Aurora

Science

When lightning is discharged it emits broadband emissions from a few hertz all the way to tens of megahertz. Similar emissions are expected on Titan from naturally occurring lightning there. There is strong evidence of naturally occurring lightning on Titan, both from organic molecules present as well as a ELF signal measured by the Huygen's probe during its descent. Since Titan's ionospheric electron density is much lower than Earth's we will be able to see RF emissions greater than around 400 kHz. In addition, a recent discovery has shown that certain, and maybe all strikes, emit a gamma ray flash due to the bremsstrahlung effect caused by relativistic electrons accelerated from lightning [1]. These gamma ray flashes are detectable from Low Earth Orbit.

Since Titan has a much denser atmosphere, we will orbit Titan at 1400 km, which has a corresponding density to a typical Low Earth Orbit of around 300-500 km [3] . An orbit this close to the surface should allow the detection of lightning. Furthermore, with a distributed network equal to the number of GPS satellites on earth, we should be able to triangulate the position of lightning similar techniques [4].

On earth, all of lightning's broadband emissions are reflected from the ionosphere due to the much higher electron density when compared to Titan's. The density of Titan's ionosphere is only about 1.8e3/cm^3 [5] compared to 1e6/cm^3 on earth. From this electron density maximum we can find the resulting plasma frequency where the ionosphere is no longer able to block emissions from propagating through [6].

(1)

This results in a cutoff frequency of about 400 kHz. This will allow any satellite within range to detect any emissions above 400 kHz present.

When lightning occurs, some relativistic electrons are accelerated by the same electric fields that cause the lightning stroke. These electrons in turn encounter particles in the atmosphere and can emit a gamma ray after "braking" or bending by the bremsstrahlung effect illustrated in Figure 2 [8]. The energy of the gamma ray emitted is equal to the lost relativistic energy of the electron. These gamma ray flashes are emitted with a angle between 30-60 degrees [9] which will allow detection in much of the visible sky of titan at the proposed orbit.

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Parent to Earth Communication

The parent satellite will have two main functions, to be the carrier of the child satellites to Titan, and as a communication relay from the child satellites to earth station.

Link Budget

Table 1 below details the link budget of the parent to earth communication channel.

We will use the Deep Space Network system which works at X band frequency, according to DSN Telecommunication Links Design Hand book [10]; the bands are 7.145-7.19 GHz for uplink (satellite to earth) and 8.4-8.45 for downlink (earth to satellite).

The orbital Velocity for Saturn and Earth is 9.68 km/s and 29.78 km/s respectively. We assume both earth and Saturn are in the same orbital plane, which results in the relative velocity difference of 20.1 km/s. The satellite orbit is perpendicular to earth orbit. So the largest frequency shift of 478 kHz, is negligible.

For the transmit antenna we use the following equation:

(2)

We assume an efficiency of 70%, a physical size of the antenna in parent satellite of 10m, a minimum wavelength of 0.042m. These parameters result in a gain of 61.9 dB. Using DSN's 70-meter Antenna and assuming 70% efficiency our resulting receive antenna gain is 79 dB.

When calculating the free path loss, we note that,

(3)

where d is the distance between earth and Saturn, which is roughly 1.2×10¹² m, and the minimum wavelength is 0.0355m. This results in a worst case path loss of 292.6 dB.

Table 2 shows the earth latitude and longitudes of the three DSN receiver locations. The three stations are in zones B, H and K , respectively.[11] So the highest precipitation rate is the Canberra complex, which has more than 42mm/hour during 0.01% of year. So maximum attenuation is 0.647*Leff. Therefore we assume Leff=2km, which results in A=1.1 dB.

According to DSN Telecommunication Links Design Hand Book [10], the attenuations for three station are 0.069 dB, 0.134 dB and 0.146 dB for the worst cases. So we will assume this attenuation to be 0.1 dB.

Since Saturn is in higher sun orbit than earth, the noise seen by the receive antenna would be mostly come from atmosphere of earth. From the DSN Telecommunication Links Design Handbook [10], the noise temperature is around 2.806K at X band in normal case. So the noise power would be −138 dB, according to:

(4)

Parent to Earth Data Link

This section describes the design of the parent satellite to Earth communication link. This is the link that will ultimately relay all of the data collected on Titan back to Earth. This section starts by establishing the data and networking requirements of the lightning monitoring system. It then uses these calculations as a basis for designing the communication link parameters of the parent-Earth link.

Timing

The precise location of each lightning strike is a crucial part of the data that the mission will generate. To locate each strike, a system similar to the current GPS orbiting Earth is proposed. To accurately perform such calculations, the time that lightning events occur must be known accurately. The calculations will also depend on accurately knowing the orbital positions of the child satellites. It is proposed that these orbital positions be calculated based on the parameters known at the times that they are launched from the parent satellite; calculating the correct future orbital positions would also be dependent on having precise timekeeping.

To accurately track the timing of events throughout the mission, it is proposed that the 25 MHz clocks of the electric field probes be used to measure time in samples since launch. All events that require timing will be stamped with a 64 bit number representing how many sample periods have elapsed since the launch of the mission. 25 Msamples per second is roughly equivalent to 7.89×10¹⁴ samples per year. A 64 bit timestamp would be able to record 264 samples or roughly 1.84×10¹⁹ samples total, meaning the 64 bit timestamp would be able to keep time until about 23400 years after launch, which will, obviously, more than accommodate the lifep of the mission.

In order to account for clock drift of the 24 separate child satellite clocks, it is proposed that a master 25MHz clock be maintained on the parent satellite. When a child-parent communication is initiated, the current master clock time will be transmitted to the child and checked against the child's clock so that it can correct the timestamps of any transmitted datagrams.

Data Generation

To detect lightning, we will measure electric fields on Titan in the 100 kHz − 10 MHz band. According to the Nyquist-Shannon sampling theorem, the minimum sampling rate for a 10 MHz signal would be 20 MHz. To ensure there is no signal aliasing and minimal quantization noise, sampling of the electric fields will be done with 12-bit quantization @ 25MHz. In order to triangulate the source of a field reading, readings of each field polarization must be gathered, increasing the sample count by a factor of 3.

When detecting lightning events, it is not necessary to report the continuous electric field waveform; instead, only the large spikes in electric field corresponding to lightning events need be reported. To accomplish this, electric field readings will be taken constantly at 25 MHz and discarded until the field magnitude spikes up above a detection threshold. Electric field samples will then be buffered until the field settles below the threshold once again. After being buffered, the samples will be grouped into a datagram with a timestamp marking the sample number of the first sample in the datagram, as well as the total number of samples in the datagram and stored in flash memory to be relayed to the parent satellite at the next opportunity. Before storage, the datagrams will be compressed with a yet to be determined lossless algorithm. While better performance may be possible with a well-designed scheme, general purpose lossless compression offers a compression ratio of about 2:1 [12]. Datagrams will be assuming Titan's lightning strikes are similar to Earth's, each lightning event should last 1ms on average [13].

Based on the above specifications, each Titan lightning event will generate roughly 450,000 bits that need to be stored and relayed on average, as seen in equation 5.

(5)

On earth, some lightning strikes generate a massive gamma ray flash known as TGFs, from which satellites equipped for gamma ray detection will detect on average of about 25 particles from low earth orbit [9]. We will assume a best case value of detecting 100 particles. As with the electric field readings, the gamma detector will have an obvious spike in activity if there is a gamma ray flash from a lightning event; photon energies corresponding to that event will be timestamped and recorded, and packed into a datagram along with the number of photon energy samples. The gamma ray datagrams would then be compressed as before and stored until they are relayed to the parent satellite. As seen from equation 6, each Titan lightning event will generate at most 6432 bits that need to be stored and relayed to the parent.

(6)

Parent to Earth Data Rate

It is estimated that lightning events occur on Titan at a rate of about 1 event per hour across the entire moon [13]. Based on the operation of GPS satellites around Earth, there are between 5 and 9 satellites in view of every position on earth. Carrying this over to Titan, for any given lightning event, 6 child satellites will be able to log the event. As established in the section on orbital paths of the satellites, the parent satellite aligns with Titan three times in the course of a single revolution of Titan about Saturn. Therefore, there are 5.315 Earth days in between each child-parent communication session in which the child satellites accumulate data and the parent has time to relay data back to Earth. Equation 7 calculates the average total data accumulated between each child-parent session that would need to be stored on the parent satellite until it can be relayed to Earth; this total data per contact is about 350 Megabits.

(7)

Equation 8 calculates the average net bit rate of the system. This average net bit rate is the rate at which data accumulates across all children and the rate at which data must be relayed to Earth to keep up with this rate of accumulation. The average net bit rate of the system is a mere 760.8 bps.

(8)

To accommodate for networking overhead, higher peak data rates, and downtime in the parent-Earth link, the parent-Earth link data rate will be chosen as 5 kbps, a margin of about 4.24 kbps or 557%.

Parent to Earth Modulation Scheme

According to the DSN Telecommunication Links Design Handbook, X-band channels are assigned in blocks of 1.4 MHz [14]. Since the target rate of 5 kbps is much smaller than the assigned channel bandwidth, spectral efficiency can be sacrificed to achieve lower SNR requirements. To that end, the parent-Earth link will be modulated using Binary Phase Shift Keying (BPSK), ½ rate Turbo Code forward error correction, and Raised Cosine pulse shaping with a roll-off factor of 0.5. It's estimated that ½ rate Turbo Coding gives a coding gain of about 7 dB at 10-5 BER [15]; this value would be higher for the target 10-6 BER and is used as a pessimistic bound. Table 3 shows a breakdown of the link performance based on the SNR calculated (parent sat section). As can be seen, the link should be able to operate as specified with a healthy SNR margin of 4.81 dB.

Figure 3 shows a block diagram depicted the path of data through the parent satellite. Data is received from the child satellites on a link that will be described in the child-parent communication section. Once received, data is stored in the parent satellite until it can be relayed to earth via the communication link specified above.

Child to Parent Communication

The child will search for lightning with three orthogonal antennas that detect broadband emissions similar to Cassini's system [16]. The child will then relay detected lightning strikes and gamma ray flashes back to parent when in view.

Link Budget

Table 4 below details the link budgets of each child to parent communication channel.

For our communication channels we will operate in the S band. We will use 2.11 GHz to 2.12 GHz for parent to child communication and 2.12GHz to 2.20GHz for child to parent.

Titan orbits Saturn at 5.57 km/s, while the parent satellite orbits at 7.02 km/s in opposite direction. The child satellite has a speed 1.5 km/s. So the maximum velocity difference would be 14.09 km/s, which leads to doppler shift of around 100 kHz, which is negligible.

A 0.5m diameter dish antenna is used in the child satellite to communicate with the parent satellite. With a minimum wavelength of 0.142m and 70% efficiency, the minimum antenna gain will be 25.3 dB. For the parent to child satellite receiver, we have chosen a 3m diameter dish antenna which results in a 40.9dB gain.

The distance between parent and child is 452,570 km at their closest point. With a minimum wavelength of λ = 0.136m, the path loss of the link is 212.4 dB.

In our orbit the only noise is galactic noise. Since little research has been done on Titan, we will assume the galactic noise is the same as for Earth: 10K at S-band frequencies when looking directly at the galactic center [10]. The proposed link occupies a bandwidth of 7.89 MHz, yielding a noise power of -149.6 dB.

Child to Parent Data Link

This section describes the design of the child satellite to parent satellite communication links. These links will be responsible for bursting each child's data collected on Titan to the parent satellite whenever the child comes in view of the parent. This section starts by describing the timing and view angle constraints on child to parent communication. It then uses these timing specifications in conjunction with the data generation figures established in the Parent to Earth Communication section to design the communication link parameters of the child-parent links.

View Angle and Timing Window

In order to achieve high signal to noise ratios on the child-parent links, dish antennas were specified for both sides of the child-parent links. Using these dish antennas to achieve high gain comes of at the price of directivity, however. The half-power beamwidth of a parabolic dish can be calculated using equation 9. For the child-parent link, λ = 0.1415m, D is the child and parent antenna diameters, and k is a performance factor equal to 70 degrees for a typical dish [17]. For the child and parent antennas specified above, these beamwidths are 19.8° and 3.3°, respectively.

(9)

For each of the 3 parent-Titan alignments per Titan revolution, the parent receiver antenna will be pointed directly towards Titan. To avoid excessive steering of the child satellites, the child antennas will be pointed parallel to the alignment of the parent satellite and Titan. Figure 4 shows a diagram of the worst-case satellite angle off boresight at the time of alignment. From this, it can be seen that the worst-case angle off boresight (WAOB) at alignment is 0.453°, which is well within the beamwidths described above.

Figure 5 shows the evolution of the WAOB as the parent satellite and Titan orbits pass each other. As seen above, a 0.5° difference in orbital angles yields a 1.1° difference in WAOB. This angle off boresight is still within the beamwidths described above, and with an adequate link signal margin communication would be possible with a 0.5° difference in orbital angles. It is of note that the communication window afforded by this difference in orbital angles can be utilized both while Titan and the parent approach each other as well as on retreat, so the total usable orbital angle is actually 1°.

The orbital period of Titan is 15.945 earth days, and the orbital period of the parent satellite is 7.9725 earth days. These periods corresponding to angular velocities of 22.58°/day and 45.15°/day. Since the two orbits are in opposite directions, the relative angular velocity is 67.73°/day. Based on this rate, the 1° orbital angle window calculated above corresponds to a communication timing window (CTW) of roughly 21 minutes.

Since the CTW is much smaller than the orbital periods of the child satellites around Titan, it will only be possible to communicate with some of the children at each alignment. Since the child orbits will spread equally around Titan, roughly 12 children should be visible at each alignment. Assuming that any given child is equally likely to be visible or not at each alignment, the chance of a child missing the communication link four times in a row is (50%)⁴ or 6.25%. Therefore, to ensure no data points are lost, the child satellites will need to be able to store at least four alignment periods worth of data, and the child-parent links must be able to burst this data to the parent within one twelfth of a CTW.

Child to Parent Data Rate

Equation 10 shows the calculation of maximum storage required for each child. Equation 11 shows the calculation of the required data rate for the child-parent links. These calculations assume an unrealistic worst-case scenario in which each child satellite sees every lightning event on Titan. The data rate calculation also includes a 20% networking overhead. Using such a high standard will ensure that the peak data rate of any child does not cause the network to fail. It will also ensure that the average performance of the network is such that communications should take place well within the CTW.

(10)

(11)

Child to Parent Modulation Scheme

Since the child-parent communications occur in orbit around Saturn, far from any other communications, there is no mandated channel spectrum to limit communication, meaning spectral efficiency is not a concern. To that end, for simplicity's sake, the child-parent links will be modulated using the same BPSK, ½ rate Turbo Code forward error correction, and Raised Cosine pulse shaping as the parent-Earth link. Table 5 shows a breakdown of the link performance based on the SNR calculated (child sat section). As can be seen, the link should be able to operate as specified with a very large SNR margin of 10.11 dB. Since operating any link with both antennas at half-power beamwidth only incurs an SNR loss of 6 dB, this margin should ensure that even if communications take place on the edge of a CTW, the link will not fail.

Figure 6 shows a block diagram depicting the path of data through the child satellite. Data is generated by the sensor array as described in the data generation section. Once generated, data is stored in the child satellite until it can be relayed to the parent via the communication link specified above.

Network Protocol

To coordinate the transmission of each child's data using a single parent receiver on a single communication channel, a simple protocol for time-division multiplexing is described in Figure 7 below (click to view source image). Assuming negligible processing delay on either side of the link, the network delay T_net due to path distance for the link is 1.51 seconds. This networking protocol will add 3T_net per child satellite, or roughly 1.81 minutes or 8.5% of a CTW. The 20% overhead factor added in the data rate calculation should more than make up for this.

Orbital System

For system launch and arrival at Titan, see Launcher and Path to Titan.

After the parent satellite arrives at Titan, it will orbit a few times around Titan to place the child satellites. The satellite configuration will follow that of GPS on earth. As it orbits, it will drop a child satellite off at 60 degree angles. When all 24 satellites are deployed, it will move into its final orbit location around Saturn.

In order to manage power requirements to communicate back to earth, the parent satellite will act as the middle man and communicate the findings back after everybody is in their place. Titan's orbit is slightly elliptical taking 15.945 days to complete an orbit around Saturn. The parent will follow the same angle of inclination, but with a circular orbit, such that it's period is 2x what Titan's is. this will cause Titan and the parent to have their closest approach 3x each period of Titan's orbit or around every 5.315 days.

(12)

Which results in an orbital radius of 769,300 km from the center of Saturn, where Ms is the mass of Saturn. Since the orbit is circular the velocity reduces to:

(13)

Therefore the speed of the parent's orbit around Saturn will be constant at 7 km/s.

The children will orbit at 1400 km around Titan because that is the lowest orbit where the atmospheric density will allow a long life. From equation 13 we get a constant velocity of 1.5 km/s.

Launcher and Path to Titan

Procella will hire the services of Titan IVB launch vehicle - the launch vehicle employed by NASA for its Cassini-Huygens mission to Saturn. The Centaur engines, now obsolete, powering the launch vehicle used for Cassini will be replaced by upgraded engines developed by SpaceX corporation. These new, more efficient engines will lower launch costs considerably as opposed to Cassini-Huygens.

Our mission will follow a similar path towards Titan. There will be two gravitational assist fly-bys of Venus followed by a flyby of Earth and a last assist of Jupiter to propel the payloads towards Titan. Owing to concerns of citizens over the earth flyby, we will try to avoid it and explore alternate trajectories.

Power Considerations

After being launched into space, the mission will depend on the fuel equipped in the parent spacecraft for all large changes in momentum required to navigate the correct trajectory to Titan.

Once the parent reaches Titan, it will jettison the each child in their predetermined orbits around the moon and place itself in an orbit around Saturn with respect to Titan as shown in Figure 8.

The parent and each child are equipped with ion thrusters each to make minor attitude modifications and maneuvers - namely appropriate orientation of the parent and sub-modules for communication.

Ion Thrusters

Ion thrusters were envisioned by Robert. H. Goddard in 1905 [18] as a means for propulsion in near vacuum conditions. Ion thruster is a form of electric propulsion. Although they generate a paltry thrust of 1 Newton per ton, ion thrusters outdo regular chemical combustion in specific impulse. Specific impulse is a yardstick of output that can be derived per unit of fuel. With an efficiency of 60-80%, they are perfect for orbital transfers, attitude modifications, etc. They are also ideal for generating much larger velocities required in deep space travel as opposed to feasible chemical rockets. This is achieved by operating a continuous thrust albeit over a long duration of time. The possibility of speeding up the time to reach Titan using the ion thrusters on the parent is being explored by Procella.

NASA's Jet Propulsion Lab is being approached as the designer for the ion thrusters required in the parent and also scaled down variants fitted on each sub-module. NASA's Evolutionary Xenon Thruster (NEXT) [19] ion engine may be a possible candidate as the thruster on the parent satellite.

Electric Power

Titan is about 10 A.U from the sun. At that distance, the solar insolation is way too less to be feasibly harnessed for generating electric power required for the mission. They also need batteries to supply energy when the solar panels are shielded. The only feasible source of energy for a deep space mission like Aurora is - Radioisotope Thermal Generator, more commonly known as RTG.

RTGs generate electricity by the Seebeck effect. Heat released by the decay of radioactive materials is converted into electricity by a thermocouple array. Although the efficiency of the some of the best RTGs is under 10%, the amount of energy that it can supply makes it worth the manufacturing.

²³⁸Pu, ⁹⁰Sr, ²¹⁰Po, ²⁴²Cm, ²⁴⁴Cm, ²⁴¹Am are some of the radioisotopes that have been studied as fuels for RTGs. 238Pu was used as the fuel to power the RTGs aboard Cassini. A radioisotope for RTG should need minimum shielding, emit mostly alpha radiation (easier to block and convert to heat), as little gamma as possible, and half a half life long enough for a constant power output for most of the mission duration. 238Pu satisfies the above criteria reasonably well.

As seen in Figure 10, the RTGs also act as general purpose heat sources, which will also prevent the spacecrafts from freezing and ceasing to function.

An RTG pack of 900W capacity will power the parent spacecraft. This power will be directed towards the ion thrusters or as power used for communication.

Each of the child satellites will be fitted with a low power RTG rated at 50W. This power will be used for the payloads, communication, momentum wheels used for fine attitude control and the ion thrusters as and when needed.

It is however very likely that RTG technology will be obsolete by the time this mission is realized. New generation radioisotope powered sources called Stirling radioisotope generators or SRGs would have matured enough to be commercially manufacturable. They have already been included in several mission proposals and will likely be included in this one as well. SRGs share the advantage of being free of movable parts along with their ancestor RTGs. However, at present, they are nearly 4 times more efficient than an RTG. SRGs are the future of radioisotope powered, non-reactor energy sources.

Power from the generators is conditioned using high efficiency DC-DC converters. It is then passed through load protection modules which will protect the communication modules, payloads, etc. from undervoltage or overcurrent events. Adequate sensors will provide the power system processor with data to make decisions. Procella will aim to make its power systems over 95% efficient so that almost all the power generated is conditioned and safely provided to the energy loads.

Note: Due to complexities and possible failure modes associated with using batteries in space, none of our spacecrafts will carry batteries.

There is concern among several citizens in the world over the use of radioisotopes and the possible radiation exposure due to an accident occurring with Aurora. Please refer to NASA's environmental impact statement when it launched Cassini. Owing to advancements since 1997, Procella will ensure lower failure odds than Cassini. Also refer to [21] for further clarification of possible concerns. Procella assures the citizens, that Aurora is as safe as things can be in a feasible realm of allowance and that this mission could provide priceless data in understanding the formation of life on earth as well as the next possible destination of life in our solar system.

Attitude Control

Deep space communication requires highly directional point to point communication for it to be successful. It is almost impossible to know how much the parent has to reorient to point its dish exactly towards the earth. This is harder in a mission like this because, not only are Saturn and Earth moving with respect to each other, the spacecraft is orbiting Saturn as well. To solve this problem, we take help of the cosmic positioning system - the position of the stars. We use sun sensors to locate any two stars to reproducibly orient the spacecraft. For Aurora, we will select the Sun and Sirius. Once the satellite can stably identify the sun, the next step is finding the earth with respect to the sun. This information is easily known and can be preloaded in the intelligence onboard the parent. Thus the parent will know where the earth is accurately and initiate point to point communication with Earth.

Coarse modifications in attitude control of the parent can be achieved by using ion thrusters or control moment gyros (CMGs). Any orbital correction required will be achieved using the ion thruster.

The child satellites will use ion thrusters for orbital correction. Unlike the parent, the children will be equipped with reaction wheels for attitude control.

Note: The thrusters serve as redundancy in attitude control to the momentum wheels thus improving the mean time before failure (MTBF) of the Aurora mission.

Resilience of Electronics

Space is polluted with several radiation sources. The most prominent radiation pollutants for our mission are cosmic rays, particle in the solar wind, ejections from events such as solar flares and coronal mass ejections as well as the RTG/SRGs equipped in our spacecrafts.

Digital electronics are the most susceptible to damage by radiation effects. Single event effects (SEL, SEU, etc) are the primary result of radiation damage. They lead to data corruption by causing bit flips. The other prominent radiation damages occur as frequency drifting in crystal oscillators and latching of CMOS switching mosfets in either on off states.

There are two broad methods of protecting semiconductor devices from radiation damage - namely physical and software.

Physical protection includes manufacturing chips on insulating substrates instead of silicon wafers. Such chips are called silicon on insulator (SOI) and sapphire (SOS) chips. These chips can withstand much higher radiation doses as compared to regular silicon devices [22]. These devices are called radiation hardened or rad-hard devices. CMOS devices can be replaced with bipolar IC counterparts. The IC package can be made radiation resistant or the chip itself can be made radiation resistant by using a depleted boron layer that can capture neutrons and prevent them from damaging the devices under the layer.

Software techniques such as error correction codes can help mitigate the effects of data corruption due to bit flips. Highly critical functions can be implemented in hardware rather than software. Data redundancy can be added to ensure that critical information is not lost.

Watchdog timers are almost a prerequisite in any spacecraft computer system. Watchdog timers are counters which perform a hard reset on the microprocessor once it counts down to zero. An OS needs to keep writing into a register in the watchdog timer before it counts down to zero in order to prevent repetitive resetting. If radiation causes the an error and the OS fails to write in the timer, the timer will reset the OS. The corrupted code will be replaced with the original code which is stored in a non-volatile memory and normal operation will be resumed.

Aurora will have a watchdog timer in the parent and every child satellite. We will also be using rad-hardened components in our electronics.

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Budget

1. Includes just main engineers employed by Procella Inc. Labor charges of subcontractors included in the cost of the subcontracted parts.

2. Includes spare components as well as testing of electronic modules.

Timeline

We estimate that development should be read to start by February, 2015, and conclude 25 months later in March, 2017.

Using the trajectory of the Cassini approach as a benchmark, we estimate that if Aurora is launched in March, 2017, the mission will arrive at Titan in November, 2023. From this point, lightning mapping will proceed until mission failure, with an intial mission target of 10 years.

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