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DARPA Zenith: Advancing Space Situational Awareness(SSA) with Liquid Mirror Telescopes

Introduction:

The exponential growth of space objects, including orbital debris, poses an increasing risk of collisions in Earth’s orbit. With over 100 million objects less than 1 cm in size, the safety of operational satellites is a significant concern.

Any of debris objects can cause harm to an operational spacecraft, where a collision with a 10-cm object could entail a catastrophic fragmentation, a 1-cm object will most likely disable a spacecraft and penetrate the satellite shields, and a 1-mm object could destroy sub-systems on board a spacecraft.

Additionally, the militarization of space has led to the development of microsatellites and anti-satellite weapons, emphasizing the need for robust Space Situational Awareness (SSA). SSA involves monitoring natural and artificial space objects, understanding changes over time, and predicting collision risks.  Telescopes play a crucial role in SSA, observing and tracking objects to ensure safe navigation, prevent collisions, and protect space infrastructure.

For deeper understanding on Space situational awareness (SSA) please visit: Space Domain Awareness (SDA): Ground-Based Radars, Telescopes, and Space-Based Sensors for Countering Space-Based Threats

Telescopes in Space situational awareness (SSA)

A telescope is an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects, or various devices used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.

Telescopes are essential tools for space situational awareness (SSA) as they allow us to observe and track objects in space. SSA involves monitoring objects such as satellites, debris, and other objects in orbit around the Earth to prevent collisions, ensure safe navigation of spacecraft, and protect critical infrastructure in space.

Telescopes located on the ground and in space provide important data on the size, shape, position, and motion of objects in orbit. This information is used to predict their future orbits and potential collision risks. Telescopes are also used to identify and track new objects entering orbit, including debris resulting from space collisions or decommissioned satellites.

In addition, telescopes are used to characterize the properties of objects in orbit, such as their reflectivity, composition, and physical structure. This information is critical for understanding the behavior of objects in space, such as how they interact with the Earth’s atmosphere and the effects of solar radiation on their orbits.

Telescopes also play a key role in supporting international efforts to manage space debris. By tracking and monitoring debris, telescopes help to reduce the risk of collisions and prevent further debris from being created.

Overall, telescopes are essential tools for space situational awareness and play a crucial role in maintaining the safety and security of objects in space.

For deeper understanding of telescopes and applications please visit: Telescopes: From Astronomy Enthusiast to Space Observer – A Comprehensive Guide

Limitations of telescopes in space situational awareness (SSA)

While telescopes are essential tools for space situational awareness (SSA), there are some limitations to their use. Here are some of the main limitations:

  1. Limited field of view: Telescopes have a limited field of view and can only observe a small portion of the sky at a time. This means that they may miss objects that are outside of their field of view.
  2. Weather conditions: Weather conditions such as clouds, fog, and atmospheric turbulence can affect the performance of telescopes. This can make it difficult to obtain clear and accurate observations.
  3. Interference: Light pollution from cities and other sources can interfere with telescope observations. Radio frequency interference can also be a problem, especially for telescopes that operate in the microwave frequency range.
  4. Cost: Telescopes can be expensive to build and maintain, especially those located in space. This can limit the number of telescopes available for SSA and the frequency of observations.
  5. Detection limits: Telescopes have detection limits that depend on factors such as the sensitivity of the detectors and the brightness of the objects being observed. This means that they may not be able to detect very small or dim objects in orbit.

 

The size of available telescope optics is a limiting factor for both astronomy and Space Domain Awareness (SDA). This limitation is primarily due to the high cost of manufacturing large, high-quality glass primary mirrors for telescopes. As the size of the primary mirror increases, the cost of manufacturing it increases exponentially. This relationship between cost and size is known as the “cost-size scaling law” of optics.

 

Because of this scaling law, it is difficult to produce large primary mirrors for telescopes. This limits the ability to mass-produce large telescopes, which in turn limits the proliferation and commercialization of telescopes at Very Large-aperture Telescope (VLT) sizes. As a result, the ability to observe and study objects in space is also limited by the size of available telescope optics.

 

Overall, while telescopes are essential for SSA, they are not perfect and have limitations that must be taken into account when using them to monitor objects in space. Therefore, a combination of observational techniques and technologies such as radar and lidar are used to supplement the limitations of telescopes in SSA.

 

Space-based mirrors are additionally vulnerable to catastrophic damage from hypervelocity debris impacts, which are anticipated to drastically increase in the near-future; and limited by payload fairing size, forcing cannot-fail, unfolding, segmented mirror designs. It is possible that liquid mirrors (LMs) may present a solution, but they have different limitations based on ground or space operation.

 

Liquid mirror telescope (LMT)

A liquid mirror telescope (LMT) is a type of telescope that uses a rotating liquid as a mirror instead of a solid mirror. The liquid used is typically mercury, although other liquids such as gallium or a mixture of molten metals can also be used. The liquid is spun at a high speed, creating a parabolic shape due to the centrifugal force. This shape reflects light like a solid mirror, allowing the telescope to capture images of the sky.

 

LMTs have several advantages over traditional solid mirror telescopes. One of the biggest advantages is cost. LMTs can be much cheaper to build and maintain than traditional telescopes because they do not require expensive precision grinding and polishing of a solid mirror. They are also easier to construct, transport and install. LMTs can be particularly useful for observations of large areas of the sky, such as the Milky Way.

 

However, LMTs also have some limitations. Because the liquid mirror is constantly rotating, it can only observe objects near the celestial equator, where the rotation of the Earth is in the same direction as the liquid. Objects near the poles are not visible because the rotation of the Earth is perpendicular to the rotation of the liquid mirror. The liquid also has a limited lifespan and must be periodically replaced, which can be costly and time-consuming.

 

Overall, LMTs are a promising technology for astronomy that has the potential to provide high-quality observations at a lower cost than traditional telescopes. However, their limitations must also be taken into account when considering their use in astronomical research

 

A 3m diffraction-limited LMT has been demonstrated for orbital debris observations.
However, fundamental limitations remain. Traditional LMTs can only point straight upward (zenith-restricted) and are, therefore, limited in the imagery they can gather. Since any movement out-of-plane disrupts the light focusing shape, they have only been applied to isotropic sky surveys, a limitation that has prevented LMTs from being widely used. Space-based LMTs have never flown for a number of reasons, to include complexities around spin-rotation and thrust-for-gravity required to form a coherent liquid mirror surface.

 

Today’s LMTs possess three advantages over traditional glass telescopes: (1) they are easier to
“fabricate”, since they are merely spun up without grinding or polishing, and do not require
custom facilities for manufacturing or periodic re-polishing, (2) they can be up to two orders of
magnitude cheaper than conventional parabolic glass telescopes , and (3) they can be relatively
impact damage-resilient (if in the space environment, potentially “self-healing” in the aftermath
of an orbital debris impact). Considering this, a breakthrough in LMTs away from the zenith
restriction may allow today’s limitations, for both astronomy and SDA applications, to be
addressed.

 

DARPA launched the Zenith program will investigate and develop modeling tools, materials, surface and field controls, and structures to eliminate these limitations to facilitate demonstration of a 2 m diameter liquid mirror telescope system (LMT) and a 1 m diameter segmented liquid mirror (LM) that may be pointed at off-zenith (out-of-plane tip/tilt) angles, and determine if the
described advantages of LMs persist. These designs will target primary mirror fluid control
methods that do not intrinsically require motion or gravity to create the mirror surface for future
applications in the space environment. If successful, Zenith can result in LMs and LMT systems
that can be used in a way more analogous to traditional point-and-slew telescopes, at
significantly larger aperture sizes, and have applicability to both ground and space.

 

Program Description/Scope

The primary goals of the Zenith program are to design, build and prove ground-based, tiltable,
scalable, damage-resilient liquid mirrors to support future ground and space systems. Zenith
solutions are to operate anywhere in the spectral band from 350 nm – 950 nm.

1. Develop the requisite surface and field control techniques to allow an LM to be tilted off-axis and slew, while maintaining imaging quality wavefront error levels, without a functional dependence on rotation, gravity, or thrust.

2. Develop the open-source modeling tools required to successfully design and predict LM
optical performance for a pointable LMT.

3. Develop both monolithic (single primary mirror) and segmented liquid mirror designs
with associated surface control, modeling, out-of-plane tip/tilt/slew, and imaging quality
wavefront error levels.

4. Demonstrate ground-based laboratory and on-sky LM imaging, with a 2 m diameter lens
and 25 Degree out-of-plane tilt and tip angles.
5. Demonstrate ground-based laboratory and on-sky LM imaging, with a 1 m diameter
segmented lens and 25 Degree out-of-plane tilt and tip angles.

Additionally, performers should develop the modeling and simulation (M&S) tools and
numerical approximations to evaluate the hydrodynamic motion of a liquid material while under
the influence of an electromagnetic field (or other actuating/control force). The technical
challenge is to understand how to model, simulate, and predict the evolution of the liquid mirror
surface as a function of the near-field source, and then apply that to a changing vector gravity
scenario given changing tip/tilt angles and slew motion of the mirror.

 

Conclusion:

The DARPA Zenith program represents an exciting endeavor to advance Space Situational Awareness through innovative Liquid Mirror Telescopes. By addressing limitations of traditional telescopes, Zenith aims to create tiltable, damage-resilient, and cost-effective LMTs capable of providing high-quality observations for both ground and space-based systems. Successful development and implementation of these cutting-edge telescopes hold the potential to enhance SSA, bolster space security, and protect critical space infrastructure for the future.

About Rajesh Uppal

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