Navigating The Cosmos With Space GPS

Image via: DiscoveryMagazine.com

Spacecraft could one day navigate through the cosmos using a particular type of dead star as a kind of GPS.

German scientists are developing a technique that allows for very precise positioning anywhere in space by picking up X-ray signals from pulsars.

These dense, burnt-out stars rotate rapidly, sweeping their emission across the cosmos at rates that are so stable they rival atomic clock performance.

This timing property is perfect for interstellar navigation, says the team.

If a spacecraft carried the means to detect the pulses, it could compare their arrival times with those predicted at a reference location. This would enable the craft to determine its position to an accuracy of just five kilometres anywhere in the galaxy.

"The principle is so simple that it will definitely have applications," said Prof Werner Becker from the Max-Planck Institute for Extraterrestrial Physics in Garching.

"These pulsars are everywhere in the Universe and their flashing is so predictable that it makes such an approach really straightforward," he told BBC News.

The proposed technique is very similar to that employed in the popular Global Positioning System, which broadcasts timing signals to the user from a constellation of satellites in orbit.

Currently, mission controllers wanting to work out the position of their spacecraft deep in the Solar System will study the differences in time radio communications take to travel to and from the satellite. It is a complex process and requires several antennas dotted across the Earth.

It is also a technique that is far from precise, and the errors increase the further away the probe moves.

For the most distant spacecraft still in operation - Nasa's Voyager probes, which are now approaching the very edge of the Solar System, some 18 billion km away - the errors associated with their positions are on the order of several hundred km.

Even for a probe at the reasonably short separation of Mars, the positioning uncertainty can be about 10km.

"It becomes possible with the development of lightweight X-ray mirrors," said Prof Becker.

"These are on the way for the next generation of X-ray telescopes. Current mirrors have a 100 times more weight and would be completely unusable.

"In 15-20 years, the new mirrors will be standard and our device will be ready to be built."

The scientist believes his navigation solution will certainly find use on Solar System probes, providing autonomous navigation for interplanetary missions and perhaps for future manned ventures to Mars where high performance systems will be an absolute requirement for safety reasons.

Via: "The BBC"

Cosmic Journeys : The Largest Black Holes In The Universe

A supermassive black hole is the largest type of black hole in a galaxy, on the order of hundreds of thousands to billions of solar masses. Most--and possibly all--galaxies, including the Milky Wa y(see Sagittarius A), are believed to contain Supermassive black holes at their centers.

Supermassive black holes have properties which distinguish them from lower-mass classifications. First, the average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the density of water in the case of some supermassive black holes.

Donald Lynden-Bell and Martin Rees hypothetized in 1971 that the center of the Milky Way galaxy would contain a supermassive black hole. Sagittarius A was discovered and named on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory. They discovered a radio source that emits synchrotronic radiation, also it was found to be dense and immobile because of its gravitation. Therefore, the first discovered supermassive black hole existed in the center of the Milky Way.

Supermassive Black Holes Outside The Milky Way

It is now widely accepted that the center of nearly every galaxy contains a supermassive black hole.

It is believed that black holes and their host galaxies coevolved between 300-800 million years after the Big Bang, passing through a quasar phase.

The nearby Andromeda Galaxy, 2.5 million light-years away, contains a (1.1–2.3) × 108 (110-230 million) solar mass central black hole, significantly larger than the Milky Way's.

The largest supermassive black hole in the Milky Way's neighborhood appears to be that of M87, weighing in at (6.4 ± 0.5) × 109 (~6.4 billion) solar masses at a distance of 53.5 million light years.

On 5 December 2011 astronomers discovered the largest super massive black hole yet found to be that of NGC 4889, weighing in at 21 billion solar masses at a distance of 336 million light-years away in the Coma constellation.

Some galaxies, such as Galaxy 0402+379, appear to have two supermassive black holes at their centers, forming a binary system.

Binary supermassive black holes are believed to be a common consequence of galactic mergers. The binary pair in OJ 287, 3.5 billion light years away, contains the previous most massive black hole known (until the December 2011 discovery, with a mass estimated at 18 billion solar masses.

A supermassive black hole was recently discovered in the dwarf galaxy Henize 2-10, which has no bulge. The precise implications for this discovery on black hole formation are unknown, but may indicate that black holes formed before bulges.

On March 28, 2011, a supermassive black hole (SMBH) was for the first time seen tearing a mid-size star apart. That is, according to astronomers, the only likely explanation of the observations that day of sudden X-ray radiation and the follow-up broad-band observations.

The source was previously an inactive galactic nucleus, and from study of the outburst the galactic nucleus is estimated to be a SMBH with mass of the order of a million solar masses. This rare event is assumed to be a relativistic outflow (material being emitted in a jet at a significant fraction of the speed of light) from a star tidally disrupted by the SMBH. A significant fraction of a solar mass of material is expected to have accreted onto the SMBH. Subsequent long-term observation will allow this assumption to be confirmed if the emission from the jet decays at the expected rate for mass accretion onto a SMBH.

Info via: "Wikipedia"