How geodesy created a vision of the Earth through the jewelled LAGEOS satellites

Image courtesy of NASA

Could this be the most beautiful satellite ever made? In fact it is one of twins, as there are two of these jewelled spheres in Earth orbit. They're the LAGEOS satellites, essentially inert reflectors to bounce lasers off. The jewels are fused silica, except for four made from germanium. The interior is not a void filled with instruments: it's a solid brass cylindrical core, covered in a thick aluminium shell, as you can see in the video below.

(Hang on, you say. Fused silica? Isn't that just glass? Well yes and no. It's amorphous silica, but without any of the other ingredients that make up the glass we use, like lime, soda and potash).

LAGEOS stands for  LAser GEOdynamic Satellite. The first was launched by the US on May 4 1976, and the second, made by the Italian Space Agency, was launched in 1992.  This means in 2017 the 60 cm sphere - harking back to the spherical satellites of the early space age, such as Sputnik, Vanguard and Echo, achieved 41 years in orbit. It's a veteran of space science.

And because they're completely passive, with no power, fuel, solar panels or instruments, their missions are not ending any time soon. They should be up there for about 8.4 million years according to the original prediction! They can just keep going until their reflectors are too weathered by exposure to the space environment to bounce a laser back to Earth. The glass eyes are meant to be dust and radiation-resistant, but, just like the famous space shuttle windows, they could be damaged by hypervelocity particle impact. Perhaps the quality of the return signal could even be a way of measuring deterioration in the surfaces since launch, and hence gauging something about taphonomy in space. I wonder if this is possible?

The two satellites orbit at around 6000 km in a circular polar orbit. The information they have provided has contributed to new perspectives of the Earth, as former LAGEOS project scientist David E. Smith explains: 

Today, we see Earth as one system, with the planet’s shape, rotation, atmosphere, gravitational field and the motions of the continents all connected. We take it for granted now, but LAGEOS helped us arrive at that view.

Even more importantly, the two LAGEOS define the centre point, based on the Earth's centre of mass, for the terrestrial reference system used in navigation.

One of the things they're used for is to measure the speed and direction of tectonic plate movement. Because of this, LAGEOS-1 was the recipient of one of Carl Sagan's time-travel interspecies communications.  He conceived a design - drawn by Jon Lomberg who also worked with him on the Voyager Golden Records - engraved on a thin steel plate that was wrapped around the brass cylinder core - depicting continental drift at three points in time: 268 million years ago, 1976, and in 8.4 million years. You'd have to crack the satellite open like an egg, though, to get at the message. It's precisely the sort of alien mystery object that science fiction writers imagine falling to a planet and catalysing personal and social revelations, even when the object is impenetrable. Who knows who or what might find it in 8.4 million years? Will it melt in re-entry, fall into the ocean unnoticed and unmourned, or slam into the Australian outback like Skylab, to lie under the stars for another few million years?

There's another little mystery too. LAGEOS 1 has the most precisely know orbit of any orbital object. In 1978, LAGEOS-1 began descending at a much greater rate than it should have. In 1983, as David E. Smith described in a Nature paper, the satellite rather unexpectedly began to ascend again. It's still losing altitude - 1.33 mm each day - but because it is mostly beyond the reach of the atmosphere which drags objects in lower orbits back in, the cause of the decay is still a matter of debate.

And here's another interesting fact. The Optus B series of GEO telecommunications satellites have laser retroreflectors mounted on them too. My colleague Owen Mace (one of the pioneering Australis Oscar V team) had the contract to make them. Optus B1 is in the GEO graveyard orbit; Optus B2 exploded on launch, and Optus B3 was replaced by Optus D1. This means it is still in GEO but technically classified as space junk. Is anyone ranging to the two reflectors? I don't know....


References
Dey, Uijal, Kar, Samanwita, and Amitabha Ghosh 2016 Possible effect of the Earth's intertial induction on the orbital decay of LAGEOS. Journal of Astrophysics and Astronomy 37(3):1-9

Fitzmaurice, M.W., Minott, P.O., Abshire, J.B. and H. E. Rowe 1977 Prelaunch testing of the Laser Geodynamic Satellite (LAGEOS). NASA report

Smith, David E. 1983 Celestial mechanics: Acceleration on LAGEOS spacecraft.  Nature Vol 304 p 15


Note: after I began writing this blog post, it morphed into an article for The Conversation.

Random thoughts about gravity and science

Saturday night. A quiet one in - I have a number of graduate students' thesis and report drafts to work through, and I'm thinking I might catch a bit of Graham Norton later in the evening. Debating whether to open the bottle of red I ended up with at the Flinders Archaeological Society's Quiz Night at the Adelaide Gaol, Wednesday last week. Might not help me appraise the students' data analysis with acuity, though.

On Thursday I got a new book, Robyn Arianrhod's Seduced by Logic. Emilie du Chatelet, Mary Somerville and the Newtonian Revolution (University of Queensland Press, St Lucia, 2011). I'm a sucker for a bit of mathematics, and with some girlie brainboxes thrown in, a good read is sure to be had by all. (Also, it features Voltaire).

A few years back, I was cheeky enough to write about gravitation myself (The Gravity of Archaeology, 2009, Archaeologies 5(2): 344-359). I'm sure I'd never get anything about gravity published in anything other than an archaeology journal, despite an invitation from the Journal of Modern Physics to contribute to a volume on cosmology and gravitation! I guess what fascinates me most about gravity is just how mysterious it really is.

In high school physics, you learn about mass, gravity, velocity, acceleration, entropy, etc, and have to solve endless tedious problems about them. As a kid, you don't really question any of it. Years (and years) later, I find myself troubled by these questions. I mean, what is mass, really? And force? If you called them something different, would you think differently about them?

I suppose one of the values of literary excursions into the history and philosophy of science is that they afford you new ways to conceptualise things and make the links between them.  Reading Arianrhod's book, I found myself musing that one of the virtues of Einstein's general relativity was that it explained the spooky action-at-a-distance aspect of Newtonian universal gravitation.

(And it was Newton who made space empty - because he had to get rid of ether as an explanation for the motions of the celestial bodies. But what are stupid gravity particles if they're not ether? Anyway I don't think gravity can be a particle. It has to be a movement).

And that thought recalled to my mind a book I haven't finished reading yet, by famous Australian postmodernist Elizabeth Grosz. Can't remember what it was called even, it's buried under a pile of papers in my office. (Actually it might be this one: Space, Time and Perversion: Essays on the Politics of Bodies, 1995).

In one passage, Grosz is talking about scientific theories. She describes how Einsteinian gravity replaced Newtownian gravity, as if they were just so many Kuhnian paradigms. And I thought (earlier in the year as I was reading it), that's not right. One didn't replace the other, that's not how we think about it. We don't study Newton as a historical curiosity. (And I was already constructing myself as part of the 'we', unconsciously, no Lizzie, you're not one of us, you don't understand).

Then I thought more about it and wondered if I was wrong. That there was some fundamental incompatibility between the two, rather than perhaps a slightly uneasy sliding of frames of reference sometimes. I always thought it was just that you used the equations or the approach that suited the problem at hand, you didn't just declare Newton was WRONG and get all quantummy about it. And, by implication in Grosz's book, science was also wrong, as we just couldn't get anything right the first time.

OK, so this is a caricature of Grosz' description; but the main thing that struck me, assuming the understanding I had of these things since a kid wasn't itself totally off the mark, was that she just didn't get how it worked.  Maybe you don't question it at school, but if you grow up in the sciences, then there are ways of approaching things that you don't even know you have until something calls it into question. From this I conclude that it can be really hard to understand science if you have not already been steeped in it. 

Lordy. I think I will open that bottle of red after all.

Zombiesat joins the ranks of space junk

Zombiesat! What's Next for the Out-of-Control Galaxy 15 Satellite

By Clara Moskowitz
SPACE.com Senior Writer
posted: 04 May 2010

The Galaxy 15 commercial satellite that recently lost contact with the ground has joined the ranks of a boatload of other debris adrift in space. It's now termed a "zombiesat" by engineers who have a better sense of humor than you might have imagined. So what's next for this 4,171-pound (1,892-kg) zombiesat?

This defunct satellite will probably drift to one of two "gravity wells" that catch most out-of-control spacecraft, experts say. Galaxy 15 could threaten nearby satellites because its communications package is stuck on and it may start interfering with its neighbors by siphoning off their signals. It's the first time such an event has ever occurred, and it sent Orbital Sciences, the satellite's builder, on a dash to figure out how to stop the satellite-run-amok.

Galaxy 15, like many communications satellites, was circling Earth about 22,369 miles (36,000 km) high in what's called geosynchronous orbit, meaning that it orbited at the same speed the Earth rotates, so that it sat perched above the same part of Earth all the time. "There are two points in geosynchronous orbit called geopotential wells," explained Nicholas Johnson, chief scientist for Orbital Debris at NASA's Johnson Space Center in Houston. "These are perturbations in Earth's gravity field. Typically when satellites lose control they will drift toward the nearest geopotential well and just oscillate around it." The two spots, also called libration points, are located at longitudes of 105 degrees west and 75 degrees east. There are already between 150 and 200 objects oscillating around these points, Johnson said.

Still a large place

In that sense, the new zombiesat doesn't significantly increase the space debris problem or pose a serious risk of colliding with an operational satellite.

"Space is still a very large place," Johnson told SPACE.com. "There are a lot of objects that are drifting back and forth. Galaxy 15 really just kind of joins a relatively large number of objects – it's not a significant new hazard from a global standpoint. But if your satellite happens to be near where Galaxy 15 is drifting then it's of more concern."  Eventually, everything in low-Earth orbit will eventually fall back down toward Earth because of atmospheric drag. The small amount of atmospheric particles in space create friction with spacecraft, causing their orbits to decay. The time it takes for an object's orbit to decay depends on its altitude.

For example, the International Space Station orbits at about 250 miles (400 kilometers) above Earth, while the Hubble Space Telescope circles much higher, at 353 miles (569 km).  "When things fall off the International Space Station, they typically fall back within a couple months, but where Hubble is, it typically takes several years to fall back to Earth," Johnson said. "At 800 km you're talking many decades or even hundreds of years."

To prevent the buildup of dead spacecraft in heavily trafficked areas of geosynchronous orbit, guidelines recommend that when a satellite reaches the end of its life it is boosted to a higher orbit out of the way. This "graveyard orbit" is about 186 miles (300 km) above where most satellites orbit. "The whole idea is to get to an altitude so they don't drift back into the operational region for a very, very long time – over 100 years," Johnson said.

It's actually easier to boost a spacecraft up just this much higher than to maneuver a craft down to where it would immediately fall back to Earth and burn up in the atmosphere, he said.

Getting rid of space junk

To actually go and collect defunct spacecraft to remove the collision risk altogether is currently beyond our ability.  "Unfortunately we haven't found a concept which appears to be both technically feasible and affordable," Johnson said. The best way to remove spent rocket stages and other large objects from orbit is to simply send up another spacecraft to rendezvous and dock with it and drag it back down to earth. This method would be extremely expensive and time-consuming, and isn't viable for the vast number of objects already in space. Some more exotic measures involving tethers and other props have been proposed, Johnson said, but aren't yet feasible.

For getting rid of very small pieces of space junk, there are two favorite ideas, he said. One involves shooting lasers at the objects to push them into lower-altitude orbits so they fall back down to Earth more quickly. "That has technical, economic, as well as policy issues," Johnson said. Another concept is to fly up a structure with a large area but low mass so that when particles strike the surface they will penetrate and lose some of their orbital energy, causing them to fall back to Earth more quickly. This option would also need many technical issues ironed out. "If it was easy we'd already be doing it," Johnson said of tackling the debris problem. "But it's prudent to be working the issue now before it becomes a serious impediment to space operations."

http://www.space.com/missionlaunches/zombiesat-out-of-control-satellite-100504.html

The gravity of archaeology

Another conference paper on the horizon. I seem always to be writing these bloody things and never doing anything. Anyway, this time it's on how gravity structures the archaeological record in orbit and on earth, and I chose gravity because the pun in the title (see above) was too tempting, and I will get to say satisfying things like the argument of the perigee and fundamental epoch to a room full of archaeologists.

Chimerastone points out that gravity may not be the most appropriate quantity? variable? to use, and I think s/he's right. I'm now combining a dynamical systems approach to geomorphology (which has a lot to do with how artefacts/sites get to be where they are) and celestial mechanics (which is how space junk gets to be where it is), and using energy instead, a la Lagrange.

I'm not sure yet if it's going to be simply profound, or merely simple.