When we heard about the July 18 debris-generating event involving DigitalGlobe’s WorldView-2 commercial imaging satellite, we immediately launched our own investigation. Over the past year, Applied Defense Solutions — the aerospace engineering firm where I work — has assembled a multi-disciplinary team of commercial and academic sensor operators, astrodynamicists, and researchers to address the growing need for non-governmental space situational awareness (SSA) services. With this team and a network of sensors at our disposal, we were ready for an event like this.
Shortly before 5:00 p.m. EDT on July 19, the Joint Space Operations Center at Vandenberg Air Force Base in California, posted a little noticed tweet to its 1,800 followers: “JSpOC ID’d debris causing event related to @DigitalGlobe WorldView-2; 8 debris pieces but WV2 confirmed operational & maneuverable!”
We were first alerted to the event at 8:25 a.m. the next morning when SpaceNews sent out its First Up newsletter. At 8:38 a.m., I put out the first request for data from the team. Collection analysis was underway with the University of Arizona’s Space Object Behavioral Sciences, SRI, Geost, Lockheed Martin, Pacific Defense Solutions, University of Hawaii and LeoLabs within the hour. By 10:00 a.m., we had reports of previous radar collection passes for LeoLabs. By 6:00 pm, LeoLabs was taking its first radar measurements of WorldView-2 and the debris objects trailing it.
As collection opportunity results were being reported, the importance of a multi phenomenology approach — incorporating data from both electro-optical and radar providers — was obvious; WorldView-2 is a sun-synchronous satellite designed to pass overhead in the daylight, limiting our opportunities for collection from optical sensors.
Between July 20 and July 27, LeoLabs took 11 separate collections on WorldView-2 and what seems to be evidence of debris. ADS and its team used the data to perform orbit determination and characterization on the detected objects with in-house software.
The orbital geometry of WorldView-2 is such that it is quite challenging to collect electro-optical data of the satellite when it is physically visible. To take a collection with most optical telescopes, three conditions must generally be met: 1. it must be between sunset and sunrise at the telescope site; 2. the satellite must be in view of the telescope; and 3. the satellite must also be in view of the sun (if it is behind the shadow of the Earth there will be no light reflected off of the object for the telescope to detect). WorldView-2 is in a high-inclination low-Earth orbit and only meets these conditions for most sites in the United States for roughly 2-10 minutes at a time. Nevertheless, our partners at the University of Arizona’s Space Object Behavioral Sciences were able to take advantage of a few short passes to collect electro-optical data of WorldView-2 and the associated objects. In the data, we detected the WorldView-2 satellite and two other distinct objects with a sufficiently high probability of detection.
We performed analyses to determine when a possible debris generating event might have occurred and whether or not the orbit of WorldView-2 was noticeably changed. Based upon Two-Line-Elements (TLEs) the U.S. Air Force publishes on its space-track.org online catalog of orbiting objects, we hypothesize that a break-up event occurred July 18 between 6:30 p.m. and 10:30 p.m. EDT (18:30 and 22:30 UTC). Comparing the orbit of WorldView-2 before and after this event, there was a semimajor axis change of about 60 meters and virtually no change in orbital inclination. This suggests a mostly head-on collision which imparted a change in WorldView-2’s orbital velocity of approximately 3.1 centimeters per second. This estimate was later confirmed by DigitalGlobe based on their internal analysis of the spacecraft telemetry.
Finally, we did a survey of objects in the space-track.org catalog to assess which objects are likely to have been in the vicinity of WorldView-2 at the time of the event. It is our belief that no object came closer than 24 kilometers on the night of July 18th, based solely on TLE propagation. However, WorldView-2 did pass through a region of space with a higher space object population density, containing debris clouds from both the 2007 Fengyun anti-satellite test and the 2009 Iridium-Cosmos collision, as well as debris from a number of spent rocket bodies. NASA’s Orbital Debris Program Office reports that these two known break-up events account for approximately 36 percent of objects in low Earth orbit. The debris has spread over a very large area encircling the globe, with highest concentrations between 750 and 900 kilometers altitude. It is not known how many untracked small objects from these events exist. When the Air Force brings the new S-band Space Fence into operations in 2018, we will know much more about objects that are today categorized as untrackable. Nevertheless, the risk of operating in these altitudes remains very low but operators of active satellites remain vigilant against conjunction events. The exact origin of the object in the WorldView-2 event remains unknown.
The message to take away from this activity and our response to a reported event is that via a collaborative consortium mechanism, much can be achieved in terms of gathering and delivering a sufficiently timely and accurate body of evidence of space object behaviors that is meaningful to a variety of entities, enabling safe civilian, commercial, and government space operations.
In looking at the forward thinking of U.S. Rep. Jim Bridenstine’s Space Renaissance Act and discussions surround Space Traffic Management, this collaborative consortium model should be added to those being considered amongst possible solutions to this problem. Dedicated sensors and operational equipment will always be needed by certain organizations, but can be easily complemented and augmented with community-wide capabilities from an aggregate of less expensive and less manpower-intensive systems. There is a growing community desire to see the development and manifestation of an international space traffic monitoring system that is easily accessible and highly transparent to the global community, with global governance much like the International GNSS Service.
This WorldView-2 exercise is just one example of what could be achieved given the proper advocacy and dedicated collaborative exploration.
Thomas Kubancik is vice president of advanced systems at Applied Defense Solutions. He wrote this commentary with Christopher Roscoe, an aerospace engineer at ADS, and Moriba Jah, director of Space Object Behavioral Sciences at the University of Arizona.