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There has been an exponential growth of space objects, including orbital debris that has increased the in-orbit collision risk. NASA estimates there are 21,000 objects orbiting Earth that are larger than 10 cm, 500,000 between 1 and 10 cm, and more than 100 million that are less than 1 cm. The large number of debris around Earth is a risk for the safety of operational satellites. 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.
Space is becoming increasingly militarized many countries are developing killer microsatellites and other antisatellite weapons (ASAT) that could be used to damage other satellites. There is also thrust on space robots which can perform repair of satellites and which could also put to deorbit adversary’s satellites. They could provide complete awareness of adversary’s activities in space so that one can take counter actions.
Space Situational Awareness
Space situational awareness (SSA) is the foundational element of space security, and it entails keeping track of all natural and artificial space objects, energy and particle fluxes and understanding how the space picture is changing over time. Space Situational Awareness entails detecting, tracking, and identifying all natural and artificial space objects, energy and particle fluxes in Earth orbit using a network of sensors and systems.
SSA encompasses surveillance of all space objects, activities, and terrestrial support systems (satellites & debris), more detailed reconnaissance of specific space objects assets (mission identification, capabilities, vulnerabilities, etc.), discerning the intent of others who operate in space, knowing the status of our own forces in real-time, and analysis of the space environment and its effects (solar storms, meteor showers, etc.).
SSA is a system of systems dealing with space surveillance, space weather and NEOs. Comprehensive SSA requires a networked system of radars and electro-optical sensors. Low altitude debris are usually observed by radar ground stations while high altitude debris are observed by optical ground stations. Recently the trend is to use space based sensors to provide timely detection, collection, identification and tracking of man-made space objects from deep space to LEO orbits.
Optical telescopes
Electro-optical sensors consist of telescopes linked to video cameras and computers. The video cameras feed their space pictures into a nearby computer that drives a display scope. The image is transposed into electrical impulses and stored on magnetic media. Thus, the image can be recorded and analyzed in real-time or later
Optical telescopes by gathering and focusing electromagnetic radiation reflected by the telescope target generate a magnified image of the object. Three primary types of
optical telescopes are used; (i) Refractors, which use lenses, (ii) Reflectors, which use
mirrors and (iii) Catadioptric telescopes which use a combination of lenses and mirrors.
At altitudes above 5,000 km, it becomes increasingly time-consuming and difficult for radars to search for objects. Optical telescopes are able to perform this function faster and easier than ground-based radar systems, although they are limited to clear weather and night-time observations.
Adaptive Optics
Recently, Adaptive Optics (AO): “measuring distortions in a wavefront and compensating for them in the light detection system”, is increasingly being used for SSA applications. This technology enhances the performance of a telescope by effectively “cancelling out” the distortions of Earth’s atmosphere.
When a telescope on the ground looks to the cosmos and takes a picture, the light it captures has first travelled through air in the atmosphere. If that air is at all turbulent, it blurs the light. This is termed “atmospheric distortion.” It’s the reason stars seem to “twinkle” when we look at them from Earth, and also the reason many ground-based telescopes can’t take super-sharp images of objects in space. Newer, more advanced ground-based telescopes implement adaptive optics, a technique to sharpen blurry images right as they’re being taken.
An adaptive optics system uses lasers, supercomputers, and an array of mirrors to correct atmospheric distortion. To sharpen an image with adaptive optics, astronomers will pick a point of light, called a “guide star.” They’ll measure and monitor how its light waves bends as they move through Earth’s atmosphere, and then send that information, via supercomputer, to the telescope’s mirrors. In response, the mirrors bend in a way that straightens out the light waves as they are reflected onto the telescope. Of course, there isn’t always a guide star near enough to the object that an astronomer is studying, and, in that case, astronomers use lasers as a guide star instead.
Space-based telescopes
An emerging solution is to put radars and telescopes into orbit. Space-Based Sensors (SBS) consist of a space segment, primarily consisting of a constellation of optical satellites, and a ground segment including networked ground stations to control the satellites.
SBS having the ability to track space objects from space, offer advantages over ground based systems since they are not affected by weather or atmosphere. This leads to improved sensor sensitivity and allows for the detection of faint objects including microsatellites and space debris. This in turn increases the probablity of collision event detection by improving the timeliness of detection for maneuver. Space based sensors will provide timely detection, collection, identification and tracking of man-made space objects from deep space to low-earth orbits.
For debris observations radar and optical stations are active, but also in situ measurements (e.g. optical observations from satellites) are efficient. Low altitude debris are usually observed by radar ground stations while high altitude debris are observed by optical ground stations.
US Air Force’s Small Telescope
Previously classified adaptive optics technologies enabled the U.S. Air Force Research Laboratory (AFRL) to capture an image of an asteroid’s moon using a telescope measuring just 1.5 meters across.
The images of the moon were captured at the Starfire Optical Range at Kirtland Air Force Base in New Mexico. The small moon, known as Linus, measures around 30 km in diameter and orbits the asteroid 22 Kalliope at a distance of around 1100 km every 3.6 days. The asteroid and its moon are found in the main belt of asteroids which stretches between Mars and Jupiter. The 1.5m telescope is only one of several telescopes at the base, all of which are claimed by AFRL to be capable of tracking satellites in low Earth orbit (LEO).
The key technology enabling this milestone is known as adaptive optics, a system that enables scientists to reduce the blurring or distortion caused by atmospheric interference when viewing objects in space. There are a few different types of adaptive optics in use, ranging from using deformable mirrors that can ‘cancel out’ atmospheric interference to using lasers to project artificial “guide stars” that can be used as reference points to measure and subsequently reduce atmospheric interference. AFRL did not state which particular adaptive optic system was used in the recent imaging of Linus but did say that the images were made without the use of a laser guide star.
According to an AFRL fact sheet, the site facility “operates one of the world’s premier telescopes capable of tracking low-earth orbiting satellites.” Dr. Jack Drummond, a Starfire astronomer, says that refining advanced imaging methods such as those used to view 22 Kalliope’s moon, Linus, could help not only in viewing celestial objects but also in detecting satellites in Earth’s orbit.
“The reason we even look at the asteroids at the [Starfire Optical Range] is because they are a good proxy for manmade satellite observations,” Drummond told the Albequerque Journal last year. “Nobody wants to be near each other’s satellites. We can’t tell two satellites to get together. Instead, we look at asteroids and their moons. It’s a perfect analogy, a perfect proxy.”
