Copyright (c) 2002 by Ron Garret (f.k.a. Erann Gat), all rights reserved.
This is the story of the rise and fall of Lisp at the Jet Propulsion Lab as told from my personal (and highly biased) point of view. I am not writing in my official capacity as an employee of JPL, nor am I in any way representing the official position of JPL. (This will become rather obvious shortly.)
I came to JPL in 1988 to work in the Artificial Intelligence group on autonomous mobile robots. Times were different then. Dollars flowed more freely from government coffers. AI Winter was just beginning, and it had not yet arrived at JPL. (Technology at the Lab tends to run a few years behind the state of the art :-)
JPL at the time was in the early planning stages for a Mars rover mission called Mars Rover Sample Return (MRSR). In those days space missions were Big, with a capital B. The MRSR rover was to weigh nearly a ton. The mission budget was going to be in the billions of dollars (which was typical in those days).
Against this backdrop I went to work for a fellow named David Miller, who also happened to be my thesis advisor. Dave had the then-radical idea of using small rovers to explore planets instead of big ones. In 1988 that was a tough sell. Very few people believed that a small rover could do anything useful. (Many still don't.)
Using some creatively acquired R&D funding, Dave hired Colin Angle (then a grad student working for Rod Brooks at MIT, now CEO of IS Robotics) as a summer student. Colin built a small robot named Tooth, which stood in very stark contrast to the 2000-pound Robby, which was the testbed for the MRSR mission.
At the time it was more or less taken for granted that AI work was done in Lisp. C++ barely existed. Perl was brand new. Java was years away. Spacecraft were mostly programmed in assembler, or, if you were really being radical, Ada.
Robby had two Motorola 68020 processors running vxWorks, each with (if memory serves) 8 MB of RAM. (This was considered a huge amount of RAM in those days. In fact, the Robby work was often criticized for, among other things, being too memory-hungry to be of any use.) Tooth, by contrast, had two Motorola 68hc11 8-bit microcontrollers each with 256 bytes of RAM and 2k bytes of EEPROM. (In later robots we used 6811's with a whopping 32k bytes of RAM.)
(As an indication of just how fast and how radically attitudes can change, the other day I heard an engineer complain that "you can't do anything with only 128 megabytes.")
Both Robby and Tooth were programmed using Lisp. On Robby we actually ran Lisp on-board. The Lisp we used was T 3.1 which was ported from a Sun3 to vxWorks with a great deal of help from Jim Firby, who came to JPL from Yale. Jim also wrote a T-in-common-lisp compatibility package called Common T. Using Common T we could do code development on a Macintosh (using Macintosh Common Lisp and a Robbie simulator, also written by Jim) and then run the resulting code directly on Robby with no changes.
Tooth's processors didn't have nearly enough RAM to run Lisp directly [1] so instead we used a custom-designed compiler written in Lisp to generate 6811 code. At first we used Rod Brooks's subsumption compiler, but later I decided I didn't like the constraints imposed by the subsumption architecture and wrote my own for a language called ALFA [2]. ALFA was subsequently used to program a whole series of rovers, the Rocky series, which eventually led to the Sojourner rover on the Mars Pathfinder mission. (Sojourner had an Intel 8085 processor with 1 MB of bank-switched RAM. It was programmed in C. More on this decision later.)
Tooth, Robby, and the Rocky series were among the most capable robots of their day. Robby was the first robot ever to navigate autonomously in an outdoor environment using stereo vision as its only sensor. (NOTE: The stereo vision code was written in C by Larry Matthies.) Tooth was only the second robot to do an indoor object-collection task, the other being Herbert, which was developed a year or two earlier in by Jonathan Connell working in Rod Brooks's mobot lab at MIT. (But Tooth was vastly more reliable than Herbert.) The Rocky robots were the first ever microrovers to operate successfully in rough-terrain environments. In 1990, Rocky III demonstrated a fully autonomous traverse and sample collection mission, a capability that has not been reproduced to my knowledge in the twelve years since.
The period between 1988 and 1991 was amazingly productive for autonomous mobile robot work at JPL. It was also, unfortunately, politically turbulent. Dave Miller's group was, alas, not part of the organizational structure that had the "official" charter for doing robotics research at JPL. The result was a bloody turf battle, whose eventual result was the dissolution of the Robotic Intelligence group, and the departure of nearly all of its members (and, probably, the fact that Sojourner was programmed in C).
In 1993, for my own robotics swan song, I entered the AAAI mobile robot contest. My robot (an RWI B12 named Alfred) was entered in two of three events (the third required a manipulator, which Alfred lacked) and took a first and second place. (Alfred was actually the only robot to finish one of the events.) But the cool part is that all the contest-specific code was written in three days. (I started working on it on the plane ride to the conference.) I attribute this success to a large extent to the fact that I was using Lisp while most of the other teams were using C.
Also in 1993 I used MCL to help generate a code patch for the Gallileo magnetometer. The magnetometer had an RCA1802 processor, 2k each of RAM and ROM, and was programmed in Forth using a development system that ran on a long-since-decommissioned Apple II. The instrument had developed a bad memory byte right in the middle of the code. The code needed to be patched to not use this bad byte. The magnetometer team had originally estimated that resurrecting the development environment and generating the code patch would take so long that they were not even going to attempt it. Using Lisp I wrote from scratch a Forth development environment for the instrument (including a simulator for the hardware) and used it to generate the patch. The whole project took just under 3 months of part-time work.
In 1994 JPL started working on the Remote Agent (RA), an autonomous spacecraft control system. RA was written entirely in Common Lisp despite unrelenting political pressure to move to C++. At one point an attempt was made to port one part of the system (the planner) to C++. This attempt had to be abandoned after a year. Based on this experience I think it's safe to say that if not for Lisp the Remote Agent would have failed.
We used four different Common Lisps in the course of the Remote Agent project: MCL, Allegro, Harlequin, and CLisp. These ran in various combinations on three different operating systems: MacOS, SunOS, and vxWorks. Harlequin was the Lisp that eventually flew on the spacecraft. Most of the ground development was done in MCL and Allegro. (CLisp was also ported to vxWorks, and probably would have been the flight Lisp but for the fact that it lacked threads.) We moved code effortlessly back and forth among these systems.
