Orwell Astronomical Society (Ipswich)
Peregrine Lunar Lander, 09-26 January 2024
Astrobotic launched its Peregrine lunar lander on 08 January 2024. The launch was via the inaugural flight of United Launch Alliance's Vulcan rocket with a Centaur 5 upper stage. Peregrine is the first private lunar lander launched from the US. The Centaur set Peregrine on a trajectory towards the Moon and was then discarded into heliocentric orbit. Unfortunately, a fuel leak from the lander shortly thereafter meant that a lunar soft landing became impossible; instead, NASA and Astrobotic steered the craft towards a safe crash landing in a remote area of the South Pacific at 21:00 UT on 18 January.
Throughout the journey of the short-lived lander, I was able to make observations on the following nights.
The sky cleared during the night, providing my first opportunity to image Peregrine. The craft was not bright. It was moving at around 7 arcseconds/minute, so I initially settled on exposures of 30 s duration. Peregrine is just about detectable in a single frame, but you need to know where to look and have a good imagination! The below image is a composite showing the motion of the craft.
After the craft crossed the meridian, its motion slowed to less than 3 arcseconds/minute, so I increased the exposure time to 60 seconds. The resulting image showing the motion of the craft is below. At this time, the craft was at around magnitude 17.2-17.4 at a range of ~250,000 km.
The JPL Horizons ephemeris models the motion of Peregrine. The propellant leak acted to deflect the trajectory of the craft, necessitating an update to the ephemeris, in turn producing changed predictions of its position. I used the predictions to align images of Peregrine, and then stacked them to produce the still and video shown below. Peregrine is now much more evident.
On 09 January, I had no predictions for the location of the Centaur, but was confident that it was outside my field of view.
The sky remained clear on the following evening and, by this time, I had obtained predictions, from Project Pluto, of the location of the Centaur. By now, it had entered heliocentric orbit and was at a distance of some 600,000 km; although much further away than Peregrine on the previous evening, it was considerably brighter, at around magnitude 15.2. It was therefore much easier to make a still and video of its motion: see below.
The Centaur was sufficiently bright to enable me to assess whether its brightness fluctuated periodically (which would indicate that the object was tumbling). I recorded a sequence of many exposures of 10 s duration, and formed them into a video. There is no evidence of periodic fluctuations in brightness: see below.
I was unable to image the lander. On later investigation, this turned out to be through the erroneous use of positional data for the previous day. Doh!
Peregrine then looped out to apogee at a distance from Earth of approximately 400,000 km on 13 January, then approached Earth again. During this time, the sky remained cloudy, and I was unable to make observations.
Conditions on 14 January were much improved, and I set out to observe both Peregrine and the Centaur stage. As previously, I consulted two sources of predictions: JPL Horizons and Project Pluto. For no particular reason, I started by using JPL Horizons. As Peregrine was faint, I had to stack images on its predicted motion; yet this revealed no trace of the craft. I checked Project Pluto and found that it predicted a significantly different position to Horizons for the craft at 23:00 UT:
The difference amounts to an angular separation of 0.35°. With my telescope centred on the JPL Horizons position, the position given by Project Pluto lay outside the field-of-view!
Only Project Pluto predicted the position of the Centaur, and I imaged it as below.
I had most faith in the predictions by Project Pluto, but had no measure of how accurate they were. Peregrine had been venting propellant and the effect of this might have have been difficult to model, causing the ephemeris to be inaccurate. My doubts were reinforced when I found that the difference between the two ephemerides for 23:00 UT on 15 January, at 0.41°, was larger than that for for 23:00 UT on the previous night:
I decided to hedge my bets and centre my images on the mid-point of the two predictions. Peregrine was a few arcseconds distant from the prediction by Project Pluto (although it was hard to be sure as the craft was not visible in a single frame and could only be discerned in an image stack). My images of Peregrine and the Centaur are below. The looping action seen in the sky is due to the rotation of the Earth.
The sky was again clear. By this time, Peregrine was falling towards the Earth, with re-entry expected two days later. I was able to observe both Peregrine and the Centaur as below.
This was the final night on which Peregrine was visible from the UK. By midnight (00:00 UT on 18 January), the craft was moving at around 6 arcseconds per minute, but by 04:00 UT had speeded up to over 20 arcseconds per minute. I elected to use 30 second exposures. They reveal a degree of brightness variation as Peregrine plunged earthwards. In the final part of the video, the motion of the lander within each exposure was sufficient that the trail became dash-like.
The Centaur, at some 2 million km distant, continued to recede.
By 21:00 UT, Peregrine had splashed down in the Pacific. The Centaur however, remained visible as below.
A video and still image of the Centaur, taken on 19 January, are below.
A check of the positions given by the two ephemerides for 23:00 UT on 15 January showed that they were now much more accordant, at an angular separation of only 0.07°, no doubt reflecting the benefit of many observations of the craft around the time in question:
During the early part of the night, the sky was clear and the almost-full Moon had not yet reached a troublesomely high altitude. I made one last, mad attempt to photograph the Centaur stage even though, by now, it was 3.8m km distant and circa magnitude 19.
I left the camera running for three hours, terminating the run when the telescope reached the meridian; by that time I would either have captured the Centaur or not, and there was no point in taking further images.
On initial processing of the images, the limiting magnitude appeared to be short of what was required, and I was pessimistic about detecting the Centaur. Conscious that such a faint object could be lost in the glare of a star, and that a hot pixel could masquerade as the object, I compiled a stack of all available images and also several partial stacks, revealing changes in the field of view over time.
The below video shows a stationary object, initially lost in the glare of a star, that becomes visible approximately half-way through. I believe that the object is the Centaur. Originally, the video also showed several random hot pixels masquerading as the Centaur - I tracked them down to the individual frames in which they appeared, and removed them. (I left the passing "space-worm" on the LHS in peace. Genus: pixeles calientes.)
The following still shows two consecutive 60-minute stacks side-by-side. In the centre of each is the Centaur: the motion of the background stars relative to the object is obvious.
The following still shows the above two frames combined. The position of the Centaur predicted by Project Pluto is indicated by the red markers, some 70 arcseconds distant from the object itself.