Blog Posted By Emily LakdawallaHow Curiosity will land on Mars, Part 1: Entry
Much of the space world, including my blog, is increasingly shifting its view toward the incredibly important event that will unfold on August 5 and 6: the landing of the gigantic Curiosity rover on Mars. For some of us, it's the culmination of years of anticipation. But for an awful lot of people out there, some moment in the next six weeks will be the first time that they've heard of this mission and how it's going to land. And nearly all of those people are going to ask the same question: Are they nuts?
When you watch the marvelous computer animation of Curiosity's landing, or today's viral video about the "Seven Minutes of Terror" (see video below) you gain an appreciation for just how many little events in a seven-minute Rube Goldberg sequence must go perfectly in order for landing day to be a good one. With that appreciation comes fear. What if something goes wrong? Why is it so complicated?
I've decided that the best thing I can do to help people understand what it takes to land on Mars is to explain the landing in excruciating detail, copying from and explaining the content of this paper by Ravi Prakash and coauthors, with some additions from the launch press kit. I can't do it all in one post, so I'm going to split it over several, probably three or four. It helps that the rover's engineers make their own distinctions between different phases. Being acronym-ophiles, they refer to it all as "EDL," which stands for Entry, Descent, and Landing. Today, I'll talk about Curiosity's approach to Mars and its entry into the atmosphere.
Curiosity Approaches Mars
Right now, Curiosity is dressed up as an interplanetary spacecraft. The rover and its jetpack descent stage are enclosed in a clamshell that consists of a heat shield under the rover and a backshell topping it. That stack is connected to a cruise stage, which is covered with solar panels (for providing power during the cruise to Mars). The cruise stage is also covered with a fluid-filled cooling system; the fluid is piped into the clamshell and back out again to collect the heat that is being thrown off by Curiosity's radioisotope thermoelectric generator and radiate that heat to cold space.
The cruise stage contains one other important system: rockets. These are used to guide the spacecraft to a specific spot at the top of Mars' atmosphere in a series of five "trajectory correction maneuvers". The last trajectory correction maneuver is scheduled for two hours before the landing. After that, the work of the cruise stage is done, and it will separate from the rover to crash to its own landing on Mars.
All of this (except for the bit about needing to vent the waste heat from the radioisotope power source) is virtually identical to the approach phases for all other recent landed Mars missions, including Mars Pathfinder, the two Mars Exploration Rovers, and Phoenix. The cruise stage will deliver the spacecraft to a spot at the top of Mars' atmosphere with amazing precision, to within a couple of kilometers of the aimpoint.
But what happens next is very different. Those previous landers entered ballistically; their fat cone-shaped heat shields were pointed with the tip of the cone in the direction of their motion, they spun around their symmetry axes, and their paths were controlled by the whim of Mars' atmosphere. Winds, areas of less dense or more dense air, and other atmospheric variability affected the rate of those spacecraft's descent. So those craft could land anywhere in a fairly large region, a skinny oval more than 100 kilometers long, landing short if the atmosphere braked them quickly or long if the atmosphere decelerated them more slowly. (In fact, every previous Mars lander landed "long.")
To prepare for its very different, non-ballistic atmospheric entry, Curiosity will despin, slowing from 2 revolutions per minute to halt at a specific orientation, in a condition that navigators call "three-axis stabilized." The spacecraft will then rotate to point the tip of its blunt heat shield "nose" in the direction of its travel. Five minutes before entry, it will jettison two 75-kilogram tungsten "balance masses" from one side of the capsule, shifting its center of gravity off-center. This will tip the nose upward by about 20 degrees, a tilt it will maintain (give or take a couple of degrees) through most of the entry phase. Atmospheric entry is defined as the moment that the spacecraft reaches an altitude of 3522.2 kilometers from the center of Mars -- about 125 kilometers above Mars' average, but closer to 131 kilometers above the landing site, which is at very low elevation. Remember this low-elevation detail -- I'll have something to say about that at the end of this post.
Curiosity enters Mars' atmosphere
Curiosity fires thrusters during the guided entry phase of its arrival at Mars. With its heat shield nose tilted upward 20 degress with respect to its direction of motion, it actually flies in Mars' atmosphere, banking in S turns to aim it at a target position and velocity for parachute deployment.
In its tilted state, the spacecraft does not fall like a rock. It actually generates lift. It will literally be flying in Mars' atmosphere, not falling. It can, in theory, even rise in elevation at some points during the deceleration phase (though it probably won't need to, given how low-elevation Gale Crater is). It will use an inertial measurement unit (which contains gyroscopes) to detect its path through the atmosphere, and will change its pitch (the angle of the cone) and also bank left or right as needed, making S-shaped curves, in order to deliver the spacecraft to a targeted spot at a targeted velocity for parachute deployment. This guided flight phase is what makes Curiosity's landing ellipse so much smaller than previous landers'. The atmosphere will still have an opportunity to move the rover around while it's under parachute, but Curiosity will be able to fly out almost all other sources of landing inaccuracy in this guided phase.