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ICE - International Cometary Explorer

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Leith
697787.  Fri Apr 16, 2010 4:36 pm Reply with quote

One of my colleagues was recently giving us a talk on the Giotto mission to study Halley's comet, and mentioned in passing an American cometary visitor.

As Halley's comet approached Earth in the 1980s, ESA's Giotto spacecraft was launched to intercept it, as were two Russian probes (Vega 1 and Vega 2) and two others sent by the Japanese (Suisei and Sakigake). NASA had planned it's own mission, but was forced to abandon it due to budget cuts, potentially leaving the US unrepresented for the historic encounter.

NASA was not to be left out, however. Between the Sun and Earth lies a point named L1 - a 'Lagrange point' at which the gravitational fields of the two bodies are in equilibrium, such that a spacecraft located here can orbit the Sun in synchronization with the Earth. The first spacecraft to be placed in such an orbit was the International Sun-Earth Explorer 3 (ISEE-3) which, in 1982, had just completed a four year mission to study the interaction between solar wind and the Earth's magnetic field.

On a recommendation from the ISEE science team, NASA decided to send ISEE-3 on a new mission to intercept another comet, named Giacobini-Zinner, and continue on to an encounter with Halley. This required a few adjustments to the satellite's orbit:

(from http://en.wikipedia.org/wiki/International_Cometary_Explorer)

This elaborate trajectory was devised by the project flight director Robert Farquhar, using another helpful property of L1 points. The gravitational equilibrium at L1 is unstable, and an object that slips off the point will drift away. Low on fuel, ISEE-3 exploited this instability to make large course corrections with small thruster burns. First breaking loose from its Earth/Sun L1 orbit on a heading for the equivalent point betwen the Earth and Moon, ISEE-3 made five lunar flybys, each pass visiting the Earth/Moon L1 point to make further course corrections.

Manoeuvres completed, ISEE-3 was renamed the International Cometary Explorer (ICE). ICE passed through the tail of Giacobini-Zinner in June 1985, becoming the first spacecraft to visit a comet, and went on to join the 'Halley Armada' in 1986.

Links & sources:
The ICE plan cometh - International Cometary Explorer (Science News, 31st Aug 1985)
NASA Planetary Data System: ISEE-3 Mission Profile
NASA ISEE-3/ICE mission page
The Internet Encyclopaedia of Science: ICE

 
bobwilson
697822.  Fri Apr 16, 2010 8:21 pm Reply with quote

Why can't they deploy this technology to get trains and buses to coincide with each other?

 
britishsm
718309.  Fri Jun 11, 2010 7:41 am Reply with quote

I read that as International Cemetry Explorer .. hence the graphic confused the beejaysus out me XD

B.

 
CB27
718318.  Fri Jun 11, 2010 8:20 am Reply with quote

In Case of Emergency

 
Flash
718424.  Fri Jun 11, 2010 11:44 am Reply with quote

That's very damn clever. I wonder how much computer power he had at his disposal in 1982? Not much, probably.

 
Jenny
718629.  Fri Jun 11, 2010 4:38 pm Reply with quote

Flash - I was told (rotten source - sorry!) by my first husband, who was a computational chemist and knew a lot about computers, that the power of the computers that lay behind the Apollo moon landings was about equivalent to the old BBC Model B. Now I can imagine that things moved on in a decade, but I doubt it was anything like what's available now.

 
Leith
718683.  Fri Jun 11, 2010 7:54 pm Reply with quote

Flash wrote:
That's very damn clever. I wonder how much computer power he had at his disposal in 1982? Not much, probably.

I'm beginning to appreciate just how clever, after reading a bit more on the theory behind the ISEE-3 / ICE trajectory planning.

I found this related article on the 'Interplanetary Transport Network' interesting:
Next Exit 0.5 Million Kilometers - Douglas L. Smith

It describes how spacecraft can make complex journeys between the planets of the solar system and their moons, by following 'Lyapunov tubes'. These tubes run between pairs of bodies and beyond and are described by the set of paths an object can take when it drifts away from an orbit around a Lagrange point. The tubes relating to different pairs of bodies can intersect, forming a network of orbital paths along which spacecraft can travel, slowly but with very little propulsion required.

Lyapunov tubes also form paths which comets and asteroids can traverse as they drift around the solar system. The article mentions a comet named Oterma with a curious orbit. Oterma periodically jumps from a solar orbit inside Jupiter's to one outside Jupiter's, and back again. The explanation appears to be that Oterma's inner orbit passes the Jupiter/Sun L1 point, causing the comet to fall into a Lyapunov tube that takes it out past Jupiter. This outer orbit passes another Lagrange point (L2), on the far side of Jupiter which sends the Oterma back to its inner orbit.

More related links:

The Interplanetary Superhighway Dr. Shane D. Ross, Caltech

http://en.wikipedia.org/wiki/Interplanetary_Transport_Network
http://en.wikipedia.org/wiki/Halo_orbit

 
bobwilson
718737.  Fri Jun 11, 2010 11:57 pm Reply with quote

Jenny wrote:
Flash - I was told (rotten source - sorry!) by my first husband, who was a computational chemist and knew a lot about computers, that the power of the computers that lay behind the Apollo moon landings was about equivalent to the old BBC Model B. Now I can imagine that things moved on in a decade, but I doubt it was anything like what's available now.


This keeps coming up - and it's just plain wrong in the same way that "aerodynamics shows that the Bumble Bee can't fly" is wrong.

Today's computers do pretty much the same job as computers of 1969 - they do it quicker, they use less power and they're smaller - but the similarities are greater than the differences.

The mathematical number crunching (mnc) capacity of an Acorn computer is distinguished from the mnc of a modern laptop only by speed of calculation. Or indeed of the mnc of a team of clerks from the 19th century.

Getting an answer to the question of "how do we get the crew of Apollo 13 back safely" would take just as long now as it did then. It is possibly true that the question "is it safe to land the Eagle" on Apollo 11 would have been answered minutes earlier than the answer available in the actual original.

But there has been no seismic shift.

It may make a good anecdote that the electronic computing power available in 1969 is a million times less than the computing power now available - but it wouldn't have made more than a jot of difference to the outcome.

 
samivel
718742.  Sat Jun 12, 2010 1:43 am Reply with quote

So is it "plain wrong" or just not relevant to the outcome? As far as I can see, all Jenny's first husband said was that computer power had significantly increased in the time since the Moon landings, to the point where a BBC Model B had the same power as the stuff NASA had back then. You then say that's wrong, but the rest of your post suggests it's right, but "it wouldn't have made more than a jot of difference to the outcome" (not that anyone had claimed anything else).

 
Guest
718900.  Sat Jun 12, 2010 11:10 am Reply with quote

Jenny wrote:
Flash - I was told (rotten source - sorry!) by my first husband, who was a computational chemist and knew a lot about computers, that the power of the computers that lay behind the Apollo moon landings was about equivalent to the old BBC Model B. Now I can imagine that things moved on in a decade, but I doubt it was anything like what's available now.

I've heard a few such anecdotes, comparing either the Apollo or the Space Shuttle flight computers to a PC / car engine management system / washing machine / wrist watch etc, but have never had occasion to actually look into the details before.

This particular comparison appears to relate to the Apollo Guidance Computer (AGC), as opposed to the ground-based suite of mainframe and mini computers used by NASA for mission planning and control.

In terms of the specifications by which we compare home PCs, there are certainly similarities. Both machines have a 2 MHz processor, running an operating system hard-coded in 30K to 40K of Read Only Memory.

The AGC was based on a 16-bit processor architecture, while the BBC B was an 8-bit machine. The AGC had 2K of RAM to the BBC B's 32K, and here we can start to see where we're not comparing like with like.

The BBC B was designed to sit a in a classroom or living room and, via a keyboard and screen, to provide general purpose computing power to employ in any manner the user's imagination could come up with (mostly playing Granny's Garden in my experience). Consequently, it needed relatively large amounts of RAM for running arbitrary programs.

The AGC was built for a far more specialized task - to provide guidance and navigation functions for the Command Module and Lunar Module, collecting data from their gyroscopes, sextant and alignment telescopes, receiving commands from NASA mission control and from the astronauts, and processing that information to control the spacecraft's thrusters. It needed to run multiple tasks in parallel, in strictly predictable time frames and operate reliably after being subjected to the shock of launch and, potentially, to extremes of temperature and radiation. All this pushes the design towards hard-coding as much of the control logic as possible, with a small amount of RAM for running a restricted set of custom operations, thus minimizing the potential for memory corruption errors due to electrical interference or ionizing radiation.

In modern spacecraft design (at least as far as my experience goes), the requirements for absolute raw processing power tend to be modest. The first flight software I worked on was launched last year, running on a 17 year old processor chip. The challenge is in building a system that delivers the required processing power whilst being temperature, radiation, vibration and shock tolerant, weighing as little as possible and consuming minimal electrical power.

In contrast, the AGC actually required what was, for the time, a great deal of processing power for a unit of its size. Its design was ground-breaking, being one of the first computers to make use of a recent invention - the Integrated Circuit (IC), or "microchip". The Apollo space programme, and the AGC in particular where instrumental in driving forward the development of the IC. According to Dr Dobb's Journal, "in the early stages, a significant proportion of all ICs manufactured in the world were going to the AGC".

So, in summary, the AGC and the BBC B were very different machines, but their computing power was comparable in some ways and certainly in the same order of magnitude compared to modern computers. The development of the AGC played a significant role in the technological breakthroughs that lead to machines like the BBC B becoming household items a decade or so later.

Links and sources:
Dr Dobb's Journal: One Giant Leap: The Apollo Guidance Computer
http://en.wikipedia.org/wiki/Apollo_Guidance_Computer
http://en.wikipedia.org/wiki/BBC_B

 
Leith
718902.  Sat Jun 12, 2010 11:13 am Reply with quote

Curious - I appear to have become "Guest" in the post above.

 
Spud McLaren
718919.  Sat Jun 12, 2010 1:01 pm Reply with quote

Leith wrote:
Curious - I appear to have become "Guest" in the post above.
Well - welcome, then.

 
Posital
718928.  Sat Jun 12, 2010 1:14 pm Reply with quote

Misread this as "International Cemetery Explorer" - perhaps something created by the Mormons?

(ooo - I'm not alone in this)

PS: Welcome guest... e-tea and e-bikkies all round!!

 
Leith
719978.  Tue Jun 15, 2010 5:27 pm Reply with quote

Thanks for the re-welcome - I've already made myself at home :)

A little more on the ICE mission, relating to Flash's earlier ponderings on the computing resources available to Robert Farquhar and his team:

I've not been able to access any of Farquhar's own papers on-line, but I found some relevant historical notes in the documentation for a currently used orbit modelling software application. These notes mention that the ISEE-3/ICE mission planning was done at NASA's Goddard Space Flight Center (GSFC), using the Goddard Mission Analysis System (GMAS) and a modified version of the General Maneuver program (GMAN).

Both GMAS and GMAN were software applications written for the GSFC's mainframes which, in the late 70s / early 80s, would probably have been machines like the IBM S/370 and the Amdahl 470 V/6. These machines appear to have had 32-bit processors and were able to address 16 Mb to 2 Gb of RAM. I've no further information on their performance, but my guess is they were comparable with the early Pentium PCs of the mid-90s. This would give them a fair bit of number crunching power for orbit modelling, albeit a lot less than today's PCs.

However, processing power is only as useful as the software you can run with it.
[The following is based mainly on my relatively superficial understanding of the Douglas L. Smith article linked in post 718683.]

The existing tools available to Farquhar and his team would have been designed for planning much simpler trajectories - elliptical, hyperbolic and parabolic orbits, described by the mathematics of Newton and Kepler. Modelling the orbits around, and paths to and from, Lagrange points is a far more complex affair. Here, the relevant maths comes from late 19th century work of Henri Poincaré on the three-body problem, which laid the foundations of chaos theory.

It is the chaotic nature of the path taken by a spacecraft traversing a Lyapunov tube that made the ICE mission trajectory possible. The location and heading at which the spacecraft ends up can vary dramatically with tiny variations in its starting location and heading, thus allowing major course changes to be performed with small manoeuvres on setting off towards, or away from, a Lagrange point.

While others later showed how to use chaos theory to precisely plot such courses, Robert Farquhar's pioneering work used numerical analysis techniques to identify the path to ISEE-3's initial L1 halo orbit, and (I assume) to plan the subsequent ICE mission trajectory.

 

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