Thursday, November 17, 2011

Getting Ready for the Cape

MSL Arrives at Launch Complex 41 inside its 5m fairing just before dawn on November 3. In the days ahead, this fairing will be mounted atop an Atlas-V 541 EELV for a launch to Mars no earlier than November 25. 

After some consideration, I've decided to bite the bullet, take a chance and head on down to Cape Canaveral next week to help see off Mars Science Lab (MSL). Better known as the Curiosity Rover, MSL is an exciting mission to Mars that will take a close-up look at the km-thick layered deposits inside Gale Crater. It's an exciting place to try for and the rover team is looking forward to a successful launch, landing and the scientific results that will be returned. Unfortunately, I was unable to attend the spectacular night launch of Phoenix in 2008. So this will be my first launch, assuming that delays don't push the launch back until after I need to fly home.  While I have some idea what to expect from video and images, I have no idea what the experience will be like in person.



Currently the launch is planned for 10:25 AM EST on Friday, November 25. That's when the first launch window opens. On that day we will get 1 hour and 43 minutes before that daily window closes (for details on the windows, see this excellent table from spaceflightnow.com). At the next level, within each daily window there are individual launch opportunities approximately every five minutes. If there are issues that prevent a launch on the 25th, the good folks at ULA will try again the next day and the next and so forth until December 18. On that day, the launch window closes entirely shut and the orbital mechanics of the positions of Earth and Mars are no longer viable for our spacecraft.

Dare I add a brief word on launch windows and their good friends landing elipses? For Earth to Mars trajectories, the preferred orbit is called a Hohmann transfer orbit. It's an ellipse with the Earth at launch, the sun in the middle, and Mars at arrival aligned along the major axis. It's a special trajectory because it requires the least amount of velocity change ("delta-V") and hence the least energy/fuel to complete. Looked at another way, this is the trajectory that allows us to get the greatest amount of Mass and hence the largest number of scientific instruments (payload) to Mars.

MSL's Hohmann Transfer Orbit: note the alignment of the Earth at departure, Mars at Arrival and the sun in the centre.

But in order to take advantage of a Hohmann transfer, Mars and Earth need to be properly aligned and that only happens a little over every two years or so. If the path of the spacecraft was completely ballistic from launch to Mars Atmospheric Entry, the launch window would be very short, perhaps no more than ~280 seconds - the time required for Mars to move through the point in space where the spacecraft would arrive. But, every spacecraft has onboard fuel for course corrections and this widens the usable launch window to about a month.

Why do we have daily launch windows? To answer this, you need to recall that the Earth is a spinning planet. Ideally, the sequence initiated by main engine ignition will proceed directly from liftoff to the Hohmann transfer orbit without delays. This suggests launching when the rotation of the Earth carries the launch location past the start of the Hohmann transfer orbit (see the animation shown on this site for a graphical representation). Technically, however, only the final burn needs to take place in this orientation. In theory, you could launch, circularize the spacecraft orbit into LEO and add in a hold of up to 90 minutes until the spacecraft orientation is appropriate for the Hohmann transfer. This would allow you to launch at any time of day, but practically speaking this may not be easily achievable depending on how propellant is distributed between the various rocket stages.

The unfortunate Russian Phobos-Grunt spacecraft is currently using a variation of this emergency technique which allows it an opportunity to head for Mars once every 90 minutes or so. Of course, the ground crew needs to sort out why the spacecraft's rockets aren't firing first.

Oh and what about landing ellipses? A landing ellipse expresses the uncertainty that we have about where the spacecraft is going to touch down on Mars. It is elliptical because the spacecraft has a velocity with respect to the planet and a value for this uncertainty translates into a larger error in space along that trajectory rather than perpendicular to it. But a little known property of landing ellipses is that each launch window has a different one! As the relative positions of Earth and Mars change over the launch window, we can compensate in latitude and longitude using corrective maneuvers (burns) in deep space. But the landing ellipse will also tend to rotate irreversibly as the launch window proceeds, producing a bow-tie-like shape when all ellipses are superimposed.

Landing ellipses for the Phoenix Spacecraft showing how the ellipse changes over the course of a launch window. Superimposed, all the different ellipses form a shape referred to as a butterfly or bow-tie. (I prefer bow-tie since I imagine that the consumption of "pork chop" diagrams from which launch windows are created is a formal affair)

Hopefully all will go as planned. The Atlas-V launcher that MSL sits atop has a good track record with planetary science missions lately with Juno successfully launching within its first daily launch window. LRO similarly launched on its very first attempted day (the launch was delayed one day due to conflicts with the space shuttle). However, MRO had a launch delay of two days and New Horizons was delayed for 8 days. Add to that the fact that the specific Atlas-V configuration variant being used, the 541*, has never been launched before, and you can see why this is a bit of a gamble. My own personal launch window only extends for two days, so I will have my fingers crossed!

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* How to read Atlas Configurations: For an Atlas-V ABC, A = diameter of the payload fairing (the "nose cone") in meters, B = Number of solid boosters, C = number of upper stages. So a 541 has the biggest payload fairing (because MSL is pretty darn large), four solid boosters and one centaur upper stage. Juno was launched aboard the similar 551. Hopefully, one less solid booster will not make a huge difference and our launch can be as picture-perfect as was that one!

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