A Method for Creating Enormous Space Telescopes

By Paul Gilster on March 4, 2021

As we follow the progress of the James Webb Space Telescope through performance tests in preparation for launch, Robert Zubrin has been thinking of far larger instruments. The president of Pioneer Astronautics and founder of the Mars Society thinks we can create telescopes of extremely large aperture — and sharply lower cost — by using the physics of spinning gossamer membranes, a method suitable for early testing as a CubeSat demonstration mission. In today’s essay, Dr. Zubrin explains the concept and considers how best to deploy next generation space telescopes reaching apertures as large as 1000 meters. We can’t know what new phenomena such an instrument would find, but the Enormous Space Telescope fits the theme of breakthrough discovery outlined in his latest book, The Case for Space: How the Revolution in Spaceflight Opens Up a Future of Limitless Possibility (Prometheus, 2020).

by Robert Zubrin

Abstract

This paper presents a method for creating Enormous Space Telescopes (ESTs). The EST employs a hoop to deploy a slack reflector membrane, such as solar sail material or radio dish. When the EST is simultaneously rotated around its center and accelerated along its axis of rotation, the membrane will assume a parabolic shape, thereby creating a reflector for a very large aperture telescope. The EST reflector can be accelerated along its linear axis by tethering its deployment hoop to a tug spacecraft. The tug can exert force on the hoop several methods, including direct thrust, centrifugal rotation of the tethered tug-reflector assembly, or by lowering the reflector from a high altitude balloon or more massive tug positioned in a higher orbit. A force equivalent to linear acceleration can also be generated to shape an EST without a tug using electrostatic means. ESTs can be used for astronomy across a wide spectrum of frequencies, ranging from the ultraviolet, through optical and infrared, down to radio. A demonstration EST with an aperture larger than the Webb Space Telescope could be flown on a CubeSat mission in low Earth orbit. ESTs with apertures of hundreds of meters could be delivered to heliocentric space in single flights of existing launch vehicles.

Background

There is no better place to do astronomy than space. Therefore, since the dawn of the space age, it has been the ardent ambition of astronomers to place ever more capable telescopes there. The largest such operational instrument, the 2.4 m diameter aperture Hubble Space Telescope, has benefitted from its location above the Earth’s atmosphere to make many great discoveries, and astronomers hold high hopes for more breakthroughs from the long-awaited 6.5 m diameter Webb Space Telescope. As the light gathering power of a telescope increases with the square of their aperture, still larger space telescopes are greatly to be desired. However, as the cost (>$10 billion) and quarter century long development schedule of the Webb telescope have demonstrated, new techniques will be required if construction of much larger observatories is to be made practical. This is the purpose of the Enormous Space Telescope (EST) concept.

The Enormous Space Telescope (EST)

The EST exploits the principle that if a flexible material subject to a linear acceleration is spun, the balance of linear acceleration and centrifugal acceleration forces will shape the material into a parabolic geometry. This technique has been used on Earth to spin cast liquid glass into parabolic dishes for use in telescopes up to several meters in diameter. The EST can employ similar physics with a properly tailored sheet of gossamer material in space to create parabolic reflector dishes with dimensions of hundreds of meters while keeping system masses well within existing launch vehicle limits.

In order to understand how the EST works, let us start by considering it in its smallest and possibly initial form, as a CubeSat demonstration mission. Consider a 13 kg, 12U CubeSat in a circular orbit 400 km above Earth. A one kilometer long tether is extended down from the satellite, and used to suspend a 13 m diameter (twice that of Webb) hoop, whose central axis aligns with the tether. Lines from the circumference of the hoop attach to the tether by a frictionless magnetic bearing, allowing the hoop to rotate freely. The interior of the hoop contains a slack solar sail material, properly tailored to accept a parabolic shape without folds, which is attached to the hoop like the skin of a slack-topped drum. Aluminized balloon film can be used to create solar sail material with a mass density of 6 grams/m2. Taking the hoop mass into account, we will assume 10 gm/m2 as the net mass density for our hoop/film combination, resulting in a mass estimate of 1.3 kg for that subsystem.

At an altitude of 400 km, the CubeSat will be moving with a velocity of 7668.63 m/s, generating a centrifugal acceleration of 8.6762 m/s2, exactly matching the Earth’s gravitational acceleration at that altitude. The reflector, however, hanging 1 km below the CubeSat will only be moving at 7667.50 km/s, generating a centrifugal acceleration of 8.675 m/s2. The Earth’s gravitational acceleration at that altitude will be 8.679 m/s. Thus the hoop will experience a downward acceleration of 0.004 m/s2, or 0.4 milliGees. This will make the sail film in the hoop sag. But if we rotate the hoop with an edge velocity of 0.1 m/s, the film material will also experience an outward acceleration, ranging from 0 at its center to 0.0015 m/s2 at its edge. Taken in combination with the linear acceleration, this will shape the film into a perfect parabola.

Fig. 1 An EST suspended by a tether in LEO. The telescope parabolic dish is spinning around the axis of the tether. Earth (down) is on the right.

This little demonstration EST, with a total mass less than 20 kg, including optics that would be positioned along or suspended from the tether at the parabola focal point, would have four times the light gathering capacity of Webb (about thirty times that of Hubble), while costing on the order of 1/1000th as much.

An even cheaper flight demonstration could be done suspending an EST from a high altitude balloon. Since a balloon moves with the wind, the payload would feel no wind. At 100,000 ft it would be above 99% of Earth’s atmosphere. A triangle of long spars could be employed with a balloon attached to each vertex, to keep the balloons out of the field of view of the telescope.

Such systems would have limitations, since they would be constantly pointing directly away from the center of the Earth. But we can do better.

Let us therefore scale our unit up in diameter by a factor of ten, to a 130 m diameter reflector dish, increasing the mass of the hoop, the optics and spacecraft by a factor of 100. It would still be a quite manageable mass though, about 2000 kg, easily launchable into interplanetary space by a Falcon 9 medium lift booster. In this case. there would be no gravity gradient available to stretch the tether. So we need to use an alternative technique.

One approach might be to spin the hoop around the spacecraft, in the manner of a tethered artificial gravity system, having a second hoop counter-rotating along with the one suspending the dish in order to neutralize gyroscopic effects. But such a system would still need to constantly change its pointing direction, making long duration exposures impossible.

Tugs for ESTs

A more effective approach would be to simply employ a spacecraft as a tug. Sunlight has a pressure of 9 micronewtons per square meter, which would add up to 0.12 Newtons over the whole body of the 130 m diameter sail. If that were the only linear acceleration of the sail, it would shape it into a parabolic reflector with its concave side pointing towards the Sun. As we want to be able to point the telescope the other way, we need to generate more thrust than that. This could be done using either electric propulsion or larger solar sails with a lower mass density then the hoop, or magnetic or electric sails, pulling its tether outward from the Sun.

Let us first consider electric propulsion. If we had an 70% efficient ion engine using argon propellant and a Isp of 7000 s, 50 kWe would be required to produce 1 N of thrust. Assuming a typical solar electric propulsion system mass to power ratio of 20 kg/kWe, that would require 1000 kg. The tug would thus accelerate at a rate of 0.001 m/s2. If the reflector was made of sail material with the minimum mass density of 6 gm/m2, its material would self-accelerate away from the Sun with an acceleration of 9e-6/0.006 =0.0015 m/s2, which is greater than the self-acceleration of the tug, and therefore unsatisfactory. However, the remedy for this is simple: just make the reflector material much thicker. For example, if we tripled its thickness to 18 gm/m2, its self-acceleration would only be 0.0005 m/s2, i.e. half that of the tug. So it would lag behind the tug and the net pull on it of 0.0005 m/s2 would make its center sag back towards the Sun. If we then set it spinning with a velocity of 0.1 m/s at its edge, an edge centrifugal acceleration of 0.00015 m/s2 would be created, shaping it into a 130 m diameter parabolic dish.

Fig. 2 Electric Propulsion tug pulling on an EST.

Operating at 1 N thrust, the thruster would consume 0.014 gm/s of propellant, or about 1.2 kg per day of thrusting (i.e. observing time). Thrust and thus propellant requirements would drop if the telescope were positioned further out in the solar system, since solar light pressure would drop as the inverse square of the telescope’s distance from the Sun. Thus, for example at 3.1 AU, it would only need to use 0.12 kg/day of propellant to generate adequate acceleration.

We could also use solar sails as tugs. In this case no propellant would be needed. Positioning the tug behind the EST would allow it to eclipse solar pressure, as shown if Fig. 3. If the tug is pulling the EST, making tug acceleration greater than reflector material self-acceleration could be assured simply by having the tug sails be larger than the reflector sail, and using a heavy gauge material for the reflector sail.

Fig. 3 Using solar sail tugs to accelerate an EST, by pushing from behind. The EST spins around the central axis. The Sun is on the left. An alternative design would send the mast through the sunward pusher sail, allowing it to deliver its thrust to the base of the mast by a set of shrouds.

A pusher sail telescope would need to point (generally, but not necessarily exactly) outward from the Sun all the time. However if a nuclear electric tug were used, and the telescope were positioned in the shadow of a planet, sunlight impinging on the rear side of the reflector would not be an issue and the telescope could be pointed in any direction.

In the case of radio telescopes, all of this becomes much easier, as there would be no solar light pressure on the rear face of the dish. In that case any kind of tug – solar electric, nuclear electric, or solar sail- could be used, with the EST pointable in any direction simply by maneuvering the tug. The amount of acceleration required from the tug could also be much less.

The Electrostatic EST

An alternative to physical acceleration to impose linear force on the dish is to use electrostatic attraction, In this case the reflector sail would be charged one way, while another sail positioned behind it and held off at a distance by a structural system would be given an opposite charge. The sails would thus attract each other, much as if by gravity, and when the assembly was spun up, both sails would assume parabolic shapes, with their concave sides pointing in opposite directions.

Let us consider the case of two 50 m radius dishes held 25 m apart by structure, with a potential difference between the two of 10 kV, creating a field of 400 volts/m. From electrostatics we have EA = Q/𝜺, so Q, the charge on each dish will be given by Q=(400)(7854 m2)(8.85e-12) = 2.8e-5 coulombs. The electrostatic force on each sail will be given by F=QE, so the total electrostatic force between the sails will be F=400(2.8e-5) = 0.0112 N. Assuming the sail materials have a mass density of 6 gm/m2, this will result in a self-acceleration each sail towards the other of 0.0112/(0,006)(7854) = 0.00024 m/s2. It may be observed that the field will actually be greater near the center because the dishes would sag towards each other. This, however, could be compensated for by varying the thickness of the sail material, making it thicker towards the center and thinner towards the edge, thereby keeping the linear self-acceleration of the two sails towards each other equal over their entire surfaces.

Fig. 4. An Electrostatic EST. The sails have opposite charges and are held separate from each other by a compressive structure. The mutual attraction of the sails can substitute for linear acceleration of the system

Size Limits of ESTs

There does not seem to be any theoretical limit to the potential size of an EST. However, as we have seen, using current materials, the mass required the create an EST system goes approximately as:

Where M is the EST system mass in kilograms and R is the aperture radius in meters. Thus our 6.5 m radius EST demo unit has an estimated mass, including its associated spacecraft, on the order of 20 kg, while our 65 m radius operational EST would be expected to have a mass on the order of 2000 kg.

Currently the largest operational launch vehicle is the Falcon Heavy, with a capability of about 60,000 kg to low Earth orbit (LEO). More powerful vehicles, including the NASA SLS and the SpaceX Starship system are expected to become operational within the next few years, with capabilities of up to 120,000 kilograms to LEO. Since an EST tug could propel itself out of LEO and into heliocentric space, this may also be taken as the limit of the size of an EST system, deliverable into space with a single launch. If we plus 120,000 kg into equation (1), we find that a practical size limit for relatively near-term EST systems would be an aperture diameter of about 1000 meters. The discoveries that might be enabled by such systems are beyond reckoning.

Conclusion

We find that the EST concept offers a practical path towards creating space telescopes with capabilities dwarfing conventional systems by many orders of magnitude. We also find that ESTs could be used to create space telescopes with comparable capabilities to conventional systems, but with several orders of magnitude lower cost. Furthermore, the EST concept holds these benefits for space astronomy across a wide range of frequencies, from ultraviolet down to radio. We therefore recommend that the concept be studied further, and that a demonstration mission be flown at an early date.

Acknowledgement; The author wishes to acknowledge the assistance of Heather Rose, who provided the illustrations for this paper.

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