[caption id=”attachment_7256” align=”aligncenter” width=”300”] Mars Science Lab Atlas V Launch - 26 November, 2011[/caption]
Everyone was really excited for the landing on Mars Science Lab (MSL) Sunday night. I worked for the Launch Services Program as the trajectory analyst for MSL. The landing was exciting for me because, even though the Atlas V launch vehicle and MSL spacecraft separated eight and a half months ago, this journey to the surface of Mars is one the rocket put it on. The landing place and the time of landing is what dictates where and when the spacecraft is dropped off by the rocket.
Let’s say you and your best friend decide, Saturday morning, August 4, you want to drive from Kennedy Space Center to New York City to see the Mars Science Lab landing broadcast live in Times Square on Sunday night (August 6, 1:31am EDT).
[caption id=”” align=”aligncenter” width=”300”] East Coast of USA - Kennedy Space Center in Florida to New York City[/caption]
You have to decide if you want to get there on Sunday
- at 3pm to catch a Broadway matinee
- at 7pm for dinner
- at 11pm when the coverage starts
- at 1am just before landing
You have figure out where you’re going to park your car, and then if you’re going to walk or take a taxi or the subway to Times Square.
- leave early Sunday morning and drive straight through
- leave Saturday and stay in a hotel
- leave Saturday and stay at your cousin Vinny’s house in Virginia.
You have to decide on which highways you want to take based on a metric (i.e. the fewest tolls or fastest time). The route might include a quick side trip to the beach (assuming the beach is on the Atlantic Ocean; go to the Pacific Ocean and you probably won’t make it in time). You have to give yourself margin for bathroom breaks, food breaks, and, of course, traffic.
These all factor into when you leave Florida. Because you only get one shot to be in Times Square when Curiosity tells us she’s reached the surface of Mars.
Designing a trajectory to Mars isn’t exactly the same, but you have to make a lot of tradeoffs and build in margin, just like your road trip to New York. One of the biggest determents when going to Mars is that, every 26 months, the orbits of Mars and Earth line up for the fastest, lowest energy trajectory to Mars. If you miss that opportunity, like we did in 2009, you have to wait for the next one.
Trajectory design is easiest when you can launch the same trajectory, regardless of time or day. That was not the case for MSL. When you go to another planet or join a train of other spacecraft or if the sun is a factor, you have to adjust your trajectory for each day and time you could launch.
There were multiple designs of trajectories to Mars. And each design needed to be run for 5-9 times for each day, depending on how close to launch we were. There was a 24 day launch period, so United Launch Alliance (ULA: Atlas V launch vehicle provider) had to create hundreds of trajectories for each design.
One thing that affects the design is the landing site. The landing site wasn’t chosen until August 2011, but we had to start modeling trajectories years beforehand. So we had to be ready for landing sites in the Northern Hemisphere, the Southern Hemisphere, and near the equator of Mars.
[caption id=”” align=”aligncenter” width=”461”]3 The landing sites with white text show the final four landing sites in the selection process. Gale Crater was the selected site.[/caption]
Another design element is that trajectories work differently based on whether, on your way to Mars, you go more or less than halfway around the sun; it’s one or the other, based on what day you launch. Mars Climate Orbiter had a Type II trajectory; Mars Reconnaissance Orbiter, like MSL, had a Type I trajectory.
Landing on Mars is incredibly risky, with a 40% success rate. So you want to be able to have data for the entire journey through the atmosphere. That way, if something goes wrong, you know when in the descent it happened. And even if you’re successful, the data will help with planning the next landing.
[caption id=”” align=”aligncenter” width=”392”] Mars Mission Success Rate (for all missions; not just landers)[/caption]
What day and time you plan on landing is going to affect whether the Mars orbiters are directly overhead and available to collect the data from MSL, or if the Earth is in view and we might be able to do Direct to Earth communication. Both methods utilize the Deep Space Network. Ideally, you get both the entire time you descend, but that’s another tradeoff.
With orbiters, you have to worry that, between deciding to depend on them and the time you land, something could happen to the orbiter and it isn’t available. With direct to Earth communication, we can’t get as much data during landing as when we go via the orbiters.
[caption id=”” align=”aligncenter” width=”200”] Mars Reconnaissance Orbiter[/caption]
Now sometimes you decide to leave a certain time to start the road trip, but something happens and the plan changes. You get 20 minutes away from your house and you realize the video camera is on the kitchen counter. You go back to get it and take out the side trip to Jekyll Island State Park in Georgia. May be halfway through the trip, you realize you’re ahead of schedule and so you stop at a sit down restaurant instead of a fast food restaurant as originally planned for lunch.
Just like you make adjustments to the trip, the MSL had six trajectory correction maneuvers (TCMs) scheduled for along the way for two different reasons. The first reason is that the launch vehicle puts the spacecraft on a trajectory aimed just a little off of Mars. This is for planetary protection; in case the spacecraft isn’t working properly after launch/at some point during the cruise to Mars, we don’t want it to automatically hit Mars and contaminate the planet if it won’t be able to do science. So the TCMs gradually move the spacecraft’s aim closer to Mars.
[caption id=”” align=”aligncenter” width=”448”] Mars Science Lab during the Cruise[/caption]
The second reason for TCMs is margin against errors. The launch vehicle aims for a specific place to drop off the spacecraft, but the engineers plan for bubble around that place that the spacecraft might actually end up. Issues could come up during flight, such as the thrusters giving more or less thrust than anticipated (this didn’t actually happen). And the JPL mission design and navigation manager for MSL said “This was among the most accurate interplanetary injections ever” regarding the launch trajectory. So MSL didn’t have to use all the TCMs planned.
On your trip to New York, even though both you and your best friend are intelligent and have taken many road trips in your lifetimes, you guys might want to stop at different restaurants or disagree on whether one highway is quicker than the other. The engineers at JPL have different ideas on what is most important when designing the route to Mars and they have to work together to agree of the best solution for the spacecraft. This is when the art of engineering and space travel happens.
Because just like you and your friend will only make this particular trip once, MSL only has one journey to Mars.
[caption id=”” align=”aligncenter”] Trajectory from Earth to Mars[/caption]
Note: credit for the image of people viewing the MSL landing from Times Square (in the banner) belongs to space.com. http://i.space.com/images/i/20214/iFF/times-square-msl1.jpg?1344282586