Payload Review

Team 3Jeff Anderson, Thomas Blachman, Andrew Fallon, John Franklin, Samuel Gaultney, David Habashy,

Brian Hardie, Brandon Hing, Zujia Huang, Sung Kim, Jonathan Saenger

OverviewMission Objectives

The objective of this mission is to determine the origin and composition of Mars’s moon Deimos. Current hypotheses from the decadal survey speculate that Deimos is either a captured asteroid from the asteroid belt, or accreted from the collision between Mars and another large body. This mission will analyze the surface and sub-surface composition of Deimos to answer these questions.

Mission Overview

For this mission, a cubesat will share the launch and interplanetary bus of the Mars 2020 mission to gain a trajectory to Mars. After the interplanetary burn of Mars 2020, the cubesat will detach and remain in hibernation mode for the majority of the cruise. Towards the end of the cruise, the cubesat will perform intermittent burns with the main engine in order to place it on an

optimal intercept path with Deimos. The impactor will separate from the main body once 72 hours out from Mars. The impactor will then execute a separation burn with cold gas thrusters in order to arrive at Deimos 50 mins ahead of the mother cubesat. After a duration of 71 hours, the impactor will strike the moon’s surface at 3.75 km/s ejecting a plume of 5,720 kg into space above Deimos. The plume will reach its maximum altitude 50 mins later; at this time the cubesat will arrive and analyze the ejected material with the intent of the determining the surface and subsurface composition of Deimos. The cubesat will continue on its flyby of Mars while transmitting the collected data to a pre-existing Mars orbiter, which will relay the information back to Earth.

Mission RequirementsIn order to consider the mission a complete success the following requirements must be

met. The impactor shall collide with the surface of Deimos and generate a plume sufficient enough, in size, for the cubesat’s pointing accuracy and spectrometer’s imaging cone to detect. The impactor shall penetrate the surface of Deimos deep enough to expose volatile compounds. The cubesat shall analyze the plume with a spectrometer and determine the 1.3µm absorption levels, as well as the absorption levels of volatiles.

Key Requirements for Instrument SelectionSpectrometer The spectrometer must be able to detect the subsurface volatiles. A lack of these volatiles

would indicate a hot birth of Deimos, and would point to it being ejecta from Mars. The subsurface volatiles are made of chemicals such as: Water, CH4 ,CO2 CO. In order to accomplish this, the spectrometer must be able to make measurements within the range of 1.0 and 2.25 µm, as seen below in Figure 1.

Fig. 1. Absorption strengths of volatiles (Argus)

The spectrometer analysis must be able to determine if Deimos is a captured asteroid. Objects from the asteroid belt are primarily categorized as C and D class. These asteroids have very weak absorption spectra in the 1.3µm range, as shown in Figure 2 below.

Fig. 2. Absorption strengths of C and D class asteroids (Cloutis)

The spectrometer is required to operate in these ranges if it is to determine the origin of Deimos.

ImpactorThe impactor needs to be able to create not only a plume big enough to be seen by the

spectrometer, but also eject material deep enough to expose any subsurface volatiles. Additionally, the pointing accuracy of the cubesat is 0.003° and the field-of-view (FOV) of the Argus spectrometer is 0.15°. This means that from the instrument’s 600 km maximum range the plume must be greater than 0.78 km wide to guarantee that the plume will be detected.

The equations above are the non-dimensional equations used to estimate the yield of the Deep Impact mission from small scale models (Richardson). From these equations we can determine the size of the crater and therefore the amount of material ejected. Using the values for the impactor speed and mass, 3.75 km/s and 4 kg respectively, it can be found that the plume’s width is 4 km and that material from as deep as 1.57 meters is ejected. These results produce a margin

over the specified requirements, however these values have already been budgeted and the extra capability will be kept as a descope option.

The velocity of the plume will be fairly slow. The equation below was used to determine the velocity distribution of the plume as a function of location in the crater.

Using this equation it was found that only 5% of the ejecta moves faster than the escape velocity of 5.56 m/s. The particles with the highest altitude will began to fall back to Deimos 50 minutes after impact, marking the maximum size of the plume. Because of this, a driving requirement is that the impactor must be able to reach Deimos 50 minutes in advance of the cubesat.

Spectrometer Trade StudyThe spectrometers that we evaluated were the ARGUS, mini-INMS, and BIRCHES

spectrometers. For the selection of the spectrometer, a trade study was done in order to choose between these three different spectrometers within the allocated 2U size constraint. The criteria between the spectrometers included mass, volume, power, spectrum ranges, operating range, data collection rate, pointing, and cost. Figure 3 below shows the comparison of the spectrometers with the selected criteria. The cells that are highlighted in green are the most desirable values for the criteria. With the smallest mass, volume, and lowest power requirements, the ARGUS spectrometer provides the greatest flexibility and size efficiency for the mission. In addition to these advantages, this spectrometer has a high TRL. Therefore, ARGUS will be chosen as the main science instrument due to its overall benefit to the mission, especially since it can detect spectral bands in the 1000 to 2500 nanometer range.

Figure 3: Mass Spectrometer Trade Study Table

Instrument ARGUS mini-INMS BIRCHES

Mass (g) 230 600 2000

Volume (U) 0.18 1.1 1.5

Power (W) 1.4 1.8 5

Spectrum (nm) 900-2500 0-2000 0-4000

Range (km) 600 675.924 100

Data Rate (Mb/s) 1 0.0013 10

FOV (degrees) 0.15 20 12

Cost ($) 49,500 15,000 200,000

TRL 8/9 6/7 8/9

Impactor Trade StudyThe impactor was already evaluated in a trade study in our mission planning phase. However some options related to its operation needed to be evaluated. The first was the amount of time spent in its separation period where it gains its 50 minute gap between itself and the cubesat. As seen in the figure below the earlier it detaches the less fuel it needs for the separation burn, but the more power will have to be supplied. The risk also increases the more time it is required to navigate by itself. The decision was made to release it earlier despite the risk, as it limits the wet mass of the impactor that isn’t transferred to Deimos.

Time Released Before Impact (hours)

ΔV Required (m/s)

Weight of Fuel (kg)

Approximate Power Required (Wh)

Risk (1-10)

24 130 1.2 60 7

48 65 0.7 120 6

72 30 0.43 180 6

A second decision we made regarded the power source of the impactor. Solar was considered as it would provide power indefinitely. However it was found to be much larger and heavier than batteries. So large that it would require folding solar panels that would raise risk. Batteries were chosen due to their density, and because the impactor will only need power for 72 hours no matter what.

Power Source Size (cm) Weight (kg) TRL

Solar 38.1 x 31.5 x 10.2 2.35868 8

Battery 7 x 3 x 2 - 7 x 3 x 6 0.032 - 0.096 9

Risk and MitigationsThis cubesat mission is of moderate risk.

1. Spectrometer Out of Range

2. Spectrometer Performance Failure

3. Impactor Misses

4. Impactor Fails Separation

5. Plume Size Failure

6. Trajectory MishapThe spectrometer is is a sensitive instrument, and thought the ARGUS has a high TRL, the chance of failure must be assessed. In the case of the spectrometer being out of range, which is the most probable type of failure, the trade study allowed extra range, granting a large degree of error. If the spectrometer where to electronically fail, the entire mission is lost. However the high flight heritage makes it very unlikely.

The impactor is a low TRL payload comprised of high TRL components. The greatest risks involve missing the target and failure to separate. The impactor will have it’s own system dedicated to flight trajectory, complete with propulsion and ADCS, this greatly reduces the chance of this happening. The separation failure would lead to loss of primary objective, but would still allow for spectrometry of the surface of Deimos.

Plume size isn’t an exact science, however models can be made that are very good at approximating what can happen. If the plume were not to disperse as expected, and not reach the volume or altitude needed, the spectrometer is capable of a wide margin of error that may make up for the dispersion failure. If the trajectory of the rocket launch is off, the cubesat failure will not even make the news, as Mars 2020 would have been lost.

Citations

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Thoth Technology Inc. Argus 1000 IR Spectrometer. Release 1.03. Pembroke, Ontario, Canada. 2010.

Clark, Pamela E. NASA NextSTEP Lunar Ice Cube Mission. NASA. PDf

Richardson, J. E., Melosh, H. J., Artemeiva, N. A., & Pierazzo, E. (n.d.).

Impact Cratering Theory and Modeling for the Deep Impact Mission: From Mission Planning to Data Analysis. Deep Impact Mission: Looking Beneath the Surface of a Cometary Nucleus, 241-267. doi:10.1007/1-4020-4163-2_10

Rodriguez M., N. Paschalidis, S. Jones, E. Sittler, D. Chornay, P. Uribe, T. Cameron.Miniaturized Ion and Neutral Mass Spectrometer for CubeSat Atmospheric Measurements.digitalcommons.usu.edu/cgi/viewcontent.cgi?article=3524&context=smallsat. Powerpoint Presentation.

Cloutis, E. A., & Gaffey, M. J. (1993). The Constituent Minerals in Calcium-Aluminum Inclusions: Spectral Reflectance Properties and Implications for CO Carbonaceous Chondrites and Asteroids. Icarus, 105(2), 568-579. doi:10.1006/icar.1993.1150