Emily LakdawallaSep 22, 2008

Ulysses is in peril

The Ulysses mission is nearing its end, as steadily declining available power is threatening the spacecraft's ability to keep its fuel from freezing. A creative plan to bleed fuel to keep it from freezing has prolonged the spacecraft's life. Now, though, their luck may be running out. Here's the latest update on Ulysses' status, from operations manager Nigel Angold, which I grabbed from the Ulysses website:

Sun sensor anomaly

Date: Fri, 19 Sep 2008 02:08:18 -0700
rom: Nigel Angold
ubject: Ulysses Status

Dear Ulysses colleagues,At the beginning of our last Ulysses pass (DOY 263, 03:20 UTC), telemetry indicated that the spin rate was ~9.9 rpm. The spin fail detect (set for 5 +- 0.2 rpm) showed "failed" and the latching valves in the hydrazine lines were closed. Further investigations revealed that sun sensors MS-1 and XBS-2 were off. We don't know why the sun sensors switched off but a chain of events will have occurred when they did:

  1. Without the Meridian Slit sensor data, the on-board spin rate is calculated erroneously due to double-crossings. In fact, the spin rate is calculated as twice the actual spin rate of ~4.85 rpm. So we are confident that the spacecraft is not actually spinning at 9.9 rpm.
  2. The spin fail detect program sensed that the spin rate was outside the failure deadbands and closed the latching valves to prevent thruster firings.
  3. Any subsequent manoeuvres, whether loaded in the time tag buffer, in the CONJ program or simply commanded in real-time have not taken place.
Since the received signal strength appears to be nominal, it can be assumed that the last Earth-pointing manoeuvre at 262.21:20 UTC executed successfully and the sun sensor switch-off occurred after that. However, it is not known if any fuel bleeding has occurred since then. Consequently, there has been a significant increase in risk of fuel freezing and the initial recovery actions have focused on reestablishing fuel bleeding.

Additional uplink time from DSS-54 was obtained (uplink in the blind because the spacecraft S-band transmitter was off) and the on-board CONJ program was reloaded to execute fuel bleeding every 2 hours with the spin fail detect logic disabled. Further commands were sent to open Latching Valve 1, select the redundant MS sun sensor and perform a real-time fuel bleed manoeuvre. None of these commands will be verified until the next downlink pass. We will try to obtain an additional 70m pass during the local daytime tomorrow.

Keep your fingers crossed.

Best regards,

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