DOY 281 (October 8): TCM 10 (or not), Separation, Centaur Observation and Braking Burn
The last 24 hours of the mission, bridging DOY 281 and 281 (October 8 and 9), will be a flurry of activity. Here is the sequence of events. I’ve provided both UTC and Pacific Daylight Time references:
Snip:
* 03:30 UTC/20:30 PDT: Shift Handover. Shift A (my shift) hands control over to Shift B. Shift B will oversee the Impact event. We’ll review the status of the spacecraft, in particular the dynamic behavior following Separation, and any last-minute items.
* 04:30 UTC/21:30 PDT: Final Orbit Determination Delivery. The Navigation team delivers its final estimate of the spacecraft and Centaur orbit. The spacecraft’s orbit can be measured directly, while, without a communications transponder aboard the Centaur, we have no direct measure of the Centaur’s orbit after Separation. This final orbit determination will become the basis for Impact command sequences, in particular the spacecraft attitude sequence to maintain pointing on the Impact site, and the Impact timing.
* Final Impact Planning and Command Generation: The Mission Design team will re-plan Impact using the latest orbit data from the Navigation team. The changes between preliminary and final Impact plan will be very subtle. The plan involves literally hundreds of Shepherding Spacecraft orientation changes to keep the onboard science instruments pointing at the expected Centaur impact point as we approach the moon. The new orbit estimate will change all of these orientations very slightly. The Sequencing Engineer will re-implement the command sequences, then pass his results to the Engineering Analyst and Simulation Engineer for final checking.
* 6:30 UTC/23:30 PDT: Disabling LCROSS Fault Management. Shift B will begin configuring LCROSS for the Impact. One of the first steps is to nearly completely disable the LCROSS onboard fault management system. Fault Management responds automatically to correct problems it detects onboard. Sometimes these are benign responses, like switching from a primary sensor to a backup sensor. Other times, the responses can be all-encompassing. It might seem strange to disable this function right before our most important phase of the mission. However, the last thing the Flight Team wants is for a problem onboard the spacecraft to interrupt our Impact observations. Some fault management responses are designed to throw LCROSS into a Survival State, turning off all power to the science payload, and disabling any onboard command sequences. This could mean disaster for the Science Team, since there would not necessarily be sufficient time to recover and return to the pre-Impact configuration. So, only minor fault management is enabled, but the more severe responses are disabled. In preparation for Impact, aside from disabling fault management, Shift B will also coordinate with the Deep Space Network to transfer our downlink path from a 34 meter diameter antenna (DSS-24) to the Goldstone complex’s 70 meter dish (DSS-14). The 70 meter antenna enables LCROSS to return science data at 1 megabit per second (1 Mbps).
* 8:30 UTC/01:30 PDT: Impact Command Approval Meeting (CAM). Shift B will review the final Impact plan and the associated onboard command sequences and ground commanding products. This is our last chance to get things right. Since the team is focused on a very specific set of checks, and for lack of time, this CAM lasts only 30 minutes. Then Shift B goes back to the MOCR to perform Impact.
* 9:00 UTC/02:00 PDT: Loading Impact command sequence to LCROSS. Shift B loads the final command products to the Shepherding Spacecraft, including a set of contingency command sequences to cover off-nominal scenarios. In the event of a building fire or an earthquake, our team even has a command sequence that would allow Shift B to leave the building and have the entire Impact sequence and observation be automated. The Deep Space Network has dedicated four antennas to this period of time, three from the Goldstone complex in California, and a fourth located at Madrid in Spain. Shift B, with the help of DSN operators at JPL, will coordinate those antennas as LCROSS changes its communications configuration. Hours earlie
* 10:00 UTC/03:00 PDT: Start of Impact onboard command sequence. Its first commands will perform a reorientation of the Shepherding Spacecraft to point the science instruments towards the expected Centaur impact point on the moon. The cameras and other instruments will not yet be on. This reorientation will also point the –Z Medium Gain Antenna (MGA) towards the Earth, enabling the team to switch the LCROSS downlink path from the omni-directional antenna to this MGA, in preparation for high-rate science data transmission.
* 10:10 UTC/03:10 PDT: Switch to –Z MGA. Shift B will command the switch from omnidirectional to the –Z MGA antenna. This is a potentially critical step in achieving full-rate science data transmission after the Centaur impact. However, since we did our combined Cold-Side Bakeout #3/MGA Test on September 24, we’re pretty confident this will work again.
* 10:15 UTC/3:15 PDT: Transitioning to Science Rate. The Flight Team will now command a transition from a standard downlink data rate of 64 kbps to our full science rate, 1 Mbps. This is another very important step to achieving full science return. However, we do have backup procedures that would allow us to transmit science data at a lower rate, 256 kbps, if the DSN 70-meter dish were to fail, or if the MGA was non-functional.
* 10:36 UTC/3:36 PDT: Payload powers on. The onboard Impact command sequence powers on and enables the DHU and science instruments. At 10:41 UTC, the command sequence also starts DHU NVM sequence 1, a sequence of instrument commands that tests each instrument in the LCROSS payload, save the Total Luminescence Photometer (TLP). The MOCR at NASA Ames begins to receive data from the science instruments, and the Payload Team and Science Team begins analyzing the preliminary data to make sure everything is working. This is still nearly one hour from Impact, but it’s the team’s last chance to find a problem in our suite of payload instruments that might otherwise foil our Impact observation. The team continues checking the instruments, and via the Flight Controller and Flight Director, commanding small adjustments to exposure settings, for 35-40 minutes.
* 11:10 UTC/4:10 PDT: TLP Instrument powers on. The Total Luminescence Photometer (TLP) instrument powers on for the first time since before launch. This instrument is very sensitive, and can only be powered on a limited number of times. The Science Team has been very careful not to overuse the instrument in tests. However, if the instrument powers on as expected, this is a major success on the road to the Impact event. The TLP, which gathers light measurements at 1000 times per second, will “catch” the Impact flash as the Centaur hits the moon, and is hence a very important instrument for water detection.
* 11:30:20 UTC/4:30:20 PDT: Flash Mode begins. One minute prior to Centaur impact, the DHU will command NVM command sequence 2, which begins Flash Mode. For the next 1 minute 3 seconds, Flash Mode will run the TLP and other instruments to capture the flash of light coming from the impact event.
* 11:31:20 UTC/4:31:20 PDT: Centaur Impact. Centaur impacts the moon at Cabeus. The energy of impact emits a brief, intense flash of light. A plume of lunar debris will rise in a pattern similar in shape to an inverted conical lampshade.
* 11:31:23 UTC/4:31:23 PDT: Curtain Mode begins. The DHU will switch from Flash Mode to Curtain Mode, which is a sampling sequence optimized to observe the evolution of the debris plume as it rises from the lunar surface. With this debris rising above the altitude of the Shepherding Spacecraft, our side-looking spectrometer will look towards the sun to measure light as it is transmitted through the debris. The remainder of the payload will be pointed down towards the impact point. This mode lasts for 3 minutes.
* 11:34:23 UTC/4:34:23 PDT: Crater Mode begins. At this late stage, the DHU will now switch from Curtain Mode to Crater Mode, which is designed to capture data about the properties of the new crater generated by the Centaur impact. The Shepherding Spacecraft now has less than one minute of time to capture and transmit data before it also hits the moon. With the Centaur impact point now off to the side, LCROSS will continue to try and track that point until its own contact with the moon.
* 11:35:39 UTC/4:35:39 PDT: Shepherding Spacecraft impact. The Shepherding Spacecraft will also hit the moon at roughly this time. The Flight Team will abruptly stop receiving telemetry a few seconds later, as the photons from LCROSS’s last transmission travel back to Earth to be received by the DSN 70 meter antenna. The LCROSS flight mission will be over.
* 11:36:02 UTC/4:36:02 PDT: You all die, you die and go to hell for pissing off the moon. We will be safe in our FEMA bunker and can't wait to take over all the stuff you leave behind.
This will be my last post until after Impact. I hope you enjoy the show tomorrow – it should be very exciting. Though we won’t have immediate feedback for water detection, I hope to report good news to you on Friday regarding the accuracy of our impact, and the collection of the science data. Then, over the coming weeks after Impact, the Science Team will review their data and interpret the observations. I’m sure you’ll be hearing news one way or the other.
Thanks for reading!
http://blogs.nasa.gov/cm/blog/lcrossfdblogOk, so I added that last one myself....But the link does have a great breakdown of events.
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