Slashdot Mirror
Mars Odyssey Completes Aerobraking
Cally writes: "Space.com reports that
Mars Odyssey has
completed aerobraking and is ready to begin its
main science mission. As the spacecraft has already produced exciting results before the start of the science mission proper, interesting data on the quantities of water in the Martian crust may be expected soon - not to mention that Odyssey provides another datapoint in the study of Gamma Ray bursts."
Here's the article:
January 11, 2002
MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIFORNIA 91109. TELEPHONE (818) 354-5011
http://www.jpl.nasa.gov
Mars Odyssey Mission Status
January 11, 2002
Flight controllers for NASA's Mars Odyssey spacecraft sent commands overnight to raise the spacecraft up out of the atmosphere and conclude the aerobraking phase of the mission.
At 12:18 a.m. Pacific time Jan. 11, Odyssey fired its small thrusters for 244 seconds, changing its speed by 20 meters per second (45 miles per hour) and raising its orbit by 85 kilometers (53 miles). The closest point in Odyssey's orbit, called the periapsis, is now 201 kilometers (125 miles) above the surface of Mars. The farthest point in the orbit, called the apoapsis, is at an altitude of 500 kilometers (311 miles). During the next few weeks, flight controllers will refine the orbit until the spacecraft reaches its final mapping altitude, a 400-kilometer (249-mile) circular orbit.
"The successful completion of the aerobraking phase is a major milestone for the project. Aerobraking is the most complex phase of the entire mission and the team came through it without a hitch," said David A. Spencer, Odyssey's mission manager at JPL. "During the next month, we will be reconfiguring the spacecraft to begin the science mapping mission." The science mission is expected to begin in late February.
During the aerobraking phase, Odyssey skimmed through the upper reaches of the martian atmosphere 332 times. By using the atmosphere of Mars to slow down the spacecraft in its orbit rather than firing its engine or thrusters, Odyssey was able to save more than 200 kilograms (440 pounds) of propellant. This reduction in spacecraft weight enabled the mission to be launched on a Delta II 7925 launch vehicle, rather than a larger, more expensive launcher.
JPL manages the 2001 Mars Odyssey mission for NASA's Office of Space Science, Washington, D.C. Principal investigators at Arizona State University in Tempe, the University of Arizona in Tucson, and NASA's Johnson Space Center, Houston, Texas, operate the science instruments. Additional science investigators are located at the Russian Space Research Institute and Los Alamos National Laboratories. Lockheed Martin Astronautics, Denver, Colo., is the prime contractor for the project, and developed and built the orbiter. Mission operations are conducted jointly from Lockheed Martin and from JPL, a division of the California Institute of Technology in Pasadena. NASA's Langley Research Center in Hampton, Va., is providing aerobraking support to JPL's navigation team during mission operations.
After a nearly perfect launch, 2001 Mars Odyssey is on its 400-million-mile, six-month journey to the red planet. The spacecraft will primarily search for water on Mars but it will also seek 19 other chemical elements and measure radiation. NASA, just barely holding the budget-cutters at bay, needs to recover from two previous Mars failures: the Mars Polar Lander and the Mars Climate Orbiter. If everything works, Mars Odyssey will spend two years circling the planet while taking measurements and readings. The mission was already providing remarkably sharp and dramatic views before and during lift-off with two cameras attached to the Delta 2 rocket, one facing up and one down.
NASA:
http://mars.jpl.nasa.gov/odyssey/
Space.com: Space.com
Given the following photograph of Mars, and knowing that the estimated albedo of the Earth's atmosphere, estimated from its composition, is of 0.75, try to predict the value of atmospheric pressure on the surface of the Red Planet.
To start with, an overall albedo of Mars should be estimated, upon observation through a telescope (a photograph is not good enough in this case) and measurements of the intensity of the light reflected off the surface of Mars as compared . It is important to understand that in the calculation process students need to assess which of their assumptions and estimations is more inexact and thus impacts more the accuracy of the model. This assigned value of a global albedo of Mars with the given elements is, of course, quite uncertain, but it does not invalidate the method.
In order to provide a working example, let us accept an overall albedo for Mars of 0.25.
The next step is to appraise what percentage of the planet is occupied by the dark green vegetation like areas and how much of it is made up of the desert type of soil. Once this is determined we have to approximate a value for the albedo of each type of terrain.
For example:
- Dark area: 60% of the planet albedo 0.15
- Desert area 40% of the planet albedo 0.06
Weighted average 0.6*0.15+0.4*0.06=0.114
If we estimated in this example that the overall albedo of Mars was 0.25, then the part of light absorbed by the atmosphere should be 0.25-0.114=0.136
Considering then the albedo as a measurement of the ability of the atmosphere to scatter sunlight and as such proportional to the specific mass of the atmosphere, we could relate the mass of the Terrestrial atmosphere to that of the Martian atmosphere:
Specific mass ratio = albedo of Martian atmosphere / albedo of Terrestrial atmosphere
= 0.136 / 0.75 = 0.1813
Now we can finally relate to atmospheric pressure. The atmospheric pressure can be defined as the force per unit area exerted by the mass of air on the surface, or in other words, the weight of the column of air on a unit of area. Having estimated the mass of the Martian atmosphere as a fraction of that of the Earth's atmosphere, we can easily relate it to the weight of the atmosphere by multiplying by the Martian gravity (0.38g)
Atmospheric pressure on Mars = 0.1813 * 0.38 g = 0.0689 of the atmospheric pressure on Earth
It is interesting to notice that from merely observing the planet we can go as far as to predict that the atmospheric pressure on Mars is approximately 7% of that on Earth.
Even though this last value is not more than a rough estimate, it still yields very important information that allows us to conjecture about the climatic situation in the Red Planet. Regarding temperatures, if the atmosphere is quite thin and thus unable to sustain any warming greenhouse effect, given that Mars lies in a much colder region in the Solar System than the Earth, it must follow that Mars should be a very cold world.
The apparently low atmospheric pressure has a profound impact on the climate of Mars and, in turn, on the possibilities of finding life on the surface. At such low atmospheric pressure added to very low temperatures, it would be very unlikely that liquid water could exit, even in trace amounts, on the surface. In effect, as pressure goes down the boiling point of a liquid descends, and even at very cold temperatures any liquid, including water, would vaporize instantly. Bearing in mind that liquid water constitutes, in a way, the operational definition for life as we know it, we can say that it would be improbable to find life on the surface of the Red Planet.
I would say, if humans could live on mount Everst without a shirt (provided the weather is not cold enough), then they could do the same on the surface of Mars. But, at the same time, I would say that they'd die very soon. Not of bends, or anything to do with pressure. But of radiation overdose. (Read Red, Blue, Green Mars).The experiment you refer to involved a tethered satellite. It was performed on February 25, 1996, and as miles of tether were unrolled, the dynamo current grew just as predicted. The tether was almost entirely unrolled when it broke near the shuttle's end, whipping off into the void. The shuttle crew tracked the satellite by radio for several minutes, then lost contact.
After the mission, the tether was examined on Earth, and was found to have been melted through. Turns out the core of the cable was a porous material that had atmospheric-pressure air trapped in it during manufacture. The air leaked out through pinholes in the outer insulation and was quickly converted, by the high voltage (~3500V) of the tether, to a plasma far denser and more conductive than the surrounding ionosphere. Instruments indicated that the plasma diverted a full ampere of current (at 3500 volts) through the insulator pinholes, enough to melt through the cable.
That's why they don't let astronauts EVA any more without gloves.
OffTopic: That last line was a feeble joke, similar to the one in my original post. That post was modded up three points (by those who took "regenerative aerobraking" seriously) before being modded down five (by those who take mismoderation seriously). Is there a record for the number of mod points, both up and down, assigned to a single comment?
As the basic rule, all measurements are metric this time. Some of the mechanical components are in US measurements simply because the aerospace industry has always made them that way and a special order of metric components would be much more expensive. However, every time a US measurement is used it is explicitly accompanied by it's metric conversion in the spacecraft's documentation.
Wrong, the ISS is not needed nor is it even designed to be a spaceport or deep space launching platform. NASA already has a plan to get to mars "Mars Direct". They just need the funds to implement it. Check out the Mars Society's web site http://www.marssociety.org and read "A Case for Mars" By Dr. Robert Zubrin. The book is a great read for anyone interested in Mars exploration and explains why we don't need tons of space infrastructure to get to mars.