ESA Galileo satellite set for new orbit

Galileo satellites are placed in medium orbits, at 23 222 km altitude along three orbital planes so that a minimum of four satellites will be visible to user receivers at any point on Earth once the constellation is complete. 

The fifth and sixth Galileo satellites, launched together on 22 August 2014, ended up in an elongated orbit travelling out to 25 900 km above Earth and back down to 13 713 km. 

In addition, the orbits are angled relative to the equator less than originally planned. 

Credit: ESA-P. Carril

ESA's fifth Galileo navigation satellite, one of two left in the wrong orbit this summer, will make a series of manoeuvres this month as a prelude to its health being confirmed.

The aim is to raise the lowest point of its orbit, its perigee, to reduce the radiation exposure from the Van Allen radiation belts surrounding Earth, as well as to put it into a more useful orbit for navigation purposes.

Should the two-week operation prove successful then the sixth Galileo satellite will follow the same route.

The Galileo pair, launched together on a Soyuz rocket on 22 August, ended up in an elongated orbit travelling out to 25 900 km above Earth and back down to 13 713 km.

The target orbit was a purely circular one at an altitude of 23 222 km. In addition, the orbits are angled relative to the equator less than originally planned.

The two satellites have only enough fuel to lift their altitude by about 4000 km, insufficient to correct their orbits entirely.

But the move will take the fifth satellite into a more circular orbit than before, with a higher perigee of 17 339 km.

"The new orbit will fly over the same location every 20 days," explains Daniel Navarro-Reyes, ESA Galileo mission analyst.

"The standard Galileo repeat pattern is every 10 days, so achieving this will synchronise the ground track with the rest of the Galileo satellites.

Orbits of the fifth and sixth Galileo satellites launched together by Soyuz on 22 August 2014, in red, compared to their intended position, in dashed green, and the position of the four satellites launched in 2011 and 2012 in solid green. 

This view looks down over Earth’s South Pole, helping to illustrate how the two satellites’ orbital inclination relative to the equator is less than was intended. 

In addition, the satellites are in an elliptical rather than circular orbit, with a maximum altitude of about 25 900 km and a minimum altitude of about 13 700 km. 

This compares to a planned circular orbit of 23 222 km. 

The satellites are in a safe state, correctly pointing towards the Sun, properly powered and fully under control. 

Credit: ESA

"In addition, from a user receiver point of view, the revised orbit will reduce the variation in signal levels, reduce the Doppler shift of the signal, and increase the satellite's visibility.

"For the satellite, reducing its radiation exposure in the Van Allen radiation belts will protect it from further exposure to charged particles.

"The orbit will also allow Galileo's Earth Sensor to hold a stable direction for the satellite's main antenna to point at Earth.

"Right now, when the satellite dips to its lowest point, Earth appears so large that the sensor is unusable. The satellite relies on gyroscopes alone, degrading its attitude precision."

Present orbits of the fifth and sixth Galileo satellites launched together by Soyuz on 22 August 2014 , in red, compared to their intended position, in dashed green, and the position of the four satellites launched in 2011 and 2012, in solid green. 

This view looks side on to the two satellites’ orbital plane, which is off-centre relative to Earth. 

The targeted orbit was circular, inclined at 55º to the equator at an altitude of 23 222 km. 

The satellites are instead in an elliptical orbit, with a maximum altitude of around 25 900 km, a minimum altitude of around 13 700 km and a lower inclination. 

The satellites are in a safe state, correctly pointing towards the Sun, properly powered and fully under control. 

Credit: ESA

The recovery is being overseen from the Galileo Control Centre in Oberpfaffenhofen, Germany, with the assistance of ESA's Space Operations Centre, ESOC, in Darmstadt, Germany.

France's CNES space agency is providing additional ground stations so that contact can be maintained with the satellite as needed.

The two satellites were previously Sun-pointing. "On 3 November that changed for the fifth satellite, as it transitioned to normal Earth-pointing mode," adds Daniel.

During November, some 15 manoeuvres will take the satellite into its new orbit. Once there, it can formally begin in-orbit testing. The host satellite's health is checked first, followed by more detailed navigation payload testing.

ESA Venus Express climbs to new orbit post aerobraking

Venus Express aerobraking. 

Credit: ESA - C. Carreau

ESA's Venus Express spacecraft has climbed to a new orbit following its daring aerobraking experiment, and will now resume observations of this fascinating planet for at least a few more months.

The orbit-raising followed a month of aerobraking that saw the spacecraft surf in and out of the atmosphere at altitudes typically between 131 km and 135 km for a couple of minutes on each of its closest approaches to the planet.

Before, normal operations involved an elliptical orbit every 24 hours that took Venus Express from 66 000 km over the south pole down to around 250 km at the north pole, just above the top of the atmosphere.

But, after eight years and with propellant running low, the Venus Express team began a daring aerobraking campaign, dipping the craft progressively lower into the atmosphere on its closest approaches.

The experiment directly explored previously uncharted regions of the atmosphere, while also providing information on how a spacecraft responds when encountering the tenuous upper reaches of an atmosphere at high speed.

Aerobraking can be used to reduce the speed of a spacecraft approaching a planet or moon with an atmosphere, allowing it to be captured into orbit, and to move from an elliptical orbit to a more circular one.

Less fuel has to be carried, yielding benefits all round.

The technique will be used on future missions and the Venus Express experiments will help guide their design.

"We have collected valuable data on the Venusian atmosphere in a region difficult to characterise by other means," says Håkan Svedhem, ESA's Venus Express project scientist.

"The results show that the atmosphere seems to be more variable than previously thought for this altitude range, but further analysis will be needed in order to explain these variations properly."

Between altitudes of 165 km and 130 km, the atmospheric density increases by a factor of roughly a thousand, meaning that the forces and stress encountered by Venus Express were much higher than during normal operations.

It also experienced extreme heating cycles, with temperatures rising by over 100°C during several 100 second-long passages through the atmosphere.

In addition, the atmospheric drag at these lower altitudes was so great that the spacecraft's orbital period was reduced by more than an hour.



The Venusian cloud tops during nearly a full orbit of ESA’s Venus Express around the planet. 

The inset shows the corresponding position and relative speed of Venus Express as it approaches from its furthest distance of 66 000 km above the south pole, swooping down to 250 km above the north pole.

The images were captured by the Venus Monitoring Camera on 7–8 January 2012 and are shown in false colour.

Credit: ESA Venus Express

"The spacecraft has proven to be very robust and has apparently experienced no substantial degradation in any area, but a detailed evaluation is still to take place," says Joerg Fischer, Venus Express operations engineer.

At the end of the campaign, 15 thruster burns raised the craft's altitude, preventing it from dropping into the atmosphere.

The last was executed on Thursday evening, boosting Venus Express to a new altitude of 460 km at its closest and 63 000 km at its furthest. This new orbit takes 22 hours 24 minutes to complete.

"During the 15 manoeuvres, each thruster fired more than 8000 pulses and burned a total of about 5.2 kg of propellant to raise the spacecraft to this new altitude," adds Joerg.

This orbit will slowly decay again under gravity, but with only a few kilograms of fuel at most now remaining further altitude-raising manoeuvres may not be possible.

If no further corrections are made, Venus Express will probably reenter the atmosphere again in December, but this time for good, ending the mission.

In the meantime, having survived not only the aerobraking experiment but also the most recent orbit-raising manoeuvres, all of the science experiments will be reactivated, continuing their detailed study of Venus for at least a few more months.

"We are delighted with the success of the experimental aerobraking campaign, and are looking forward to assessing the details over the coming months," says Patrick Martin, mission manager.

"Meanwhile, we are enjoying the view from our new orbit around Venus, and plan to continue augmenting the scientific return of this exciting mission."