Posted on 08/15/2003 6:36:49 AM PDT by bedolido
The Hubble Space Telescope, astronomy's vaunted time machine, was granted a conditional reprieve yesterday when an expert panel recommended that NASA consider sending a crew of astronauts at the end of the decade to extend its career, rather than dropping it out of orbit, as has been planned.
But the committee said its recommendation should be carried out only if the science to be performed in those additional years was able to beat competing proposals for new NASA science projects.
For the last 13 years, floating above the Earth's murky atmosphere, the telescope has beamed down crisp images of galaxies still forming at the dawn of cosmic time, peering into the hearts of galaxies and quasars in search of black holes, and investigating the mysterious "dark energy" that seems to be wrenching the cosmos apart.
"By any standards the H.S.T. has been a spectacular success — one of the most remarkable facilities in the entire history of science," said the committee, whose chairman is Dr. John Bahcall of the Institute for Advanced Study in Princeton, in a report posted yesterday on the Web site of the National Aeronautics and Space Administration.
The committee members and other astronomers pointed out, however, that since the breakup of the Columbia shuttle, the telescope's future has been threatened because it is hostage to the ability of a space shuttle to pay periodic visits for maintenance and to replace old instruments with new ones.
Those repair missions would take the shuttle too far from the orbit of the International Space Station in case of trouble. As a result, the space agency should be prepared for a range of possibilities, the report said, from no more shuttle missions to two.
More is likely to be heard on that score in a couple weeks when the report of the Columbia Accident Investigation Board is released. But ultimately, Dr. Bahcall said, Congress, the White House and the NASA administrator will decide whether the shuttle may visit the telescope.
Dr. Anne Kinney, who is in charge of astronomy and physics in the space agency's office of space science, said that the Bahcall committee's report was "a good report."
"It reminds us that we need to be flexible," Dr. Kinney said. But she added that there was no budget for the extra mission and no precedent for the kind of competition that Dr. Bahcall and his colleagues had proposed.
"It's going to be a challenge," she said.
Astronomers were generally pleased with the report. Dr. Wendy Freedman, director of the Carnegie Observatories in Pasadena, Calif., called it "balanced and thoughtful."
Dr. George Rieke, an infrared astronomer at the University of Arizona, said the idea of a competition was "a sensible way to deal with limited resources."
Dr. Steven V. W. Beckwith, director of the Space Telescope Science Institute, praised the report, saying, "I couldn't be happier."
He added:"Everyone here is tremendously grateful to NASA for its support of the Hubble. We're delighted to have the chance to compete to continue this extraordinary story."
The story of the $1 billion Hubble, launched in 1990, is one of the great comeback stories in modern science. It was designed to take advantage of an orbital vantage point above the Earth's atmosphere, which smears images and blocks some wavelengths of light from reaching ground-based telescopes.
Once it was in orbit, however, astronomers were devastated to discover that the telescope had a flawed mirror.
The flaw was corrected in 1993 by sky-walking astronauts who, in effect, fitted the telescope's instruments with corrective lenses, enabling Hubble to attain the glory for which it was designed.
NASA has long planned to end Hubble's spectacular run and bring it down to make way in the budget for the James Webb Space Telescope, now scheduled to be launched in 2011. But some astronomers have urged that Hubble's life be extended, arguing that the telescope has grown even more productive in its years in orbit, and that the Webb could be delayed.
Moreover, the Webb is being designed for the infrared wavelengths that very distant galaxies would be emitting as they sped away in the expanding universe, not the visible wavelengths that Hubble sees so exquisitely.
The panel that NASA asked to review the issue also includes Dr. Barry Barish of the California Institute of Technology, Dr. Jacqueline Hewitt of the Massachusetts Institute of Technology, Dr. Christopher McKee and Dr. Charles Townes, both of the University of California, and Dr. Martin Rees of Cambridge University in England.
One of the main elements in their thinking, Dr. Bahcall said, was the realization that NASA might have to mount a mission to the telescope anyway at the end of the decade to attach a rocket to bring it out of orbit safely. The telescope is too big to be left to tumble out of orbit by itself.
The space agency had originally hoped to grab it with the shuttle, bring it back to Earth and put it in the Smithsonian Air and Space Museum, but the Columbia disaster scuttled that option.
NASA is looking into developing a robotic spacecraft that could attach itself to the telescope, but many astronomers argued that the job could be done more reliably by astronauts. And if the astronauts are there anyway, they said, the telescope could be spiffed up for another few years of science.
Another element was the inspirational qualities that Hubble has both for astronomers and for the public, Dr. Bahcall said.
Indeed you are making sense. Sorry for taking so long in responding, was so tired I took a quick nap. LOL!
Ok back to your question. I believe what you are attempting to say is can we build a telescope that can see further than the Hubble. Here is a nice site that can explain it far better than I can in a paragraph or two. :-)
And no, I do not see you as "stupid"!
If I may offer my two cents...
If the purpose in boosting to geosync orbit is to prevent uncontrolled reentry, why not use a reboost to change the HSTs orbit to one that the Shuttle can get to and still make it to ISS? That would keep the HST servicable. It takes a lot more fuel to change altitude to geosync than to change inclination with a minor altitude adjustment.
Lets start with a little background. When we launch a satellite (such as the Hubble) into orbit around the Earth, not only do we need to know its precise position, we also need to know where it (antennas, sensors, solar arrays, etc.) is pointing.
First lets talk a bit about “where it is”. An orbit is a nothing more than an object falling around another object. Both Kepler and Newton came up with a set of laws that describe this phenomenon.
Kepler’s 3 laws of planetary motion:
1) The orbit of a planet is an ellipse with the sun at one of the foci.
2) The line drawn between a planet and the sun sweep out equal areas in equal times.
3) The square of the periods of the planets is proportional to the cubes of their mean distance from the sun.
So what is that telling us? In a nutshell, all orbits are ellipses, the close to the body you are orbiting the faster you go (e.g. if you have a highly elliptical orbit the satellites velocity will increase as it approaches the object being orbited and decrease as it get further away), and the further away an orbit is, the slower the object moves.
These laws not only apply to planets, but to any orbiting body.
Note: Super geek alert #1: (took that term from Physicist hehehe)
For an orbiting body this is not entirely correct. It turns out that both bodies end up orbiting a common center of mass of the two-body system. However, for satellites, the mass of the Earth is so much greater than the mass of the satellite, the effective center of mass is the center of the Earth.
Newton’s three laws (and law of gravitation)
1) The first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. (Commonly known as inertia)
2) The second law states that force is equal to the change in momentum (MV) per change in time. (For a constant mass, force equals mass times acceleration F=ma)
3) The third law states that for every action there is an equal and opposite reaction. In other words, if an object exerts a force on another object, a resulting equal force is exerted back on the original object.
Newton’s law of gravitation states that any two bodies attract one another with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Note: Super geek alert #2:
Actual observed positions did not quite match the predictions under classical Newtonian physics. Albert Einstein later solved this discrepancy with his “General Theory of Relativity”. In November of 1919, using a solar eclipse, experimental verification of his theory was performed by measuring the apparent change in a stars position due to the bending of the light buy the sun’s gravity.
So what is all this trying to tell us? Planets, satellites, etc orbit their parents in predictable trajectories allowing us to “know” where they will be at any given time. A set of coordinates showing the location of these objects over a period of time is called its ephemeris.
From here out let us stick to satellites in orbit about the Earth. Since the Earth is not a perfect sphere (its an Oblate Spheroid), satellites drift from their predicted position due to the Earth’s non-spherical shape. Also at low Earth orbits, the atmosphere creates a drag on the satellite also causing a drift (perturbation) in its orbit. At higher altitudes, such as a geosynchronous orbit, the solar wind and effects from the moon are more pronounced.
This requires us to update the ephemeris periodically.
Now that we have a better understanding of its orbital position, we need to concentrate on its pointing (Attitude Control).
Why do we need to worry about pointing? If the satellite has solar panels (arrays), they need to point towards the sun to provide power. Sensors need to point at their respective targets, such as a star sensor, sun sensor etc. Thermal and possible contamination consideration must be taken into effect when pointing also.
Remember for every action there is an equal and opposite reaction. So if I spew mass (jet of gas out of a thruster nozzle), the satellite will move in the opposite direction. Also if I spin a wheel onboard the satellite, the result will be the satellite spins in the opposite direction.
Satellites (and spacecraft) are incredibly precise machines with exquisite craftsmanship. The life of a satellite is often computed by the onboard fuel requirements. For geostationary satellites, periodic maneuvers (delta-Vs) must be accomplished to keep them on station. This is also required for many lower orbiting satellites as well. For an orbit plane change (move it into a different orbit), mass must be ejected to move the satellite.
Note: Super geek alert #3:
The Hohmann transfer orbit is the most energy efficient (minimum energy solution) way of getting from one circular orbit to a higher or lower circular orbit. This type of transfer orbit is used by the interplanetary spacecraft to travel to the other planets in our solar system.
Since fuel is precious and usually cannot be replenished (called consumables), other methods of pointing were devised that did not require mass ejecta from the satellite. Spinning reaction wheels were one. If you have orthogonal reaction wheels, just by spinning them you can provide precise pointing. Unfortunately, external forced (perturbations) adds unwanted momentum to the wheels. To compensate (unload momentum from the wheels) for this, I have seen both low-level monopropellant jets or torque rods used for this purpose.
Note: Super geek alert #4:
A monopropellant is one that does not require an oxidizer to function. Usually monopropellants are composed of a liquid compound called Hydrazine (N2H4). When this liquid comes in contact with a platinum catalyst, it is decomposed into gaseous ammonia (Nh3), nitrogen and hydrogen. This gas is then ejected (fired thru a nozzle) out a jet to providing motion for the satellite.
An ingenious method of unloading momentum without the use of fuel was devised using simple electromagnets. Remember the Earth is surrounded by a magnetic field (why your compass works). If you attach orthogonal electromagnets on your satellite and turn them on, the resultant field interacts with the Earth’s field causing a torque on the satellite. These are what are known as Torque Rods.
Since the reaction wheels, gyros, and torque rods all work using electricity and the solar arrays provide that electricity, theoretically the life of the satellite is indefinite. Unfortunately, there are degradations of the thermal coatings, blankets, sensors, and failures of both the gyros and reaction wheels that ultimately limit the life of any satellite.
Over a period of time, these degrade to the point that the satellites can no longer function within design spec. At some point, you either have to replace the satellite, repair it, or say farewell.
For the Hubble, it was designed to be serviced by the space shuttle fleet. The tradeoff is when does the cost and risk of service out weigh the science benefit. This is what the scientific and engineering communities are wrestling with per the original post.
I sure wish they had boosted skylab. I too would like to see the Hubble maintained. I am just not sure of the cost benefit over other missions, telescopes, etc.
ROFL! You would not think so from my first post. OMG, I went back and re-read it this morning. Sigh! Horrible grammar and sentence structures.
Oh, our physicist priesthood...Hubble Huggers.
Actual observed positions did not quite match the predictions under classical Newtonian physics. Albert Einstein later solved this discrepancy with his “General Theory of Relativity”. In November of 1919, using a solar eclipse, experimental verification of his theory was performed by measuring the apparent change in a stars position due to the bending of the light buy the sun’s gravity.
It is a little appreciated fact that gravity bends light even under Newtonian gravity. The quantitative predictions of the two models, however, differ by a factor of two.
[Geek alert: Why does gravity bend light under Newton? According to Sir Isaac, the force of gravity on an object is proportional to the mass of the object. But for a given force, the acceleration of the object is inversely proportional to its mass, so the object's mass drops out of the equation. Acceleration is thus independent of the object's mass, for small masses--even for massless photons! The speed of light is large, but it's finite, so the bend angle is nonzero.]
5.56mm
When the space telescope is in orbit, where do these external forces come from? How much of a net torque is exerted by the solar wind? How long does it take for the reaction wheel angular momentum to build up to a level that is unacceptable, or is there some reason the wheels must be absolutely still? Also, once you start adding angular momentum to a wheel, isn't it hard to subtract exactly the same amount of momentum? (Hmmm... maybe that is why the wheels need to be normally still, so you can just apply a brake to stop a turn of the telescope).
A monopropellant is one that does not require an oxidizer to function. Usually monopropellants are composed of a liquid compound called Hydrazine (N2H4). When this liquid comes in contact with a platinum catalyst, it is decomposed into gaseous ammonia (Nh3), nitrogen and hydrogen. This gas is then ejected (fired thru a nozzle) out a jet to providing motion for the satellite. otherwise known as an "EXOTIC FUEL"
An ingenious method of unloading momentum without the use of fuel was devised using simple electromagnets. Remember the Earth is surrounded by a magnetic field (why your compass works). If you attach orthogonal electromagnets on your satellite and turn them on, the resultant field interacts with the Earth’s field causing a torque on the satellite. These are what are known as Torque Rods.
Since the reaction wheels, gyros, and torque rods all work using electricity and the solar arrays provide that electricity, theoretically the life of the satellite is indefinite. Unfortunately, there are degradations of the thermal coatings, blankets, sensors, and failures of both the gyros and reaction wheels that ultimately limit the life of any satellite.
Over a period of time, these degrade to the point that the satellites can no longer function within design spec. At some point, you either have to replace the satellite, repair it, or say farewell.
For the Hubble, it was designed to be serviced by the space shuttle fleet. The tradeoff is when does the cost and risk of service out weigh the science benefit. This is what the scientific and engineering communities are wrestling with per the original post.
Thanks...For the review in the Newtonian laws, will LaGrange Points help in this matter, or are they too far out?...wrong positions...they'er still on same elipic (sp?) plane...aren't they?... :|
But what I really want to know is...........where are the posts and updates, specifically, new PHOTOS of Mars (?).....this is such an incredible month for viewing this beauty, and of course, Indy has been muggy, overcast and humid beyond belief since Aug started. :^(
Now if you want to get really picky ...
The earth has a non-trivial moon, which makes it wobble slightly in its solar orbit. Thus, the real center of mass of the earth-moon system is a bit off from the geographical center of the earth. Since the satellite also feels the tug of the moon, it is basically orbiting the earth-moon center of gravity.
The difference is, What?...In the total Volume of the cosmos?...I know, a miss, is a miss...no flames, Please :/...as a skink, I do like it warm. :))
If the satellite has solar panels (arrays), they need to point towards the sun to provide power. Sensors need to point at their respective targets, such as a star sensor, sun sensor etc.
Couldn't these be made in spherical form, so that they'd always be "pointed" in the right direction? I guess a flat panel has more surface area exposed to the target than a small sphere, but a larger sphere should do just as well.
LOL, thanks for the additional info, RA.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.