Austin Barker

ASEN 5050

December 12, 2002

The goal of the Mars Exploration Rover (MER) mission is to determine whether Mars was ever conducive to life. Two spacecraft will be independently launched, each carrying an identical rover, which must land on the surface of Mars. This mission must deal with the complications of interplanetary transfers, as well as those of landing. An overview of the mission will presented, as well as an overview of planetary entry and a detailed analysis of the specific MER landing plan. In addition, this landing will be compared to the landing of a possible manned mission on Mars.

The idea of life on another planet is thought by many to be only in the realm of science fiction. However, scientists of the Mars Exploration Rover (MER) mission plan to seek evidence that liquid water once existed on the surface of Mars, a possible indication that there was life as well. The MER mission is part of the Mars Exploration Program which plans to take advantage of every launch opportunity to send robotic explorers to Mars. The MER mission will send two rovers, MER-A and MER-B, on separate launches in the upcoming launch window of mid-2003. The rovers will land on sites selected based on the likelihood that they once held liquid water. The rovers will be armed with cameras and an array of scientific instruments to take detailed pictures and analyze rock and soil samples. The panoramic view afforded by the rovers’ camera will allow scientists to drive the rover to promising locations around the landing site. The two rovers will collect data for approximately 90 days and hopefully contribute to the four major science goals of the MER mission: 1) Determine whether life ever existed on Mars, 2) Characterize the climate and 3) Geology of Mars, and 4) Prepare for human exploration [1].

While the science of MER is very exciting, many engineering challenges, namely getting there, must be overcome to make the scientific goals of the mission attainable. The two current launch windows of the MER spacecraft are May 30-June 16, 2003 for MER-A and June 27-July 14, 2003 for MER-B [1]. After launch, the spacecraft will be placed in a parking orbit about Earth until it is in proper position to initiate the transfer to Mars. This action is demonstrated in Figures 1. The transfer orbit to Mars will be similar to a Hohmann transfer, as

seen in Figure 2. For the sake of comparison with a familiar transfer, we will look at the flight time using a Hohmann transfer assuming Earth and Mars and are in circular orbits about the Sun, and the radii of these orbits are given by a_E = 149 598 023 km and a_M = 227 939 186 km. The semimajor axis of the transfer ellipse is given by

We can now compute the time of transfer by

where t is the period of the orbit and m _s = 1.327 124 28 km³/s² is the gravitational parameter of the Sun [3]. Both of the rovers have a constant arrival date: January 4 for MER-A and January 25 for MER-B. Assuming MER-A is launched in the middle of its launch date, June 7, it will take 211 days to reach Mars, about seven weeks faster than the Hohmann transfer. This indicates that the two spacecraft will travel less than 180° around the Sun, and are therefore on a type I transfer [4].

After making the initial transfer, there are three scheduled trajectory correction maneuvers (TCM) during the cruise phase of the mission. Based on the information for the similar Mars Pathfinder mission, the first TCM will remove the injection bias which will keep the spacecraft from actually reaching Mars. This is required in order to prevent the upper stage from entering the Martian atmosphere and contaminating it. The second TCM is to correct errors in the first TCM. Finally, the third burn will be performed closer to approach, and will target for the necessary entry geometry. This burn will remove an additional bias that keeps the spacecraft itself from impacting Mars at too great a velocity in case control is lost after TCMs 1 or 2 [5].

When the spacecraft is 45 days from the Martian atmosphere, the mission enters its approach phase. During this time, the final three TCMs are made to correct for TCM 3 errors and make any small corrections necessary to precisely land the rover at the selected site. After being (hopefully) properly aligned the rovers will enter the atmosphere and land on the surface in a process that will be described later. And finally, after a long journey, they will be able to begin exploring.

After looking at the flight plan of the MER mission, we can see it is a very complex process just getting the spacecraft in position for landing. Then comes the final task of actually putting the spacecraft on the ground. Once the spacecraft is in low Earth orbit, the effects of drag are, for the most part, negligible. Therefore, we can rely completely on the equations of astrodynamics, with possibly some extra perturbation terms. However, this is not possible during entry. During this phase, the spacecraft has moved out of the phase where only the equations of astrodynamics are used and into an intermediate phase where astodynamics is still very important, but now the equations of aerodynamics become significant. Finally, as the density of the atmosphere increases, the equations of aerodynamics dominate. When we introduce the equation of aerodynamics, we introduce the concepts of lift and drag. In addition, engineers must determine how to decrease the kinetic energy of the spacecraft enough to safely land without generating more heat than it can handle. Also, the deceleration loads must be limited to what can be safely handled by the payload (humans, scientific instruments, etc.) [6]. Because it is a hurdle that must be cleared before the spacecraft can touch down, an understanding of planetary entry is an essential part of spaceflight dynamics for MER and any other mission that endeavors to land on a planet with an atmosphere.

One of the simper approaches to the problem of landing on a planet is that of ballistic entry. Because this strategy quickly penetrates deep into the atmosphere, gravitational forces are negligible compared to the drag force which, assuming hypersonic flight and an exponential atmosphere, is given by

where C_D is the drag coefficient, A is the cross-sectional area of the spacecraft, v is its velocity, H is the altitude, r is the atmospheric density at H = 0, and H₀ is the scale height. This entry method favors the greater accuracy of a steep flight path angle to the gentler deceleration loads and heating rates of a shallow angle entry [6]. Because an object in ballistic entry will reach the dense lower atmosphere before significantly reducing its speed, the heating rate will be very high. However, due to the short duration of the deceleration period, the heat transferred to the vehicle is limited. A blunt-nosed ballistic entry object will depend on the strong shock wave formed by its hypersonic passage through the atmosphere to divert much of the heat generated in addition to a thick heat shield that will act as a heat sink. The disadvantage of this method is the excessive deceleration loads and the weight penalty of the thick heat shield [7].

Another method, "double dip" entry, takes advantage of the lift of the vehicle to make two decelerating dips into the atmosphere and was used by the Apollo capsules during their return from the Moon. The first phase is made with the lift of the spacecraft pointing away from the surface of the planet. The vehicle swoops down, decelerating, then begins to ascend. Before leaving the atmosphere, the vehicle rolls over, thus generating negative lift. This allows the vehicle to remain in the atmosphere and complete the deceleration and landing [8]. Using this entry method, the heat transferred is more gradual, allowing the use of a thinner heat shield that relies on radiating the heat, rather than acting as a heat sink [7].

Yet another entry method is aerobraking. This method involves making multiple passes through the atmosphere to gradually slow the vehicle until it can safely complete the landing. Since this entry strategy uses multiple orbits of the planet, the gravitational effects cannot be ignored, and must be combined with atmospheric drag to produce the following equation of motion

where the atmosphere is assumed to be exponential, B* is C_DA/m, m is the mass of the spacecraft, and R is the radius of the planet. When the spacecraft is making its initial approach of the planet, it is in a hyperbolic trajectory. Therefore, the spacecraft must enter the atmosphere deeply enough to allow for sufficient slowing to enter an elliptical orbit about the planet. However, if the speed is decreased too much, then the spacecraft will not have sufficient velocity to maintain orbit, causing it to land. Assuming the velocity of the spacecraft is appropriately reduced, it will then be able to make multiple passes through the atmosphere which will degrade the apoapse of the orbit while leaving periapse relatively unchanged. One external concern with this orbit is due to radiation belts. This is a major concern for Earth entry because the intensity of the Van Allen belts can be significant. However, this is not a problem on Mars because Mars lacks the global magnetic field to trap the energetic particles that make up radiation belts [9]. An additional difficulty results from the dynamic nature of planetary atmospheres, resulting in uncertainty about the state of the atmosphere at the time of entry. This difficulty can be lessened by using a spacecraft with lift, which affords more control during braking [8].

The entry plan of the MER mission is closest to the ballistic method. The spacecraft will make a direct entry of the atmosphere, relying on atmospheric drag to slow it down and its heat shield for protection. After the vehicle has been sufficiently slowed, it will deploy a parachute to further retard the velocity. Before impact, airbags are inflated around the vehicle, then the spacecraft will fire RAD (Rocket Assisted Descent) rockets to further slow the descent. Finally, the parachute is cut and the rover will bounce along the Martian surface and eventually roll to a stop. This sequence is pictured in Figure 3.

The basic strategy of the MER entry is that of the ballistic method, but it also employs a variety of techniques other than the drag of the spacecraft to reduce the speed enough to safely land. This strategy is adapted from that of Mars Pathfinder. One of the primary objectives of the Pathfinder mission was to demonstrate that this "low-cost" landing was feasible. It accomplished this objective when it successfully landed on Mars on July 4, 1997 [10]. This direct entry strategy eliminates the need for costly burns to put the spacecraft in orbit about the planet, and then to enter into an entry trajectory. In addition, this entry method allows for the high degree of accuracy necessary to land the rovers within the approximately 120 ´ 120 km ellipses[11].

As was indicated above, one of the four primary goals of the MER mission is to prepare for human exploration of Mars. In order to do just that, it is necessary to look at a possible entry scenario for a manned mission to Mars. Looking at the MER entry strategy, we immediately see that it would not be possible to use the same strategy for a manned mission. The most obvious reason is that human astronauts would likely object to being dropped from a 20 meter height to bounce and roll to a stop inside any number of airbags. In addition, it is unlikely that the direct entry method used by MER would be sufficient to adequately decelerate a far more massive manned landing vehicle. Also, the double dip method employed by the Apollo missions, followed by a parachute descent into the ocean would not be as effective on Mars. This is primarily because Mars’ atmosphere increases in density much more gradually that Earth’s. The upper atmosphere of Mars is more dense than that of Earth, but the lower atmosphere far less dense, making the parachute much less effective [6]. However, one useful feature of Mars is its lack of significant radiation belts. This fact would allow the use of aerobraking without the hazard of radiation belt exposure. Aerobraking would also incur much gentler deceleration forces that the ballistic entry. Once the spacecraft enters, I would recommend a sequence similar to that of the MER mission. While the parachute will not be as effective as it would be on Earth, it is still an economical way to supplement the RAD rockets. Finally, rather than encasing the lander in airbags, I would suggest dropping the vehicle from a much lower height and onto an inflatable object that will collapse when impacted by the lander, which would cushion the landing but would not cause the tumbling of the MER airbags.

The MER mission will land two rovers on the surface of Mars to study rock and soil samples, looking for information about liquid water on the surface. In order to allow the rovers to do their job, a multitude of obstacles must be overcome. These include launching the spacecraft from Earth, navigating through interplanetary space, precisely aligning the spacecraft for entry, and finally, landing safely on the surface. There are a variety of entry techniques which all attempt to decelerate the vehicle enough to land while preventing the forces experienced and the heat absorbed from being excessive. The MER mission utilizes a direct entry, followed by a parachute and rocket slowed descent and an airbag-cushioned landing. This strategy was successfully demonstrated by the Mars Pathfinder mission and is both efficient and accurate. Making the transition to a manned mission adds a number of complications, including increased vehicle mass and less rugged payload. Though I have provided an entry strategy that takes into account these new constraints, it also introduces additional errors in targeting. Therefore, NASA’s engineers have their work cut out for them in moving to the next step of putting a man on Mars.