Fobos-Grunt ends its misery via re-entry

by Chris Bergin

Russia’s failed Fobos-Grunt spacecraft has re-entered on Sunday, following its failure to leave Low Earth Orbit (LEO) on what was supposed to be a sample return mission to the moon of Phobos. An accurate prediction of the spacecraft’s re-entry time and location will require some time to confirm, although Russian officials claim the spacecraft should have re-entered over the Pacific Ocean.

The Mission:

Fobos-Grunt was an ambitious sample-return mission to Mars’ larger natural satellite, Phobos. With a mass of 13,500 kilograms, Fobos-Grunt was the largest planetary spacecraft ever built in the former Soviet Union and was to be the first sample return mission to the natural satellite of another planet, and the first such mission to be conducted by Russia.

Fobos-Grunt – which was also hosting China’s first Mars probe, Yinghuo-1 as a passenger – enjoyed a nominal launch via a Zenit-2 launch vehicle, which occurred at 02:16 local time on Wednesday, November 8, 2011 – from the Baikonur Cosmodrome.

The two-stage Zenit lofted Fobos-Grunt into orbit via the first stage’s single RD-171M engine and second stage’s RD-8 vernier engine and RD-120 main engine – both performing as advertised.

Following shutdown of the second stage main engine, Fobos-Grunt separated, and solid rocket motors on the second stage fired to increase the separation distance between the spent stage and the payload.

Fobos-Grunt was set to perform an orbit-raising manoeuvre two and a half hours after launch, prior to a second burn 126 minutes later, which would have taken it into heliocentric orbit to begin its journey to Mars. Both burns failed to materialize.

Had the burns taken place, Fobos-Grunt would have taken eleven months to reach Mars, performing three course corrections along the way. Orbital insertion was planned for 9 October next year, when the spacecraft was to enter an orbit with a periareion of about 800 kilometres, and an apoareion of around 80,000 kilometres.

Following insertion, Yinghuo-1 would have separated from Fobos-Grunt and begin its mission. By January 2013, Fobos-Grunt would have been in a 10,000 kilometre circular orbit around Mars, and would have entered a quasi-orbit around Phobos in early February, before landing on the satellite later that month.

In either late February or March, the spacecraft’s return module would have been expected to have lifted off from Phobos, and return to heliocentric orbit for the journey back to Earth. This would have been expected to arrive at Earth in August 2014.

The reason for the lack of a burn from the cruise stage – derived from the Fregat stage, powered by an S5.98M engine using unsymmetrical dimethylhydrazine as propellant and nitrogen tetroxide as an oxidiser – is still not fully understood.

The lack of fault information is mainly due to the failed efforts by Russian controllers to re-establish a link with the spacecraft, in order to receive vital telemetry, or indeed send commands to aid such a process.

Among the challenges associated with communicating with Fobos-Grunt during passes over ground stations was believed to be a potential blockage by the yet-to-be-used fuel tank of the low gain antennas.

This tank – located on the aft of the cruise stage – would be expended and released in the event of both burns being completed. It is understood the spacecraft was never designed to be commanded prior to these two burns.

The Attempt To Recover:

There were hopes of a a breakthrough in communicating with the stricken spacecraft received the help of European Space Agency (ESA) assets. Reported by the European Space Operations Centre (ESOC) in Darmstadt, Germany, contact – the first observed since Fobos-Grunt’s launch – was received by ESA’s tracking station at Perth, Australia later in November.

It was the modification to the dish, a last gasp effort to establish a link with Fobos-Grunt, which broke the silence, after previous attempts – including those of ESA – to communicate with the spacecraft. However, this initial success did not include the downlink of telemetry.

With very short windows of opportunity to send communications to Fobos-Grunt as it raced overhead in Low Earth Orbit (LEO), controllers only had a matter of minutes to send commands, which related to switching on the the spacecraft’s transmitter and send a confirmation signal back..

Some telemetry was gained, along it was understood to be incomplete and “garbled”.  Russian media also reported that a second dish – at the Baikonur Cosmodrome – had made a level of contact with the spacecraft, although this was believed to be only a short term success.

While it appears clear that no useful commands were successfully received during the short period contact was established, the spacecraft soon went silent again, leading to ESA officially giving up after numerous attempts to talk to Fobos-Grunt without success.

The Upcoming End of Fobus-Grunt:

With the spacecraft’s fate now certain, the remaining challenge was to predict where an when the spacecraft will return to Earth for a destructive re-entry. These predictions, as per usual, were refined several times in the final hours.

No major items of hardware on Fobos-Grunt were listed as potentially surviving entry, while the aluminium tanks containing the large amount of propellant mass are likely to be some of the initial hardware to succumb to the destructive forces of entry.

The spacecraft’s orbiter was observed via amateur and professional groups, allowing for a level of modelling to be carried out on its decaying orbit, although the actual confirmation/sighting of re-entry will require confirmation. However, Russian military officials on RIA Novosti claimed the spacecraft should have re-enter over the Pacific Ocean, as much as an official entry point remains outstanding.

(Images: Via Roscosmos and ESA)

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