How Curiosity Will Land on Mars, Part 1: Entry
Posted by Emily Lakdawalla
22-06-2012 19:19 CDT
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" (which I embedded 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.
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 downward 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.
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.
There are a few other things about Curiosity's atmospheric entry that are different. It is larger than previous missions, and has that unusual tilted angle of descent. It has a relatively high entry velocity of 5.9 kilometers per second. These combine to make Curiosity's atmospheric entry hotter than previous missions'. That, in turn, required Curiosity's heat shield to be made of a different, tougher material than was used before. Curiosity's heat shield is covered with Phenolic Impregnated Carbon Ablator, also known as PICA.
This is the fireball part of Curiosity's arrival at Mars. Just 80 seconds after passing through that entry interface altitude, the surface of the heat shield it reaches its peak temperature of about 2100 degrees Celsius. It's hard to put this number in context; it's way hotter than the melting temperature of basalt lava, which varies from 1000 to 1200. It's hot. But the PICA prevents that heat from propagating to the interior. Ten seconds after peak heating, the spacecraft reaches peak deceleration.
The heat shield has done its job when the spacecraft slows to about Mach 2.0. The supersonic parachute cannot be opened at higher than Mach 2.2. When it has flown to its target altitude, position, and velocity for parachute deployment, the spacecraft ejects six 25-kilogram masses on the opposite side of the spacecraft from the previous ejected masses, rebalancing it. The engineers call this the "Straighten Up and Fly Right" maneuver (and, inevitably, abbreviate that SUFR). The spacecraft also rotates 180 degrees at this point to aim its ground-sensing radar properly. If you're keeping score, that's 300 kilograms of dead weight that the spacecraft brought to Mars and dropped on the way to the ground!
The spacecraft is ready for parachute deploy. And that part I'll cover in a later entry!
But first, a comment about the guided entry. This new entry method was needed for two things. One was to shrink the size of the landing ellipse. That opened up more possible landing sites, because it's hard to find places that are flat and safe for landing that are 150 kilometers wide. Only 25 kilometers wide is much easier; and with Curiosity's driving capability, it was expected to be able to rove out of the ellipse to go to interesting rocks in landscapes too dangerous for landing. We're doing that with Gale crater, landing in a flat place next to a very tall mountain, at least some of which Curiosity will climb.
The other reason for this guided entry, flying through Mars' atmosphere, was to permit landing at much higher elevations than before. This, too, opened up lots of new possible land sites. As it turned out, though, the landing site selection committee picked a spot at very low elevation -- so low that it was actually on the list for a possible Mars Exploration Rover landing until their ellipses got too big. In a sense, then, this is one capability of the new guided entry system that is kind of, well, wasted. Let's hope we get another chance to use a system like this to explore some higher-elevation spot on Mars!
It's all going to be very exciting. If you can possibly come to Pasadena, we're hosting a great party over the weekend of the landing.