Quasar to guide ESA's Mars landing

One of the brightest objects in the universe will deliver pinpoint accuracy to navigation over 150 million kilometres, scientists say. Bill Condie reports.

The ESA's deep space tracking station in Argentina with, inset, Quasar P1514-24 on the guidance map.
ESA/D. Pazos

Quasars, the brightest objects in the universe, will be used to pinpoint the positioning of the European Space Agency’s ExoMars Trace Gas Orbiter (TGO) as it is inserted in Mars orbit ahead of delivering its Schiaparelli landing module.

The light from quasars comes from the disc of gas and stars that surround supermassive black holes at the centre of galaxies. The black holes are believed to power the quasars.

The ESA will use them as “calibrators” in a technique known as Delta-Differential One-Way Ranging, or delta-DOR to determine the ExoMars spacecraft’s location to within just a few hundred metres at a distance of more than 150 million kilometres.

Quasars can emit 1,000 times the energy of the Milky Way galaxy but are only about the size of our solar system. Because they are so far from Earth, they appear as fixed points in the sky and their positions can be mapped with high precision.

This combined with their extreme brightness, make them excellent reference points for navigating in space.

The ESA explains the technique.

Quasar P1514-24 marked with two blue lines near the centre of this guidance map with the comparison star marked with a solid circle and the dashed circle showing the control star. The latter is used to check possible problems with the comparison star and the overall quality of the photometry.
Rami Rekola, Univerity of Turku, 2001

“In the delta-DOR technique, radio signals from ExoMars/TGO are being received by two widely separated deep-space ground stations, one, say, at New Norcia, Western Australia, and one at Cebreros, Spain, and the difference in the times of signal arrival is precisely measured.

“Next, errors due to current conditions in Earth’s atmosphere (which affect all radio signals passing through) are derived by simultaneously tracking radio signals from a quasar.

“Engineers can apply these as corrections to the signal received from ExoMars/TGO, delivering a significantly more accurate fix on its position.”

ESA ground stations have begun the first of many delta-DOR observations that will be used to precisely locate ExoMars/TGO, using quasar P1514-24.

There will be more frequent observations as the mission enters the crucial phases ahead of inserting the ExoMars/TGO craft in orbit around the Red Planet.

“In October, in the final critical week before Mars arrival, teams will be conducting two delta-DOR observations daily,” says Mattia Mercolino of the ESA’s operations centre in Darmstadt, Germany.

“It’s an excellent example of critical, real-time teamwork between the flight dynamics experts, the ground station operators, the ExoMars mission controllers and our delta-DOR team, and it would be much more difficult to get to Mars without this expertise.”

The technique is so precise, the current delta-DOR observations will enable location of the spacecraft to less than 1,000 metres when it’s near Mars, a distance of about 150 million kilometres from Earth.

“This is comparable to detecting from the location of an object in Singapore from Darmstadt, to about five centimetres precision,” says Mercolino.

With more observations, the location will become more accurate.

“We should be able to get the accuracy down to just 150 metres at 150 million kilometres.”

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