Phobos, its exploration and the ‘Hedgehog’

Technology

Akhilesh Agarwal Electrical Engineering

PESIT Bangalore, India

akhilesh.ab@gmail.com

Abstract – The development of

technology jointly by NASA and

Stanford has caused excitement in the

space exploration field. Using this

technology it is possible to explore the

surface of Phobos by launching spiked

beach ball shaped pods onto its surface

from a ‘mother spaceship’ by pre-

mapping the surface of Phobos using

the Phobos Surveyor orbit. The main

purpose of exploring Phobos is for it to

be a hub for future missions to Mars.

I. INTRODUCTION Humans, for many years now, have tried to explore the surface of Mars through various means such as the rovers Spirit and Opportunity and more recently, Curiosity. Some of these mission have been fruitful and others, a little less so. The main issue that underlies a manned mission to Mars is the difficulty of coming back from its surface. To overcome this issue an alternative approach to Martian exploration is being thought of – using Phobos as a station between Earth and Mars.

A novel idea to start off the construction process of a docking station on Phobos is to explore its surface using small, spiky, spherical rovers that NASA, Stanford University and the Massachusetts Institute of Technology have been speculating on working on. [1] This paper will give a brief overview of all the missions that have been launched by NASA and Europe to survey the orbit of Mars and in-turn provide data on its moon, Phobos. In addition to Mars missions the Phobos Surveyor mission [2] and the accompanying ‘Hedgehogs’ in development will be discussed.

II. MARS EXPRESS SPACECRAFT

The Mars Express is the first mission to be launched by Europe to the Red Planet [3]. This orbiter has an on-going investigation

of Mars and its moons, Phobos and Deimos. It was originally launched to study the atmosphere and geology of Mars but it has also studied its closest moon, Phobos, in detail. The images shown are computer generated graphic depictions of the spacecraft [4].

The components of this spacecraft from clockwise are: (1) High-gain antenna (2) Spacecraft Subsystems (3) Beagle 2 lander (4) Orbiter Instruments (5) Folded Solar Panel and in the centre there is the propellant tank.

III. OPPORTUNITY

Photographs from the Mars rover, Opportunity, have shown evidence of regular transits of Phobos across the Sun during which the

moon’s shadow is cast on the surface of Mars. This reveals that Phobos is too small to cover the solar disk due to which a total eclipse is not possible [5]. Opportunity gives us evidence that

Phobos could be a second generation solar system body that coalesced in the Martian orbit after the formation of the planet. It denies the claim that Phobos formed concurrently out of the same cloud that gave birth to Mars [6].

IV. MARS GLOBAL SURVEYOR

NASA’s Jet Propulsion Laboratory launched the Mars Global Surveyor (MGS) in November, 1996. Through this project the Unites States returned to the Mars exploration scene after 10 years. The Mars Global Surveyor did considerably more than just study Mars. It wasn’t uncommon for the MGS to perform rolls and pitches to take images off its nadir track. The roll maneuvers are called Roll Only Targeting Opportunities (ROTO) and

allow it to shoot images from as far off as 30° from nadir. The relative motion between the MGS and the planet was compensated by controlling the pitch of the spacecraft. NASA called this the Compensation Pitch Roll Targeting Opportunity (CPROTO) [7,8]. Due to its maneuverability and flexibility it allowed for very high resolution imaging by onboard Mars Orbiting Camera (MOC). The image below shows the camera system [9].

In addition to shots of Mars the Surveyor was capable of taking images of its moons. The Mars

Global Surveyor took a spectacular shot of the Phobos Monolith in 1998 using the Mars Orbiter Camera [10]. Below is shown a more recent image of the Phobos Monolith from the HiRISE camera [11].

V. MARS RECONNAISSANCE ORBITER AND HiRISE

Under the supervision of the Jet Propulsion Laboratory, Lockheed Martin built this $700 million spacecraft designed for the exploration of Mars from orbit [12]. This spacecraft was put into Martian orbit on March 10, 2006. The landforms, minerals, stratigraphy and ice on Mars are analyzed using a plethora of scientific instruments such as radars, spectrometers and cameras aboard the Mars Reconnaissance Orbiter [13]. The MRO monitors the daily weather and surface conditions on Mars studying potential landing sites, paving the way for future spacecraft. It is intended to serve as a highly capable relay satellite for future missions by sending back data to Earth using its new telecommunications systems. The MRO carries on board the HiRISE (High Resolution Imaging Science Experiment) camera which has photographed hundreds of targeted swaths of Mars’ surface in detail [14]. Although the camera operates in the wavelengths of the visible spectrum the images produced by the telescopic lens of the HiRISE camera are taken at resolutions never before seen in planetary exploration missions. The purpose of these high resolution images is to help scientists study the morphology of Mars in a more

comprehensive manner than before. The HiRISE gathers information on the mineral groups present though observations at near-infrared wavelengths. These new, high-resolution images are providing unprecedented views of layered materials, gullies, channels, and other science targets, in addition to characterizing possible future landing sites. Areas for close-up HiRISE imaging are selected on the basis of data returned from Mars Global Surveyor, Mars Odyssey and regional surveys conducted by the Mars Reconnaissance Orbiter's own instruments. The MRO can only ever see one face of Phobos because it orbits Mars far below the orbit of Phobos. Below is the image of a clear shot of the surface of Phobos showing the Stickney crater from the HiRISE [15].

The area around the crater appears to be graying in colour as opposed to the colour of the rest of the body. This suggests that the gray surface is relatively younger.

VI. STRUCTURE OF PHOBOS The surface of Phobos has been bombarded by asteroids crashing into its ancient surface over the years. It has the look of a massive potato. It was also thought of as a fragment of Mars which was blasted off by a large asteroid. An image taken by the HiRISE camera is shown below.

The mean radius of Phobos is 11.1 km which is 7 times bigger than Deimos. Its dimensions are 26.8kmx22.4kmx18.4km which makes its too small for it to be rounded under the force of gravity [16]. The force of gravity exerted by this moon is about 1/1000th than that of the Earth’s [17]. It is irregular and small with its orbit being 9400 km from the center of Mars [18]. In fact, Phobos is so close to Mars’ surface that it produces an unusual effect - it revolves around Mars faster than Mars rotates about its own axis. This happens because

Phobos orbits Mars below the synchronous orbit radius. Phobos is one of the least reflective and most heavily cratered bodies in our Solar System. On one end it has a large impact crater called the Stickney crater – named after the wife of the scientist who discovered Phobos (Asaph Hall). An interesting feature about Phobos is that its orbital radius is reducing because of its short orbital period and tidal interactions. It is estimated that because of this phenomena it will crash into Mars in the next 10-12 million years [19].

VII. SURFACE OF PHOBOS It is believed that the entire body is made up of material with composition similar to that of carbonaceous chondrite, covered with a layer of fine grained regolith at least 100 meter thick as recent images from the Mars Global Surveyor tell us. Phobos’ density (1.876 g/cm3) is too low for it to be a solid rock [20]. Due to the high porosity of the surface of Phobos it is speculated that the body may contain a reservoir of ice. Spectroscopically, there is evidence that the surface of Phobos lacks hydration but this does not throw the presence of ice below the regolith off the table. Amidst all these features there is the Stickney crater, located just above the equator, which takes up a large proportion of the surface of Mars [21]. The impact that happened

millions of years ago came close to shattering the moon. There have been analyses of results from the Mars Express Spacecraft which have revealed that the grooves that run down the circumference of Stickney are not radial but centered on the leading apex of Phobos in its orbit [22]. This moon is speculated to be one of the best studied satellites in the solar system.

VIII. PHOBOS VS. DEIMOS The study of Phobos is considered at a higher priority over Deimos because of three reasons: (1) Phobos’ orbit is significantly closer (a = 2.76RM) to Mars than Deimos’ (a = 6.90RM) which makes it a better positioned platform for remote sensing and monitoring of Mars. (2) Opportunities for discovery and scientific enquiry are enhanced since Phobos’ dimensions are larger. (3) The abundance of boulders due to the lack of regolith mantling offer ready access to consistent rock outcrops allowing for easy harvesting of samples [23].

IX. PHOBOS SURVEYOR Marco Pavone of Stanford University along with the Massachusetts Institute of Technology and the NASA Jet Propulsion Laboratory is the pioneer behind the architecture of the mission to Phobos. This project is a part of NASA’s Innovative Advanced Concepts program [24].

The Phobos Surveyor is an orbiter around the Martian moon, Phobos, which is designed to gather measurements from its surface and the chemical composition. The Phobos Surveyor mission would be fruitful to future manned missions on Mars by investigating the surface and in turn constructing a base on Phobos first before the construction of one on Mars. The Surveyor orbiter would help determine the best landing sites on the moon for manned missions. If successful this technology could be applied to missions to more solar system bodies that are considerably small.

X. HEDGEHOGS ‘Hedgehogs’ are roughly spherical rovers which measure roughly 2 feet across that will be deployed by the Phobos Surveyor which will serve as

the ‘mother ship’ to these sea urchin-like robots. They are so designed in order to adapt to the low-gravity environments in which they are required to function. A full mission using the Phobos Surveyor would take about two years, assuming five or six hedgehogs are sent with the spacecraft. During this time the mother ship could spend a few months analyzing the potential landing sites before releasing the hedgehogs. The mother spacecraft (Phobos Surveyor) would deploy these spiked robots one at a time. These small robots will provide us with information about Phobos’ soil, and its other surface features using microscopes. The robots working on the surface of Phobos would then send out a signal to the Phobos Surveyor to release a new hedgehog [25].

Concept of the Phobos Surveyor Mission

Since the gravity on Phobos is 1/1000th that of the Earth, traditional methods of locomotion such as wheels and legs would not work as the wheels would just revolve but the robot wouldn’t be unable to move. To overcome this, the prototypes designed by NASA have three spinning disks in an orthogonal configuration which are accelerated or decelerated to adjust the speed of the hedgehog. Once accelerated the hedgehogs use the inertia to move around the surface of Phobos. Stepping away from the usual means of movement the hedgehogs will move in a series of hops or short flights that are controlled. These tiny rovers will derive their power from solar panels placed on them. The advantages of the unique design and shape of these hedgehogs are threefold: (1) Overcome difficulties in movement due to low gravity. (2) Obstacles such as canyons and crevasses can be easily hopped over as opposed to traditional rovers that would get stuck. (3) Require minimal human intervention to function. Inter-communication between rovers would allow them to determine their location and future movements. This technology is solely being developed for the exploration of Phobos although if it is successful it will be used to survey other small bodies in our solar system.

To quote Marco Pavone, he would say that this new system represents “the next level of autonomy in space” [26].

ACKNOWLEDGMENT I thank Professor Alessandro Massarotti, Stonehill College, for motivating me to write this research paper. I also thank Nidhi Angle, Birla Institute of Technology and Shreesha Muthyala, Drexel University, for their help in creating the layout for this paper.

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[25] Space.com, January 19, 2013, Elizabeth Howell [26] Mothership and her Hedgehogs: New Concept for Exploring Phobos, Space Safety Magazine, January 16, 2013, Maria Fischer