The ongoing saga of DSP Flight 23
by Brian Weeden
|The lack of information about DSP-23 creates a situation where it can be hard to separate the facts from the fiction and yet has significant implications for space security.|
The first public indications that something was amiss came from the See-Sat amateur satellite observer community on November 15, 2008. Its members have long tracked supposedly “invisible” satellites, especially those like DSP that are seven meters (22 feet) in diameter, 10 meters (33 feet) in length, have a mass of 2,381 kilograms (5,250 pounds), and typically remain mostly stationary over a single spot on the Equator. On November 6, one particular observer had not only tracked the satellite but had equipment that could measure radio signals and noted that the usually chatty DSP-23 was not transmitting. At this time it was still in its usual location at approximately 9° E longitude, putting it above a point west of Gabon and south of Nigeria on the west coast of Africa. The same observer tracked it again on November 24 and noted that it was transmitting again but was noticeably weaker than normal. More importantly, the satellite was now drifting eastward.
One can imagine the geostationary belt as a giant, circular NASCAR racetrack and the satellites in that orbit as the cars. This is because all the satellites in GEO are at almost the same exact orbit, going around the Earth in the same general direction and altitude. Although these satellites are moving around the Earth at just under 11,300 kilometers per hour (7,000 mph), they appear to be almost stationary in the sky to an observer on the surface of the Earth because they make one complete orbit in the same time it takes the Earth to rotate once. If a spectator was to stand in the middle of the infield of a circular NASCAR track and turn in place at the same rate the cars were moving around the track, you would see the same effect: the cars would appear stationary.
Of course nothing in physics is ever this simple in reality, and indeed the situation on both the racetrack and in geostationary orbit is more complex. The exact forward velocity of a satellite in orbit around the Earth is determined by the strength of the Earth’s gravitational pull and the altitude of the satellite. The Earth is not a perfect sphere so it follows that its gravitational field, which is a function of its mass, is not uniform. In fact, there are two “bulges” along the Equator at approximately 75° and 225° East longitude. These gravity “troughs” pull satellites in geostationary orbit east or west towards whichever is closest, giving the satellite an apparent east or west drift. To a spectator spinning in place in the center of the aforementioned racetrack, the same effect can be seen by the small variations in velocity as cars jockey for position on the racetrack.
Just as a car crash in NASCAR can have disastrous effects on the entire field and not just one or two cars, policymakers decided early on that keeping satellites from colliding in geostationary orbit was extremely important. To manage this limited resource, an international legal framework managed by the International Telecommunication Union (ITU) was put in place to license and distribute satellite frequencies (in 1963) and slots (in 1973) for geostationary orbit. Each state or private entity that wishes to place a satellite in a specific position over the Equator must apply for a license to the ITU and receive permission if they would like their physical position and operating frequency to be protected from interference.
The ITU specifies “slots” in GEO, usually as a fraction of a degree of longitude, that serve as a box within which each satellite owner/operator is supposed to maintain their satellite’s position through stationkeeping. This involves small, periodic maneuvers done to counter the east or west pull of the gravitational troughs as well as the north and south inclination drift due to the gravitational pull of the Moon and Sun. Once a satellite in the geostationary belt reaches the end of its operational life, which is usually determined by the amount of fuel they have onboard for stationkeeping, the owner/operator is supposed to boost it out of its slot into a higher graveyard orbit. This both frees up the slot for reuse and ensures that the now inactive satellite doesn’t collide with any other active satellites. The US military has followed all of these rules since their inception and has licenses from the ITU to operate its geostationary satellites.
The first media report of a possible issue with DSP-23 came from Reuters on November 24, 2008, which reported that the satellite had apparently stopped working in mid-September of that year. The story was prompted by a memorandum from the Pentagon to Congress requesting emergency funding for a “gap-filler” satellite to prevent a possible hole in missile warning coverage due to the failure and on-going delays in the constellation slated to replace DSP, the Space Based Infrared System (SBIRS). Aviation Week revisited the story on December 1, but again there was no official US comment.
On December 2, SPACE.com published an article on DSP-23, this time with input from Vladimir Agapov, who runs the International Scientific Optical Network (ISON). Consisting of 18 scientific and research telescopes located mainly across Europe and Asia, ISON uses spare time on these instruments to track deep space satellites in Earth orbit. Some of this data goes into the “Classification of Geosynchronous Objects”, a publication put out by the European Space Agency’s space debris office, which serves as a supplement to the public Space Track catalog maintained US military.
Since November of 2008, DSP-23 has been drifting eastward through the GEO belt at approximately one degree of longitude a week, passing by many other active satellites along the way. First and foremost was the Hotbird constellation at 13° E, a tight cluster of three satellites operated by Eutelsat which share the same GEO slot. DSP-23 is currently drifting past the SES Astra cluster at 19° E, which just added its sixth satellite only a day or two before the arrival of DSP-23. Somewhere around the beginning of April it will pass by the Astra cluster of four satellites at 28.2° E and continue towards the 75° gravitational trough over India.
|Since November of 2008, DSP-23 has been drifting eastward through the GEO belt at approximately one degree of longitude a week, passing by many other active satellites along the way.|
Like a car rolling down the side of a trough between two hills, DSP-23 will continue past the gravitational trough while slowing down, and at some point, probably near 135° E, will reverse its direction and head back through the trough towards its starting position. Over time it will continue to oscillate back and forth between 8° E and 135° E, with a reduction in amplitude each time until at some point many years from now it will settle in the trough at a roughly stable position along with other 148 tracked objects captured in the same manner.
Reuters published an updated story on DSP-23 on January 6 with few new facts. Again, there was no official public statement from the US government, the military, or the satellite’s manufacturer, Lockheed Martin. Unofficial comments related how difficult it was to do such “long distance detective work”.
On September 17, 2008, Lieutenant General William Shelton, then commander of 14th Air Force and the Joint Space Operations Center (JSpOC) at Vandenberg AFB in California, gave the opening keynote at the annual Advanced Maui Optical and Space Surveillance (AMOS) conference. One of the main themes of his talk was the extreme difficulty and importance of being able to attribute critical satellite failures to their appropriate cause, be it a manufacturing defect, collision by a piece of debris, a space weather event, or from hostile action.
The inability to answer this question undoubtedly causes many a sleepless night for general officers and their staff alike, given the importance placed on satellite capabilities and dominance. And with the recent concerns over China’s burgeoning military space power and successful low Earth orbit anti-satellite test in January 2007, there is no doubt that at least a few people in Washington and the military considered the possibility that DSP-23’s failure could have been the result of some nefarious Chinese operation.
So perhaps it should not have come as a surprise that on January 14 Spaceflight Now reported that the US government had taken action to try and answer this question. According to the report, the Defense Department had maneuvered the two Micro-satellite Technology Experiment (MiTEx) satellites, previously dormant in the geostationary belt, to do inspection flybys of DSP-23 to look for any external evidence as to what caused the failure. The first made a flyby DSP-23 on December 23 and the second on New Year’s Day.
The US Air Force’s desire to have a satellite inspection capability dates back to the very beginning of military satellite operations. The Satellite Inspection (or Interception) program—SAINT—was conceived in the late 1950s and contracted to RCA in 1961. The goal was to mount a television camera and radar in the nose of an Agena B upper stage and boost it into orbit using an Atlas rocket. SAINT would then maneuver close to an unfriendly target satellite, photograph and analyze it, and report back all the details to the US military. From the beginning the US Air Force wanted to also give SAINT the ability to destroy or disable the target satellite, but such efforts were blocked by the Eisenhower and Kennedy administrations. The program was eventually canceled in 1962 before SAINT could make it into orbit for both budgetary reasons and because the technology challenges were deemed insurmountable at the time.
|The US Air Force’s desire to have a satellite inspection capability dates back to the very beginning of military satellite operations.|
The US military finally made its satellite inspection capability reality on April 11, 2005 with the launch of XSS-11 (USA 165). This gave the US Air Force the capability to rendezvous and inspect satellites in low Earth orbit using onboard cameras and LIDAR (Light Detection and Ranging). And on June 21, 2006 the launch of the MiTEx pair enabled this same capability in geosynchronous orbit (see “Mysterious microsatellites in GEO: is MiTEx a possible anti-satellite capability demonstration?”, The Space Review, July 31, 2006). Both XSS-11 and MiTEx were officially labeled technology demonstrators and limited to rendezvous and inspection of other pieces from their respective launches or other American satellites. However, the unofficial possibilities are obvious. And while XSS-11’s position was published in the public Space Track catalog, the MiTEx satellites (cataloged under their cover names of USA 187 and USA 188) have never had their position listed publicly by the US military.