After all the excitement of yesterday’s launch, things are going to be fairly quiet until August. But Cosmoboy thought it would be fun to take a little look at Curiosity’s trip to Mars. How far does it have to go? How is the steering accomplished? How do we stay in contact?
Communicating with spacecraft is not easy. As the recent Phobos-Grunt disaster shows, once you lose contact getting it back is tough. The Earth’s rotation also means you can’t just rely upon one communication facility as every few hours it’s going to be pointing in the wrong direction. So you need a bunch of dishes dotted around the world. And that’s exactly what NASA’s Deep Space Network is, a system of three stations separated by 360/3=120 degrees apart on the Earth. There are dishes in:
Goldstone, in the Mohave Desert in California. The Goldstone facility actually contains five antennas, the biggest of which is the recently refurbished 70m “Mars” antenna. The name of this antenna actually comes from the face that it was first used to pick up the Mariner 4 spacecraft which had been lost by smaller antennas during its Mars fly-by. Since that time the dish has tracked Voyager 2 as it passed Uranus and Neptune as well as Spirit and Opportunity on Mars.
Robledo de Chavela, Spain which is about 60 km west of Madrid. (Note, the website is in Spanish, so unfortunately I couldn’t delve into as much detail as I hoped, and ironically it looks much better than the Goldstone website!). This facility also has five antennas, including a 70m dish that weighs 8000 tons. Like the other two facilities, the receivers sit in an area that is somewhat bowl shaped to shield it (as much as possible) from terrestrial radio interference.
Tidbinbilla, Australia which is about 30 km from Canberra (love the cows, this is farm land!). This station has the smallest number of active antennas, at three. Like the other two it’s largest dish is also 70m. Some film buffs probably remember the movie “The Dish” with Sam Neil, actually this facility isn’t the one referenced there. That was Honeysuckle Creek, which closed in 1981. In fact, there were once seven tracking stations across Australia.
Steering Curiosity is a bit like driving using your rear view mirror. The spacecraft relays signals back to the Earth and then course adjustments are sent back. Curiosity isn’t really looking for Mars or piloting toward it directly, instead that’s being handled through the DSN. Importantly, it has to be done with great accuracy, believe it or not sunlight will “push” Curiosity during it’s flight to Mars, and this needs to be taken into account (you don’t want to wind up hundreds of km off course). What’s more, by the time Curiosity reaches Mars and enters the atmosphere it needs to know very precisely where it is, any errors in position could be disasterous. There’s only 70 kg of propellant to make course corrections, i.e. two tanks of gas in a 350 million mile journey!
Depending upon the precise launch time, NASA had a bunch of different possible trajectories all laid out. As it happens, Curiosity launched bang on the number one launch time of 10:02 am EST. With the trajectory laid out, it’s now a matter of tracking and making sure all the thruster burns happen on time. If you’re wondering, the thrusters are actually pretty low power, they have only 1 lb of thrust. Some of the maneuvers thus take hours of burn.
But by and large, getting Curiosity to Mars is routine. It’s been done before. And as I keep reminding everyone, the landing is where things get interesting!