Autonomous Spacecraft Navigation With Pulsars
Werner Becker, Mike G. Bernhardt, Axel Jessner
An external reference system suitable for deep space navigation can be defined by fast spinning and strongly magnetized neutron stars, called pulsars.
Their beamed periodic signals have timing stabilities comparable to atomic clocks and provide characteristic temporal signatures that can be used as natural navigation beacons, quite similar to the use of GPS satellites for navigation on Earth.
By comparing pulse arrival times measured on-board a spacecraft with predicted pulse arrivals at a reference location, the spacecraft position can be determined autonomously and with high accuracy everywhere in the solar system and beyond.
The unique properties of pulsars make clear already today that such a navigation system will have its application in future astronautics.
In this paper we describe the basic principle of spacecraft navigation using pulsars and report on the current development status of this novel technology….
Read more http://arxiv.org/pdf/1305.4842v1.pdf
At the center of the Vela supernova remnant is a madly spinning neutron star. It spins *11 times per second*, which helps it whip up a magnetic field so fierce it can actually defy the gravity of the star, which is a billion times stronger than Earth’s!
All of this help it generate two beams of matter and energy that blast away from its poles. Recent observations of this beam show that it appears to be wobbly, making a corkscrew motion over a period of 120 days. This movie is made from 8 images of the pulsar and its weird beam taken by the Chandra X-ray Observatory.
Read more: http://www.slate.com/blogs/bad_astronomy.html
Pulsars, superdense neutron stars, are perhaps the most extraordinary physics laboratories in the Universe. Research on these extreme and exotic objects already has produced two Nobel Prizes. Pulsar researchers now are poised to learn otherwise-unavailable details of nuclear physics, to test General Relativity in conditions of extremely strong gravity, and to directly detect gravitational waves with a “telescope” nearly the size of our Galaxy.
Neutron stars are the remnants of massive stars that exploded as supernovae. They pack more than the mass of the Sun into a sphere no larger than a medium-sized city, making them the densest objects in the Universe, except for black holes, for which the concept of density is theoretically irrelevant. Pulsars are neutron stars that emit beams of radio waves outward from the poles of their magnetic fields. When their rotation spins a beam across the Earth, radio telescopes detect that as a “pulse” of radio waves.
By precisely measuring the timing of such pulses, astronomers can use pulsars for unique “experiments” at the frontiers of modern physics. Three scientists presented the results of such work, and the promise of future discoveries, at the American Association for the Advancement of Science meeting in Vancouver, British Columbia.
Pulsars are at the forefront of research on gravity. Albert Einstein published his theory of General Relativity in 1916, and his description of the nature of gravity has, so far, withstood numerous experimental tests. However, there are competing theories.
“Many of these alternate theories do just as good a job as General Relativity of predicting behavior within our Solar System. One area where they differ, though, is in the extremely dense environment of a neutron star,” said Ingrid Stairs, of the University of British Columbia.
In some of the alternate theories, gravity’s behavior should vary based on the internal structure of the neutron star.
“By carefully timing pulsar pulses, we can precisely measure the properties of the neutron stars. Several sets of observations have shown that pulsars’ motions are not dependent on their structure, so General Relativity is safe so far,” Stairs explained.
Recent research on pulsars in binary-star systems with other neutron stars, and, in one case, with another pulsar, offer the best tests yet of General Relativity in very strong gravity. The precision of such measurements is expected to get even better in the future, Stairs said.
Another prediction of General Relativity is that motions of masses in the Universe should cause disturbances of space-time in the form of gravitational waves. Such waves have yet to be directly detected, but study of pulsars in binary-star systems have given indirect evidence for their existence. That work won a Nobel Prize in 1993.
Now, astronomers are using pulsars throughout our Milky Way Galaxy as a giant scientific instrument to directly detect gravitational waves.
“Pulsars are such extremely precise timepieces that we can use them to detect gravitational waves in a frequency range to which no other experiment will be sensitive,” said Benjamin Stappers, of the University of Manchester in the UK.
By carefully timing the pulses from pulsars widely scattered within our Galaxy, the astronomers hope to measure slight variations caused by the passage of the gravitational waves. The scientists hope such Pulsar Timing Arrays can detect gravitational waves caused by the motions of supermassive pairs of black holes in the early Universe, cosmic strings, and possibly from other exotic events in the first few seconds after the Big Bang.
“At the moment, we can only place limits on the existence of the very low-frequency waves we’re seeking, but planned expansion and new telescopes will, we hope, result in a direct detection within the next decade,” Stappers said.
With densities as much as several times greater than that in atomic nuclei, pulsars are unique laboratories for nuclear physics. Details of the physics of such dense objects are unknown.
“By measuring the masses of neutron stars, we can put constraints on their internal physics,” said Scott Ransom of the National Radio Astronomy Observatory. “Just in the past three to four years, we’ve found several massive neutron stars that, because of their large masses, rule out some exotic proposals for what’s going on at the centers of neutron stars,” Ransom said.
The work is ongoing, and more measurements are needed. “Theorists are clever, so when we provide new data, they tweak their exotic models to fit what we’ve found,” Ransom said….
Read more: physorg.com
Pulsar are among the most exotic things in the Universe. These objects are rotating neutron stars emitting radiation from their magnetic poles. They appear to pulse because the magnetic axis is not aligned with the axis of rotation, so the pole comes in and out of view as the neutron star rotates.
But pulsars are also puzzles. The conventional view is that their magnetic field arises from the movement of charged particles as they rotate. These charged particles ought to behave like a superfluid and so should end up becoming aligned with the axis of rotation. That’s clearly not the case since.
What’s more, these kinds of superfluid currents are likely to be highly unstable, generating wobbles in the magnetic field. But pulsars are well known for being amazingly stable. How can this be?
Another problem is how pulsars end up with magnetic fields that are so strong. The conventional view is that the process of collapse during a supernova somehow concentrates the original star’s field. However, a star loses much of its material when it explodes as a supernova and this presumably carries away much of its magnetic field too. But some pulsars have fields as high as 10^12 Tesla, far more than can be explained by this process.
Today, Johan Hansson and Anna Ponga at Lulea University of Technology in Sweden suggest a clever way out of this conundrum. They point out that there is another way for magnetic fields to form, other than the movement of charged particles. This other process is by the alignment of the magnetic fields of the body’s components, which is how ferromagnets form.
Their suggestion is that when a neutron star forms, the neutron magnetic moments become aligned because this is the lowest energy configuration of the nuclear forces between them. When this alignment takes place, a powerful magnetic field effectively becomes frozen in place.
This makes neutron stars giant permanent magnets. Hansson and Ponga call them neutromagnets.
A neutromagnet would be hugely stable, just like a permanent ferromagnet. The field would be likely to align with the star’s original field, which although much weaker, acts as a seed when the field forms. Significantly, this needn’t be in the same direction as the axis of spin.
What’s more, since neutron stars all have about the same mass, Hansson and Ponga can calculate the maximum strength of the fields they ought to generate. This number turns out to be about 10^12 Tesla’s, exactly the value that’s observed in the highest strength fields around neutron stars.
That immediately solves several of the outstanding puzzles about pulsars in a remarkably simple way.
The theory is testable too. It predicts that neutron stars cannot have magnetic fields greater than 10^12 Tesla. So the discovery of a neutron star with a stronger field would immediately scupper it.
But the idea also raises some questions of its own. Not least of these is whether it is even possible for neutron magnetic moments to become aligned in the way Hansson and Ponga suggest. The Pauli exclusion principle would, at first sight, seem to exclude the possibility of neutrons being aligned in this way.
But Hansson and Ponga point to laboratory experiments which suggest that nuclear spins can become ordered, like ferromagnets. “One should remember that the nuclear physics at these extreme circumstances and densities is not known a priori, so several unexpected properties (such as “neutromagnetism”) might apply,” they say.
Hansson and Ponga are the first to say their idea is speculative. Be that as it may, it also has a certain elegance and explanatory power that makes it worth pursuing in significantly more detail.