Sunday 12 July 2020

OT: Comet C/2020 F3 NEOWISE

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The image above shows comet C/2020 F3 NEOWISE, which is currently visible low on the northern horizon after sunset and before sunrise. From my 51 degree latitude, it is circumpolar, so visible all night, although very low in the sky. For the Northern hemipshere, this is probably the best comet since Hale-Bopp in 1997.

I took the image (or rather: images, as it is an image stack) last night around 00:58 UT (2:58 CEST) from Polderpark Cronesteyn on the outskirts of Leiden. It is a stack of 45 images of 0.6s exposure eacht, at ISO's 1600 and 4000, taken with a Canon EOS 80D + Samyang 2.0/135 mm on a fixed tripod (i.e. no tracking). The comet was only 8 degrees above the N-NE horizon at that time.

Apart from the bright bent yellowish dust tail, a hint of the straight, faint blue ion (gas) tail can be seen.

It was a very nice night (with owls calling), but a bit moist, with a carpet of low fog over the meadows. I had an USB dew lint and USB battery with me, and was glad I did. My glasses fogged over at times.

The comet is visible with the naked eye (even from my urban environment) with a few degrees of tail. It is impressive in 10 x 50 binoculars. the brightness is somewhere around magnitude +2.

The 2.0/135 mm Samyang turned out to be a superb lens for this comet, by the way. I am surprised by what I can achieve with it on this comet without a tracking mount.

Saturday 4 July 2020

ISS Debris Avoidance Manoeuvre of 3 July 2020

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ROSCOSMOS has announced that the International Space Station (ISS) had to make an unscheduled orbit adjustment (a debris avoidance manoeuvre)  at 18:53 Moscow Time (15:53 UT) on July 3, in order to dodge a piece of space debris. The rocket engine of the Progress MS-14 cargoship attached to the ISS were used for the manoeuvre, burning 100 seconds giving the ISS a delta V of 0.5 m/s. The ISS orbit was raised by about 900 meters as a result.

The brief bulletin did not identify which piece of space debris was dodged. Using COLA, I could however identify it as object 27923 (1987-079AG), a piece of debris from the Russian Proton rocket that launched the Kosmos 1883 GLONASS satellite on 16 September 1987.

One of the rocket stages from this launch shed some 31 pieces of debris in 2003, most of which decayed rapidly. The object that necessitated the July 3 ISS manoeuvre is one of the larger, and one of the few remaining shed pieces on-orbit. It is is a very eccentric, 350 x 4454 km, 64.9 degree inclined orbit (it's apogee has come down considerably over the past 17 years, from almost 20 000 km in 2003). The CSpOC catalogue characterizes its size as 'medium' (i.e. an RCS of 0.1 - 1.0 m2).

Had the ISS not changed it's orbit, this piece of space debris would have made a pass to a nominal distance of ~0.5 km at 18:28:19.07 UT on July 3. Note that this is a nominal value based on two TLE's: so there is a possible error of 1-2 km. But it is clear that this larger piece of debris would have passed well within the 4 x 4 x 10 km safety box around the ISS, necessitating the debris avoidance manoeuvre.

COLA output:

DATE       UT            RANGE   dALT    ANGLE
3 Jul 2020 18:28:09.07   0.5     0.1     107.1

The encounter would have occurred at 436 km altitude over the south Atlantic some 600 km northeast of the Falklands, near 48.1 S,  51.7 W (see illustration above and movie below).

ISS debris avoidance manoeuvres like this are not very frequent: it happens maybe once per 1-2 years.