International Cometary Explorer

• 1. Introduction

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  The International Cometary Explorer was born with a different name, a different purpose, and a different destination. Launched on the 12th of August 1978 as the International Sun-Earth Explorer 3, it was designed to sit in a mathematically elegant parking spot between Earth and the Sun, measuring the invisible forces that shape our cosmic neighborhood. Nobody at the time imagined that this spacecraft, built to study Earth's magnetic bubble, would one day become the first human-made object to pass through the tail of a comet.

  What does a spacecraft do when its original job is finished? For most probes, the answer is silence. For ISEE-3, the answer was a second life. Engineers realized they could use the Moon's gravity like a slingshot, flinging the spacecraft out of its Lagrange orbit and onto an intercept course with a comet millions of kilometers away. That maneuver would require fifteen separate thruster burns and five lunar gravity assists.

  And decades later, after NASA had quietly closed the books on it, a small team of engineers and scientists crowdfunded a private effort to wake it up again. The questions that story raises have no clean answers: who owns a spacecraft the agency no longer wants? Can open-source software and a rented radio dish reach something launched in 1978? And what happens when the answer turns out to be almost?

• 2. The Libration Point Pioneer

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  On the 20th of November 1978, ISEE-3 entered orbit around the Sun-Earth Lagrange point, roughly 1.5 million kilometers from Earth, at a distance of 235 Earth radii. No artificial object had ever parked there before. The location is sometimes called a libration point, a place where the gravitational pull of the Sun and Earth balance each other so precisely that a spacecraft can hold position without constantly firing its engines.

  ISEE-3 carried no cameras. Its instruments were designed to detect things the human eye cannot see: energetic particles, plasma waves, and the structure of fields threading through space. The spacecraft rotated at 19.76 revolutions per minute about an axis perpendicular to the ecliptic plane, a spin that simultaneously stabilized its orientation, generated solar power, and kept its antenna sweeping toward Earth.

  The mission's four main scientific goals covered the outermost boundaries of Earth's magnetosphere, the shock wave where the solar wind meets that magnetic envelope, the behavior of plasma sheets inside the magnetosphere, and the study of cosmic rays and solar flare emissions near one astronomical unit from the Sun. The probe answered all four, and by the early 1980s, it had essentially completed what it set out to do. That completion opened the door for something no one had originally planned.

• 3. Stealing a Satellite

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  Frederick L. Scarf of TRW saw an opportunity when NASA rejected a proposal for a dedicated mission to Halley's Comet. Working with NASA's Robert W. Farquhar, Scarf found a path to repurpose a spacecraft that had, as they put it, already outlived its usefulness. Scarf later told a reporter, "We stole it," referring to the bureaucratic audacity of redirecting a functioning government satellite without a formal new mission program.

  On the 10th of June 1982, ISEE-3 fired its engines and left its halo orbit. What followed was a complex choreography: a series of passages looping through Earth's magnetotail, threading between the planet and the Sun-Earth Lagrange point again and again. Fifteen propulsive maneuvers were spread across more than a year. Five times, the spacecraft skimmed past the Moon and borrowed a slice of lunar gravity to bend its trajectory.

  The final lunar pass, on the 22nd of December 1983, brought the spacecraft just 119.4 kilometers above the lunar surface. That close approach was the last gravitational nudge needed to eject it from the Earth-Moon system entirely. After that pass, the spacecraft was officially renamed the International Cometary Explorer. Its new target was comet Giacobini-Zinner, and its new purpose was to prove that a comet sends out shock waves as it travels through space.

• 4. Giacobini-Zinner and Halley

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  On the 11th of September 1985, ICE passed through the plasma tail of comet Giacobini-Zinner at a distance of approximately 7,800 kilometers from the nucleus. No spacecraft had ever visited a comet before. The probe confirmed that the comet does indeed push a bow shock ahead of itself as it moves through the solar wind, a finding that changed how scientists understood cometary physics.

  Less than six months later, ICE got a second comet encounter at no additional cost. In late March 1986, while the so-called Halley Armada of spacecraft, including Giotto, Vega 1, Vega 2, Suisei, and Sakigake, completed their early-March approaches to Comet Halley, ICE transited between the Sun and the comet from a much greater distance. Its minimum distance to Halley's nucleus was 28 million kilometers, a figure that puts it in perspective when compared to the Earth's own closest approach to that comet in 1910, which was 20.8 million kilometers.

  ICE was not designed as a cometary probe, and its distant Halley flyby was an opportunistic bonus rather than a designed encounter. Still, its data added a thread to the larger tapestry of observations gathered that spring. A heliospheric mission update approved by NASA in 1991 kept the spacecraft operating, focusing on coronal mass ejections and cosmic ray studies in coordination with the Ulysses probe. By May 1995, ICE was running on a low-duty cycle, with some analytical support coming from the Ulysses project team.

• 5. Shutdown and a Lingering Signal

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  On the 5th of May 1997, NASA ended the ICE mission, leaving only a carrier signal broadcasting into space. The downlink bit rate tells the story of a spacecraft slowly fading: it had started at 2,048 bits per second, dropped to 1,024 bits per second during the Giacobini-Zinner encounter, then continued falling: 512 bits per second on the 9th of December 1985, 256 on the 5th of January 1987, 128 on the 24th of January 1989, and finally 64 bits per second on the 27th of December 1991. NASA donated the craft to the Smithsonian Museum, though it remained in a 355-day heliocentric orbit with an aphelion of 1.03 AU and a perihelion of 0.93 AU.

  In September 2008, NASA used the Deep Space Network, with help from the company KinetX, to locate and check on the spacecraft. The status check produced a surprise: 12 of its 13 experiments were still functioning. The probe still carried enough propellant for 150 meters per second of delta-v. Scientists began discussing the possibility of reactivating it in 2014, when the spacecraft would again swing close to Earth, and potentially sending it to observe more comets in 2017 or 2018. The carrier signal that NASA had left running since 1997 had, in an indirect way, kept the option open.

• 6. The Reboot Project

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  In April 2014, a group of engineers, programmers, and scientists formally announced the ISEE-3 Reboot Project. Their stated plan was to command the spacecraft to fire its engine, steer it into an orbit near Earth, and then restart its original science mission, sharing all incoming data through crowd-sourcing. The effort had a hard deadline: the orbit correction needed to happen by late May or early June 2014, or at the latest in early July using more fuel, to allow the Moon's gravity to pull the craft into a useful halo orbit.

  A complication emerged immediately. The Goddard Space Flight Center confirmed that the NASA Deep Space Network hardware built to transmit signals to the spacecraft had been decommissioned in 1999 and was too costly to rebuild. The reboot team solved this by finding the original technical documentation and replicating the modulator and demodulator electronics in software, using software-defined radio techniques and open-source programs from the GNU Radio project. They installed an off-the-shelf SDR transceiver and a power amplifier on the 305-meter Arecibo dish antenna on the 19th of May 2014.

  On the 15th of May 2014, the project reached its crowdfunding goal of US$125,000 on RocketHub, covering the cost of software development, archival research, and dish antenna time. A stretch goal of US$150,000 was also met, with a final total of US$159,502 raised. NASA, though not funding the effort, provided advisors and on the 21st of May 2014 signed a Non-Reimbursable Space Act Agreement with the project, described at the time as the first such agreement for a spacecraft the agency no longer planned to use.

  On the 29th of May 2014, the team successfully commanded ICE to switch into Engineering Mode and broadcast telemetry. On the 2nd of July 2014, the thrusters fired for the first time since 1987. Six days later, on the 8th of July, a longer thruster sequence failed. The nitrogen gas needed to pressurize the fuel tanks appeared to be gone. On the 24th of July 2014, the team officially announced that all attempts to change the spacecraft's orbit had failed. On the 10th of August 2014, at 18:16 UTC, ICE passed about 15,600 kilometers from the surface of the Moon and entered a heliocentric orbit. Contact was lost on the 16th of September 2014, with the probe's exact position uncertain enough that no one could say whether it would be possible to reach it again. The spacecraft is expected to return to the vicinity of Earth in 2031.