And will you then ask them to visit the site every week to remove the changes made by people who aren't experts in the field?
That's why I gave up on it -- it's like trying to build a sandcastle too close to the water's edge. I'd rather use my time to create something that won't be destroyed after a month or two.
> Can this be programmed into cheap telescopes for well known light sources?
No. The technology required to combine two light beams in a coherent way is wa-a-a-y more expensive than a "cheap" telescope. One must be able to control the length of the two paths of light to a small fraction of wavelength of the light. In the case of ordinary visible light, that means "a small fraction of about 500 nm". That's the hard part:-(
> Is this the answer to light pollution?
Again, no. If you can perform interferometry, you can in effect reduce the size of the field of view, if you wish, and therefore reduce the noise contributed by background light; but for most purposes, you still want to see more than just point sources, which means a reasonable field of view, which means that there is still plenty of noise from the background.
A body moving in a circle of radius R
at a uniform speed V experiences an
acceleration a = (V*V)/R towards the
center of the circle. In neither of the
cases you mention does any centripetal
acceleration come close to the local
gravitational acceleration at the surface of
the planet.
Case 1: The Earth: orbital speed V = 30 km/s,
and R = 150 million km, so (V*V)/R is of
order (10^8)/(10^11) m/s^2, or about 10^(-3)
m/s^2. The local gravitational acceleration
is about 10 m/s^2, of course.
If you speak of the Earth's rotational motion
at the equator, then very roughly V = 500 m/s
and R = 6,400,000 m, so (V*V)/R has magnitude
roughly (2.5 x 10^5) / 6.4 x 10^6 = 0.03 m/s^2;
again, much less than 10 m/s^2 due to the
gravitational pull of the Earth.
Case 2: The new planet. Its orbital radius
is about 2 billion meters, so the circumference
is about 7 billion meters; if it travels that
distance in a period of 2 days = 170,000 seconds,
then it speed is about V = 40,000 m/s.
The orbital centripetal acceleration is
therefore of order (16 x 10^8)/(2 x 10^9) =
0.8 m/s^2. That's much larger than the
Earth's orbital centripetal acceleration,
but still far less than the likely gravitational
acceleration at the surface (or cloudtops)
of this planet.
Gravitational lenses have been seen many times before, but never so complete...
Way back in 1989, radio astronomers found a
gravitational lens near the galaxy
MG1643+1346 which creates two images,
one of which is a nearly complete circular
ring. Take a look at this radio image
from Langston et al., AJ 97, 1283 (1989):
The key to this idea is that, given a particular set of initial conditions for the perturbations of density after the Big Bang, matter becomes concentrated in long, thin, filamentary structures. When those structures collapse under the influence of gravity, the result is group of galaxies -- in this, one big one and several small ones -- stretched out along the axis of the early filament(s). So, rather than being distributed all around the big galaxy in a spherical cloud, the little galaxies are arranged in a very loose flattened bunch.
Now, all this depends, of course, on the particular distribution of density perturbations in the early universe. All we astronomers can do is pick some particular model and follow it to see how it evolves. I'm not aware of good reasons for requiring any particular distribution from first principles; people just pick reasonable models that are somewhat easy to describe.
The issue is not resolution. The issue is sensitivity. In order to pick up the very, very faint signal from Voyager, you need a large collecting area. That means a big dish -- 70 meters or more in diameter. They are not cheap. It would take 100 7-meter dishes to equal the collecting area of a single 70-meter dish, and 100 7-meter antennae aren't cheap, either.
There are already relatively cheap launch vehicles for sub-orbital and orbital missions. Orbital Sciences Corporation offers several vehicles; the Pegasus places a payload in LEO for about $30 million. Eurockot (no, that's not a typo) uses Russian SS-19 ballistic missiles to send objects into LEO; the Canadian MOST satellite, for example. In the near future, the SpaceX Corp. will offer vehicles with launch costs between $6 and $20 million.
For sub-orbital flights, NASA (and others) offer sounding rockets for just a few hundred thousand dollars per flight.
University researchers already have a number of options. The astronomical research (in which I am involved) will certainly NOT be helped by adding a human to the payload, so this news story is irrelevant to us.
First, the direct detection of gravitational waves would confirm certain aspects of the theory of general relativity, as other posters have noted.
Second, gravitational wave detectors will provide us with a new window to the universe. Ordinary stars emit mostly visible light, so ordinary optical telescopes are well suited to their study. Cold clouds of interstellar gas emit mostly radio waves, so radio telescopes are the best choice to study them. We know of certain objects --- relatively uncommon ones -- which ought to produce a good deal of gravitational radiation: very massive objects moving very quickly, such as pairs of black holes or neutron stars orbiting around each other at small distances. Gravitational wave detectors will allow astronomers to study the properties of these objects more precisely than we can with ordinary telescopes (since they do not emit much electromagnetic radiation).
Finally, it is possible (though I suspect unlikely) that the universe may contain a whole class (or classes) of objects which are currently unknown to us, but which will appear as strong sources of gravitational radiation. Almost every time astronomers have added a new type of telescope to their toolkit, they have stumbled across previously unknown phenomena. The first gamma-ray telescopes, for example, revealed gamma-ray bursts, which were completely undetected (and unexpected) by other means in the late sixties and early seventies.
One last note: LIGO and other gravitational wave detectors provide very poor angular resolution, compared to ordinary optical telescopes. They will tell us something like "a source of gravitational waves is over there, about 10 degrees above the horizon at 5 degrees south of East." The "error circle" for a typical detection will be a few degrees in size. It may be quite a challenge for astronomers to identify the optical counterpart to a new source of gravitational waves, since there will usually be thousands to millions of optical sources within the error box of a gravitational wave detection.
Look at the entire series of HST images over time
on
A Star of Space and Film
·
· Score: 5, Informative
If you go to the HST web site, you can see an entire series of images of V838 Mon over the past three years.
Although the series _appears_ to show a shell of gas expanding outwards from the star, it does not. Instead, what we see is the expanding echo of light reflecting off gas and dust in the interstellar medium, between V838 Mon and the Earth. It might help to look at a nice diagram of the "light echo" effect provided by space.com:
The fact that no material is actually shooting outwards into space as fast as the pictures appear to indicate -- that we are simply seeing a reflection of light as it moves through the gas cloud, like the beam of a flashlight swept through the air in a dusty room -- explains how the shell can _appear_ to expand outwards faster than light.
There are plenty of events and areas of study which aren't directly experimentally verifiable but which are considered science. Like evolutionary biology and big bang cosmology.
Both of which contain some testable statements (e.g. in cosmology, inflation predicts certain properties in the microwave background on specific angular scales), and some untestable statements. Scientists (ought to) ignore the latter.
Science is not as easy to define as most people (including most/.ers) like to imagine. If it were, the philosophy of science wouldn't be a very interesting discipline.
I'm a practicing scientist. I don't find the philosophy of science intereresting at all; it annoys me. I wonder what fraction of practicing scientists do enjoy the philosophy of science...
You can read the entire paper in PDF or PS at astro-ph, a web site which collects preprints in the physical sciences. See
http://xxx.lanl.gov/abs/astro-ph/0501589
I read the paper quickly. The authors have to come up with a model which has virtually no observable consequences (otherwise, we would have seen this source of matter by now), but which can also be tested experimentally in the not-too-distant-future (or else it wouldn't be science). They predict that some of the cosmic-ray shower telescopes may be able to detect the little cloudlets of dark matter. We'll see.
Transits of Venus not only way to measure AU
on
Venus Transit Finished
·
· Score: 2, Interesting
Though it is certainly true that astronomers of the eighteenth and nineteenth centuries spent a great deal of time and energy travelling to the far corners of the Earth to observe transits of Venus, these rare events were NOT their only chances to measure the absolute size of the solar system. Simultaneous or near-simultaneous measurements of Mars or certain asteroids also allow one to derive absolute distances via parallax; although the targets are more distant than Venus, they provide significantly better observing conditions and references for astrometry. Cassini, for example, used measurements of Mars in 1672 to calculate the Astronomical Unit (the distance between Earth and Sun) to better than 10 percent.
Still, transits of Venus were certainly a major focus for the astronomical community. I wrote up material on the geometry and history of transits for a seminar:
read it for yourself .
There are links to other good sites at the end of my lecture.
Because it was originally written in Latin by Claudius in his epistles. It is an ancient truism, said well before there was a NASA, before there was an English language, well before the Earth was known to be round.
Greek astronomers had figured out that the Earth was round several centuries before Cladius. They drew on several lines of evidence: the shape of the Earth's shadow during lunar eclipses, the change in constellations visible at different latitudes, and the fact that the masts of ships sailing away from port disappear long after their hulls.
Any Roman who paid attention in school would have known that the Earth was round, too.
Transits of Venus -- in which the planet crosses the face of the Sun as seen from Earth -- are rare events. They occur in pairs, eight years apart, with gaps of roughly 120 years between pairs. The last pair was 1874 and 1882, so this movie shows the most recent transit.
However, the next transit is in just a few months, on June 8, 2004. It will be visible from Europe, but only the tail end can be seen from North America. If you miss this one, the next is in June of 2012.
Transits were very important to astronomers in the past because they offered an opportunity to measure the distance between the Earth and the Sun; that, in turn, yielded the distance between Earth and every planet in the solar system. I've written a document explaining how transits of Venus could be used to determine the size of the solar system. It includes a little history, too.
Look at
This is the third asteroid we've found which has an orbit tied loosely to that of the Earth. The others are 3753 Cruithne and 2002 AA29. You can see pictures and applets and read about these other bodies at Paul Wiegert's web site:
Lowell thought that very small deviations of the motion of Uranus from its calculated orbit indicated that there must be another planet ("Planet X") perturbing its motion. He estimated where it might be, started a big search for it, and then died.
Many years later, Tombaugh stumbled across Pluto while making a survey of the entire ecliptic.
Yes, the planet was very roughly in the region of the sky Lowell had predicted. But it was soon obvious that the mass of Pluto was way, way, way too small for it to be responsible for the residuals in the orbit of Uranus. It was simply coincidence that one object (Pluto) happened to be roughly in the same area that another (the hypothetical perturbing planet) was calculated to be.
An article by Standish in Astronomical Journal (1993) shows that the residuals Lowell was using were incorrectly computed, and that there is no evidence for a perturbing planet. Here's a section of the abstract:
It is shown that the alleged 'unexplained anomalies in the motion of Uranus' disappear when one properly accounts for the correct value of the mass of Neptune and properly adjusts the orbit of Uranus to the observational data.....
there remains no need to hypothesize the existence of a tenth planet in the solar system.
I'm a professor, and I've seen the same mix of praise, criticism, and just plain garbage in reviews of me published on one particular public web site. It's the same old story: any unmoderated site is soon overrun with trash.
The galling feature of all the "Rate-A-Professor" sites I've seen is the anonymity they provide. I wonder how many students would post messages like "You suck!" if they had to attach their names at the end? But they never do...
Let me put the shoe on the other foot -- suppose that someone started a "Rate-A-Student" web site, where
professors could post messages anonymously like "Mr. Smith came to class only four times all quarter, and snored his way through two of those. He showed no initiative, failed completely to understand the concept of square roots, has abysmal handwriting, and shows little sign of being able to communicate with his peers." The site could advertise to employers -- "Hey, want to check up on that guy who applied for your Network Administration opening? Check out comments made by those who worked with him for months!"
Great idea. Go 500 AU away from the Sun,
then take out your big telescope and ultra-sensitive
visible/IR detectors and point them back at...
the Sun. You'll see a blindingly bright object,
magnitude -13 or so. And your goal is to search
for planets around other stellar systems, which
might be, what, apparent magnitude 25 or so?
"But the gravitational lensing will amplify the
light from those faint little planets!" you
cry. Amplify by how much --- you need a factor
of over one trillion in order to bring these
planets up within one-millionth the apparent
brightness of the Sun. Oh, and by the way,
you'll be magnifying the STARS around which those
planets circle by this same amount, which won't
make the planets any easier to see.
Take a look at
one of my course WWW pages
describing the difficulties of direct detection
of planets to get some idea of the practical
difficulties. Using the Sun as a gravitational
lens won't help at all.
Not "science" -- "biology"
on
Who Owns Science?
·
· Score: 5, Insightful
Note that the PLoS plans to start with two journals which focus on biology and medicine. These are the fields where basic research can yield megabucks in the relatively short term. In my own field (astronomy), there's not a cent to be made by anyone; hence, I doubt we'll see a PLoS journal of astronomy or astrophysics anytime soon.
Note also that if researchers didn't care about getting money from industry, they wouldn't be chary of publishing their results for all to see. The real problems occur when scientists need big money to set up big labs employing many people to develop new medicines (or do research which has obvious applications to new medicines) which can treat "wealthy" diseases: diseases which affect many people in wealthy countries.
I don't see a way around this: investment by big pharmaceutical companies WILL speed the pace of such research (that's good), but will also lead to secrecy and higher drug prices for some time after the products first appear (that's bad).
Some problems are just plain complicated. This is one of them. I wish the PLoS the best of luck, but I don't give them much of a chance. As long as a few researchers are willing to work in secrecy, they can use the PLoS results plus their "secret" results and often beat the "public" researchers to the punch. It's not unlike the prisoner's dilemma.
As others have said already, the primary mirror is not of the right design to look back at the Earth and actually yield the
right kind of details. Hubble focuses to infinity and an earth-imaging satellite only has to focus to a distance of a few
hundred miles -- the exact altitude depends on the satellite's orbit.
HST's instruments include movable mirrors which
allow one to modify the focus. They could easily
focus on objects at the distance of the Earth's
surface. HST has taken
pictures of the Moon,
which is certainly not at infinity.
Furthermore, Hubble's optics are too sensitive to be pointed at the Earth or the Moon -- both are so bright that they'd
blow out the sensors.
Some of HST's instruments would saturate if
they took exposures of the Earth through wide
filters. Others would not. The HST calibration
team sometimes takes exposures of the Earth
or Moon to use as flatfields.
But, yes, as many have already pointed out,
HST can't take images resolving newspaper
headlines.
The parent asks about the portion of Cassini's
trajectory which passed very close to the Earth.
On August 18, 1999, the spacecraft swept past
the Earth at a minimum altitude of just over 700
miles. You can read about it here:
Why fly so close? The JPL team arranged it so that Cassini went past the "back" side of the Earth. The earth circles around the Sun at a pretty good clip (about 30 km/sec). Cassini came towards the Earth from behind in its orbit. The gravitational force of the Earth on the spacecraft pulled it forward, speeding it up as it went by. By the same token, the spacecraft slowed the Earth down a little bit, but by an insignificant amount.
This is one of the two sorts of "gravitational slingshot" manuevers the celestial mechanics can use to give spacecraft more speed without using lots of fuel.
Simple analogy: stand on a sidewalk as cars drive past at 30 mph. Just as one car is about to pass you, toss a tennis ball out in front of it. The collision will greatly increase the speed of the tennis ball in the direction of the car's motion
(and only very slightly decrease the speed of the car). We can't bounce spacecraft off the Earth in the same way:-), but we can use gravity to pull spacecraft forward in a much gentler manner.
For information on the risks associated with the flyby, please read
Slashdot Mirror
And will you then ask them to visit the site
every week to remove the changes made by
people who aren't experts in the field?
That's why I gave up on it -- it's like
trying to build a sandcastle too close to the
water's edge. I'd rather use my time to
create something that won't be destroyed
after a month or two.
> Can this be programmed into cheap telescopes for well known light sources?
:-(
No. The technology required to combine two light beams in
a coherent way is wa-a-a-y more expensive than a "cheap"
telescope. One must be able to control the length of the
two paths of light to a small fraction of wavelength of
the light. In the case of ordinary visible light, that
means "a small fraction of about 500 nm". That's the
hard part
> Is this the answer to light pollution?
Again, no. If you can perform interferometry, you
can in effect reduce the size of the field of view, if
you wish, and therefore reduce the noise contributed
by background light; but for most purposes, you
still want to see more than just point sources,
which means a reasonable field of view, which
means that there is still plenty of noise from the
background.
Alas.
No.
A body moving in a circle of radius R at a uniform speed V experiences an acceleration a = (V*V)/R towards the center of the circle. In neither of the cases you mention does any centripetal acceleration come close to the local gravitational acceleration at the surface of the planet.
Case 1: The Earth: orbital speed V = 30 km/s, and R = 150 million km, so (V*V)/R is of order (10^8)/(10^11) m/s^2, or about 10^(-3) m/s^2. The local gravitational acceleration is about 10 m/s^2, of course. If you speak of the Earth's rotational motion at the equator, then very roughly V = 500 m/s and R = 6,400,000 m, so (V*V)/R has magnitude roughly (2.5 x 10^5) / 6.4 x 10^6 = 0.03 m/s^2; again, much less than 10 m/s^2 due to the gravitational pull of the Earth.
Case 2: The new planet. Its orbital radius is about 2 billion meters, so the circumference is about 7 billion meters; if it travels that distance in a period of 2 days = 170,000 seconds, then it speed is about V = 40,000 m/s. The orbital centripetal acceleration is therefore of order (16 x 10^8)/(2 x 10^9) = 0.8 m/s^2. That's much larger than the Earth's orbital centripetal acceleration, but still far less than the likely gravitational acceleration at the surface (or cloudtops) of this planet.
The summary states incorrectly:
Way back in 1989, radio astronomers found a gravitational lens near the galaxy MG1643+1346 which creates two images, one of which is a nearly complete circular ring. Take a look at this radio image from Langston et al., AJ 97, 1283 (1989):
Click to see radio image of lensed quasar.
So, this newest system is a pretty good lens, but not the "most complete" one yet found.
By the way, if you want to understand how gravitational lensing works, you can read some lectures I wrote for an introductory astronomy class:
The key to this idea is that, given a particular set of initial conditions for the perturbations of density after the Big Bang, matter becomes concentrated in long, thin, filamentary structures. When those structures collapse under the influence of gravity, the result is group of galaxies -- in this, one big one and several small ones -- stretched out along the axis of the early filament(s). So, rather than being distributed all around the big galaxy in a spherical cloud, the little galaxies are arranged in a very loose flattened bunch.
Now, all this depends, of course, on the particular distribution of density perturbations in the early universe. All we astronomers can do is pick some particular model and follow it to see how it evolves. I'm not aware of good reasons for requiring any particular distribution from first principles; people just pick reasonable models that are somewhat easy to describe.
The issue is not resolution. The issue is sensitivity. In order to pick up the very, very faint signal from Voyager, you need a large collecting area. That means a big dish -- 70 meters or more in diameter. They are not cheap. It would take 100 7-meter dishes to equal the collecting area of a single 70-meter dish, and 100 7-meter antennae aren't cheap, either.
Nice idea. Won't work.
http://spacex.com. Sorry about that.
For sub-orbital flights, NASA (and others) offer sounding rockets for just a few hundred thousand dollars per flight.
University researchers already have a number of options. The astronomical research (in which I am involved) will certainly NOT be helped by adding a human to the payload, so this news story is irrelevant to us.
First, the direct detection of gravitational waves would confirm certain aspects of the theory of general relativity, as other posters have noted.
Second, gravitational wave detectors will provide us with a new window to the universe. Ordinary stars emit mostly visible light, so ordinary optical telescopes are well suited to their study. Cold clouds of interstellar gas emit mostly radio waves, so radio telescopes are the best choice to study them. We know of certain objects --- relatively uncommon ones -- which ought to produce a good deal of gravitational radiation: very massive objects moving very quickly, such as pairs of black holes or neutron stars orbiting around each other at small distances. Gravitational wave detectors will allow astronomers to study the properties of these objects more precisely than we can with ordinary telescopes (since they do not emit much electromagnetic radiation).
Finally, it is possible (though I suspect unlikely) that the universe may contain a whole class (or classes) of objects which are currently unknown to us, but which will appear as strong sources of gravitational radiation. Almost every time astronomers have added a new type of telescope to their toolkit, they have stumbled across previously unknown phenomena. The first gamma-ray telescopes, for example, revealed gamma-ray bursts, which were completely undetected (and unexpected) by other means in the late sixties and early seventies.
One last note: LIGO and other gravitational wave detectors provide very poor angular resolution, compared to ordinary optical telescopes. They will tell us something like "a source of gravitational waves is over there, about 10 degrees above the horizon at 5 degrees south of East." The "error circle" for a typical detection will be a few degrees in size. It may be quite a challenge for astronomers to identify the optical counterpart to a new source of gravitational waves, since there will usually be thousands to millions of optical sources within the error box of a gravitational wave detection.
If you go to the HST web site, you can see an entire series of images of V838 Mon over the past three years.
v e/ releases/2005/02/image/a
y /i mg_display.php?pic=light_echo_graphic_030326_02,0. jpg
o _0 30326.html
http://hubblesite.org/newscenter/newsdesk/archi
Although the series _appears_ to show a shell of gas expanding outwards from the star, it does not. Instead, what we see is the expanding echo of light reflecting off gas and dust in the interstellar medium, between V838 Mon and the Earth. It might help to look at a nice diagram of the "light echo" effect provided by space.com:
http://www.space.com/php/multimedia/imagedispla
The European Space Agency also has a good description of the event:
http://www.space.com/scienceastronomy/light_ech
The fact that no material is actually shooting outwards into space as fast as the pictures appear to indicate -- that we are simply seeing a reflection of light as it moves through the gas cloud, like the beam of a flashlight swept through the air in a dusty room -- explains how the shell can _appear_ to expand outwards faster than light.
Both of which contain some testable statements (e.g. in cosmology, inflation predicts certain properties in the microwave background on specific angular scales), and some untestable statements. Scientists (ought to) ignore the latter.
I'm a practicing scientist. I don't find the philosophy of science intereresting at all; it annoys me. I wonder what fraction of practicing scientists do enjoy the philosophy of science ...
You can read the entire paper in PDF or PS at astro-ph, a web site which collects preprints in the physical sciences. See
http://xxx.lanl.gov/abs/astro-ph/0501589
I read the paper quickly. The authors have to come up with a model which has virtually no observable consequences (otherwise, we would have seen this source of matter by now), but which can also be tested experimentally in the not-too-distant-future (or else it wouldn't be science). They predict that some of the cosmic-ray shower telescopes may be able to detect the little cloudlets of dark matter. We'll see.
Wrong mission. You are thinking of Stardust, which will return samples from a comet.
Genesis allowed solar wind particles to slam into polished slabs of metal; some of the particles stick and can be recovered afterwards.
I teach physics and astronomy courses at RIT. All my lecture notes are freely available to anyone. Look at
http://spiff.rit.edu/classes/
Enjoy.
Though it is certainly true that astronomers of the eighteenth and nineteenth centuries spent a great deal of time and energy travelling to the far corners of the Earth to observe transits of Venus, these rare events were NOT their only chances to measure the absolute size of the solar system. Simultaneous or near-simultaneous measurements of Mars or certain asteroids also allow one to derive absolute distances via parallax; although the targets are more distant than Venus, they provide significantly better observing conditions and references for astrometry. Cassini, for example, used measurements of Mars in 1672 to calculate the Astronomical Unit (the distance between Earth and Sun) to better than 10 percent.
Still, transits of Venus were certainly a major focus for the astronomical community. I wrote up material on the geometry and history of transits for a seminar: read it for yourself . There are links to other good sites at the end of my lecture.
Greek astronomers had figured out that the Earth was round several centuries before Cladius. They drew on several lines of evidence: the shape of the Earth's shadow during lunar eclipses, the change in constellations visible at different latitudes, and the fact that the masts of ships sailing away from port disappear long after their hulls.
Any Roman who paid attention in school would have known that the Earth was round, too.
Transits of Venus -- in which the planet crosses the face of the Sun as seen from Earth -- are rare events. They occur in pairs, eight years apart, with gaps of roughly 120 years between pairs. The last pair was 1874 and 1882, so this movie shows the most recent transit.
However, the next transit is in just a few months, on June 8, 2004. It will be visible from Europe, but only the tail end can be seen from North America. If you miss this one, the next is in June of 2012.
Transits were very important to astronomers in the past because they offered an opportunity to measure the distance between the Earth and the Sun; that, in turn, yielded the distance between Earth and every planet in the solar system. I've written a document explaining how transits of Venus could be used to determine the size of the solar system. It includes a little history, too. Look at
http://spiff.rit.edu/classes/phys235/venus_t/venus _t.html
We can test the current tectonic plate models. How? The plates move on timescales of tens of millions of years. This lake is 2500 years old.
We know how fast DNA diverges, No, we don't. Not over periods as short as 2500 years. And not very well at all over longer periods, either.
This isn't "Insightful". It's nonsense.
This is the third asteroid we've found which has an orbit tied loosely to that of the Earth. The others are 3753 Cruithne and 2002 AA29. You can see pictures and applets and read about these other bodies at Paul Wiegert's web site:
http://www.astro.uwo.ca/~wiegert/
Lowell thought that very small deviations of the motion of Uranus from its calculated orbit indicated that there must be another planet ("Planet X") perturbing its motion. He estimated where it might be, started a big search for it, and then died.
Many years later, Tombaugh stumbled across Pluto while making a survey of the entire ecliptic. Yes, the planet was very roughly in the region of the sky Lowell had predicted. But it was soon obvious that the mass of Pluto was way, way, way too small for it to be responsible for the residuals in the orbit of Uranus. It was simply coincidence that one object (Pluto) happened to be roughly in the same area that another (the hypothetical perturbing planet) was calculated to be.
An article by Standish in Astronomical Journal (1993) shows that the residuals Lowell was using were incorrectly computed, and that there is no evidence for a perturbing planet. Here's a section of the abstract:
And yes, I am an astronomer.
I'm a professor, and I've seen the same mix of praise, criticism, and just plain garbage in reviews of me published on one particular public web site. It's the same old story: any unmoderated site is soon overrun with trash.
The galling feature of all the "Rate-A-Professor" sites I've seen is the anonymity they provide. I wonder how many students would post messages like "You suck!" if they had to attach their names at the end? But they never do ...
Let me put the shoe on the other foot -- suppose that someone started a "Rate-A-Student" web site, where professors could post messages anonymously like "Mr. Smith came to class only four times all quarter, and snored his way through two of those. He showed no initiative, failed completely to understand the concept of square roots, has abysmal handwriting, and shows little sign of being able to communicate with his peers." The site could advertise to employers -- "Hey, want to check up on that guy who applied for your Network Administration opening? Check out comments made by those who worked with him for months!"
How long would a site like that last?
Great idea. Go 500 AU away from the Sun, then take out your big telescope and ultra-sensitive visible/IR detectors and point them back at ...
the Sun. You'll see a blindingly bright object,
magnitude -13 or so. And your goal is to search
for planets around other stellar systems, which
might be, what, apparent magnitude 25 or so?
"But the gravitational lensing will amplify the light from those faint little planets!" you cry. Amplify by how much --- you need a factor of over one trillion in order to bring these planets up within one-millionth the apparent brightness of the Sun. Oh, and by the way, you'll be magnifying the STARS around which those planets circle by this same amount, which won't make the planets any easier to see.
Take a look at one of my course WWW pages describing the difficulties of direct detection of planets to get some idea of the practical difficulties. Using the Sun as a gravitational lens won't help at all.
Note that the PLoS plans to start with two journals which focus on biology and medicine. These are the fields where basic research can yield megabucks in the relatively short term. In my own field (astronomy), there's not a cent to be made by anyone; hence, I doubt we'll see a PLoS journal of astronomy or astrophysics anytime soon.
Note also that if researchers didn't care about getting money from industry, they wouldn't be chary of publishing their results for all to see. The real problems occur when scientists need big money to set up big labs employing many people to develop new medicines (or do research which has obvious applications to new medicines) which can treat "wealthy" diseases: diseases which affect many people in wealthy countries. I don't see a way around this: investment by big pharmaceutical companies WILL speed the pace of such research (that's good), but will also lead to secrecy and higher drug prices for some time after the products first appear (that's bad).
Some problems are just plain complicated. This is one of them. I wish the PLoS the best of luck, but I don't give them much of a chance. As long as a few researchers are willing to work in secrecy, they can use the PLoS results plus their "secret" results and often beat the "public" researchers to the punch. It's not unlike the prisoner's dilemma.
The parent is wrong about several things:
HST's instruments include movable mirrors which allow one to modify the focus. They could easily focus on objects at the distance of the Earth's surface. HST has taken pictures of the Moon, which is certainly not at infinity.Some of HST's instruments would saturate if they took exposures of the Earth through wide filters. Others would not. The HST calibration team sometimes takes exposures of the Earth or Moon to use as flatfields.
But, yes, as many have already pointed out, HST can't take images resolving newspaper headlines.
The parent asks about the portion of Cassini's trajectory which passed very close to the Earth. On August 18, 1999, the spacecraft swept past the Earth at a minimum altitude of just over 700 miles. You can read about it here:
http://www.jpl.nasa.gov/releases/99/csearthflyby.h tml
Why fly so close? The JPL team arranged it so that Cassini went past the "back" side of the Earth. The earth circles around the Sun at a pretty good clip (about 30 km/sec). Cassini came towards the Earth from behind in its orbit. The gravitational force of the Earth on the spacecraft pulled it forward, speeding it up as it went by. By the same token, the spacecraft slowed the Earth down a little bit, but by an insignificant amount. This is one of the two sorts of "gravitational slingshot" manuevers the celestial mechanics can use to give spacecraft more speed without using lots of fuel.
Simple analogy: stand on a sidewalk as cars drive past at 30 mph. Just as one car is about to pass you, toss a tennis ball out in front of it. The collision will greatly increase the speed of the tennis ball in the direction of the car's motion (and only very slightly decrease the speed of the car). We can't bounce spacecraft off the Earth in the same way :-), but we can use gravity to pull spacecraft forward in a much gentler manner.
For information on the risks associated with the flyby, please read
http://a188-l009.rit.edu/richmond/answers/cassini. html