Emily Lakdawalla • Jul 11, 2008
"Ulysses is not dead yet."
ESA issued a statement in February saying that, as Ulysses' radioisotope thermoelectric generators were running out of power, the spacecraft would likely die some time this year. The actual death blow to the spacecraft was likely to be the freezing of hydrazine fuel in a cold spot in a fuel line. Mission controllers found creative ways to prevent the freezing, but the solution was not a long-term one, and ESA had a ceremonial send-off and wrap-up of the mission in mid-June, announcing that the spacecraft would be shut down on July 1. However, it now appears that announcement was premature. ESA issued a statement on July 3 titled "Ulysses hanging on valiantly." And on Wednesday, the following email was sent by Ulysses mission operations manager Nigel Angold to the Ulysses community, indicating that Ulysses' voyage could actually continue for some time.
Date: Wed, 09 Jul 2008 16:13:49 -0700
rom: Nigel Angold
ubject: Ulysses Status
Dear Ulysses friends and colleagues,
As you are aware, the proposed July 1st Ulysses operations end date has come and gone. And as a result of our fuel bleeding and other operations strategies, we have managed to avoid freezing the hydrazine so far!
Now we are continuing operations on a day-to-day basis until the fuel freezes. That includes fuel bleeding every 2 hours to keep the hydrazine moving through the pipes and Earth-pointing manoeuvres interleaved, when required.
When we see the fuel freezing, we will switch off the S-band and some instruments for a couple of days in an effort to thaw the hydrazine. After that, we'll try to switch on the X-band once again using some more radical (and hence more risky) procedures.
If the fuel has not frozen by the end of July, we'll try some benign X-band switching in early August and plan for radical switching in late August.
What will happen in the unlikely event that we re-establish our X-band downlink? Well, obviously we'll leave it on!!! And at the end of August we should have enough fuel remaining to continue for a number of months (assuming that we stop the fuel bleeding). The short portion of pipe that is currently close to freezing will warm up but other areas of pipework will cool down and they will get close to freezing during the last quarter of 2008. As for funding for operations beyond August, that's another question.
This coming weekend, we will be testing the redundant on-board receiver which is connected to the front low gain antenna (LGA-F). This is to validate our ability to command the spacecraft when the HGA is not pointing towards Earth e.g. if no manoeuvres have been performed for several days due to frozen fuel.
I know some of the instrument teams have held wakes to celebrate the fantastic journey that we have been on all these years. However, I make no apology for the fact that Ulysses is not dead yet. I hope that in the coming weeks we get to see the transition from fast to slow solar wind.
Very best regards,
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