Trans. JSASS Aerospace Tech. Japan Vol. 10, No. ists28, pp. To_3_1-To_3_5, 2012 Topics The Trajectory Design Strategies for Akatsuki Mission 1) 1) 1) 1) By Chikako HIROSE , Nobuaki ISHII , Takayuki YAMAMOTO , Yasuhiro KAWAKATSU , 2) 2) 3) Chiaki UKAI , Hiroshi TERADA and Masatoshi EBARA 1)Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan 2)NEC Aerospace Systems, Ltd., Tokyo, Japan 3)NEC Corporation, Tokyo, Japan (Received June 17th, 2011) The Venus explorer, Akatsuki, was launch on 20 May 2010. After 200-day journey through the interplanetary transfer orbit, it reached the Venus at the altitude of 550 km on 7 Dec 2010. However, it experienced a trouble of the explorer's propulsion system and was not able to be the Venus orbiter. It now orbits the Sun with the period of 203 days. In this paper, we discuss the trajectory design strategies for Akatsuki mission by introducing the constraints which come from the observation orbit and the spacecraft system. The details of planning and the results of orbital maneuvers are also shown in this paper. Key Words: Akatsuki, Venus, Interplanetary, Trajectory Design 1. Introduction (1) Direct transfer orbit (0.5 revolution about the Sun) in May, Jun 2010 Launch The Venus is a very similar planet as the Earth in its size in Dec 2010 Venus arrival and mass. However, the surroundings are largely different; it (2) 1.5 revolution orbit (1.5 revolutions) is covered by the thick atmosphere of carbon dioxide and in May, Jun 2011 Launch sulfuric acid clouds and it blows 100 meter per second, called in Dec 2012 Venus arrival the super-rotation. The Venus explorer, Akatsuki, carries five kinds of observation equipments in order to study these (3) Earth Swingby Transfer Orbit (1.5 revolutions) atmospheric motion precisely by orbiting around the Venus1). in Jun 2010 Launch Akatsuki was launched by the Japanese launch vehicle in Jun 2011 Earth Swingby H-IIA at 21:58:22 on 20 May 2010 (UTC). After 200-day in Oct 2012 Venus arrival journey through the interplanetary transfer orbit, it reached the Venus at the altitude of 550km at 00:00 on 7 Dec 2010 (UTC). The flight time of case (2) is longer than the one of case (1) However, it experienced a trouble of the explorer's propulsion and the arrival to the Venus will be delayed considerably. The system and the Orbit Maneuvering Engine (OME) cut the fire case (3) needs an additional one year to the Venus and also after 158-second burning time, whereas the retrograde orbital antenna stations abroad for tracking Akatsuki after the Earth maneuver of 718 seconds was necessary to insert to the Venus swingby. As a result, although we prepared three kinds of circular orbit. Currently, Akatsuki orbits the Sun with the period launch windows including backups, we decided to proceed of 203 days. with the case (1) since the probe development was also on schedule. The adopted orbit is shown in Figure 1 and 2, and its 2. The trajectory Design of Akatsuki summary is introduced in Table 1. Earth --> Venus Orbit (2010 launch) 1.5 2.1. The Transfer Orbit A rr iv al Earth The ratio of orbital period of the Earth and the Venus is 1.0 (D ec 2010) approximately 13:8. The revolution period of Venus is 224.7 days; while the Earth completes eight revolutions in eight 0.5 years, the Venus revolves 13 times. Hence, the synodic km) 0.0 8 Sun chances come every eight years. Since the Venus orbit is -0.5 V enus inclined at about three degrees to the ecliptic plane, the best (10 Y_Ec opportunities for launch are in June or in December when the -1.0 orbit crosses each other and the energy for transferring orbit La unc h (June 2010) becomes minimum. We prepared three kinds of orbit whose -1.5 launch windows start from 2010 to 2011. Ecliptic Coordinate -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 X_Ec (108 km ) Fig. 1. The transfer orbit to the Venus (J2000 Ecliptic coordinate). 1 Copyright© 2012 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved. To_3_1 Trans. JSASS Aerospace Tech. Japan Vol. 10, No. ists28 (2012) Escape Orbit from Earth 20, 000 マーカは30分毎 10, 000 0 Y (km) -10, 000 Earth -20, 000 Ecliptic Coordina te -30, 000 -80,000 -60,000 -40,000 -20,000 0 20,000 X (km ) Elliptic Orbit about Venus 80,000 マーカは30 分毎 70,000 60,000 Fig. 3. Ground track of H-IIA Flight 17. Some examples of Second 50,000 engine second ignition and cutoff are shown. 40,000 30,000 20,000 10,000 Venus 0 Y (km) -10,000 Ecliptic Coordinate -20,000 -20,000 0 20,000 40,000 60,000 80,000 X (km) Fig. 2. The Earth-escape and Venus-capture trajectory (J2000 Ecliptic coordinate, dotted every 30 minutes). Table 1. The designed trajectory. Launch Fig. 4. Excess velocity at Earth departure and Venus arrival under the Window 17 May 2010 - 2 Jun 2010 (UTC) condition where Earth-escape declination is over -30 degrees. Earth-escape direction (Declination) more than -30 degrees Venus Circular Orbit (B) The capacity of spacecraft for orbital maneuvers Pericenter Altitude Above 300 km during the mission's The capacity of orbital maneuvers for Venus Orbit Insertion period (two years) (VOI) is limited by the amount of fuel mounted to the explorer. In the case when the probe mass is 530 kg (maximum), the Apocenter Altitude Approx. 80,000 km excess velocity of Venus approach should be below 3.4 km/s Period 30 hours because of the size of fuel's and oxidizer's tanks. Figure 4 Inclination Approx. 170 degrees shows the relation between excess velocity of Earth departure and Venus arrival under the condition where the Earth-escape 2.2. The constraints on launch window declination is shallower than -30 degrees. This figure indicates There are four constraints mainly on launch windows which that the launch window must be later than 17 May 2010. come from the observation orbit and spacecraft system; (A) Capacity of launch vehicle, (C) The visibility after separation at JAXA Usuda station (B) Capacity of spacecraft for orbital maneuvers, In the case when critical operation becomes necessary for (C) Visibility after separation at JAXA Usuda station, orbital correction after separation, it is desired that more than (D) Umbra period in Venus circular orbit. four hours of visibility is ensured at the first tracking at JAXA Usuda station. The visibility from ground stations depends on (A) The capacity of launch vehicle the declination of the excess velocity. Figure 5 shows that the Akatsuki is inserted into the interplanetary orbit by H-IIA elevation from Usuda station becomes lower and the duration rocket by applying the second ignition of the second engine. becomes shorter when the declination of excess velocity Although the rocket has the capacity to insert a spacecraft becomes deeper, larger in absolute value. In order to ensure more than 1 ton into the Venus Transfer Orbit (VTO), the the four-hour visibility, the declination at the Earth departure interface point of mass is set at 530 kg at maximum. With this needs to be shallower than -30 degrees. mass, the rocket can insert a probe into the interplanetary orbit whose excess velocity is less than about 4.3 km/s. As an (D) The umbra period in Venus circular orbit additional condition, the declination of Earth-escape velocity The umbra must be less than 90 minutes in one revolution is required to be shallower than -30 degrees by the constraints because of the amount of lithium ion battery of the spacecraft. of coasting duration from the first cut off to the second The umbra duration differs according to the orbit and its ignition of the second engine as shown in Figure 32). If it is timing; it is short when umbra occurs near pericenter and is deeper, enough coasting time is not ensured and additional long near apocenter as seen in Figure 2. There is also such a parking orbit of about one revolution is required. In this case, period when no umbra occurs. For the launch cases in May or another study on additional fuel for rocket's attitude control in June in 2010, the suitable launch dates are no later than 2 will be necessary. 2 To_3_2 C. HIROSE et al.: The Trajectory Design Strategies for Akatsuki Mission Antenna Elevation from UDSC 30 Elev(dec : -30deg) Elev(dec : -35deg) Elev(dec : -40deg) 25 Elev(dec : -45deg) 20 Declination Angle -30 deg 15 -35 deg -40 deg 10 -45 deg 5 Elevation(deg) AnglefromUDSC 0 0 5 10 15 20 Time from Launch (hours) Fig. 5. Depending on the Earth-escape declination, the antenna elevation Fig. 7. The changes of pericenter altitude depending on the arrival date. changes at first tracking at Usuda station. 2.5 Launch 06/01 B-plane Angle U mbr a 190 deg Arrival 12/08 Penumbra 2.0 1.5 1.0 0.5 Shadow Dura (hours) tion 0.0 Fig. 8. Target Plane for VOI. 0 100 200 300 400 500 600 700 800 Flight Time from Venus Capture (days) 2.5 Launch 06/05 B-plane Angle Table 2. The target point when launched on 20 May 2010. Arrival 12/10 190 deg 2.0 VOI (UTC) h (km) θ (deg) 1.5 7 Dec 2010 00:00:00.000 550.0 189.5 1.0 peri- and apo-center changes gradually.