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FEATURE: Cubesats - How Small Can Satellites Get?
by Michael Thomsen

You’ve definitely heard of the, those small, 1 kg satellites called Cubesats, so named due to their physical dimensions being that of a 10 cm cube. They are extremely popular as an educational tool at universities, but is it truly feasible to create a functioning satellite within so small a package? Additionally, do Cubesats have any practical use, or are they merely just a toy?

Until the 1990s, satellites grew ever larger, typically having as mass of several tons. As a means to counter the ever increasing size and cost of space projects, the NASA administrator at that time, Daniel Goldin, pioneered the “faster, better, cheaper” approach that would allow NASA to continue to operate a wide variety of programs without exponentially increasing the costs. Minisatellites (200-500 kg) became the new point of focus. Even micro-satellites (below 200 kg) started to gain interest, one such example is the Danish 61 kg Ørsted satellite, which was launched in February of 1999 to perform measurements of the magnetic field of the Earth. Several universities have designed and built even smaller satellites called nanosatellites, which have a mass of less than 10 kg.

However, two issues in particular prevented most of these satellites from being launched. One was the cost of the launch. To reduce this heady cost, small satellites were always launched as a secondary payload. This lead to another major hurdle, namely the rigorous requirements to ensure the nanosatellite did not interfere with, or even destroy, the primary payload during launch. Imagine what could happen if a small university satellite deployed, by mistake, its solar panels or antennas before separation.

As is the case for other universities, Stanford University and California Polytechnic State University (CalPoly) had been attempting to find a solution to these problems to allow students to launch and communicate with their own satellite as an integral component of their higher education.

A team lead by professor Bob Twigs at Stanford University developed the Poly Picosat Orbital Deployer, or P-POD. This small container holds a payload of 10 x 10 x 30 cm3. The P-POD payload is used for small satellites, either with the full 10 x 10 x 30 cm3 size, or even smaller 10 x 10 x 10 cm3 cubes.

The P-POD protects the launch vehicle and other satellites from the Cubesats as well as providing a standard interface between the launch vehicle and the Cubesats. Only the P-POD needs to interface to the launch vehicle. As deployment is accomplished through the use of a spring that slides the Cubesats out along four rails inside the P-POD, the interface between the P-POD and the Cubesats is relatively uncomplicated.

The predecessor of the P-POD was the 23 kg Stanford OPAL (Orbiting Picosat Automatic Launcher) satellite, which deployed six picosatellites in January of 2000. The concept worked, and shortly afterwards, several universities around the world initiated student Cubesat projects.

The small volume and mass of Cubesat satellites was thought to result in a low launch cost of US$30,000 (nowadays, launch costs have increased to about US$50,000). Such was affordable to universities and there was an expected fast turn-around time of between one and two years from project initiation to launch of the satellite. As university students are rarely involved in a project for more than a few years, and as the motivation among the students is significantly increased when you have a chance of communicating with your own satellite in space at the end of the project, it is important to ensure the project time remains fairly short.

A problem many, particularly small and new, aerospace companies encounter, is the requirement to prove their technology works in space before they can sell their product. In particular, flight heritage is an important factor. This was meant as another big selling point for the Cubesat platform — you could test new hardware in space at a very low cost.

The first Cubesats were launched on a Eurockot on June 30, 2003. These Cubesats represented a wide variety of projects, both professional and university led, some of which had been started just two years earlier. Since then, a total of 38 Cubesats have been launched, although with varied success (see Table 1.) Fourteen Cubesats were lost during a launch failure in July 2006, four have never been contacted, and only limited contact has been possible with another five Cubesats. However, a further 15 satellites have fulfilled all of their mission goals. They cover a wide variety of missions from bus verification (most including a camera), through component testing, to complete science missions (including astrobiology, and ionospheric research).

Some of the obvious challenges of designing a Cubesat are the small volume, and small mass — due to the size issue, redundancy is rarely considered. In addition, a result of the reduced volume is the small area available for solar cells. This area is often reduced further as deployable antennas necessitate adequate room, and often sensors and payload require additional surface area.

Another major challenge, specifically for universities, is how to manage the project. Sometimes the projects are student led, and at other times a more ‘professional’ management style is used. Another key issue is the decision regarding how long a project should run and when to commit to a launch opportunity. If a project is scheduled early, there is a known deadline to work against, which can be a good motivator. However, on the other hand, delays in the project can easily result in cutting the final and critical test and verification phases to a bare minimum. If you wait until the design has been proven, you may have to wait a year or two before the satellite can be launched.

Many of the first Cubesats were designed as a test platform to verify the basic spacecraft bus, consisting of the mechanical structure, power system, onboard computer, communication, attitude control, and so on, worked properly. Some of the early ones were the Japanese CUTE-I (Tokyo Institute of Technology) and XI-IV (University of Tokyo), and the Danish AAU Cubesat (Aalborg University—also see this issue's cover image!). All of these satellites were small 1 kg Cubesats. The two Japanese Cubesats worked flawlessly — several pictures were downlinked from XI-IV (the camera on CUTE-I was used as a sun sensor, and the raw images could not be downlinked by design), and they still, more than five-years later, continue to transmit their beacon signal.

The AAU Cubesat, on the other hand, never properly worked entirely. In the beginning, the beacon signal was heard, but it was weak. The assumption was made the tracking of the ground station antenna was not correct, but that turned out not to be the case. Instead, the team managed to borrow an 8 m dish, which was available approximately one month after launch. This allowed the team to receive and decode the beacon signals. Unfortunately, the housekeeping data transmitted in the beacon indicated the battery capacity was severely reduced. A full communication link was never accomplished.

The conclusion as to the reason for the low signal strength was that two of the deployable antenna whips had not completely unfolded and were short-circuited during deployment. The reason for the loss of the battery capacity was the batteries were packed in a foil. When operated in a vacuum, the batteries could swell and would severely reduce capacity. This problem was actually known before the launch and was ‘solved’ by mounting the batteries between two aluminum plates to ensure the batteries could not expand [1].

Later launches included several missions dedicated to test a Cubesat bus, including Boing (CSTB-1) and Aero Astro (AeroCube-2). The latter contained a camera designed at Harvey Mudd College, which automatically started taking pictures shortly after the satellite was ejected from the P-POD. Those photos include the picture of another Cubesat, CP4 from California Polytechnic Institute (see photo).

Apart from missions designed to educate and generally gain experience with very small satellites, or to test components or sensors in space, a few Cubesat missions have focused on a scientific mission objective. Two are QuakeSat and GeneSat.

QuakeSat was a triple Cubesat developed at Stanford University whose mission was to study earthquake precursor phenomena from space. This is accomplished by measuring extremely low frequency magnetic signals in low Earth orbit, downloading the data to a ground station, and post processing the data on the ground. QuakeSat was launched in July of 2003 and was designed to be a single year mission. A 0.7 m deployable boom contained the magnetometer in order to minimize the magnetic disturbance from the satellite on the sensor. Like most other Cubesats, communication was facilitated through the use of a HAM frequency band at 9600 baud using the AX.25 protocol. The satellite also had deployable solar cells. Downloads of about 500 MB of data were managed during the mission, and a new, larger, 150 kg class satellite is now being developed as a result of the knowledge gained from the original Cubesat mission.

Another example of a scientific Cubesat based mission is GeneSat. This project was a collaboration between NASA Ames, industry partners, and universities. The satellite consisted of a satellite bus, which had the same dimensions as a single Cubesat, while the payload took up an additional 10 x 10 x 20 cm3. The objective was to develop a miniature life support system that could fit into a triple Cubesat and could deliver nutrient and perform assays for genetic changes in E. Coli. The satellite payload consisted of a pressurized sealed vessel, an integrated analytical fluidics card assembly, which included a media pump, valves, microchannels, filters, membranes, and wells to maintain the biological viability of the microorganisms. Optical sensors were used to detect genetic changes. During the experimental phase, which lasted for 96 hours, the payload temperature had to be regulated within 0.5 °C. The satellite monitored the external and internal Cubesat temperatures, as well as the radiation environment.

The biological experiment was initiated on December 18, 2006. Approximately two days after launch, and after 96 hours of project implementation, the biological experiment was complete and all of the baseline data had been downloaded. After the completion of one month of on-orbit operations, all primary mission objectives had been met. Control of the satellite was turned over to students at Santa Clara University, which used the satellite for educational purposes as well as to monitor the health of the satellite to determine how well the components worked as a reference for future missions.

As a final example, I have proposed a mission of a 2.5 kg 10 x 10 x 30 cm3 Cubesat, which is to measure the magnetic field of the Earth. This would be a successor to the Danish Ørsted satellite, and the European Swarm mission. The scientific justification focuses on the necessity to monitor the magnetic field of the Earth on a continuous basis in order to generate accurate navigational models that are used, for example, in deep well drilling in the petrochemical industry. The required resolution for such work is difficult to obtain from ground stations. It is also necessary to obtain magnetic field measurements from space in order to study the interaction between the solar wind and the magnetic field of the Earth, an area that is still not completely understood. The satellites will consist of a miniature 3 axis fluxgate magnetometer, which is a miniaturized version of the Ørsted magnetometer, a GPS receiver (e.g., SGR-05 from SSTL, and a small boom to ensure magnetic contamination is minimized at the sensor [2].

There are very few resources available to a payload if limited to the 1 kg Cubesat. However, many tasks can be implemented if expanding to a double or triple Cubesat. Even though the launch cost of a double or triple Cubesat is somewhat higher (although far from double or triple, as the cost for launch support from the ground personnel is a major portion of the cost), it may still offer far better results, as mass, volume, and power are less restricted.

You may well be asking yourselves, okay, how much does a Cubesat program cost? As most of the Cubesat projects are university based, it is hard to calculate their cost, as the main workload is completed by students, and professors rarely charge their expenses directly against the project budget. As an example, the 1 kg DTUsat required a budget of US$200,000, excluding salaries.

A couple of the commercial projects have released their budgets. QuakeSat had a total budget of US$1 million, including launch, but not including all salary and an additional operational cost of US$170,000 per month. The MAST mission by Tethers Unlimited (see image, previous page), consisted of three tethered picosatellites and had a price tag of about US$1 million for the entire program.

As so many projects are underway, it is hard to keep track of them all. I began compiling a list of Cubesat projects a couple of years ago, which can be viewed on the Internet [3]. When going through the list of Cubesat projects being developed, it turns out many still focus on the spacecraft bus, often including a camera and/or a radiation detector. Obviously, it is a wise decision to start with a simple project. Many already launched Cubesats with more advanced mission objectives have failed. What is surprising to me is that so many universities are working on the same type of problem instead of working together and truly coordinating their projects. Imagine what could be accomplished if they operated in an open-source manner. Knowledge others had gained could be reused and potential design problems would be more easily identified.

Certainly, engineering students would find it extremely interesting to design and build a satellite bus, launch it, and listen for it in space. In my opinion, it would be more valuable if the effort was concentrated into other areas. For instance, designing a new payload, or designing a new attitude sensor, rather than continually ‘reinventing the wheel.’

Focus will, more than likely, switch in this direction over the next few years, as so many universities soon will have a proprietary spacecraft bus they can reuse, or update, for future projects. Until Cubesats become open source, and even if a university does not already have a Cubesat bus, getting started is relatively easy. Several companies, including Pumpkin Inc., are selling Cubesat kits, which can be used as a foundation for a project. Three Cubesats have already been built and launched based on parts from such a kit.

Currently, 15 new Cubesats are on manifests to be launched in the near future. Three will be launched together with TacSat-III on a Minotaur-1, another three have been selected for a NASA flight currently scheduled for June 2009, and another nine (plus two backups) have been selected for the ESA Vega maiden flight, currently scheduled for a November 2009 launch.

[1] L. Alminde, M. Bisgaard, D. Vinther, T. Viscor, K.Z. Østergaard: “The AAU-Cubesat Student Satellite Project: Architectural Overview and Lessons Learned”, 16th IFAC Symposium on Automatic Control in Aerospace, St. Petersburg, 2004
[2] M. Thomsen, J.M.G. Merayo, P. Brauer, S. Vennerstrøm, N. Olsen, L. Tøffner-Clausen: “Feasibility of a Constellation of Miniature Satellites for Performing Measurements of the Magnetic Field of the Earth”, 6th IAA Symposium on Small Satellites for Earth Observation, Berlin, 2007
[3] M. Thomsen: “Michael’s List of Cubesat Satellite Missions”, <http://mtech.dk/thomsen/space/Cubesat.php>

About the author
Michael Thomsen holds an electronic engineering Master and Ph.D. degree from the Technical University of Denmark and a Master of Space Studies degree from the International Space University. Michael was working on the DTUsat Cubesat, which was launched on June 30th, 2003. He has since followed the Cubesat community closel, and regularly updates “Michael’s List of Cubesat Satellite Missions”. Currently Michael works at the Danish National Space Center, in the section developing the fluxgate magnetometers and star trackers for the ESA Swarm mission.