Night of December 28th to 29th, 2003
La Silla – It’s one o’clock in the morning, my night assistant comes back from the “comedor” (dining hall), and it’s my turn to go eat. Colleagues follow in silence. It’s another calm night above the Cordillera. We barely pause to look at the moonless sky. It’s said there are “papas fritas” (French fries) tonight, so everyone’s hungry.
At two o’clock in the morning, I head back to the control room, using the dimmest headlights to avoid disturbing the observations. Donkeys were spotted on the mountain this afternoon, so caution is necessary. Except for a few beams of light filtering through the blinds of the control room, there is no light from the ground, only from the sky. The urge to shout our presence on this rock to the world hits me as I walk on the terrace, facing the surrounding hills and beyond, the Pacific Ocean. It’s not the first time. Who could be thinking of us? Who would really spare a thought for those whose job is to gaze at the sky every night?
The control room light hits me in the face. Three seconds to adjust, back to work.
2:07 AM – The program I’m observing is definitely long. Long exposures of an hour, once again, for obscure and unknown “quasars.” With all these observations, which overwhelmingly focus on quasars and distant galaxies, I wonder how modern cosmology isn’t advancing faster… A sarcastic remark in the middle of the night, nothing more.
8:11 PM – Bing. An email arrives. From Hawaii. It reads:
“Dear Cédric, I hope you’re the right person to contact. I’m an astronomer in Hawaii, ‘observing’ for two space mission targets, but we’re mostly dealing with a snowstorm at the summit of Mauna Kea, and we can’t open the dome. I’m trying to find ‘some folks’ at other observatories who could take two images for me.
Target ‘Stardust’: Comet 81P/Wild2 – The probe-comet encounter is on January 4th, 2004 [in 6 days] – the Stardust mission team urgently needs astrometry now! On a 2.2-meter telescope, it should take 40 seconds.
The second target is for the Deep Impact mission: Comet 9P/Tempel1 – currently magnitude 21.9. We need its phase function, and measurements tonight would provide us with information about its surface properties, which directly impact the probe-comet encounter (in about a year). Again, on a 2.2-meter telescope, it would take 900 seconds.
Is there a chance of using one of the telescopes at La Silla to take two images with the R filter? Just in case, I’m including the coordinates and ephemerides of both comets (I know Tempel1 might be too far north?).”
9P/Tempel 1 – Deep Impact target – mag 21.9 – need R filter, S/N=50 image – guide at comet rate.
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Date__(UT)__HR:MN R.A._(ICRF/J2000.0)_DEC dRA*cosD d(DEC)/dt a-mass T-mag
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2003-Dec-29 04:00 m 06 56 17.66 +30 16 24.5 -31.06 6.76 2.111 21.96
2003-Dec-29 05:00 06 56 15.26 +30 16 31.2 -31.10 6.67 1.968 21.96
2003-Dec-29 06:00 06 56 12.86 +30 16 37.8 -31.09 6.57 2.035 21.96
2003-Dec-29 07:00 06 56 10.47 +30 16 44.3 -31.04 6.49 2.356 21.96dRA*cosD d(DEC)/dt =
The rate of change of target apparent RA and DEC (airless). d(RA)/dt is
multiplied by the cosine of the declination. Units: ARCSECONDS PER HOUR81P/Wild 2 – Stardust target – bright – need image in any filter for astrometry – used 40 sec in morning twilight through R filter.
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Date__(UT)__HR:MN R.A._(ICRF/J2000.0)_DEC dRA*cosD d(DEC)/dt a-mass T-mag
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2003-Dec-29 08:50 N 16 18 18.35 -19 04 13.5 90.12 -15.10 4.342 13.05
2003-Dec-29 09:00 N 16 18 19.41 -19 04 16.0 90.09 -15.09 3.769 13.05
2003-Dec-29 09:10 N 16 18 20.47 -19 04 18.6 90.05 -15.09 3.329 13.05
2003-Dec-29 09:20 C 16 18 21.53 -19 04 21.1 90.02 -15.08 2.983 13.05
2003-Dec-29 09:30 C 16 18 22.59 -19 04 23.6 89.98 -15.08 2.704 13.05dRA*cosD d(DEC)/dt =
The rate of change of target apparent RA and DEC (airless). d(RA)/dt is
multiplied by the cosine of the declination. Units: ARCSECONDS PER HOURThanks
Karen M.
(In charge of ground and earth-orbital observations, Deep Impact Mission)
2:16 AM – I check my programs, but it only confirms my feeling. The night is rather empty, since after the one-hour exposure of the current program, I have nothing scheduled until 4:30 AM.
2:18 AM – I reply to Karen. She can count on La Silla’s solidarity! The night springs into action!
2:20 to 2:30 AM – Preparing the “Observation Blocks” for the two comets. As usual for moving objects in the sky, the coordinates are entered at the last minute.
3:00 to 3:30 AM – While my program finishes, I read up a bit on the two missions. The Stardust mission aims to get into the tail of comet Wild2, collect particles (a comet is made of dirty snow that “evaporates” when exposed to sunlight), and, for the first time, bring them back to Earth! The Deep Impact mission, on the other hand, will approach comet Tempel1, release a large projectile that, upon impact, will eject particles from the surface and even the top layers. The main module of the probe, following the projectile, will collect and analyze them. These two missions will teach us more about the origin of our solar system, as comets contain the materials with which our system was formed.
3:36 AM – It’s around 7 sidereal time (ST). The first comet, Tempel1, is observable. The program is over, and I feverishly enter the comet’s coordinates into the Observation Block. And we’re off! The telescope points, then stops. We’re on target. It starts differential tracking to follow the comet (instead of following the stars’ movement, it follows the comet’s). 15-minute exposure. We wait…
3:52 AM – The image appears on our imager’s (SUSI2) screen. And it looks like this:
We got it! Just on the edge of the detector, but we got it… We take another image. With these images, the Deep Impact mission astronomers will be able to compare the comet’s exact position with the (known) position of the surrounding stars, using the exact time of observation and the telescope’s pointing coordinates. After a few celestial mechanics calculations, they’ll know how to determine the best orbit for the probe.
4:12 AM – We search for the second comet, whose encounter is in 6 days, while the probe was launched over a year ago! But the comet is too low, barely on the horizon. According to the coordinates, it’s rising quickly in the sky. But with the NTT, we can’t go lower than about 11 degrees above the horizon. So, we wait a bit. The right ascension coordinate of Wild 2 (usually noted as “alpha”) is just over 16h. The sidereal time is around 8h. So, we have an hour angle of about 8 hours. It’s still too much (the ideal is a zero-hour angle, meaning the object is crossing the meridian).
4:15 to 5:15 AM – I resume my service programs while waiting.
5:20 AM – We point to the second comet! The telescope is very, very low (12 degrees). The image quality is very poor due to flexion on the telescope mirror. It’s almost vertical!
5:40 AM – The sun rises in 20 minutes, the comet is 14 degrees above the horizon, so we observe despite the poor image quality.
5:50 AM – The first images come out. Nothing special is visible. We try again until the sun rises.
5:58 AM – The sun rises. We didn’t see anything in the images, but there’s nothing more we can do. I send the last images to Karen. We’ll see.
A little after 6:00 AM, I step outside. The day is dawning. The light is beautiful. I’m going to bed. I’ll learn later that we were the only ones with an available imager, and that comet Wild2 was indeed on the images after processing. A nice success. The leaders of both missions seem rather happy! They had a meeting during the day to decide on the trajectory for the Stardust probe. From what I’ve heard, it went well. And as for me, I’ve got two more comets to add to my list…
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