A key part of any story about exploring new worlds is the transport you use to get there. From the Enterprise, to Captain Nemo’s Nautilus, to Lee Scoresby’s balloon. Meet Mothra, our aerostat. An aerostat is a lighter-than-air aircraft, which in our case is a hybrid helium balloon-kite tethered to the ground. It’s not the kind people can ride in. It’s just big enough to lift our equipment up to altitudes where bats and insects are flying.
Mothra is a sixteen cubic meter Helikite. How big is that? She could fill a small one-car garage, and can carry a payload of from 8-16 kg, depending on wind speeds. It took three large tanks of helium to fill her up, and we kept another on hand for topping off over the two-week expedition.
Who controls Mothra? The fairies, of course
She’s connected by a tether line controlled by an electric winch we call Pichi. (That’s one of the names given to the fairies who controlled the kaiju Mothra). Pichi is mounted on a trailer hitch and powered by a big electrical cord hooked up to the vehicle’s battery. The tether line comes out from the winch and through a sheave (like a pulley) firmly anchored to the ground, and then is tied to the aerostat. The sheave is anchored in the center of a large inflated ring called a helibase, which protects Mothra when she is near the ground in very high winds. As you can imagine, we make sure the anchors are set securely and the knots are properly tied.
For our prototype system, we used 800 meters of super-strong Dyneema line capable of pulling 1,700 kg. This was a conservatively strong line, because we have experience with handling smaller Helikites in gusty Texas winds. We opted for safety with this much larger Helikite. On this expedition, however, we had unusually light winds. That means I was never terrified of losing Mothra in a big wind gust! On the other hand, we discovered that our heavy duty line was literally too heavy to achieve the goal of flying to 500 meters high. One night we let out most of the line. The weight of the heavy-duty line itself, combined with the other components, exceeded Mothra’s lift capacity. So, to get to 500 meters we will need either a lighter line or a larger aerostat!
A typical deployment
A typical deployment starts a couple of hours before sunset. We arrive at the hangar and position the van based on the expected wind direction. Next, we prepare all the components that we will attach to the tether line. That includes loading the detectors onto the array, making sure the tethersonde is transmitting to the base station, and checking batteries. When everything is ready, we bring Mothra out of the hangar and attach her to the tether line, and use Pichi to start letting her out. We track the amount of line out using a system of marks every 25 meters. Based on a predetermined plan, we attach the various components to the line at specific intervals. This includes flashing lights to alert pilots who might be flying in the area at night. Our goal is to be fully deployed shortly after sunset, to capture the first arrival of bats.
Time to relax
Once the line is fully deployed, we monitor the altitude and wind speed aloft, which are transmitted from the tethersonde every two seconds. If all goes well, this is the quiet part of the night. It’s great for watching stars, listening for bats with hand-held detectors, maybe checking out what’s attracted to a black light trap. However, I am very serious about some basic safety rules. Everyone must carry heavy gloves and a headlamp with them at all times after Mothra is attached to the line, in case we need to move quickly. Gloves are important because there is an amazing amount of tension on the line and a bare hand will be pinched or rope-burned.
But be prepared!
The possibility of disaster always looms in the darkness: a massive wind gust or change of wind direction, unexpected lightning or storm clouds approaching, or a winch failure with hundreds of meters of line out. I always take the time to talk through emergency procedures, just in case. For example, if the winch fails we have to “walk it down”, which means literally grabbing the line and pulling it down as we walk toward the aerostat. In high winds, the tension on the line is intense and this will require multiple people. Also, the ground is rarely clear in all directions, so we’re likely to be walking through a cornfield. We agree on a detailed plan including who will be the primary person holding the line, as well as who will take each component collected off the line as we bring it down manually. I have had the experience of walking it down, and I’ve also witnessed a big wind gust break a line! Mothra has a special “cut down” device that will burn a hole to release helium after she is more than 1.5 km from the launch site, and I hope to never test it.