Analysis of the Cosmos 1408 ASAT
The Event
On 15th November, 2021 around 2:50 AM UTC something happened above the Laptev Sea, a marginal sea of the Arctic Ocean. A defunct COSMOS spacecraft (KOSMOS 1408 - catalog ID 13552), launched in 1982 to carry out electronic intelligence activity was shot down by a Russian ASAT missile, creating a debris cloud that raised an immediate alarm in the space community (and after the Ukraine invasion happened in February 2022 this event sounds like a sinister a sinister premonitory event).
More than three months have passed from that event, and the public available information about the consequent debris cloud are enough to run a forensic analysis about what happened and in which way the situation is evolving. This article attempts to reveal the type and nature of the impact using the following information:
The SATCAT catalog
SATCAT is a text file containing one record for each of the mankind object in orbit around the Earth. It includes all objects launched from 1957 plus the catalogued debris (minimum object size is about 10 centimeters long). Each object is shown accordingly with different classifications, including the NORAD ID. The figure below shows few lines of the catalog containing some of the COSMOS 1408 debris:
This catalog plays a key role in correlating the information contained in the TLE catalog, that is daily updated and reports the orbital data of the objects orbiting the Earth at that time.
The TLE Catalog
TLE stands for Two Line Elements. In short, the orbital parameters of each object in the catalog are encoded into a two-lines string. This string is the input for a specific, medium fidelity propagator (called SGP-4) that is used to estimate the future location of the satellite in the following few days. The TLE catalog is a snapshot of the situation in space at time of its creation (differently from the SATCAT catalog, that is an historical archive). The picture below shows how the TLE catalog appears: lines 1 and 2 refer to the same object.
Tracked Debris Over Time
As first step, I collected all TLE catalogs from 15th November 2021 to 16th March 2022 and then I compared the content of each of them with the latest available SATCAT catalog, looking for any COSMOS 1408 debris record. At time of analysis, SATCAT contained 1604 individual records (this is the overall number of debris tracked over this time span, but the actual number of them is much larger, since fragments smaller than 10 centimeters are not tracked). Initially there were few debris tracked, but their number started to increase fast few days after the event. The figure below shows the number of tracked debris vs. time:
Considerations about this plot:
The video below show the current (as 17th March, 2022) debris cloud around Earth. In white is highlighted the orbit of the ISS. You can see how the two intersected each other twice per orbit, posing a serious risk for the safety of the crew.
领英推荐
The Gabbard Diagram
The Gabbard diagram is a widely used tool in the study of satellite fragmentation. It was invented by John Gabbard to study the fragmentation of Delta upper stages in orbit. Basically it is a plot of the apogee and perigee heights of the fragments against their periods. The shape we see in the plot is an asymmetrical, inclined X with the arms intersecting at the coordinates (period, altitude) of the fragmenting satellite.
The effect of the impact is a variation in the specific energy of the fragments, that implies an instantaneous variation of their velocity. If we sit on an orbital reference system, we see how these variations involve the radial, in-track and orbit normal components. Despite the orbit normal component changes the fragment's inclination, it does not have any evidence in the Gabbard diagram. Instead, what we can see is the effect of the changes in the in-track and radial directions.
Positive variations of the in-track speed will lead to an increment of the specific energy, and consequently both the period and the semimajor axis will increase. If we assume a perfectly circular orbit before the fragmentation, after it we'll have a new orbit with the perigee at impact point and the apogee higher than the initial orbit radius. As result, the Gabbard diagram will show the apogee points lying along a straight line with a positive slope in the right-hand side of the plot, while the perigee points will stay on an horizontal, straight line.
For negative variations of the in-track speed the impact point will become the apogee of the new orbit, with a perigee altitude lower than the initial orbit radius. For fragmentations in LEO orbits, the reduction of the perigee will also lead to an increasing drag force acting on the fragments, also reducing the apogee altitude over time. As result, the Gabbard diagram will show the apogee and perigee points lying along inclined, curved line in the left-hand side of the plot.
The effect of the radial speed variations is to move the apogee and perigee points above and below the apogee and perigee lines by the same distance, depending on the delta V magnitude. The figure below shows the Gabbard plot of COSMOS 1408 3 months after the event:
It is useful to compare the previous diagram with the one relative to the COSMOS/ IRIDIUM collision happened in February 2009. The following Gabbard plot (courtesy Celestrak) refers to April, 2010 (more than 1 year later):
As you can see, the left part of the diagram is pretty different. The relatively low altitude of COSMOS 1408 at event time plays an important role here: all debris that got a negative delta V in in-track or radial direction started to decay significantly. Also, the right part shows an homogeneous distribution of fragments up to the far right. This is a typical signature of hypervelocity collisions, as explained below.
Hypervelocity Collisions
Collisions between objects in space most of the time are hypervelocity collisions. Such a collision is defined as one where the velocity (V) of the impactor (relative to the target) is so great that its kinetic energy ( K = ? m V^2) is greater than the energy released in the detonation of the same mass (m) of high explosive. Implementing some formulae, it can be seen that hypervelocity collisions occur when the relative velocity between target and impactor is 4 km/sec or greater.
In LEO orbits, the velocity of a satellite is slightly higher than 7 km/sec, and the typical collision velocity between two objects in LEO is between 10 and 11 km/sec. Hypervelocity collisions spread a very large number of fragments in all directions, while non hypervelocity collisions spread fragments in preferred directions, depending on the geometry of the impact. Also, for non hypervelocity collisions, the number of fragments is not so high, especially if the collision itself is not catastrophic (this happens when the impactor is destroyed and the target is damaged but not totally destroyed). This can mean that this was not an hypervelocity collision. The ASAT missile could have hit the satellite from behind with a relatively low speed. This also matching with my first guess when I first analyzed the public available information to build an STK scenario, as shown in the video below:
Final Remarks
It is clear that missile strike events like this contribute substantially in increasing the number of space junks in orbit. They should be banned?from the international community and treated as hostile acts, as they undermine future access to space. In this case we have a reasonable hope that most of the fragments will decay in the mid-term, as also suggested by the following histogram, that shows the fragment's distribution against their period:
The histogram is left-shifted respect to the COSMO orbit period right before the strike (about 94.15 minutes), showing an increasing number of fragments falling down towards the Earth.
Retired space executive
3 年I offer that the impact is communicated to each object at the speeds of sound in the materials. (Speeds because stress waves and compressive waves have different propagation speeds.). As material properties, they don’t change much (if at all) in orbit. They are much less than orbital velocities. The impact is over before the material knows anything has happened. The bodies are “rigid”, and the consequence is a brittle fracture with subsequent deformation as the disturbance propagates in the fragments. These are the phenomena that lead to many more pieces in a hyper velocity collision.