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Trans. JSASS Aerospace Tech. Vol. 10, No. ists28, pp. To_3_1-To_3_5, 2012 Topics

The Design Strategies for 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 explorer, Akatsuki, was launch on 20 May 2010. After 200-day journey through the interplanetary transfer , 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 the 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 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 (0.5 revolution about the Sun) in May, Jun 2010 Launch The Venus is a very similar as the in its size in Dec 2010 Venus arrival and . 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 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 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 . 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 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 . Hence, the synodic

km) 0.0 chances come every eight years. Since the Venus orbit is 8 Sun -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 ) about Venus 80,000 マーカは30 分毎 70,000 60,000 Fig. 3. 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 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 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 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 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- 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 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) Angle fromUDSC

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. Figure 7 shows the

0.5 changes of altitude depending on the difference of the launch

Shadow Duration(hours) date, the arrival date to the Venus. The insertion target was

0.00 100 200 300 400 500 600 700 800 set at 550.0km in altitude so that we don't have to change this Flight Time from Venus Capture (days) number depending on the launch date and that the pericenter Fig. 6. The umbra period in the Venus circular orbit (above: launched on altitude doesn't go beneath 300 km for about 2 years (800 1 Jun 2010, below: launched on 5 Jun 2010). The below case doesn't days). satisfy the requirement: umbra must be less than 90 minutes.

(c) The umbra period in Venus circular orbit Jun 2010. If it becomes later than the date, the umbra becomes As discussed in section 2.2, the umbra must be less than 90 long as shown in Figure 6 as one example. minutes in one revolution by the amount of lithium ion battery By these constraints, the launch window was set from 17 of the spacecraft. Figure 8 shows the Target plane of VOI. The May 2010 to 2 Jun 2010 (UTC). conditions (a) and (b) determines the time and altitude of

insertion, and the angle θ determines the insertion point. 2.3. Target of Venus Orbit Insertion (VOI) Here, the angle θ is measured from x axis which is parallel As the step, the target point for VOI should be to the Earth ecliptic orbit in the Target plane which is determined. The following four conditions were considered; perpendicular to the velocity of the explorer at TCA and is (a) Time of Closest Approach (TCA) to the Venus, centered at the Venus. When θ is changed, the umbra period (b) Approaching altitude to the Venus, increases/decreases as the eccentricity vector after VOI varies. (c) Umbra period in Venus circular orbit, The umbra condition is satisfied when θ is near 190 degrees. (d) Incidence angle of sunlight. Furthermore, we optimize θ with the next condition (d).

(a) The TCA to the Venus (d) The incidence angle of sunlight To ensure longer visibility at Usuda station, TCA is set at For the thermal environment, the incidence angle of 00:00:00 (UTC), which is near central time during tracking. sunlight toward +Y-plane of the probe must be less than 13

degrees in the Venus circular orbit. (b) The approaching altitude to the Venus Under these constraints the target point when launched on The altitude of pericenter should be over 300 km during the 20 May 2010 was derived as shown in Table 2. observation period of two years. The orbital period, 30 hours, is determined so that the explorer is nearly synchronized with 3. The Plans and Results the angular velocity of super rotation in cloud layer for about

20 hours centered at apocenter. Hence, the apocenter becomes 3.1. Orbital maneuvers in Venus Transfer Orbit (VTO) about 80,000 km (13 times of the Venus's radius) in altitude. The launch of Akatsuki was initially set on 17 May 2010. Elliptical orbits are perturbed by the Sun and the altitude of The weather conditions were bad on the date and Akatsuki

was launched on 20 May. The explorer was separated almost

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Table. 4. The plan and results during VTO phase. * APH-1 / OME test maneuver (28 Jun 2010 10:00:00 (UTC)) ΔV Magnitude Direction [deg] Closest approach in Target Plane [m/s] RA Dec error Altitude [km] θ [deg]

Plan (ICV10) 11.66 91.5 -64.0 550.0 189.5

OD estimation (ICV11) 12.15 96.5 -66.2 4579.4 185.7

Error 0.49 (4.2%) 5.0 -2.2 3.1 * TRM-1 (8 Nov 2010 01:00:00 (UTC)) Plan (ICV44I) 2.97 40.5 13.5 550.0 189.5 OD estimation (ICV45I) 2.85 40.3 15.6 624.1 189.1 Error -0.12 (-3.9%) -0.3 2.1 2.1 74.1 -0.4 * TRM-2 (22 Nov 2010 00:00:00 (UTC)) Plan (ICV46I) 0.26 48.6 -31.9 550.0 189.5

OD estimation (ICV47I) 0.27 50.3 -29.2 537.2 189.5

Error 0.01 (4.9%) 1.7 2.7 3.1 -12.8 0.0

* TRM-3 (1 Dec 2010 00:00:00 (UTC))

Plan (ICV48I) 0.04 237.4 16.6 550.0 189.5 OD estimation (ICV48I2) 0.04 228.5 23.8 547.6 189.5 Fig. 9. The orbital maneuvers in VTO. Error -0.003 (-8.1%) -8.9 7.2 11.1 -2.4 0.0 exactly at the interface point and, as a result, the orbital maneuver to correct the trajectory was calculated as only 1 Venus m/s. Although several maneuvers were considered as the Closest Approach initial plan, no orbital corrections were necessary. Table 3 Post -V shows the maneuver plan before launch and result. The date / ICV time and the amount of maneuver in the plan after launch are the final values before performing maneuvers. The four Pre Delta-V Delta-V maneuvers performed during the VTO are shown in Figure 9: ICV APH-1 (aphelion), TRM-1 (trim), TRM-2 and TRM-3. Fig. 10. The evaluation method of maneuver results to the plans. (1) APH-1 / Orbit Maneuvering Engine Test Maneuver In order to obtain the characteristics of the OME (F: 476.1 The planning and its result of the orbital maneuvers are N), used for VOI, a test maneuver for 13-second burning time shown in Table 4. Each maneuver was evaluated by applying was performed. As shown in Table 4, we confirmed that the FTA method as described in Figure 10; we fit the location and velocity were corrected about 12 m/s, as expected. time of the Venus closest approach by propagating the pre- (2) TRM-1, TRM-2 and TRM-3 and post- delta-V . Figure 11 shows how the Whereas the precision requirement of the altitude of VOI Venus closest approach changed in the Target Plane after was not strict for Akatsuki mission, we performed the small each maneuver. Whereas the target altitude is 550.0 km, the orbital corrections before VOI with a view to future missions final result after TRM-3 was 547.6 km, which was 2.4 km which require more precise targeting such as aerobrake. By lower than our aim. This difference is caused by several the Fixed Time of Arrival (FTA) method3), the time and combined effects, such as the accuracy of orbit determination, orbital location shown in Table 2 was targeted and orbital orbit prediction, attitude control and thruster's capacity. maneuvers by small (F: 4x18.1 N) were conducted; Whereas 547.6 km was rather very accurate for Akatsuki 21-second burning for TRM1, 2.1 seconds for TRM-2 and 0.4 mission, the improvement of the accuracy will be desired for second for TRM-3. more highly precise orbit insertion for the future.

Table 3. The maneuver preparation before launch and plan after launch. Maneuver plan before launch Maneuver plan after launch No Date (day) ⊿V Purpose Thrusters ⊿V(m/s) Necessary Ignition Time (UTC) ⊿V(m/s) 1 L+0.5 INI-1 Injection error correction 1 RCS 20 if necessary not required 2 L+ (1 - 20) INI-1C Injection error correction 1-1 RCS 5 if necessary not required 3 L+7 INI-2 Injection error correction 2 RCS 1 if necessary not required 4 L+14 INI-3 Injection error correction 3 RCS 1 if necessary not required 5 L+ (27 - 51) APH-1 OME test maneuver OME 10 - 20 necessary 28 Jun 2010 10:00:00 11.66 Correction of orbital plane in orbiting Venus 6 VOI-30 TRM-1 Trim maneuver 1 for targeting Venus RCS 1 if necessary 8 Nov 2010 01:00:00 2.97 7 VOI-10 TRM-2 Trim maneuver 2 for targeting Venus RCS 1 if necessary 22 Nov 2010 00:00:00 0.26 8 VOI-3 TRM-3 Trim maneuver 3 for targeting Venus RCS 1 if necessary 1 Dec 2010 00:00:00 0.04

9 VOI VOI-1 Venus orbit insertion 1 (Period: 96 hours) OME 700 - 770 necessary 6 Dec 2010 23:49:00 748.31 10 VOI+2 APV-1 Altitude correction of pericenter RCS or 10 - 80 necessary (9 Dec 2010 00:14:57) (59.92) Correction of orbital plane in orbiting Venus OME 11 VOI+4 VOI-2 Venus orbit insertion 2 (Period: 48 hours) OME 110 necessary (11 Dec 2010 00:28:48) (101.44) 12 VOI+5 APV-2 Altitude correction of pericenter RCS or 5 if necessary OME 13 VOI+6 VOI-3 Venus orbit insertion 3 (Period: 30 hours) OME 110 necessary (12 Dec 2010 23:59:40) (97.18)

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4,000 Altitude 550.0km 3,000 Venus 2,000

1,000 Post TRM-1 (Nov. 8) #ICV45I 0 Post APH-1 (Jun. 28) #ICV11 -1,000 Target point #ICV11 -ICV44I -2,000 h: 550.0km θ: 189.5°

[km] Target-plane axisin Y -3,000 #ICV0 -ICV10

-4,000 -14,000 -12,000 -10,000 -8,000 -6,000 -4,000 -2,000 0 X axis in Target-plane [km] -900 Altitude 550.0km

-1,000 Post TRM-1 (Nov. 8) #ICV45I Post TRM-2 (Nov. 22) #ICV47I

-1,100 Fig. 13. The current . Post TRM-3 (Dec. 1) Y axis in Target-plane [km] Target-planeaxis in Y Target point h: 550.0km #ICV48I2 θ: 189.5° -1,200 -6,800 -6,700 -6,600 -6,500 -6,400 -6,300 X axis in Target-plane [km] Fig. 11. The Venus closest approach in the Target plane after each maneuver. (ICV is an identification of orbital elements.)

3.2. The VOI maneuvers In the VOI-1, the OME was supposed to perform a retrograde maneuver of 748.3 m/s for 718 seconds. However, it experienced a trouble of its propulsion system; the telemetry Fig. 14. The distance between Venus and Akatsuki. shows the attitude control was disturbed 152 seconds after the ignition and that, 6 seconds later, the OME cut the fire at the 3.3. The current orbit of Akatsuki same timing as the probe was changed to the Attitude Control Since VOI-1 maneuver ended up 20 % of planning, Mode. Table 5 and Figure 12 show the maneuver plans and Akatsuki couldn't be a Venus orbiter. It is in heliocentric orbit results. We evaluated the maneuver direction and duration by with the period of 203 days, whose perihelion is 0.61 AU and fitting the orbital elements after VOI-1, assuming that the aphelion is 0.74 AU. Figure 14 shows the distance between the explorer started the maneuver on schedule with force Venus and Akatsuki. Since the revolution period of Venus is 100 %. Whereas it is different from the burning duration 224.7 days, there will be chances in 2016 to 2017 to meet the obtained by the telemetry (158 seconds), delta-V caused by Venus again by performing maneuvers at some appropriate the attitude control after VOI-1 is included for this evaluation. time beforehand. Table. 5. The plan and result of VOI-1. ⊿ ⊿ Direction Error [deg] V magnitude V duration 4. Conclusions [m/s] [sec] in plane out of plane Plan (ICV48I3 (Dec. 5)) 748.3 718.0

OD estimation (Dec. 9) 134.8 142.5 -0.14 +1.14 We discussed 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 their results of orbital maneuvers in the operation phase were also shown. Since Akatsuki is in heliocentric orbit, we will study maneuver plans for the next opportunity to meet the Venus.

References

Earth 1) Nakamura, M. et al.: PLANET-C: Venus Climate Orbiter mission of Japan, Planet. Space Sci., 55 (2007), pp. 1831-1842 2) Saito, Y. et al.: Research on Launch Vehicle Interface for Sun Interplanetary Mission, Proceedings of the 22nd International Symposium on Space Flight Dynamics, 2011. 3) Lawden, D.: Optimal programme for correctional manoeuvres, Fig. 12. The trajectory plan and result. Three orbital maneuvers were Astronautica Acta. 6 (1960), pp. 195-205. planned to lower the pericenter. The VOI-1 ended up 20% of planning.

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