A view of the Mars Express mission control room at ESOC during the Mars Express orbital insertion maneuver. (credit: Georg Dittie)
Christmas on Mars
With ESA at mission control as Mars Express slips into orbit
by Daniel Fischer
Monday, January 5, 2004
So, where was the crucial, bending yellow line that had been promised to us as the visual proof that Europe’s first Martian visitor was slipping into orbit? It was already past the expected time, and yet the huge projection screen in front of us was empty! Numerous scientists, dignitaries and journalists had gathered in the wee hours of December 25, 2003, at the European Space Operations Centre (ESOC) in Darmstadt, Germany, in a festively decorated hall, just across from the actual control room of the European Space Agency where the Mars Express was commanded and where all information collected by antennae around the world was coming in as well. Earlier we had even been allowed to briefly crowd into a tiny VIP room overlooking the actual computer terminals and huge data displays on the walls.
Key flight controllers were on hand to explain the action, and there it had been: a simple graph where a tilted yellow line was plotted in real time. This was the frequency of the S-band carrier signal broadcast by Mars Express thru a small antenna as it was racing towards the planet, shifted slightly but distinctly by the Doppler effect caused by its speed relative to Earth. The spacecraft had already been put into a kind of deliberate safe mode, with most nonessential systems powered down and the fixed high gain antenna pointed off Earth: it was of utmost importance that the main engine would point in precisely the right direction during its upcoming half-hour firing, the one and only chance to insert Mars Express into orbit. While frequently circled by Soviet and especially American spacecraft over the years—and by Mars Global Surveyor and 2001 Mars Odyssey right now—there had never been a European Mars orbiter.
Without the high-gain antenna there normally would have been no way at all to monitor the orbit insertion in real time, and ESA had actually announced just weeks earlier that one would learn about success or failure only hours after the event. But in the last few weeks engineers had come up with the Doppler trick: just broadcast the clean carrier over one of two small low-gain antennae, and they—and everyone else—would be able to follow Mars Express’ braking into orbit as it happened. And now the engine should already be firing for several minutes, yet the Doppler plot was conspicuously absent from all displays in the much more spacious guest area to which we had been led back. What was going on? It was at 4:04 Central European Time (CET; 10:04 p.m. EST December 24th) when flight director Mike McKay appeared on stage, to explain what had transpired next door.
There had been tense moments in mission control, he revealed, as the carrier signal had disappeared abruptly, just when the main engine should have started to fire (plus the eight minutes of light time from Mars to Earth)! Had the spacecraft exploded, just as NASA’s Mars Observer apparently did ten years go? Did the sudden motion cripple its electrical systems? It took the flight controllers some minutes, a cool McKay explained, until they found a simple, completely harmless, but also somewhat embarrassing explanation for the loss of the carrier: it was being sent thru the wrong antenna! Six days earlier the same Doppler technique had been used to monitor the clean jettisoning of the Beagle 2 lander from the mother ship, and one had simply forgotten that the geometry was now a different one.
|Controllers found a simple, completely harmless, but also somewhat embarrassing explanation for the loss of the carrier: it was being sent thru the wrong antenna!|
So Mars Express was in all likelihood happily broadcasting its carrier signal, but away from Earth and out of reach of even the 70-meter antenna of NASA’s Deep Space Network in Goldstone, California, that ESA had been granted access to during crucial maneuvers. All we could do is wait another long hour when, long after engine cutoff and an automatic attitude adjustment, Mars Express would emerge from behind Mars. Then the carrier- broadcasting antenna should be pointing in Earth’s direction again and the success of orbit insertion would be clear almost immediately. That should happen at 5:11 CET. Some space talks were given in the meantime, but the added suspense kept everyone on the edge of their seats. Again no direct views from mission control or their displays were available to the crowd that was growing by the minute: We had to wait for word from McKay once more.
At 5:16 he walked back to the stage and spread the good news which we heard before anyone else outside mission control: the S-band signal had reappeared only seconds after the expected time, and thus Mars Express had to be at least in some orbit around Mars! How good it was remained to be determined: from the downlinked signal frequency alone one cannot measure a precise relative speed as the temperature of the transmitter could have changed during the past hour. There would be, however, another clever method to get better orbital information soon: so-called Doppler sweeps in which a signal with a well-known frequency is uplinked and returned by the spacecraft. The full telemetry and thus information about the state of Mars Express’ subsystems would not return until about 9:48 CET, though, as the spacecraft was not programmed to turn and point the high- gain back to Earth any sooner. This had been deemed necessary in case the main 400-newton engine would have failed and eight 10-newton thrusters would have been called into action to accomplish the orbit insertion over a much longer time. This contingency had not occurred, it was now obvious, and the strength of the S-band signal also demonstrated that the orbiter’s attitude was fine and as planned.
page 2: accessible controllers, inaccessible spacecraft >>