Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the drug store, but that’s just peanuts to space.
– from “The Hitchhiker’s Guide to the Galaxy” by Douglas Adams
Yesterday’s post might have come across as a bit of a downer. I don’t usually like being a downer, so let’s shift gears from that and get technical!
Last night, I talked about being separated from Earth. Some of the reasons for this are obvious: we’ll be isolated in a dome with no other direct human contact. But, there’s an additional effect that serves to separate us just a bit further: a 20-minute delay will be added to all voice communications. Why is that?
I’ve spent the last five years as a flight controller at Lockheed Martin’s Mission Support Area in Denver, Colorado, a mission control center for NASA interplanetary spacecraft. During my time here, I’ve had a chance to work with real spacecraft around Mars: Mars Odyssey, Mars Reconnaissance Orbiter, and MAVEN. To control these spacecraft, we design, program, and build commands, and then beam them to the spacecraft using the giant satellite dishes of NASA’s Deep Space Network…like DSS-14, a 70-meter dish located at Goldstone, California:
Once leaving the dish, the signals travel through space at the speed of light, about 300,000 kilometers per second! That’s fast enough that we usually think of light being almost instantaneous in our everyday lives, and that’s usually pretty reasonable – we flip a light switch, and the light from the bulb just a few meters away reaches our eyes almost immediately.
But, as Douglas Adams pointed out, space is big. Really big. Earth and Mars travel around their orbits at different speeds, and the distance between the two can vary from 54.8 million kilometers to 401 million kilometers. Doing some quick math (divide distance by the speed of light), we discover that it can take anywhere from 183 seconds (just over 3 minutes) to 1340 seconds (a bit over 22 minutes) for the signals to cross the distance. Once the spacecraft receive the commands, they send confirmation signals back to let us know that they received and understood the commands; that takes the same amount of time again! That means we’re anywhere from 6 minutes to 44 minutes of “round-trip light time” away from our Mars orbiters.
Is this a real effect? It sure is! Interplanetary flight controllers learn to think in three times simultaneously: “first-bit time”, the time the signal is sent from the dish; “SCET”, the time the spacecraft receives and executes the signal; and “ERT”, the time the confirmation signal from the spacecraft returns to Earth. Keeping track of all three times can be a bit confusing at first, but becomes almost second nature with practice. Of course, this effect isn’t just limited to Mars; I also frequently work with the Earth-trailing Spitzer Space Telescope (currently 23 minutes round-trip), and the Jupiter-bound Juno spacecraft (now over an hour and a half round-trip!).
So, our 20-minute communication delay in HI-SEAS is rooted in real-world effects. Now, as I mentioned before, the time between Earth and Mars in reality varies from 3 minutes to 22 minutes. However, choosing a constant time delay makes the effect easier to implement in our simulation. Choosing a higher-end value, though, means that, in this respect, we’ll have a somewhat more difficult experience than real visitors to Mars would encounter.
…on the other hand, things could be worse. One effect we’re not modeling is conjunction: roughly every two years, Mars disappears behind the Sun from the viewpoint of Earth. This effect lasts for a couple of weeks, and, during that time, we’re unable to reliably send commands to or receive data from the Mars orbiters. Before conjunction, we go through steps to put the spacecraft into safe configurations to weather out the long wait; afterward, we download all of the data recorded during the silence, and restore the spacecraft to their full operational states.
So, if you manage to give us a call while we’re in HI-SEAS, please don’t be alarmed if we don’t pick up straight away…