing a MoEVVMeMeMeMe sequence can be seen in Fig.

1.7. Two orbiters and a transfer module, consisting of electric propulsion and

chemical propulsion units, will be launched as a single composite spacecraft. The expected launch date is 2013, and arrival at Mercury is in 2019.

The total transfer time is about six years, two of which are spent before the first encounter of Mercury. As a comparison, a direct Hohmann transfer to Mercury takes only 105 days. A long mission time, and the need for an accurate targeting of the celestial bodies during swing-by manoeuvres demand for long and precise operations. Therefore, the gain in propellant offered by gravity assist manoeuvres comes at the cost of a higher operation cost. A trade-off between mission time and propellant mass is always required. In this sense, designing an MGA trajectory is analogous to solving a multiple objective optimisation problem, in which the two objectives are partially conflicting. In this work, it will be shown how to take into account the two merit functions.

ESA, NASA and JAXA are also studying a mission to Jupiter and the Jovian moon system. The mission is currently known as Europa Jupiter System Mission (EJSM) or Laplace [35]. With the ambitious goal of determining whether the Jupiter system harbours habitable worlds, the mission consists of two separate spacecraft, possibly to be launched in 2020. Both spacecraft will use Venus, Earth, Earth gravity assists to reach Jupiter (sequence EVEEJ). The mission will then continue around Jupiter with multi-year tours of the Jovian system, including many flybys of Io, Europa, Ganymede and Callisto. Finally, the two probes will inject around Europa and Ganymede respectively. For a more detailed overview of the phases of this mission, see Section 4.2.

These last two missions will be used as case studies in this work.

Fig. 1.7. One of the trajectories investigated for BepiColombo. Red and green arcs are thrusting arcs. From [32].

x, km

y, km