Mars Pathfinder
Frequently Asked Questions

Pre-Launch, Launch and Cruise

Why it is that 2:09 a.m. on 2 December (and earlier times on dates thereafter) is the precise time Pathfinder must launch? What is is about the Earth's position that makes this important?

When the spacecraft leaves the Earth to go to Mars, it must be going in a particular direction. Since the Earth rotates, the launch site is only lined up with this direction twice per day (for an instant in each case). Since the two opportunities are about 12 hours apart, the launch vehicle people make us choose one or the other. It is okay to launch at a time slightly different from the ideal time because the spacecraft can use it's propulsion system to correct for the error. The spacecraft has a limited amount of fuel, however, so we can't accept a very big error (up to approximately 1 minute is okay).

You can see all of Mars Pathfinder's launch opportunities at the Launch Windows Page.

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Two.

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The first burn of the second stage is used to place the spacecraft in a low parking orbit around the Earth. It then coasts until it gets to the right point in its orbit to do the burn to go to Mars. The second stage then ignites again to begin pushing the spacecraft towards Mars. The third stage finishes off the job because the second stage fuel tanks are nearly empty.

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It depends on launch date. For December 2, we go into shadow at 3:15 am and exit at 3:45 am.

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This is not quite correct. The spacecraft begins transmitting when it separates from the third stage (at about 3:25 am). The Deep Space Network station in Goldstone, California should detect this signal about five minutes later, and we should begin to get engineering data from the spacecraft. We don't actually try and send a command to the spacecraft for several more hours (about 4-5 hours after launch). All of these operations are conducted by engineers at JPL.

--Richard Cook, Mars Pathfinder Mission Operations Manager

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Because the path which the spacecraft takes to get to Mars, essentially "catching up" to the planet, it will travel approximately 312 million miles (500 million kilometers) in its seven month journey. However, when Pathfinder actually arrives at the planet, the Earth and Mars will be separated by approximately 120 million miles (200 million kilometers).

--Dave Spencer, Mars Pathfinder Trajectory and Navigation Team Member

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Technicians and engineers that work in the vicinity of the lander must wear what we call "bunny" suits (it was a joke name originally, but many years ago the name caught on). These are clean head-to-toe garments that prevent dirt and biological contamination of the lander by the workers.

At other times when the hydrazine fuel was being loaded into Pathfinder's fuel tanks, some workers had to wear "SCAPE" suits. These suits are also head-to-toe, but they also provide self-contained breathing equipment which is strapped to their backs. In fact they look a lot like space suits. Hydrazine, in addition to being highly flammable, is an extremely caustic and dangerous liquid. These suits are designed to protect the workers in the unlikely event of a hydrazine leak.

--Rob Manning

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You're correct in saying that once we're in Earth orbit, it is necessary to time the stage II/stage III burns properly to inject us onto the interplanetary transfer trajectory. It is also critical, however, that the circular parking orbit be in the same plane as the interplanetary trajectory. This allows us to take advantage of the velocity our spacecraft has from the parking orbit; the launch vehicle does not have enough energy to drastically change our orbit plane. Since the plane of the parking orbit is critical, the launch time is also critical. Think of it this way: for a "planar" launch, the launch site, the center of the Earth, and the interplanetary velocity vector must all lie in the same plane. This only happens twice each day.

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The July 4 arrival date is maintained by increasing the launch energy throughout the launch period; the Delta II gave us a higher velocity on the December 4 launch than it would have if we had launched on Dec 2. The arrival date was selected based upon a mixture of patriotism and favorable orbital mechanics--the optimal arrival date was in early July anyway, so why not pick the 4th!

-Dave Spencer, Mars Pathfinder Trajectory and Navigation Team Member

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The 23,000 mph is the speed relative to Earth (at the time of third-stage separation), but remember that that the Earth itself is travelling round the Sun at around 67,000 mph, so we are actually travelling much faster than 23,000 mph relative to the Sun.

The latest navigation solution shows our velocity with respect to the Sun is 33.516 km/sec, or around 75,000 miles per hour! Dividing that into 310 million miles will then give you 172 days, or less than 6 months. The remaining error from that calculation is caused by the fact that the Sun-relative velocity decreases (from 75,000 mph to 47,640 mph) as we leave Earth and approach Mars, so the real travel time is around 211 days.

-- Pieter Kallemeyn, Mars Pathfinder Navigation Team Chief

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Prior to ignition of the Delta rocket third stage, the second stage fires small jets to spin-up the vehicle to 70 revolutions per minute. Then, the second stage is jettisoned and the third stage burn occurs. The high spin rate is needed for stability of the vehicle. After the burn is over, however, the Pathfinder spacecraft needed to be despun to a spin rate of 12 rpm (a higher spin rate would make it difficult for the sun sensors to acquire the Sun). The despin is accomplished by deploying two weights laterally from the third stage along tethers (like a figure skater extending her arms to slow her spin). The weights are appropriately called yo's, and they are sized to despin the spacecraft to the desired rate. The yo's detach from the third stage following deployment.

--Dave Spencer, Mars Pathfinder Mission Design and Navigation

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Pathfinder is now in an elliptic orbit around the Sun. The basic laws of gravitational attraction lead to varying speeds for a body as it moves along an elliptical orbit. The body moves fastest when it is closest to its center of attraction and continually slows down as it moves away from this center. It moves most slowly when it is farthest away from the center and begins to speed up again as it moves back in towards the center to complete its orbit. For Pathfinder, the center is the Sun, the closest point in its heliocentric orbit is nearer to Earth and the farthest point would be somewhere just beyond Mars. Pathfinder is constantly slowing down on the part of its orbit that leads from Earth to Mars. If the spacecraft were allowed to continue without landing on Mars, it would begin to speed up again as it came back in towards the Earth's orbit and the Sun.

This also happens to the planets as they move in their orbits about the Sun. However it's a much smaller effect since the planets' orbits are more nearly circular than Pathfinder's. That's why Earth's and Mars' orbits look like circles in our trajectory plots while the spacecraft's trajectory looks like part of an ellipse.

--Robin Vaughan, Mars Pathfinder Navigation Team

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I've read the craft during the cruise en route to Mars will be rotating. If that's the case, how does the signal keep in touch between earth and the spinning craft's antennae without losing contact with each spacecraft revolution? In other words, how does the antenna point back steady enough to earth to send/receive signals if it's rotating?

On the way to Mars, the Mars Pathfinder spacecraft spins like a child's toy top. Visualizing a child's top, you will note that the spin axis of the top always points in the same direction (at the ground) no matter how fast you spin the top. Our Mars Pathfinder communications antenna, a simple microwave horn transmitting at 8 GHz, is located very close to the spin axis of the spacecraft. We keep the spin axis of the spacecraft pointed at Earth, and hence the antenna is always in view!

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For the first 30 days of the mission, the Deep Space Network will track Mars Pathfinder 24 hours per day, 7 days per week. After January 4, 1997, that tracking time will be reduced to 3 passes per day, 3 days per week as you noted. The Deep Space Network has one antenna at each of 3 complexes (Goldstone, California; Canberra, Australia; Madrid, Spain) that is capable of transmitting at the 7 gigahertz radio frequency that commands Mars Pathfinder. Two other projects, Mars Global Surveyor and Near Earth Asteroid Rendezvous also require the use of this antenna for commanding their spacecraft. Scheduling conflicts are not a problem currently, as the three missions are at very different places in the sky. Conflicts will increase as the Mars Pathfinder and Mars Global Surveyor orbits converge on Mars in the summer of 1997.

--Leif Harcke,Telecommunications Systems Analyst

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It is clear that the MPF must slow down to safely land on Mars. But why is there a planetary protection requirement which limits the impact velocity to less than 1000 ft/sec? This requirement is stated in the description of the trajectory correction maneuvers.

The requirement you're talking about is meant to further reduce the contamination of Mars from parts of the vehicle in the unlikely event of a crash landing. It specifically states that at any time during cruise, there should be a less than 0.1% chance that the spacecraft will impact Mars at a velocity greater than 1000 ft/sec. This, of course, assumes that we have lost control of the spacecraft completely at some point along the way to Mars, which is itself an unlikely event. This requirement affects how we execute the first two midcourse maneuvers.

-- Pieter Kallemeyn, Mars Pathfinder Navigation Team Chief

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Since the Mars Pathfinder cruise stage does not carry any imaging experiments, how is it possible, during the approach, to precisely determine the point of entry into the Martian atmosphere to ensure a landing at Ares Vallis?

Your question is the crux of Orbit Determination, which is the discipline of determining a body's flight path through space, under the influence of gravity, solar pressure, propulsive events and outgassing, and other small forces. Basically, through the Deep Space Network tracking stations, we get two types of data during every tracking pass: Doppler data and Range data. The Doppler data is a measure of the frequency change of the returned signal compared to the frequency of the signal uplinked to the spacecraft from the DSN. Doppler indicates the range rate of the spacecraft relative to the tracking station. Range data is simply (and approximately) the time it takes for a signal to reach the spacecraft from the DSN, and be retransmitted and received on the ground. This "roundtrip light time" is then multiplied by the speed of light to give the spacecraft range from the tracking station. Doppler and Range taken together can give a pretty good estimation of the spacecraft's position and velocity in space.

Doppler and Range information can also be supplemented by optical navigation, as you suggest. In fact, some sort of imaging of the planet surface would be necessary to land on a precise landing site (in a particular crater, for example). For the Pathfinder mission, our landing site is actually pretty large (200 km x 100 km), so we don't need an optical navigation system. Sample return missions (likely in 2005, or maybe sooner) will likely use optical navigation.

-- Dave Spencer, Mars Pathfinder Mission Design and Navigation

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Why does Pathfinder spin on the way to Mars? Why does it spin at different rates?

Early in the Mars Pathfinder design process we decided to use spin stabilzation as the primary attitude stabilizing mechanisim. Like a giant top, the spacecraft spins at 2 revolutions per minute during the 7 month cruise phase of the mission. Of course, like a top, the spin dynamics causes the orientation of the spacecraft to remain fixed in "inertial space" indefinately provided you don't do anything to the spacecraft - like fire thrusters. The other method of letting the spacecraft remain stationary (like the Voyager and Mars Global Surveyor spacecraft) requires that the attitude control system operate all the time to keep it from tumbling in space. (So called, "three-axis" attitude control systems typically monitors attitude using star and sun sensors and gyros and periodically fires thrusters or turns reaction wheels or other torquing devices to keep things like antennas pointed toward Earth and cameras pointed to targets.) There are advantages and disadvantages to both approaches (the Galileo spacecraft even does both! Half of it spins at up to 3 rpm and the other half remains stationary!). The advantages of a "spinner" are that the attitude control system can be turned off once the attitude of the spacecraft is where you want it - but don't try taking long duration photographs of objects from fixed cameras on the spacecraft; the images will be smeared!

Since Mars Pathfinder doesn't have cameras that can "see" outside of the spacecraft during "cruise" to Mars, we weren't constrained to have the spacecraft required to operate without some rotation. In fact the opposite was true, we had to design it so that it could spin. First of all, the Delta II rocket's upper stage had to be spun up to 70 rpm so that it could remain stable during the time of the orbital injection burn which took us out of Earth's orbit toward Mars. Since Mars Pathfinder was then bolted to the upper stage, it also had to be designed (balanced) to handle being spun up to 70 rpm then de-spun down to about 12 rpm just before it separated from the upper stage. Secondly, near Mars, once the aeroshell (with the lander and rover in it) separates from the cruise stage, the aeroshell must be spinning at 2 rpm to stay stable during Martian atmospheric entry (remember, that it is just a spinning bullet at this point, there is no active control).

So why do we spin at 2 rpm? Actually 2 rpm is just right for the entry process. If it rotated too fast, then the aeroshell would spend too much time at its entry attitude and it would be aerodynamically lifted too much (it would act like a skipping rock on a quiet lake), too slow and it could start to tumble. During cruise it would be nice if the spacecraft could spin faster, but 2 rpm is the fastest it can spin while also being able too recognize stars with our star scanner (the stars would "blip" by too fast for the scanner to see them!).

Now that the spacecraft has been "despun" down to 2 rpm, we will leave it spinning at that rate all the way until we get to Mars. It will start to slow down only after the parachute opens in the Martian atmosphere!

--Rob Manning, Mars Pathfinder Chief Flight Engineer

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Why is Pathfinder taking such a long course to Mars? Why not wait until the Earth is just ahead of Mars in Earth's orbit and fly directly away from the sun to Mars' orbit? The probe and the planet would then meet up in space and Mars would capture the craft. I know you want to take advantage of gravity assists but it seems like a few slingshots around the Earth would give you the neccessary velocity.

There are two reasons we take the long road to Mars. The first reason is launch energy--by launching into a trajectory that is in roughly the same direction as the Earth's orbit about the Sun, we take full advantage of the Earth's velocity (remember, once we leave Earth's gravity, we're in an orbit about the Sun, just like the planets). If we were to launch into a trajectory radially outward from the Sun as you suggest, we would need a launch vehicle powerful enough to negate the initial velocity that we get from the Earth [launching from Earth is similar to throwing a ball from a speeding car--if you want the ball to go straight out and not in the direction the car is moving, you have to throw the ball backwards as well as out].

Secondly, our current trajectory gives us a Mars atmospheric entry velocity of 9.3 km/s. If we were to launch onto a trajectory radially outward from the Sun, our arrival velocity at Mars would be much higher--this would cause problems with our Entry, Descent and Landing system.

--Dave Spencer

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If space is near absolute zero, why do you need to cool the craft? Is the 2 rpm rotation not sufficient to keep the sun from heating the craft?

For the same reason the rotating Earth does not cool it to the ambient deep space temperature of 3 degrees Kelvin (just above absolute zero), Mars Pathfinder adopts the "average" thermal energy of the whole "sky", including the very hot Sun. Although the Sun takes up less than a fraction of a percent of the "sky" as seen by Mars Pathfinder, it's surface is millions of degrees hotter than the surrounding deep space. That is enough to "average" the temperature of the spacecraft to a little colder than room temperature (to calculate the temperature right, you also need to factor in the spacecraft's emissivity and absorbtivity). So the spacecraft's average temperature is around 10 deg Celsius. That would be great except for another point: we have "hot spots" built into the design. So that we can keep the lander's electronics and battery cool while on the surface of Mars, we intentionally surrounded them inside a thermal enclosure (just like the rover team did with the rover electronics). The problem then is to keep the lander's electronics (especially the X-band radio transmitter) from roasting in there while we cruise to Mars. We solved this problem by pumping freon around the cool perimeter of the cruise stage (those white radiators you can see on the pictures) and then down inside the lander and rover. The freon takes the excess heat away from those hot spots and then radiates that heat into the cold of deep space.

--Rob Manning

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What is an "SEU"?

SEU = Single Event Upset (also called SEE or Single Event Effect).

An SEU occurs when a high energy ionized particle (say an ionized oxygen atom traveling at 50% the speed of light) which is generated by either a magnetic storm on the Sun or a galactic cosmic ray (the source of these particles is still somewhat unknown), hits the spacecraft, goes right through the structure and happens to hit an electronic memory cell (a "bit"). It usually doesn't do much permanent damage to the cell, but the event could easily flip the bit from a binary "1" to a "0" (or vice versa) thereby confusing the on-board software. Electronics used on spacecraft are usually as expensive as they are due to the fact they often must be specially designed and manufactured to be "hardened" to SEUs and other types of non-ionizing radiation effects.

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