A PROBE TO EUROPA

For:

Dr. W. T. Fowler

By:

Hector O. Alvidres

Department of Aerospace engineering

and Engineering Mechanics

The University of Texas at Austin

1. INTRODUCTION

1.1 Background

1.1.1 Jupiter

Some scientists have suggested that Jupiter is a "brown dwarf". These are celestial bodies which are too hot to be planets, but too cold to be stars. Jupiter emits 67 percent more heat than it absorbs from the Sun, and it has been estimated that, if the largest planet in our solar system was 100 times larger than it is now, it could ignite fusion reactions in its core. This is a characteristic found in stars. For this reason Jupiter, and its sixteen known satellites, has been regarded as a "mini-solar system" by some scientists [1,169].

Jupiter is a large ball of gas, but scientists have only been able to see 80 km. into its atmosphere. It is also thought that in the center of the planet lies a core of molten rock. Its atmosphere is composed of hydrogen and helium, and small amounts of methane, ammonia, phosphorous, water vapor, and hydrocarbons. In its 1979 encounter, Voyager discovered that the amount of helium was less than that expected, and less than that observed on the Sun, which is to some extent inconsistent with the brown dwarf concept. The atmosphere has also displayed lightning 10,000 times more powerful than that on Earth. The discovery of lightning by Voyager I, in 1979, created great excitement among scientists. In a 1952 experiment by Harold Urey and Stanley Miller, these two scientists combined a mixture of gases, like those found in Jupiter's atmosphere, and introduced an energy source, like sparks and ultra-violet light. These conditions meant to simulate the conditions prior to the existence of life in the early solar system. The result of the experiment was the creation of certain nucleotides and amino acids, which are the basic building blocks of life as we know it [1,169-172].

In 1989, Galileo was launched to study Jupiter, and it is scheduled to arrive in 1995. It will directly sample Jupiter's atmosphere, conduct close analyses of the satellites, map the structure and the dynamics of the magnetosphere, and map the thermal properties of the planet.

1.1.2 Europa

Europa, or JII ,is the smallest of the Galillean satellites. It is slightly smaller than the Moon. It has a surface made of H20 ice that is three miles thick, with no atmosphere [1,176]. The temperature of the ice on the surface is 90.±10.K [2,1011]. It is very smooth with differences no greater than 100. meters between the lowest and the highest elevations. This translates to being 20 to 30 times smoother than the Earth [1,176]. Scientist believe that this icy surface was created by gases escaping from the center of the satellite outward. It is also believed that, in its earlier stages, Europa's surface was 75. km. thick [2,1011]. However, the interior's high temperatures have melted away some of the surface from the inside. It has also been speculated that underneath the surface, there are oceans of liquid water, and that conditions may be favorable for the satellite to support some kind of life form [1,176].

1.2 Justification and Value of a Mission to Europa

If in fact Jupiter is some kind of young star, and the Jupiter Planet-satellite system is some kind of mini-solar system, a mission to Europa would provide an insight into the creation of our solar system, or any solar system. It could become a near-by laboratory in space, where scientists could try to learn more about the creation of the planet Earth. It would also create the opportunity to investigate the existence of life in another planet, other than Mars, where conditions are favorable, as it was explained in Section 1.1. It would also allow scientists to determine the likelihood of any of the Jovian satellites ability to support life. And at the very least, it could serve as an orbiting platform to study the largest planet in the solar system.

There is only so much that Earth based telescopes can see. With the development of the Hubble telescope, the increase in the amount of scientific investigation that can now be performed, as a result of a space borne telescope, is staggering. But nothing can beat the discoveries that the Voyager Project made in its first twelve years. An observer at the scene cannot be replaced by an observer millions of miles away. Voyager took a very close look at Jupiter, Saturn, Uranus and Neptune, and collected data which would have been impossible to collect from Earth. But even with the science obtained from Voyager, Jupiter still holds many mysteries. Most of them will hopefully vanish with the arrival of Galileo. Galileo will provide much more information, on Jupiter, than that provided by Voyager. However, even though Galileo will conduct experiments on the moons of Jupiter, Europa should be given a closer look because of its unique characteristics. A mission to Europa, and the Galileo mission, have the potential to give scientists an incredible amount of information on the Jupiter system, and hopefully on the rest of the solar system. Because of its physical characteristics Europa, holds the best possibility for finding life as we know it, and the more hospitable conditions for a spacecraft than any of the other Jovian satellites.

2. MISSION DESCRIPTION

2.1 Mission Objectives

The main objectives of the mission to Europa are: to study the surface of the satellite; to determine any geological activity; to determine the stability of the satellite; to determine the composition of the satellite below the icy surface, to determine the composition of its core, to study the Jupiter-satellite system, and to determine the ability of the satellite to sustain life. The secondary objective is to provide a science platform to study Jupiter.

2.2 Feasibility

As explained in the previous section, the mission to Europa has very similar objectives as previous mission to other planets. Voyager traveled from Earth to Jupiter, and on. So the propulsion technology is available. Many of the experiments planned for the surface of Europa are similar to those performed by the Viking spacecraft. The only new experiment that will be performed is the penetration of the three mile thick Europan surface. In the next section, two approaches to this problem are described. The one chosen promises to be a simple and reliable method. The simplicity of the technique selected will add to the reliability of the mission.

2.3 Mission Scenarios Explored

Under both mission scenarios presented below a double-vehicle set-up is reviewed. The spacecraft will consist of an orbiting unit and a lander, much in the same way as Viking. This has proven to be a very efficient method of performing a mission. Upon arrival to Europa, the spacecraft will take close-up pictures of the satellite. After arrival into orbit, a series of orbits will be performed to find an orifice on the surface, through which the probe can be introduced. If no such orifice is found, then the best landing site will be selected. Once the landing site has been found, the spacecraft will be prepared to launch the lander. After separation, the orbiter will continue to work as a relay station and to continue analyzing the surface of Europa, or the surface of Jupiter. The lander will descend to the surface and then anchor down. Seismometers will be deployed and measurements will be taken. Other science will be performed before deploying the inner probe. Signals will be recorded or beamed up to the orbiter for transmission to Earth.

The difference of the two scenarios are explained below:

2.3.1 Flying Spike

A high speed projectile was to be launched after a landing site was found. The projectile, or "spike", would have had explosives at the end which would have detonated after the spike had penetrated the surface and stopped moving . This would ensure that spike would penetrate through the three mile layer of ice. The lander would then arrive and drop the probe through the hole created by the spike. The probe would then beam the signals to the lander.

This method seemed highly unreliable, and inefficient. Without having first studied the surface, it is not known what effect the explosion would have on the planet. This method was discarded immediately after the next suggestion was proposed. A mission summary is shown in Figure 2-1.

2.3.2 Warm Feeler

This method promises to be simple and reliable. After a good landing spot has been selected, the lander will separate from the orbiter and land on the surface. There will be two probes which contain a Radioisotope Thermo-electric Generators which will serve to power the probe and create heat to melt the ice. As the ice melts, the probes will sink letting out cable from a reel inside each probe. Since the surface crust is calculated to be three miles deep, the cable will be 4.5 miles long. After all science has been conducted at this depth, the probes will be released so that inner core analysis can be performed. The wire will act as an antenna and collect the data for the lander.

The mission summary is shown in Figure 2-2.

Figure 2-1. "Flying Spike" Mission Scenario

Figure 2-2. "Warm Feeler" Mission Scenario

2.3 Mission Selected

The mission selected was the Warm Feeler. It is a more reliable mission than the Flying Spike, and most of the equipment and instrumentation can be obtained off the shelf. The Flying Spike would have required the lander to land on a specific area, with a very small margin of error. The Warm Feeler does not have to do this since it is self contained.

The Europa Probe will be launched from the Space Shuttle from Low Earth Orbit (LEO). It will be injected into a non-Hohmann trajectory which will take it to Jupiter in 688 days. It will then fly through the Jovian atmosphere to take advantage of aerobraking. The atmosphere will slow the spacecraft down and place it into a Hohmann like transfer which will take it to an orbit around Europa, at an altitude of 185. km above the Europan surface. The spacecraft will orbit until a suitable landing site is found. Then the lander will separate and proceed to land on the surface of the moon. After conducting seismological studies, the lander will deploy the RTG's which will study the interior of the ice crust. During this time the orbiter will relay the data to Earth, or store it for later transmission. After this portion of the analysis is completed, the RTG's will be released to study the interior of the core.

3. TRAJECTORY ANALYSIS

Perhaps the most difficult part about designing this mission is developing a trajectory which will deliver the spacecraft to its target. Jupiter is the largest, and heaviest planet in the solar system. Therefore, its gravitational field can pull celestial bodies into its sphere of influence and accelerate them at very high speeds. This is very useful when the planet is being used for gravity assist, like Voyager did, but when a spacecraft is trying to land on one of the Jovian moons, the gravitational pull can create serious problems. Tables 3-1 and 3-2, show some of the characteristics of Jupiter, and the target moon Europa. Because of the high gravitational constant of Jupiter, and the orbital velocity of Europa, it was not possible to design a trajectory which arrived directly into the target satellite. The trajectory had to be designed using the atmosphere of Jupiter to aerobrake the spacecraft. The spacecraft then continues in a Hohmann type trajectory which has enough velocity to reach orbital velocity around Europa. The following section describes the trajectory in detail.

TABLE 3-1. STATISTICS ON JUPITER

Distance to the Sun: 778. X 106 km. [3,361]

Orbital Speed: 13.06 km/s. [3,361]

Gravitational Constant: 1.268 X 108 km3/s.2[3,361]

Equatorial Radius: 71,370. km. [3,361]

Inclination to Ecliptic: 1.306° [3,361]

Inclination of Equator to Orbit: 3.07° [3,361]

TABLE 3-2. STATISTICS ON EUROPA

Distance to Jupiter Surface: 5.996 X 105 km.

Semi-major Axis of Orbit: 6.71 X 105 km/s. [2,124]

Orbital Speed: 14.38 km/s

Gravitational Constant: 3.246 X 103 km3/s.2

TABLE 3-2. STATISTICS ON EUROPA(con't)

Equatorial Radius: 1525. km. [2,994]

Orbit Inclination: 0.5° [2,124]

Orbit Eccentricity: 0.0 [2,124]

Orbital Period: 3DAYS13HRS

3.1 Trajectory Description

The Europa Probe will be launched from the space shuttle from LEO on June 14, 1999. The shuttle will be flying at a velocity of 7.728 km/s, but the probe will require a transfer trajectory velocity of 38.542 km/s relative to the Sun. This translates to a velocity of 9.71 km/s relative to the Earth. To escape the gravitational pull of the Earth, the upper stage of the probe will increase the orbital velocity by 6.849 km/s to obtain the transfer velocity required. The probe will follow a departure asymptote which will place it at an angle of .389° below the ecliptic.

On August 20, 2000, 432 days and 20 hours after launch from LEO, the probe will conduct a broken plane maneuver which will line the spacecraft up with the target planet. The increase in velocity required will be .0109 km/s.

On May 2, 2001, 688 days after departure from LEO, the probe will arrive at Jupiter at a velocity of 9.342 km/s, relative to the Sun, 7.61 km/s relative to Jupiter. However this velocity will increase dramatically as it approaches the planet. The velocity will increase so much that it will surpass the speed of Europa, and therefore, the atmosphere of Jupiter must be used to slow the spacecraft down. Upon arrival to the Jovian atmosphere, at 71,370 km from the center of the planet, the spacecraft will be flying at 60.09 km/s. It is important to note that at this distance the escape velocity is 59.609 km/s. However, the probe will only require a decrease of 1.089 km/s to acquire the necessary velocity which will place it in the elliptical orbit which will take it to Europa and deliver the probe at apjove of this transfer trajectory. The velocity of the spacecraft at apjove will be 15.758 km/s relative to Jupiter and 1.378 km/s relative to Europa, which orbits Jupiter at 14.38 km/s. The spacecraft's orbital velocity around Europa will place in a circular orbit, at an altitude of 185.2 km.

Table 3-3 shows a summary of velocities and information used to determine the trajectory just described. Figures 3-1 through 3-3, show details of the trajectory. Appendix A contains the hand calculations and some of the software that was used to determine the trajectory.

TABLE 3-3. TRAJECTORY SPECIFICS

Date of Departure: June 14, 1999

Date of Broken Plane Maneuver: August 20, 2000

Date of Arrival: May 2, 2001

Time of Flight: 688 Days

Angle Between Departure Planet

at Departure and Arrival Planet

at Arrival: 176.94°

Angle Between Departure Planet

at Departure Location of

Broken Plane Maneuver: 130.30°

Angle of Departure Asymptote

to the ecliptic: .389°

LEO Altitude: 296 km

LEO Velocity: 7.728km/s

Velocity Required for Transfer

Trajectory (Heliocentric): 38.542 km/s

(Relative to Earth): 9.71 km/s

DV Required to Achieve Transfer

Trajectory: 6.849 km/s

DV Required for Broken Plane

Maneuver: .0109 km/s

Orbital Velocity of Jupiter: 13.06 km/s

Offset for Arrival at Jupiter

(See Figure 3-2): 563,583.62 km

Arrival Velocity at Jupiter

(Heliocentric): 9.342 km/s

(Relative to Jupiter): 7.61 km/s

Velocity at Atmosphere of

Jupiter Altitude: 60.09 km/s

Velocity After Aerobraking: 58.52 km/s

TABLE 3-3. TRAJECTORY SPECIFICS(con't)

DV from Aerobreaking: 1.089 km/s

Apjove of Transfer Trajectory

to Europa: 672,710. km

Velocity of Spacecraft at Europa

(Relative to Jupiter): 15.758 km/s

Velocity of Spacecraft at Europa

(Relative to Europa): 1.378 km/s

Altitude: 185.2 km


Figure 3-1. Mission Trajectory

Figure 3-2. Offset at Jupiter Arrival

Velocities are relative to Jupiter

Figure 3-3. Arrival to Europa Detail

4. THE SPACECRAFT

The total weight of the spacecraft, not including the shuttle, is 50,036. lb. The upper stage will carry 42,578. lb. of fuel with a casing weight of 4,257. lb. The lander will weigh 2,249. lb. including landing fuel. The fuel and casing will weigh 924. lb. The orbiter's total weight will be 950. lb. a brief description of the probe's main systems is listed below. Figure 4-1 shows the spacecraft and its science instrumentation.

4.1 Spacecraft Sub-systems

4.1.1 Propulsion System

Because of the high DV requirement at departure from LEO, it was required to employ a booster with a high specific impulse. This implies that the booster would have to be liquid fueled. In this case, the fuel was Hydrogen and Oxygen. This combination has a specific impulse of 366 s [4,125]. The result will be a DV of 6.85 km/s from LEO to the transfer trajectory. A recent attempt to launch a liquid fueled booster from the space shuttle was not met warmly by the crew. Therefore, different options are discussed in the conclusion. Along the way, the spacecraft will perform a broken plane maneuver which will require a DV of .0109 km/s. This will be easily accomplished by the Hydrazine propulsion system. This system will also perform other maneuvers such as trajectory corrections. The next major maneuver requiring the use of fuel will be to separate the lander from the orbiter and land the lander on the surface of Europa. This will require a DV of 1.378 km/s and will be accomplished also by Hydrazine, which has a specific impulse of 300 s [4,112].

4.1.2 High Gain Antenna (HGA)

The HGA on the orbiter will be responsible for transmitting information to Earth. There are two channels. One channel transmits engineering and science data at a high rate, and the second channel transmits health and state data at a low rate.

The HGA on the lander only transmits one way at a high rate.

4.1.3 Attitude Control System

Prior to separation the spacecraft will be three-axis stabilized. A computer on board the orbiter will be responsible of sending any signals to the propulsion system to keep the vehicle pointed in the right direction. The gyro controls will make any special corrections, like pointing cameras, a second mode will be responsible for navigation. After separation, the orbiter will remain stabilized in the same manner.

4.1.4 Power System

The orbiter and the lander will be powered by Radioisotope Thermoelectric Generators (RTG) and batteries. The orbiter will have one, and the lander will have two. The RTG's will provide plenty of power for the entire mission. After the lander deploys its RTG's it will use battery power to transmit data to orbiter.

4.1.5 Data Storage System

The Data Storage System will be used when it is not possible to send the data simultaneously. It will be used during periods of occultation or other periods when data cannot be transmitted. The orbiter will be able to store images as well as analog data.

4.1.6 Thermal System

Temperature onboard the spacecraft will be controlled by the main computer and consists of a network of passive and active systems that will maintain instrumentation within their safe limits.

On the ground, the RTG's will provide the lander some of the heat required until the RTG's are deployed. After that the lander will go into a passive system. Some of the heat will come from Jupiter.

4.1.7 Aerobraking System

The aerobraking system must be capable of protecting the spacecraft through the aerobraking sequence in the Jovian atmosphere. It will be required to slow the spacecraft down by 1.089 km/s.

4.1.8 Computer System

The Computer System will be responsible for managing all information in the spacecraft. It will interpret signals from Earth, and issue commands as needed. It will also contain any diagnostic and safing sequences required for the survival of the spacecraft.

4.2 Science Instrumentation

4.2.1 Orbiter

Because of the mass constraints and the lack of atmosphere on Europa, the orbiter will only be equipped with two visual cameras, and a radio experiment. The cameras will assist in finding a landing site by using stereoscopic imaging, and the radio experiment well search for radio waves being emitted from Europa.

Figure 4-1. Europa Probe Science Instrumentation

4.2.2 Lander

Video Camera- The video camera will collect images from the surface of Europa, and will serve as eyes for the ground operators.

Temperature Sensor- The temperature sensor will be used to measure the temperature of the surface and the temperature surrounding the spacecraft.

Seismometer- The seismometer will check for seismic activity.

Radio science- The radio science experiment onboard the lander will consist of analyzing the properties of the matter found below the ice crust.

Mass Spectrometer- The mass spectrometer will analyze any gasses found below the ice crust.

RTG's- The RTG's can create a great amount of heat by the decay of plutonium dioxide. This heat will be used to melt the ice and penetrate the crust.

5. CONCLUSION

Jupiter contains a wealth of scientific information. After Voyager sent back pictures of the solar system's largest planet, scientists wanted to learn more about the secrets of this planet. Europa is also an enigmatic body and should be studied further. This was only a feasibility study. After Galileo arrives to Jupiter and begins sending information, it will serve to analyze a better mission. Suggestion to improve this study follow in this section.

5.1 Cost Analysis

Table 5-1 shows a brief summary of what the mission is expected to cost. This is a first round estimate.

TABLE 5-1. COST ANALYSIS

Detail Cost (In Millions)

Launch System (Space Shuttle): 183.

Upper Staging: 60.

Off the Shelf Technology: 150.

New Technology: 300.

Operations 150.

TOTAL 843.

5.2 Further Analysis

The experiments conducted during this mission are very similar to those conducted during the Viking project. It would be recommended to use the same technology; however, it would it would also be recommended to decrease the weight of the instrumentation to increase the efficiency of the system.

The DV required is high if no gravity assist is used. This DV could be reduced by performing a gravity assist. This would improve the payload capabilities and would also allow the possibility of using a solid fuel rocket instead of a liquid fuel rocket.

6. REFERENCES

1. Kohlhase, Charles, The Voyager Neptune Travel Guide, JPL Publication 89-24, Pasadena, California: The National Aeronautics and Space Administration, June 1, 1989.

2. Gehrels, T., Jupiter, Tucson, Arizona: The University of Arizona Press, 1976.

3. Bate, Roger r., Mueller Donald D., White Jerry E., Fundamentals of Astrodynamics, New York: Dover Publications, 1971.

4. Evered, Douglas S., Boris, Kit, Rocket Propellant Handbook, New York: The Macmillan Co., 1960.


APPENDIX A

CALCULATIONS

A1. Model Used to Compute Fuel Requirements

S Rule

* DV1=Isp1*g*ln(M/(M-mf1))

* DV3=Isp3*g*ln((M-mf1-s1-mf2-s2-spl)/(M-mf1-s1-mf2-s2-spl-mf3))

* M=mf1+mf2+mf3+mpl+s1+s2+s3+spl

* s1=mf1*.1

* s3=mf3*.1

Input Name Output Unit Comment

22433.126 DV1 ft/s Delta V.at Dep.

366 Isp1 s Specific Imp.1

32.2 g ft/s^2 Acc. Gravity

M 50036.605 lb Total Mass

mf1 42578.846 lb Fuel Mass Burn at Dep.

s1 4257.8846 lb Casing of Burn at Dep.

4520.28 DV3 ft/s Delta V. at Arr.

300 Isp3 s Specific Imp. 3

mf3 840.79525 lb Fuel Mass Burn at Arr.

1325 mpl lb Lander Mass

s3 84.079525 lb Casing of Burn at Arr.

950 spl lb Orbiter Mass

A2. Model Used to Compute Offset at Jupiter

S Rule

* E=v3^2/2

* E=vp^2/2-mu/rp

* y=(rp/v3)*sqrt(v3^2+2*mu/rp)

Input Name Output Unit Comment

E 28.95605 km^2/s^2 Energy of Orbit

7.61 v3 km/s S/C Vel. Rel to Planet

vp 60.093476 km/s Vel. Perijove

126800000 mu km^3/s^2 Jupiter Grav. Const.

71370 rp km Perijove

y 563583.62 km Offset

A3. Hand Calculations