ARRL

CSI Space

CubeSat Investigators: What's Going On with FUNCube?

FunCube_1_Model.jpgBy: Mark Spencer, WA8SME mspencer@arrl.org

The FUNCube (AO-73) satellite is an excellent resource that allows your students to be real space sleuths and hone their forensics skills by investigating what is happening to the satellite on orbit…just like the TV crime scene investigators do in the popular crime solving shows. The FUNCube (FC) satellite is a satellite that just keeps on giving and is full of surprises.  

Suggestions on how to access the telemetry signals being transmitted by the FC and a description of the Materials Science Experiment that is carried on the FC are summarized in the Pragmatic Guide for Using the FUNCube (AO-73) Materials Science Experiment in the Classroom.

I have been monitoring the FC telemetry since it was first launched in November 2013, and some interesting inconsistencies between what I expected and what actually is happening on orbit became evident almost immediately. This resulted in my own CSI investigation to try and figure out what happened, what is happening, and what can we expect in the future with this satellite. This investigation included steps similar to those taken by a TV CSI team; secure the scene and collect data, evaluate the data to gain some perspective as to the who, what, where, how, and why surrounding the scene, scrutinize the data to develop theories, test those theories with the data that was collected, discredit, advance, adjust, and/or make new theories based on the investigation and the data, and hopefully discover that one scrap of evidence (data) that leads to the ah-ha moment that solves the mystery…or sometimes, leave us hanging with a mystery unsolved.  

While evaluating the temperature data generated from the Materials Science Experiment, I also looked at the rotation rate of the satellite. Around December 28, 2013, something happened to the satellite that resulted in an abrupt and unexpected change in the rotation rate that warranted a second and more detailed look. The mystery continued from there.  

The rotation rate slowed down to a minimum of about 8.5 minutes per revolution, then started to spool up again to the current (as of this writing) of about .92 minutes per revolution, but now there is a hint in the data that the spin rate is going to slow down again. So what is going on?  

  • The FC is basically a gyroscope that is spinning about its Z-axis. [Why do they want a satellite to spin?] 
  • There is no mechanism built into the satellite to impart and maintain a rate of spin. [Discovered by reviewing the available construction details of the satellite. Is this a good thing? What do other satellites use to impart and maintain rotation about the Z-axis?]  
  • What forces are there on orbit that would cause the satellite rotation rate to change? [Air resistance? Solar pressure?  Micro-meteorites, dust, junk, etc that can collide with the satellite?   Something else?] 
  • Is there a relationship between the rotation rate of the satellite and the temperatures of the Materials Science Experiment bars? [If there is a relationship, what is it? What would be the cause of this relationship? Are the bar temperatures the same or are they proportionally different? ] 

It is up to your CSI team to come to their own conclusions. For me, I am looking at a couple of interesting theories, and learning a ton of things along the way.  

First, there was an obscure reference in a scholarly document about gyroscope behavior (that unfortunately I can’t find again) that speculated, “If a precession force were applied for less than 1/4th the period of a gyroscope’s rotation, this precession force pulse might result in an increase in the rotation speed.” The fixed magnet within the satellite that is used to align the Z-axis of the satellite with the lines of the Earth’s magnetic field “flips” the satellite end-over-end twice during a single orbit. The magnet imparts a precession force that is perpendicular to the Z-axis to accomplish this maneuver. Could it be that at the low rotation rate, this flipping force is less than 1/4th the period of the slowest rate of rotation and therefore is imparting a small force that increases the rotation speed?  

Second, there is a theory concerning the force mechanism that causes small asteroids (as small as 10 centimeters) to rotate. This theory is called the Yarkovsky-O’Keefe-Radzievski-Paddack Effect (YORP). Basically the YORP effect speculates that small temperature differences across the mass of an asteroid along with differences between the emissivity and absorptivity of the material that makes up the asteroid impart tiny forces that over time cause the body to rotate. The theory is that these tiny forces are due to the differences in pressure that result from differences the way that heat energy is absorbed and emitted from the surfaces of the asteroid.   But it can take thousands of years between visits to Earth by an asteroids making the comparison of rotation rates from one pass to the next impossible. Could it be that the FC is a manmade object that is exhibiting the YORP Effect? Could it be that the FC will allow us to verify the YORP Effect? 

The fascinating thing about all this is that no one really knows what is going on with FC? Your CSI team can come up with theories and speculations that are just as valid as anyone’s. And perhaps your CSI team might discover that ah-ha data moment that solves the mystery. 

If you would like some additional details about my investigation, let me know. The participants in this year’s teachers institutes can expect the opportunities afforded by collecting and analyzing FC telemetry to be covered in more detail.