PASADENA, Calif. -- NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has successfully deployed its lengthy mast, giving it the ability to see the highest energy X-rays in our universe. The mission is one step closer to beginning its hunt for black holes hiding in our Milky Way and other galaxies.
"It's a real pleasure to know that the mast, an accomplished feat of engineering, is now in its final position," said Yunjin Kim, the NuSTAR project manager at NASA's Jet Propulsion Laboratory, Pasadena, Calif. Kim was also the project manager for the Shuttle Radar Topography Mission, which flew a similar mast on the Space Shuttle Endeavor in 2000 and made topographic maps of Earth.
NuSTAR's mast is one of several innovations allowing the telescope to take crisp images of high-energy X-rays for the first time. It separates the telescope mirrors from the detectors, providing the distance needed to focus the X-rays. Built by ATK Aerospace Systems in Goleta, Calif., this is the first deployable mast ever used on a space telescope.
On June 21 at 10:43 a.m. PDT (1:43 p.m. EDT), nine days after launch, engineers at NuSTAR's mission control at UC Berkeley in California sent a signal to the spacecraft to start extending the 33-foot (10-meter) mast, a stable, rigid structure consisting of 56 cube-shaped units. Driven by a motor, the mast steadily inched out of a canister as each cube was assembled one by one. The process took about 26 minutes. Engineers and astronomers cheered seconds after they received word from the spacecraft that the mast was fully deployed and secure.
The NuSTAR team will now begin to verify the pointing and motion capabilities of the satellite, and fine-tune the alignment of the mast. In about five days, the team will instruct NuSTAR to take its "first light" pictures, which are used to calibrate the telescope.
Why did NuSTAR need such a long, arm-like structure? The answer has to do with the fact that X-rays behave differently than the visible light we see with our eyes. Sunlight easily reflects off surfaces, giving us the ability to see the world around us in color. X-rays, on the other hand, are not readily reflected: they either travel right through surfaces, as is the case with skin during medical X-rays, or they tend to be absorbed, by substances like your bone, for example. To focus X-rays onto the detectors at the back of a telescope, the light must hit mirrors at nearly parallel angles; if they were to hit head-on, they would be absorbed instead of reflected.
On NuSTAR, this is accomplished with two barrels of nested mirrors, each containing 133 shells, which reflect the X-rays to the back of the telescope. Because the reflecting angle is so shallow, the distance between the mirrors and the detectors is long. This is called the focal length, and it is maintained by NuSTAR's mast.
The fully extended mast is too large to launch in the lower-cost rockets required for relatively inexpensive Small Explorer class missions like NuSTAR. Instead NuSTAR launched on its Orbital Science Corporation's Pegasus rocket tucked inside a small canister. This rocket isn't as expensive as its bigger cousins because it launches from the air, with the help of a carrier plane, the L-1011 "Stargazer," also from Orbital.
NuSTAR is a Small Explorer mission led by the California Institute of Technology in Pasadena and managed by JPL for NASA's Science Mission Directorate in Washington. The spacecraft was built by Orbital Sciences Corporation, Dulles, Va. Its instrument was built by a consortium including Caltech; JPL; the University of California, Berkeley; Columbia University, New York; NASA's Goddard Space Flight Center, Greenbelt, Md.; the Danish Technical University in Denmark; Lawrence Livermore National Laboratory, Livermore, Calif.; and ATK Aerospace Systems, Goleta, Calif. NuSTAR will be operated by UC Berkeley, with the Italian Space Agency providing its equatorial ground station located at Malindi, Kenya. The mission's outreach program is based at Sonoma State University, Rohnert Park, Calif. NASA's Explorer Program is managed by Goddard. JPL is managed by Caltech for NASA.
Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.
A small X-ray telescope was boosted into orbit at noon EDT today by an air-launched Pegasus XL rocket Wednesday, the first step in an ambitious low-cost mission to study supermassive black holes believed to be lurking at the cores of galaxies like Earth's Milky Way and to probe the creation of heavy elements in the cataclysmic death throes of massive stars.
The innovative telescope, built around an extendable 33-foot-long Tinkertoy-like mast with nested X-ray mirrors on one end and sensitive detectors on the other, also will study the mechanisms responsible for stellar explosions and look for clues about what powers the energetic jets of particles blasted away from some black holes that apparently can disrupt star formation and even galactic evolution.
While X-ray telescopes sensitive to lower energies have been operated with great success, the $180 million Nuclear Spectroscopic Telescope Array, or NuStar, is the first space telescope designed to focus higher-energy X-rays like those used for medical imaging and dental X-rays.
NuSTAR images are expected to be "10 times crisper and a hundred times more sensitive than any we've had of the cosmos to date," said Fiona Harrison, the principal investigator at the California Institute of Technology. "This will enable Nu-STAR to study some of the hottest, densest and most energetic phenomenon in the universe."
The mission got underway with a dramatic pre-dawn launch from an L-1011 jet at an altitude of about 40,000 feet above the Pacific Ocean some 120 miles south of the Kwajalein Atoll in the Marshall Islands. Tucked into the nose cone of a three-stage solid-fuel Pegasus XL rocket, the NuSTAR spacecraft was dropped like a bomb at 12 p.m EDT (GMT-4; 4 a.m. Thursday local time). After a five-second fall, the first stage of the winged Pegasus booster ignited with a rush of flame to begin the steep climb to orbit.
Orbital Sciences Corp. of Dulles, Va., provided the carrier aircraft, the Pegasus XL booster and the NuSTAR satellite to NASA and the mission operations team at the California Institute of Technology under a "Small Explorer" program contract valued at nearly $180 million. The Pacific Ocean launch zone was selected to enable the spacecraft to reach a scientifically favorable orbit tilted just six degrees to the equator.
All three stages of the Pegasus booster operated normally, falling away as planned as their propellants were exhausted. Thirteen minutes after launch, NuSTAR was released into its operational 375-mile-high orbit. A few minutes after that, the telescope's transmitter was activated and telemetry confirmed the successful deployment of its five-segment solar array.
This was the 41st launch of a Pegasus rocket and the 31st using the more powerful XL version. Overall, Pegasus rockets have launched more than 70 satellites since 1990, with 27 successful missions in a row over the past 15 years.
"Today was a great day for NuSTAR, a great day for Pegasus, a great day for the entire launch team," said Tim Dunn, NASA's assistant launch director. "We thank Orbital Sciences for the ride, and we're ready to get into the science operation of the NuSTAR mission."
Over the next six days, engineers plan to activate and check out NuSTAR's attitude control system, star trackers, solar array drive electronics and instruments. Then, seven days after launch, commands will be sent to deploy the critical open-framework mast that provides the required separation between the telescope's X-ray mirrors and its detectors.
Unlike optical telescopes that use mirrors to bounce starlight to detectors, penetrating X-rays are focused by cylindrical, nested mirrors that cause a very slight deviation in the trajectory of the incoming radiation. A relatively long focal length is required to achieve the sensitivity astronomers need.
NuSTAR's ability to detect high-energy X-rays is the result of improved mirror and detector technology. But its ability to be launched by a small, relatively low-cost rocket is the result of an innovative design incorporating an extendable mast, built by ATK Aerospace Systems, that was originally developed for a shuttle radar mapping mission.
Earlier X-ray telescopes, sensitive to lower energies, were built around fixed structures and required large launch vehicles. NASA's Chandra X-ray Observatory, for example, weighed more than six tons and was launched by the shuttle Columbia. NuSTAR weighs just 770 pounds. The mast providing the required separation between mirror and detectors was designed to fit inside a 3.3-foot-tall canister at launch.
Assuming the 25-minute mast deploy sequence works properly next week, engineers will carefully align the optics and detectors. Science operations are expected to begin in about 30 days.
"One of NuSTAR's primary science goals is to study black holes (and) the extreme physics, the fascinating physics that occurs very close to the black hole where spacetime is severely distorted and particles are accelerated close to the speed of light," Harrison said. "And also to understand how black holes are distributed throughout the universe."
While galactic blacks holes initially were thought to be rare, "in the last 15 years, we understand that there is a very massive black hole at the center of every galaxy like our Milky Way," Harrison said. "And not only that, these black holes influence the way these galaxies grow and form."
While black holes are, by definition, invisible due to gravity so intense not even light can escape, they can be detected by looking for the radiation that is generated as gas and dust are sucked in and heated to extreme temperatures by friction with other in-falling material.
"What you're actually seeing is the dust and gas, the material in the galaxy, that's attracted by the black hole's gravity," Harrison said. "Close to the black hole, it organizes itself into a disk and friction turns this gravitational energy into heat. The material heats up such that when you're closest to the black hole, just a few times further away than the event horizon itself, the material is radiating high energy X-rays."
Combining NuSTAR's images with those captured by other, lower-energy X-ray telescopes, scientists will be able to "study the entire X-ray spectrum," Harrison said, "we can watch atoms circulate in the closest orbits near the black hole, we can observe how spacetime distorts our view of these objects and tell things like how fast the black hole is spinning."
NuSTAR will also focus on a variety of other energetic phenomenon, including the mechanisms responsible for the creation of heavy elements in supernova explosions and those powering the unimaginable jets of particles boosted to near light speed in the vicinity of black holes and other collapsed bodies like neutron stars and spinning pulsars, the left-over cores of stars destroyed in supernova blasts.
NuSTAR's state-of-the-art mirrors and detectors will help astronomers bring their views of the high-energy universe into much sharper focus.
"It's like you're trying to read a book without your glasses," Harrison said. "You know there's text there, but you can't make out the letters. Currently, we can make out about 2 percent of this cosmic text. But with NuSTAR, we'll be able to make out the majority of the story, we'll be able to image the sky, read the story and understand things like how galaxies form, how black holes grow and the history of the high energy universe."
The mission is expected to last at least two years.
› Full image and caption
|Most Distant Object Titleholders|
|UDFj-39546284||Galaxy||2011 —||z=~10.3||Announced January 26, 2011 also based on studies of images captured earlier in the Hubble Ultra Deep Field survey. (Not spectroscopically confirmed)|
|Progenitor of GRB 090429B||Gamma-ray Burst||2009-2011 —||z=~9.4||Announced for the first time at the American and Astronomical
Society meeting in January 2010. Discovered by Cucchiara et al. via
photometric redshift analysis of a J-band drop-out .
Data include ground based facilities like the Gemini telescopes and the Hubble Space Telescope. Not spectroscopically confirmed, but photometric redshift measure exclude at high confidence a z < 7.7 presence of a dusty galaxy which would mimic the observation.
|UDFy-38135539||Galaxy||2010 − 2011||z=8.55||Announced October 20, 2010 based on studies of images captured earlier in the Hubble Ultra Deep Field survey.|
|Progenitor of GRB 090423 / Remnant of GRB 090423||Gamma-ray burst progenitor / Gamma-ray burst remnant||2009 − 2010||z=8.2|||
|IOK-1||Galaxy||2006 − 2009||z=6.96|||
|SDF J132522.3+273520||Galaxy||2005 − 2006||z=6.597|||
|SDF J132418.3+271455||Galaxy||2003 − 2005||z=6.578|||
|HCM-6A||Galaxy||2002 − 2003||z=6.56||The galaxy is lensed by galaxy cluster Abell 370. This was the first non-quasar galaxy found to exceed redshift 6. It exceeded the redshift of quasar SDSSp J103027.10+052455.0 of z=6.28|
|Quasar||2001 − 2002||z=6.28|||
|Quasar||2000 − 2001||z=5.82|||
|SSA22-HCM1||Galaxy||1999 − 2000||z=5.74|||
|HDF 4-473.0||Galaxy||1998 − 1999||z=5.60|||
|RD1 (0140+326 RD1)||Galaxy||1998||z=5.34|||
|CL 1358+62 G1 & CL 1358+62 G2||Galaxies||1997 − 1998||z=4.92||These were the remotest objects known at the time of discovery. The pair of galaxies were found lensed by galaxy cluster CL1358+62 (z=0.33). This was the first time since 1964 that something other than a quasar held the record for being the most distant object in the universe.|
|PC 1247-3406||Quasar||1991 − 1997||z=4.897|||
|PC 1158+4635||Quasar||1989 − 1991||z=4.73|||
|Q0051-279||Quasar||1987 − 1989||z=4.43|||
|Quasar||1987||z=4.04||This was the second quasar discovered with a redshift over 4.|
|Quasar||1986 − 1987||z=3.80||This is a gravitationally-lensed double-image quasar, and at the time of discovery to 1991, had the least angular separation between images, 0.45 ″.|
(QSO J2003-3251 , Q2000-330)
|Quasar||1982 − 1986||z=3.78|||
|Quasar||1974 − 1982||z=3.53|||
|Quasar||1973 − 1974||z=3.408||Nickname was "the blaze marking the edge of the universe".|
|4C 05.34||Quasar||1970 − 1973||z=2.877||Its redshift was so much greater than the previous record that it was believed to be erroneous, or spurious.|
|Quasar||1968 − 1970||z=2.399|||
|Quasar||1967 − 1968||z=2.225|||
(Q1116+12 , PKS 1116+12)
|Quasar||1966 − 1967||z=2.1291|||
(Q0106+01 , PKS 0106+1)
|Quasar||1965 − 1966||z=2.0990|||
|3C 147||Quasar||1964 − 1965||z=0.545|||
|3C 295||Radio galaxy||1960 − 1964||z=0.461|||
|LEDA 25177 (MCG+01-23-008)||Brightest cluster galaxy||1951 − 1960||z=0.2
|This galaxy lies in the Hydra Supercluster. It is located at B1950.0 08h 55m 4s +03° 21′ and is the BCG of the fainter Hydra Cluster Cl 0855+0321 (ACO 732).|
|LEDA 51975 (MCG+05-34-069)||Brightest cluster galaxy||1936 -||z=0.13
|The brightest cluster galaxy of the Bootes cluster (ACO 1930), an elliptical galaxy at B1950.0 14h 30m 6s +31° 46′ apparent magnitude 17.8, was found by Milton L. Humason in 1936 to have a 40,000 km/s recessional redshift velocity.|
|LEDA 20221 (MCG+06-16-021)||Brightest cluster galaxy||1932 -||z=0.075
|This is the BCG of the Gemini Cluster (ACO 568) and was located at B1950.0 07h 05m 0s +35° 04′|
|BCG of WMH Christie's Leo Cluster||Brightest cluster galaxy||1931 − 1932||z=
|BCG of Baede's Ursa Major Cluster||Brightest cluster galaxy||1930 − 1931||z=
|NGC 4860||Galaxy||1929 − 1930||z=0.026
|Using redshift measurements, NGC 7619 was the highest at the time of measurement. At the time of announcement, it was not yet accepted as a general guide to distance, however, later in the year, Edwin Hubble described redshift in relation to distance, leading to a seachange, and having this being accepted as an inferred distance.|
(Dreyer nebula 584)
|Galaxy||1921 − 1929||z=0.006
|At the time, nebula had yet to be accepted as independent galaxies. However, in 1923, galaxies were generally recognized as external to the Milky Way.|
|M104 (NGC 4594)||Galaxy||1913 − 1921||z=0.004
|This was the second galaxy whose redshift was determined; the first being Andromeda - which is approaching us and thus cannot have its redshift used to infer distance. Both were measured by Vesto Melvin Slipher. At this time, nebula had yet to be accepted as independent galaxies. NGC 4594 was originally measured as 1000 km/s, then refined to 1100, and then to 1180 in 1916.|
|Star||1891 − 1910||160 ly
(this is very inaccurate)
|This figure is wrong, originally announced in 1891, the figure was corrected in 1910 to 40 ly (60 mas). From 1891 to 1910, it had been thought this was the star with the smallest known parallax, hence the most distant star whose distance was known.|
(Alpha Ursae Minoris)
|Star||1847 -||50 ly
(this is very inaccurate)
|Star (part of a double star pair)||1839 - 1847||7.77 pc
|61 Cygni||Binary star||1838 − 1839||3.48 pc
|This was the first star other than the Sun to have its distance measured.|
|Uranus||Planet of the Solar System||1781 − 1838||18 AU||This was the last planet discovered before the first successful measurement of stellar parallax. It had been determined that the stars were much farther away than the planets.|
|Saturn||Planet of the Solar System||1619 − 1781||10 AU||From Kepler's Third Law, it was finally determined that Saturn is indeed the outermost of the classical planets, and its distance derived. It had only previously been conjectured to be the outermost, due to it having the longest orbital period, and slowest orbital motion. It had been determined that the stars were much farther away than the planets.|
|Mars||Planet of the Solar System||1609 − 1619||2.6 AU when Mars is diametrically opposed to Earth||Kepler correctly characterized Mars and Earth's orbits in Astronomia nova. It had been conjectured that the fixed stars were much farther away than the planets.|
|Sun||Star||3rd century BCE — 1609||20x Earth-Moon distance (this is very inaccurate)||Aristarchus of Samos made a measurement of the distance of the Sun from the Earth in relation to the distance of the Moon from the Earth. The distance to the Moon was described in Earth radii (20, also inaccurate). The diameter of the Earth had previously been calculated. At the time, it was assumed that some of the planets were further away, but their distances could not be measured. The order of the planets was conjecture until Kepler determined the distances of the four true planets from the Sun that were not Earth. It had been conjectured that the fixed stars were much farther away than the planets.|
This list contains a list of most distant objects by year of discovery of the object, not the determination of its distance. Objects may have been discovered without distance determination, and were subsequently found to be the most distant known at that time.
|Year of record||Distance (Mly)||Object||Type||Detected using||First record by (1)|
|964||2.5 ||Andromeda Galaxy||Spiral galaxy||Naked eye||Abd al-Rahman al-Sufi|
|1654||3||Triangulum Galaxy||Spiral galaxy||Refracting telescope||Giovanni Battista Hodierna|
|1779||68||Messier 58||Barred spiral galaxy||refracting telescope||Charles Messier|
|1880s||206 ± 29||NGC 1||Spiral galaxy||Dreyer, Herschel|
|1959||2,400 ||3C 273||Quasar||Parkes Radio Telescope||Maarten Schmidt, Bev Oke|
|1960||5,000 ||3C 295||Radio galaxy||Palomar Observatory||Rudolph Minkowski|
|2009||13,000 ||GRB 090423||Gamma-ray burst progenitor||Swift Gamma-Ray Burst Mission||Krimm, H. et al.|
- (1): Object must have been named or described. Objects like OJ 287 are ignored, because though they were detected as early as 1891 using photographic plates, they were ignored until the advent of radiotelescopes.
by Jason Major on May 19, 2011 from www.UniverseToday.com
Three Views of Saturn's Northern Storm.
ESO/University of Oxford/L. N. Fletcher/T. Barry
First seen by amateur astronomers back in December, the powerful seasonal storm that has since bloomed into a planet-wrapping swath of churning clouds has gotten some scrutiny by Cassini and the European Southern Observatory’s Very Large Telescope array situated high in the Chilean desert.
The image above shows three views of Saturn acquired on January 19: one by amateur astronomer Trevor Barry taken in visible light and the next two by the VLT’s infrared VISIR instrument – one taken in wavelengths sensitive to lower atmospheric structures one sensitive to higher-altitude features.
While the storm band can be clearly distinguished in the visible-light image, it’s the infrared images that really intrigue scientists. Bright areas can be seen along the path of the storm, especially in the higher-altitude image, marking large areas of upwelling warmer air that have risen from deep within Saturn’s atmosphere.
Normally relatively stable, Saturn’s atmosphere exhibits powerful storms like this only when moving into its warmer summer season about every 29 years. This is only the sixth such storm documented since 1876, and the first to be studied both in thermal infrared and by orbiting spacecraft.
The initial vortex of the storm was about 5,000 km (3,000 miles) wide and took researchers and astronomers by surprise with its strength, size and scale.
“This disturbance in the northern hemisphere of Saturn has created a gigantic, violent and complex eruption of bright cloud material, which has spread to encircle the entire planet… nothing on Earth comes close to this powerful storm.”
– Leigh Fletcher, lead author and Cassini team scientist at the University of Oxford in the United Kingdom.
The origins of Saturn’s storm may be similar to those of a thunderstorm here on Earth; warm, moist air rises into the cooler atmosphere as a convective plume, generating thick clouds and turbulent winds. On Saturn this mass of warmer air punched through the stratosphere, interacting with the circulating winds and creating temperature variations that further affect atmospheric movement.
The temperature variations show up in the infrared images as bright “stratospheric beacons”. Such features have never been seen before, so researchers are not yet sure if they are commonly found in these kinds of seasonal storms.
“We were lucky to have an observing run scheduled for early in 2011, which ESO allowed us to bring forward so that we could observe the storm as soon as possible. It was another stroke of luck that Cassini’s CIRS instrument could also observe the storm at the same time, so we had imaging from VLT and spectroscopy of Cassini to compare. We are continuing to observe this once-in-a-generation event.”
– Leigh Fletcher
A separate analysis using Cassini’s visual and infrared mapping spectrometer confirmed the storm is very violent, dredging up larger atmospheric particles and churning up ammonia from deep in the atmosphere. Other Cassini scientists are studying the evolving storm and a more extensive picture will emerge soon.
Updrafts of Large Ammonia Crystals in Saturn Storm
May 19, 2011
This false-color infrared image, obtained by NASA's Cassini spacecraft, shows clouds of large ammonia ice particles dredged up by a powerful storm in Saturn's northern hemisphere. Large updrafts dragged ammonia gas upward more than 30 miles (50 kilometers) from below. The ammonia then condensed into large crystals in the frigid upper atmosphere. This storm is the most violent ever observed at Saturn by an orbiting spacecraft.
Cassini's visual and infrared mapping spectrometer obtained these images on Feb. 24, 2011. Scientists colorized the image by assigning red to brightness detected from the 4.08-micron wavelength, green to brightness from the 0.90-micron wavelength, and blue to brightness from the 2.73-micron wavelength. Large particles (red) reflect sunlight well at 4.08 microns. Particles at high altitude (green) reflect sunlight well at 0.9 microns. Particles comprised of ammonia -- especially large ones -- do not reflect 2.73-micron sunlight well, but instead absorb light at this wavelength.
The storm here shows up as yellow, demonstrating that it has a large signal in both red and green colors. This indicates the cloud has large particles and extends upward to relatively high altitude. In addition, the lack of blue in the feature indicates that the storm cloud has a substantial component of ammonia crystals. The head of the storm is particularly rich in such particles, as created by powerful updrafts of ammonia gas from depth in the throes of Saturn’s thunderstorm.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency (ASI). NASA’s Jet Propulsion Laboratory in Pasadena, Calif., manages the mission for NASA's Science Mission Directorate at the agency's headquarters in Washington. The Cassini orbiter was designed, developed and assembled at JPL. The visual and infrared mapping spectrometer was built by JPL, with a major contribution by ASI. The visual and infrared mapping spectrometer science team is based at the University of Arizona, Tucson. JPL is a division of the California Institute of Technology in Pasadena.
Credit: NASA/JPL/Univ. of Arizona
The leading edge of Saturn's storm in visible RGB color from Cassini raw image data taken on February 25, 2011. (The scale size of Earth is at upper left.) NASA / JPL / Space Science Institute. Edited by J. Major.
by Steve Nerlich on May 14, 2011 from www.UniverseToday.com
Could assessing the orbital motions of distant red dwarf binaries offer support for a branch of fringe science? Well, probably not... Credit: NASA.
The Sloan Low-mass Wide Pairs of Kinematically Equivalent Stars (SLoWPoKES) catalog was recently announced, containing 1,342 common proper motion pairs (i.e. binaries) – which are all low mass stars in the mid-K and mid-M stellar classes – in other words, orange and red dwarves.
These low mass pairs are all at least 500 astronomical units distance from each other – at which point the mutual gravitation between the two objects gets pretty tenuous – or so Newton would have it. Such a context provides a test-bed for something that lies in the realms of ‘fringe science’ – that is, Modified Newtonian Dynamics, or MoND.
The origin of MoND theory is generally attributed to a paper by Milgrom in 1981, which proposed MoND as an alternative way to account for the dynamics of disk galaxies and galactic clusters. Such structures can’t obviously hold together, with the rotational velocities they possess, without the addition of ‘invisible mass’ – or what these days we call dark matter.
MoND seeks to challenge a fundamental assumption built into both Newton’s and Einstein’s theories of gravity – where the gravitational force (or the space-time curvature) exerted by a massive object recedes by the inverse square of the distance from it. Both theories assume this relationship is universal – it doesn’t matter what the mass is or what the distance is, this relationship should always hold.
In a roundabout way, MoND proposes a modification to Newton’s Second Law of Motion – where Force equals mass times acceleration (F=ma) – although in this context, a is actually representing gravitational force (which is expressed as an acceleration).
If a expresses gravitational force, then F expresses the principle of weight. So for example, you can easily exert a sufficient force to lift a brick off the surface of the Earth, but it’s unlikely that you will be able to lift a brick, with the same mass, off the surface of a neutron star.
Anyhow, the idea of MoND is that by allowing F=ma to have a non-linear relationship at low values of a, a very tenuous gravitational force acting across a great distance might still be able to hold something in a loose orbit around a galaxy, despite the principle of a linear F=ma relationship predicting that this shouldn’t happen.
Left image: The unusual flat curve (B) of velocities of objects in disk galaxies versus what would be expected by a naive application of Kepler's Third Law (A). Right image: A scatter plot of selected binaries from the SLoWPoKE catalogue (blue) plotted against the trend expected by Kepler's Third Law (red). Credit: Hernandez et al. (Author's note - Kepler's Third Law of Planetary Motion fits the context of the solar system where 99% of the mass is contained in the Sun. Its applicability to the motion of stars in a galactic disk, with a much more even mass distribution, is uncertain)
MoND is fringe science, an extraordinary claim requiring extraordinary evidence, since if Newton’s or Einstein’s theories of gravity cannot be assumed to universal, a whole bunch of other physical, astrophysical and cosmological principles start to unravel.
Also, MoND doesn’t really account for other observational evidence of dark matter – notably the gravitational lensing seen in different galaxies and galactic clusters – a degree of lensing that exceeds what is expected from the amount of visible mass that they contain.
In any case, Hernandez et al have presented a data analysis drawn from the SLoWPoKES database of widely spread low-mass binaries, suggestive that MoND might actually work at scales of around 7000 astronomical units. Now, since this hasn’t yet been picked up by Nature, Sci. Am. or anyone else of note – and since some hack writer at Universe Today is just giving it a ‘balanced’ review here, it may be premature to consider that a major paradigm of physics has been overturned.
Nonetheless, the concept of ‘missing mass’ and dark matter has been kicked around for close on 90 years now – with no-one seemingly any closer to determining what the heck this stuff is. On this basis, it is reasonable to at least entertain some alternate views.
Dhital et al Sloan Low-mass Wide Pairs of Kinematically Equivalent Stars (SLoWPoKES): A Catalog of Very Wide, Low-mass Pairs (note that this paper makes no reference to the issue of MoND).
Hernandez et al The Breakdown of Classical Gravity?
Tagged as: Modified Newtonian dynamics