Even with some mishaps, humans have successfully landed robotic spacecraft on the surface of Mars. Any landing tries to do two things. First, you want the spacecraft to decrease in speed before coming in contact with the surface (high speed contact would be called a crash). Second, you want your system to be low mass. You don't want to bring a giant rocket to Mars just for the landing—that means you have to expend even more energy getting it to Mars in the first place.

To consistently accomplish these things, NASA is building what it's calling a low-density supersonic decelerator. Now let me ask and answer the questions you might have about LDSD.

What is different about LDSD and previous spacecraft that have landed on Mars?

Previous missions have relied upon a parachute for the bulk of the decent. A spacecraft will enter Mars' atmosphere and go through an entry process similar to how spacecraft reenter on Earth (it's not called reentry on Mars because the spacecraft is entering for the first time). However, the density and thickness of the atmosphere on Mars is much lower. This means the atmosphere doesn't slow the craft down all the way. The next step is to open a parachute. Once the spacecraft is at a low enough speed, the parachute continues slowing the spaceship.

The LDSD is a little different. For the entry, the LDSD system has an extra step. As the spacecraft enters the atmosphere, it has an inflatable donut-shaped object around the spaceship. This increases the surface area and thus drag. This airbag-like device is called a supersonic inflatable aerodynamic decelerator (SIAD). With the SIAD, the spacecraft will slow down more than a normal spacecraft and then move into the second part of the landing.

Illustrations of the three new drag devices being developed by the LDSD project: 100-foot (30-meter) Supersonic Ringsail

(SSRS) Parachute (at top), 20-foot (6-meter) SIAD-R (bottom

left), 26 foot (8-meter) SIAD-E (bottom right). NASA

The next landing phase includes a high speed parachute. With LDSD, NASA has changed the parachute so it can be opened earlier and at a higher speed.

Where does the name Low-Density Supersonic Decelerator come from?

Let's break it down. First, there is "low-density." This refers to the lower density atmosphere of Mars. The "supersonic" is there because the spacecraft will use it while moving at between Mach 4 and 2. "Decelerator" means the spacecraft will slow down.

What's the difference between deceleration and acceleration? Nothing really. Acceleration is defined as the rate that the velocity changes. This includes speeding up, slowing down, changing direction, or any combination of the three. Then why did they use "decelerator"? Perhaps they liked the acronym LDSD instead of LSDA. Just between you and me, every time I see "LDSD" I think of Captain Kirk in Star Trek IV saying that Spock had taken too much LDS.

Why do we need the LDSD?

Here is the problem. The Curiosity lander had a mass of 900 kg (1 ton). What happens when you want to have a larger mass payload? Maybe you want to land humans on Mars. If you double the mass of the payload, you need to double the area of the parachute. Curiosity used a parachute 15 meters in diameter. If you doubled the mass, you would need a parachute 60 meters in diameter. Perhaps you can see the problem. Bigger payloads need a better landing system. This is exactly the goal of the LDSD.

Does this mean that we will now be sending humans to Mars?

Not really. If we want to send humans to Mars, we will need to figure out a new landing method. This is one option that could be used, but it's still in the testing phase. No one wants to build a spacecraft mission with an untested landing system.

How are they going to test the LDSD?

The problem with Mars is it has a much lower density atmosphere than Earth. As you go higher in Earth's atmosphere, the density of air decreases. So, to test the LDSD, engineers will send a test vehicle up on a balloon to about 180,000 feet. At that point, the vehicle will be released. It will fire some thrusters that will spin the vehicle. After that, it will fire a solid rocket engine to bring up to Mach 4. At this high speed, the SIAD will be deployed. If this works as planned, it will bring the speed of the vehicle down to around Mach 3. Once the vehicle gets to a little bit slow speed, it will deploy the supersonic parachute.

At the end, the vehicle will remain under the parachute until it reaches the surface in the ocean off the coast of Hawaii. The primary focus of this test will be to examine the performance of the redesigned supersonic parachute.

The flightpath of the LDSD test mission. NASA

Why did NASA make this UFO?

It's not a UFO. A UFO is an Unidentified Flying Object. This vehicle is identified as the LDSD. You could say it's saucer shaped if you like. Maybe that will make you happy. The vehicle is a circular disk shape because that will be the shape of the bottom of a spacecraft that would land on Mars.

When will NASA run this latest LDSD test?

A window opened this week, but the weather has not cooperated. The launch will depend on weather conditions — especially the ocean wave height. If you want to keep up with the exact launch details, I suggest following NASA on twitter (@NASA) or check out the NASA blog. Also, you will be able to watch the test and press info on the NASA ustream channel.

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