The two questions that I get asked about most frequently when describing the STO-2 mission are “why are you putting a telescope on a balloon?” and “why do they need to launch from Antarctica?” These are great questions, but to answer them I need to give you a brief overview of some terminology. You can get a lot of information about the STO-2 mission in my previous blog post here.
The terminology I’ll be talking about is the payload. The payload is the main piece of equipment that is hanging from the balloon, so pretty much everything that launches except the balloon and the parachute. The payload part of the balloon is shown above. The payload has three primary components: the telescope (primary and secondary mirrors sitting within the tube), the instrument (which is the 3 cameras sitting inside the helium tank [dewar], and associated support electronics), and the gondola structure and support packages (the frame of the telescope, the solar panels, navigation cameras, the guidance/communication system, and more).
The reason for putting a telescope is the same as why we put them on satellites; you are looking through less air while in space, so the image quality is better. The atmosphere blocks some of frequencies of light from making it to the ground, so you can’t see that light from the ground. You are probably familiar with this in terms of the Ozone layer of the atmosphere shielding us from the sun’s UV rays, and the terahertz frequencies we observe for the STO-2 mission similarly blocked. Specifically, terahertz light is blocked by water vapor. The altitude our balloon can reach (125,000 ft = 24 miles) is above 99% of the atmospheric water vapor, so we can see the terahertz light shielded from the ground. Even on the driest places on the Earth’s surface, there is too much water vapor in the air to take images over a large region of the sky.
Another advantage of balloon missions is that they are recoverable payloads. When a satellite has reached the end of its mission, it is either burned up in the atmosphere or sent adrift into deep space. At the end of a balloon mission, the payload is separated from the balloon and returns to Earth on a parachute. A team goes to recover the payload, it gets sent back to the US to make upgrades and fix minor damage that occurs during the landing, and the mission can fly again. The 2 in the STO-2 mission name means that the STO-2 instrument has already flown once before and is on its second flight. The telescope can have different instruments attached to it, and this telescope and gondola structure are on their 6th flight! Previously, the telescope was used for cosmic ray and comet-observing missions. The STO-2 instrument was upgraded and expanded from the STO-1 flight, which flew in 2011. The ability to make quick upgrades and reusing flight-proven hardware is a significant advantage over satellite missions.
The final advantage that balloon missions have over satellites is their cost. A satellite mission costs anywhere from a hundred-million dollars to over one BILLION dollars. Balloon missions cost between 1-10 million, depending on the complexity of the payload, mission length, etc. For emerging technology, like terahertz cameras, it makes sense to build and use small-scale balloon payloads to do really cutting-edge science before spending the money to dive into the finer scientific details with large-scale satellites. Balloon missions serve as a great pathway to developing robust technology that can survive the harsh space environment and still achieve the primary science objectives.
Now that I have converted you to balloon-spacecraft enthusiasts, I can address the question of why we fly them out of Antarctica rather than out of ASU’s backyard. Believe it or not, the answer has a lot to do with the weather! Space-weather, that is.
The maximum height the balloon reaches is ~125,000 ft, which is within the stratosphere. The atmospheric pressure at that height is less than 1% of what it is at sea level, but there is still enough air up there to have wind. Near the solstices, June 21 and December 21 each year, the winds at that altitude circle both the Arctic and Antarctic poles. In order for a balloon payload to be recoverable, as discussed above, we want to launch the balloon somewhere with stable weather conditions where we can reliably predict its flight path and plan on where it will land. Since these polar winds are seasonal, we know approximately when they will occur each year, and can plan accordingly.
We also use the circularity of the winds to our advantage. Since the winds circle the pole, we can launch the payload from a clear site, and wait for the balloon to make one or more rotations and land it near the launch site. You may not think this is important, but when you launch balloons at the poles, there aren’t roads to get to just any point on the continent to go pick up your payload. The outdoors are harsh even in the summer, and the nearest fully-functional hospital is over 1000 miles away. The closer you can land your payload to the station, the easier and safer it is for the recovery team. You can see the flight path of the STO-1 mission in the picture above.
We launch balloons from Antarctica, but not many get launched from the Arctic. Why? If you look at the picture of Antarctica, what is missing? There aren’t any big towns on Antarctica, in fact, no person lives here permanently. NASA is concerned about the possibility of the balloons landing somewhere unsafe, since once the payload separates from the balloon there is no way to maneuver it for a controlled descent. The northern latitudes are more populous than the southern latitudes, so the risk of the payload causing damage is less in Antarctica. In the Arctic, the flight path of the balloons goes over many countries, some of which have restrictions on what spacecraft they allow to fly over them. Overall, it is safer to launch from Antarctica only.
Finally, we only launch from Antarctica during the Antarctic summer (which is winter in the northern hemisphere). The main reason for this is because at 77 degrees south, the sun never sets at McMurdo in the summer. This is good for the telescope since it has 24 hour access to sunlight to get collected by the solar panels to power the various systems on the payload. It also makes the payload recovery easier, as well as easier for the scientists and engineers to set everything up.