Mysterious microsatellites in GEO: is MiTEx a possible anti-satellite capability demonstration?by Ryan Caron
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| Information on the microsatellites themselves is virtually nonexistent. Seeing as similar previous missions were conducted largely within the public eye, one has to wonder what MiTEx is doing that must remain so secret. |
While MiTEx itself is shrouded in secrecy, the launch on June 18 publicly touted a new upper stage built by NRL. As a “technology demonstrator”, the upper stage is equipped with lightweight, high-capacity propellant tanks and thrusters that use a long-lasting alloy. It also has solar panels, lithium-ion batteries, and a star tracker. Compared to traditional upper stages, which typically consist of an unadorned solid-fuel rocket motor and little else, this elaborate contraption is quite novel and is designed to do a lot more than boost the microsatellites from their transfer orbit to GEO.
Yet while this upper stage is unusual, every one of the individual technologies listed below are themselves tested and well-established. Since DARPA released a widely published list of all the technologies used in the upper stage, it was possible to analyze the key components:
Platinum/rhodium bi-propellant attitude control thrusters: Thrusters that use platinum/rhodium are designed to be fired tens of thousands of times, equating to more than 50 hours of total burn time over the life of the thruster. These types of thrusters have been used on a variety of long-endurance spacecraft, and are an evolution of the common columbium thrusters that went all the way to Jupiter on the Galileo mission. The platinum/rhodium alloy, while a fairly recent development, is already being used in Astrium’s 10N thruster line. Such a long-lasting thruster is most unusual for a conventional upper stage, whose functional life is typically measured in hours at most.
High-performance coated columbium delta-V thruster: Columbium-based thrusters were introduced in the 1970s. In this case, the delta-V thruster provides the burn necessary to transfer from geostationary transfer orbit (GTO) to GEO. This is a mission-critical event, and a proven technology is desirable. This motor would be even more important if the MiTEx’s hypothesized orbital maneuverability was indeed the case.
The only potential for new technology is the high-performance anti-oxidant coating. These coatings, usually a platinum alloy of some sort, traditionally chip and degrade after heavy use and time in orbit. It would be assumed that any new coatings would improve on these limitations. However, the need for coatings would be negated altogether if the platinum/rhodium alloy, used in the stage’s attitude control system, were implemented. As such, this thruster was apparently selected for its proven reliability, not as a technology demonstrator.
Inconel-718 composite overwrap pressure vessels and titanium propellant tanks with internal propellant management devices : Simply put, the lightest tank for a given amount of propellant is spherically-shaped, since that is the minimum surface area for a given volume. However, when additional propellant is needed, the diameter of the spherical tanks is often constrained by the size of the satellite and the launch vehicle shroud. This requires changing the tank geometry from spherical to cylindrical. This comes at a significant mass penalty due to the additional titanium. However, less titanium is required when the cylindrical tank is wrapped with an Inconel composite, which mitigates the increase in mass.
| So, if MiTEx is a technology demonstrator, and the upper stage has proven hardware, what exactly are the technologies that this mission is demonstrating? |
This tank construction is consistent with the inferred additional propellant onboard the NRL Upper Stage beyond what is required to simply boost the microsatellites to GEO. This extra propellant is surmised from the payload capabilities of the Delta 2 7925 booster used to launch MiTEx. With each microsatellite weighing 225 kilograms, over 600 kilograms is still available for the upper stage. This is far more than what is necessary, providing room for as much as an estimated 200 kilograms of additional propellant for other, undisclosed maneuvers. The additional propellant storage is also consistent with potential uses of the micro-satellites for inspection and rapid maneuvering.
Triple-junction solar cells: This is only the second time that solar cells have been present on an upper stage for GEO delivery. The Air Force’s Integral Apogee Boost System (IABS) launched the Defense Satellite Communications System 3 (DSCS 3) spacecraft between 1991 and 1998 and was equipped with solar panels. However, IABS resembles the NRL Upper Stage in its use of solar panels only. Since IABS does not appear to be as capable a platform, the true advantages of solar panels on upper stages were not used to their fullest extent (so far as we know) until now.
Triple-junction solar panels have increased efficiency over previous generations because they can take greater advantage of the sun’s spectrum. While fairly new, these solar cells do not need additional flight testing: they have already been to Mars on the rovers Spirit and Opportunity.
All of these technologies have flight heritage. So, if MiTEx is a technology demonstrator, and the upper stage has proven hardware, what exactly are the technologies that this mission is demonstrating? The technology goals of the MiDSTEP program provide a huge range of untried technologies for DARPA to investigate.
Microsatellite deployment in GEO raises serious concerns. At such distances, ground-based detection via visual observation or radar is extremely difficult if not impossible. Currently, only the US Space Surveillance Network is capable of detecting these satellites reliably. This effectively gives MiTEx stealth capability. MiTEx is classified as a technology demonstrator, and it may be simply that. However, the possibility remains that the satellites are intended to do much more than demonstrate technology. Either way, the technologies used on MiTEx have substantial implications.
The MiTEx technologies detailed under the parent MiDSTEP program are applicable to a wide variety of military missions. The propulsive and lifetime capabilities of the NRL Upper Stage enable the satellites to go anywhere in GEO, and even perform proximity operations around other satellites. Such proximity would enable detailed reconnaissance of a satellite: identifying weaknesses, taking photographs, and collecting all the satellite’s incoming and outgoing radio traffic. More hostile acts, such as denying ground communications, depleting propellant reserves, and even inflicting permanent damage to the satellite cannot be ruled out.
| MiTEx is classified as a technology demonstrator, and it may be simply that. However, the possibility remains that the satellites are intended to do much more than demonstrate technology. |
More benign applications are certainly possible, such as using the satellite as a synthetic aperture radar platform. Using both microsatellites and the mobility of the upper stage, a dynamic and versatile radar system is possible. Although radar small enough to fly on satellites has a relatively limited range, on the order of a thousand kilometers or less, the radar fence between the two microsatellites could create a precise map of critical orbital slots within GEO.
MiTEx could be demonstrating technologies that have not yet been tested in the harsher GEO environment. Or it could indeed be operational, performing any number of possible clandestine missions. We simply do not know.
