As described in another blog post, CYGNSS is a constellation mission with eight satellites. Typically, when you launch a single satellite, you use a single rocket to put it into orbit. With eight satellites, you might think that we would use eight rockets to get them into orbit. While this would be really nice, it is not practical simply due to cost considerations.
Rockets are pretty expensive. This is primarily because there are not many of them made each year, which means that there is not a mass production facility set up to drive the prices down. There are only a few rocket launches each year, which means that each rocket is almost a custom build. The Pegasus rocket, which is one of the smallest and cheapest rockets you can get, costs about $40,000,000. Considering that the total cost of all eight satellites of CYGNSS is about $100,000,000 to design, build and test, the cost of the rocket is a lot. This means that we can’t launch each of the satellites on individual rockets, but have to launch all of them together.
In order to do that, Sierra Nevada Corporation is making a deployment module for the mission. This is a essentially a big tube that will hold all eight satellites in two rows with four satellites around the tube. When the rocket gets to just the right altitude, the deployment module will start up and deploy pairs of satellites in opposite directions. If you check out this video, you can see the deployment of the satellites. It’s a great video!
Each satellite will come off the deployment module with a speed of about 1 m/s (about 2 miles per hour) with respect to the deployment module, which will be orbiting with a speed of about 7,600 m/s (about 17,000 MPH). From one satellite’s point of view, the other satellites are moving quite slowly, while the whole constellation is moving extremely fast. For example, it will take the whole constellation about 90 minutes to orbit the Earth. For the fastest satellite to lap the slowest satellite, it will take over six months. In that time, the constellation will have gone around the Earth about 3500 times.
If you look at this video, you can see how the satellites spread out, how they lap each other, and how the global hurricane coverage changes as a function of time (as explained in this blog post). As satellites lap each other, and bunch together, the coverage dips down a little bit. While it is not a great deal, we would like to minimize this. We want to put the satellites into an evenly spaced constellation to make the coverage stay constant with time.
If the satellites were a lot bigger, they would carry propulsion, which would allow them to modify their speed. We could then space the satellites out as we would like and keep them spaced out throughout the mission. Since they are tiny microsatellites, they don’t have propulsion. But, it is possible to use the atmosphere to help up out a bit.
I recently wrote a blog post about drag and terminal velocity. In that post, I explain how air can act as a drag force on objects. Well, the atmosphere extends up into the region in which satellites orbit. In fact, the primary force that satellites feel (besides gravity) in low Earth orbit, is atmospheric drag. Satellites that are below about 500-600 km orbit, will feel such a strong drag force that they will de-orbit and burn up in the atmosphere in less than about 20 years unless they have propulsion to keep them in orbit. Above about 600 km, the lifetime of the satellites grows exponentially. (In fact, the second US satellite ever launch, in 1958, Vanguard, is still in orbit around the Earth, nearly 60 years later.)
CYGNSS will be put into orbit at about 500 km, where the drag force is quite weak (compared to here on the surface), but still exists. The satellites will de-orbit and burn up in the atmosphere in about eight years, from our calculations. Interestingly, we can change the drag force on the satellites by changing their orientation. This is because the satellites are shaped sort of like a bird – a thick center with wings. If you fly the satellites like a bird, with the wings parallel to the ground, the drag force is minimized. If you tip the satellites up, then the bottom of the wings face into the “wind” and the drag force will be increased by about a factor of eight. We can use this tipping to control the speed of the satellites and therefore the spacing. The idea is that we will tip the satellites up for a few days to change their speed a bit, then look at the constellation spacing and maybe tip a different satellite up for a while. After about six months or so of “playing” with the satellites’ orientation, drag, spacing, and speeds, they will be roughly evenly spaced around the orbital plane. Take a look at this video to see how the spacing and coverage changes when we use this concept to space the satellites out.
Using techniques like this, it is possible to launch constellations of very small satellites on a single rocket to accomplish great science!