On June 18, 2006, a Delta II 7925 rocket launched a pair of Micro-satellite Technology Experiment (MiTEx) microsatellites into geostationary orbit (GEO). Due to their classified nature, the function and technological specifications of the satellites are unknown; even their orbital position is classified. One thing that is known is that Lockheed Martin and Orbital Sciences each built one, weighing 225 kilograms apiece.
The MiTEx microsatellites are a part of Microsatellite Demonstration Science and Technology Experiment Program (MiDSTEP), a joint program between the Defense Advanced Research Projects Agency (DARPA) and the Air Force. MiDSTEP, together with a program known as System F6, falls under DARPA’s Program Element 0603287E. $43 million was allocated to both these programs from fiscal year 2005 (FY 05) to FY 07. In analyzing the open literature, the technologies to be demonstrated by the MiTEx satellites appear to be consistent with some proposals for anti-satellite weapons. As such, there is concern about the United States developing anti-satellite capability when there has yet to be a full public discussion of the ramifications.
NRL Upper Stage
Although most information about MiTEx remains shrouded, some intriguing details have been released about the new upper stage that took the microsatellites to GEO. Built by the Naval Research Lab (NRL), this stage is equipped with solar panels and a star tracker – unusual since stages normally rely on the payload for such functions – as well as thrusters made of a special long-life platinum/rhodium alloy.
The NRL hardware is a drastic departure from conventional upper stages, whose lifetime is typically measured in hours. The hardware on the NRL Upper Stage, at a minimum, could last weeks. In fact, only one other U.S. upper stage has ever had solar panels, the Integrated Apogee Boost Subsystem (IABS). IABS was used to carry the Defense Satellite Communications System satellites to GEO during the mid 1990s. Beyond this, no other upper stage manufactured for transfer to GEO remotely resembles what the NRL has built.
With the potential for additional fuel to be carried in high-volume, Inconel-composite wrapped tanks, the Upper Stage seems designed to do much more than simply take the microsatellites to GEO. Large, fairly rapid changes in orbital position are possible, allowing MiTEx to freely traverse what has become the most economically viable and strategically important locale in space.
Yet while the upper stage is unique, none of the technologies used in it are actually as novel as the Department of Defense’s (DOD) press reports imply. Sufficient information was released to allow a detailed analysis of the individual technologies which are onboard. This analysis, which is at the end of this document, verifies that these technologies not only already have a flight history, but are not even exclusive to the U.S. aerospace industry, with the platinum/rhodium thrusters in particular being used in several European missions.
Possible MiTEx Objectives
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 U.S. 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 being 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 – 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 causing permanent damage to the satellite cannot be ruled out.
MiDSTEP Technologies
MiDSTEP investigates the use of microsatellites from Low Earth Orbit (LEO) to orbits beyond geosynchronous. The following technologies are listed for research under MiDSTEP’s budget sheet:
- optical space surveillance
- situational awareness sensors
- lightweight power systems
- chemical and electric propulsion systems
- advanced lightweight structures
- radio technology including micro crosslink
- active radio frequency sensor technology
- commercial processor and software
- miniature navigation technologies
- autonomous operations
- high thrust solar thermal propulsion
o responsive orbit transfer
o radiation resistant electrical power
- ultra-stable payload isolation and pointing
- miniature communication systems
- responsive fabrication and integration
These technologies are diverse, with some requiring only minor evolution of current technologies (power systems, navigation) to those necessitating dramatic reaches in technical capabilities (solar thermal propulsion, autonomous operations). Given the sheer number of technologies and their difficulty, it is unlikely that a pair of 225 kilogram microsatellites could test all of these concepts.
NRL Upper Stage Technologies
The following is a list of the widely published technologies being demonstrated on the NRL Upper Stage. Each technology is followed by an analysis of its probable application.
Platinum/rhodium bi-propellant attitude control thrusters
Thrusters that use platinum/rhodium are designed to be fired tens of thousands of times, equating to 50-plus hours of total burn time over the life of the thruster. These thrusters have been used on a variety of long endurance spacecraft, and an older generation of this thruster has gone all the way to Jupiter on the Galileo mission. Such a long-lasting thruster is most unusual for a conventional upper stage, whose functional life is typically measured in hours at most. With platinum/rhodium thrusters, the Upper Stage can operate for weeks or even months of heavy usage.
High-performance coated columbium delta-V thruster
Columbium-based thrusters were introduced in the 1970s. In this case, the delta-V thruster provides the “kick” necessary to transfer from the initial transfer orbit to GEO. This is a mission-critical event, and a proven technology is desirable. It would be even more critical to the MiTEx mission if the hypothesized orbital maneuverability was indeed the case. The only potential for new technology is the high performance anti-oxidant coating. These coatings traditionally chip and degrade after heavy use and time in orbit. It would be assumed that any new coatings would be improving on these aspects. However, the need for coatings would be negated altogether if the platinum/rhodium alloy, used above in the attitude control system, was implemented. As such, this thruster was selected for its proven reliability, but as a technology demonstrator.
Commercial off-the-shelf manual valve tested to aerospace standards
This is too ambiguous to assess in detail, but potential benefits of using commercially available valves are obvious if one intends to produce hardware en masse.
Lightweight Inconel-718 composite overwrap pressure vessels, and
Lightweight titanium propellant tanks with internal propellant management devices
The lightest tank for a given amount of propellant is a sphere 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, requiring the use of a cylinder. 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, mitigating the increase in mass.
This tank construction is consistent with the inferred additional propellant onboard the NRL Upper Stage beyond what is required to simply get the microsatellites to GEO. This extra propellant is surmised from the payload capabilities of the Delta II 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 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. As mentioned above, the Air Force’s Integral Apogee Boost System (IABS) launched the Defense Satellite Communications System (DSCS III) between 1991 and 1998 and was equipped with solar panels. However, IABS resembles the NRL Upper Stage in its use of solar panels only; it does not appear to be as capable a platform; as such the 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 advantage of more 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.
Lithium-ion batteries
This is too ambiguous to assess in detail, but compared to other battery technologies rated for space operations, lithium-ion is far superior. As such, lithium-ion batteries have flown on many missions and are practically standard on all microsatellites. A conventional upper stage would use simpler, non-rechargeable batteries for its short mission. Combined with the solar panels, this Upper Stage is designed to last much longer what is traditionally expected.
A low-cost/high-performance star tracker
This is also too ambiguous to assess in detail, since hardware is always driven to cost less and perform better. Typically upper stages either rely on the payload for guidance and navigation functions. When an upper stage does carry these functions, it is typically in the form of inertial measurement units, which are very reliable over typical upper stage lifetimes. Inertial systems tend to “drift” and as such have to be recalibrated if used for a long duration. Star trackers do not suffer from drift, and as such can be used indefinitely.
Conclusion
The lack of mission specifics for the MiTEx satellites has shrouded this program in suspicion. The positions of the satellites are not known and could change dramatically over time. The unusual characteristics of the NRL Upper Stage imply a far greater capability than what is required for its disclosed mission.
Previous military technology demonstrators, namely Experimental Satellite System (XSS)-11 and the Demonstration for Autonomous Rendezvous Technology (DART), have performed autonomous proximity operations and reconnaissance on obsolete U.S. satellites. In these instances, a modest amount of information was publicly available, despite the controversial nature of the missions. Since information available on MiTEx is very limited in comparison, it is reasonable to assume that the satellites are testing more controversial technologies than did XSS-11 or DART. Otherwise, DOD wouldn’t be losing any ground by disclosing the functions of MiTEx.
Alternatively, MiTEx could indeed be operational, performing any number of possible clandestine missions. We simply do not know.
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