Sunday, November 22, 2020

A space junk bestiary: yo-yo de-spin weights

Among the oldest pieces of space junk in Earth orbit are de-spin weights from the US series of TIROS weather and TV satellites. I've been noticing them in debris catalogues for years, and decided it was time to find out what they were really all about.  

TIROS 1 was launched in 1960 - just three years after Sputnik 1. The satellite is now 60 years old! TIROS stands for Television Infrared Observation Satellite Program. One aim of the satellite was to see if Earth observation from space would work, and could be used for weather reporting and prediction. The other was to test television broadcast potential - so you can see that it is one of the progenitors of two very significant satellite-based industries today.

The satellite was a cylindrical drum covered in solar panels with short, angled antennas. It returned one of the earliest pictures of Earth from outside.

Image courtesy of NASA

There were 10 satellites in the first TIROS series, after which they continued as TIROS-N in collaboration with the US National Oceanic and Atmospheric Administration (NOAA). Of these 10, five are still in Low Earth Orbit as pieces of space junk.

So what about these de-spin weights?

When rockets are launched, a key consideration is keeping them stabilised so that they don't pitch, roll or yaw, and end up in a crater in the ground instead of in orbit. One way to stabilise them is to spin them perpendicular to their long axis. This method of stabilisation was frequently used for the launch of small, lightweight spacecraft (Cornille 1962:1). The problem with that is when the satellite is released, it has the same spin as the rocket, which is generally too high for it to function. So you have to reduce the spin and reset the satellite. This is what the de-spin weights are for. There might be one or two weights. Two weights are called yo-yo weights, and one is just a yo-weight. 

Fortunately, I did not have to figure this out all by myself, as the Practical Engineer has done a pretty great job of explaining the principles, as you will see in the video.

This is a description of the mechanism from Fedor (1961:1):

The yo-yo de-spin mechanism is essentially two pieces of wire with weights on the ends ...  These wires are symmetrically wrapped around the equator of the satellite and the weights are secured by a release mechanism. At a pre-selected time after satellite spin-up and release from the launching vehicle, the weights are released, thus discarding enough momentum to reduce the spin of the satellite to the desired value.

The weights are not only released, but discarded as mission-related debris. This kind of discard is now discouraged in guidelines for mitigating space debris. 

Something I wonder about is whether the weights still in orbit belong to the TIROS satellites remaining in orbit, or whether they belong to a re-entered TIROS? Did the de-spin weight tend to re-enter with corresponding satellite? Is the orbit an indicator? 

So many questions! Did the USSR use this mechanism for satellite stabilisation? There are certainly no USSR de-spin weights catalogue from this period that I could find.

And what did they look like? Those illustrated in Fedor (1961) look (from the grainy image) to be rectangular, maybe about an inch long. Fedor (1961) also uses a unit of measurement called the slug-ft/2 and I don't even know what it is. OK, I looked it up and this is what it is.

Where else can we find de-spin weights? The Explorer series used them. Here is a marvellous illustration of them working on Explorer 11, launched in 1961 (from Cornille 1962).

This is a de-spin weight from 1962 British satellite Ariel 1. Note that this one doesn't have a rigid cord, but a tightly coiled spring (also from Cornille 1962). The 'stretch' yo-yo design was patented by Cornille and Fedor in 1970. This example has, to my mind, a rather sinister snake-like aura as if it might suddenly spring to life and start seeking you out with its eyeless head and whipping tail.


The Dawn mission to Vesta and Ceres, launched in 2007, used de-spin weights, as the launch rocket's 3rd stage was spin-stabilised. I wanted to know where the weights ended up. Fortunately my friend Ady James knew where to look for them and located this blog post by Dawn Mission Director and Chief Engineer Marc Rayman for me:

After the third stage has finished firing, it remains securely attached to Dawn for another 4 minutes 50 seconds. Although the stage is stabilized by spinning, the spacecraft does not operate that way; yet by this time, they would be spinning together at 46 rpm, too fast for the latter’s control system. Therefore, starting 5 seconds before separation, the third stage activates a surprisingly simple system to slow its rotation rate. Wrapped around the Delta are two cables, each 12.15 meters (39 feet 10 inches) long. At the end of each is a 1.44-kilogram (3-pound-3-ounce) weight made of aluminum and tungsten. When the cables are released, the spin causes them to unwind. As they carry the weights farther and farther out, the spin slows down because of the same principle that makes an ice skater spin faster by pulling her arms in or slower by extending them to her sides. After 4 seconds, when they are fully unwound, the cables unhook from the spacecraft. With their weights still attached, they enter independent orbits around the Sun; perhaps one of them will be studied by a future solar system archeologist.

Well there you go, he was right as that's exactly what I'm doing! This post adds some interesting details. The weight is let go by releasing the cable, so the cable is still attached to it. I wonder if it remains taut after release? The cables are 12 metres, more than the length of two tall women end-to-end. No information about the shape of the weights, but we do have materials - aluminium and tungsten. Why tungsten, a metal in short supply on Earth? There is no need to make these weights durable as their only purpose is to be heavy. Perhaps tungsten adds the required weight for size. How does this alloy react to space weather, plasmas and bombardment? Is it contributing aluminium particles to the space environment? Could they combine with the atomic elements so common in the space environment to form new compounds? Al2O3 (aluminium oxide) or tungsten oxides?

The weights would, one presumes, behave like a meteorite as they are solid. And more like a metal-rich meteorite too. Would they be distinguishable through a telescope or by spectroscopy?

So many questions.

How many of these weights are circulating among all the space junk? Jonathan McDowell's catalogue of space objects has 286 de-spin weights listed. How many of these are still in orbit is something I'll have to leave for a future calculation.

What about the damage a collision with a de-spin weight would cause? These are dense, heavy, solid objects, and I'm going to guess for that reason would be far more destructive than something of a similar size, cross-sectional area or velocity, but made of different materials. It should also be possible to model how an impact crater from collision with one of them might differ from other pieces of space junk. This would help identify liability for the damage, as the launching state is responsible for this under UN treaties.

Comparatively, there are not that many de-spin weights, compared say to rocket bodies, but perhaps the greater damage they can do would merit being selectively targeted for active debris removal. 

As Fedor and Cornille are rapidly becoming my yo-yo gurus, I thought I should learn a little more about them. Sadly Henry Cornille died in 2019 at the age of 80. He went on to work on the Apollo programme. I couldn't find anything about Fedor online so this clearly requires deeper archival research. If you think about it, they are the pioneers of today's space tether technology.

I've got another loose end to tie up here - the relation to the popular children's toy called the yo-yo. I realise I don't know anything about its origins. *goes and looks up* Well, this was an eye-opener, as my assumptions that it was 1960s invention turn out to be extremely wrong! The yo-yo is very ancient technology, originating in China (as indeed does rocket technology). 

This is a fetching red figure vase showing a boy playing with a yo-yo in the 5th century BCE, from the Antikensammlung museum in Berlin.

The yo-yo despin weights are a lovely thread linking physics from the past to the future. I've become quite captivated by them.


Cornille, Henry J.Jnr 1962 A method of accurately reducing the spin rate of a rotating spacecraft. NASA Technical Note D-1420 

Fedor, JV 1961 Theory and design curves for a  yo-yo de-spin mechanism for satellites. NASA Technical Note D-708