Brightest beacons

Oct 02, 2013
Credit: NASA/ESA, Jeffrey Kenney (Yale University), Elizabeth Yale (Yale University)

Deep-space missions require precise navigation, in particular when approaching bodies such as Mars, Venus or a comet. How precise?

It's necessary to pinpoint a spacecraft 100 million kilometres from Earth to within 1 km. To achieve this level of accuracy, ESA experts use '' – the most luminous objects known in the Universe – as beacons in a technique known as Delta-Differential One-Way Ranging, or delta-DOR.

Quasars are fascinating objects that can emit 1000 times the energy of our entire Milky Way galaxy. This prodigious luminosity originates from a region only about the size of our Solar System. They are fuelled by – which might be billions of times as massive as our Sun – feeding on matter at the centre of their host galaxies.

The image shows one such quasar galaxy, NGC 4438, 50 million light-years from Earth.

Because quasars are extremely bright and distant, they can be used as reference points for spacecraft navigation.

In the delta-DOR technique, radio signals from a spacecraft are received by two separate , one, say, in New Norcia, Australia and one in Cebreros, Spain, and the difference in the times of arrival is precisely measured.

Next, errors due to the radio signals passing through Earth's atmosphere are corrected by simultaneously tracking a quasar – the coordinates of which are precisely known.

"For delta-DOR to work, the quasar and the spacecraft should be within 10º as seen from Earth," says Markus Landgraf, from ESA's Mission Analysis team.

"There are around 200 000 quasars known in the Sloan Digital Sky Survey, and almost any of them are potential candidates to be used in delta-DOR tracking."

Once the location of the spacecraft derived from the ground stations is compared to the known location of the quasar, engineers can apply corrections, delivering a significantly more accurate fix on its position.

"Quasar locations define a reference system. They enable engineers to improve the precision of the measurements taken by ground stations and improve the accuracy of the direction to the spacecraft to an order of a millionth of a degree," says Frank Budnik, a flight dynamics expert at ESA.

Using the results of the delta-DOR processing together with the range and Doppler measurements, which are also derived from the spacecraft signals received on ground, ESA can achieve an accuracy in location of just several hundred metres at a distance of 100 000 000 km.

Explore further: What does the next generation telescope need to detect life?

add to favorites email to friend print save as pdf

Related Stories

Rosetta correctly lined up for critical Mars swingby

Feb 15, 2007

ESA mission controllers have confirmed Rosetta is on track for a critical 250-km Mars swingby on 25 February. Engineers have started final preparations for the delicate operation, which includes an eclipse, ...

Dead stars could be the future of spacecraft navigation

Oct 09, 2012

Scientists at the National Physical Laboratory (NPL) and the University of Leicester have been commissioned by the European Space Agency (ESA) to investigate the feasibility of using dead stars to navigate ...

Goodbye Herschel: Closing views of an icon

Jul 02, 2013

(Phys.org) —This tiny dot against the streaking star field is one of the last views that ground-based observers will see of ESA's iconic Herschel space observatory.

Recommended for you

New window on the early Universe

23 hours ago

Scientists at the Universities of Bonn and Cardiff see good times approaching for astrophysicists after hatching a new observational strategy to distill detailed information from galaxies at the edge of ...

Chandra's archives come to life

Oct 22, 2014

Every year, NASA's Chandra X-ray Observatory looks at hundreds of objects throughout space to help expand our understanding of the Universe. Ultimately, these data are stored in the Chandra Data Archive, ...

New robotic telescope revolutionizes the study of stars

Oct 22, 2014

In the last 8 months a fully robotic telescope in Tenerife has been carrying out high-precision observations of the motion of stellar surfaces. The telescope is the first in the SONG telescope network and ...

User comments : 0