ICE: The First Comet Flyby

In recent years it seems that NASA regularly extends the missions of its long-lived planetary spacecraft sometimes far beyond their original primary missions. The armada of NASA spacecraft roving across the surface and orbiting high above Mars have had their missions regularly extended for a decade or more. The Cassini mission currently orbiting Saturn has had its original four-year primary mission, which ended in 2008, extended repeatedly and now will not end until 2017 when the spacecraft will be intentionally destroyed by entering Saturn’s atmosphere.

In addition to extending missions at their targets, NASA has also redirected existing spacecraft with plenty of life still left in them to new targets. In the last decade, NASA redirected its Deep Impact spacecraft, which had completed its primary mission to comet 9P/Temple 1 in 2005, towards a 2010 encounter with comet 103P/Hartley 2 under the new mission name EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation). Likewise, NASA’s Stardust mission, which had returned samples of comet dust after its encounter with comet 81P/Wild 2 in 2003, was directed to reobserve Comet Temple 1 in 2011 and the crater Deep Impact left as part of its extended NExT (New Exploration of Temple 1) mission.

A closeup view of the four-kilometer long active nucleus of Comet Hartley 2 as seen during the EPOXI encounter on November 4, 2010. (NASA/JPL)

Recycling an existing spacecraft that has already completed its original mission makes good sense for a number of reasons. The spacecraft has already been designed, built, and launched in the course of its primary mission. These are typically the most expensive parts of any mission. For the price of a little on-board propellant needed to nudge a craft into a new trajectory as well as a modest amount of additional funding for mission operations and science, an existing spacecraft can sometimes be sent to another target of interest. In the case of EPOXI, for example, that amounted to $40 million for the new mission to Hartley 2 – a significantly reduced price tag compared to the hundreds of millions of dollars a new, dedicated mission would have cost. Deep Impact was certainly not the first spacecraft to be reused by NASA for a comet mission. Coincidentally, the first spacecraft to have a close encounter with a comet three decades ago, called ICE (International Cometary Explorer), was itself a recycled spacecraft that had already completed its original mission much closer to home.

The Original Mission

ICE started its existence as the third spacecraft in the ISEE (International Sun-Earth Explorer) program. The purpose of this cooperative program between NASA and ESA (European Space Agency) was to observe the interaction between the Earth’s magnetosphere and the Sun-dominated interplanetary environment. The first pair of satellites in the ISEE series— one supplied by NASA and the other by ESA— were launched together by a single American Delta 2914 rocket on October 22, 1977, into an elongated 340 by 137,900 kilometer Earth orbit. This pair of co-orbiting satellites, whose separation could be varied as needed to meet its science objectives, allowed observations to be made simultaneously from two points inside the Earth’s magnetosphere.

Diagram showing the ISEE (International Sun-Earth Explorer) constellation whose objective was to study Earth’s magnetosphere and its interaction with the Sun.

The American-built ISEE 3 was launched by a Delta 2914 on August 12, 1978, but instead of entering Earth orbit like its predecessors, it was placed into a halo orbit about the L1 Sun-Earth Lagrange point 1.5 million kilometers from the Earth towards the Sun—the first spacecraft to take up station there. From this unique vantage point, ISEE 3 could monitor the properties of the solar wind about an hour before it reached Earth’s magnetosphere where ISEE 1 and 2 were making their measurements. This allowed scientists to determine if various observed phenomena were caused by the Sun or by activities in Earth’s magnetosphere.

The launch of NASA’s ISEE 3 spacecraft on August 12, 1978 using a Delta 2914 rocket. (NASA)

Managed by NASA’s Goddard Space Flight Center (GSFC), ISEE 3 was a spin-stabilized spacecraft with a mass of 478 kilograms at launch. It was a cylinder 1.7 meters in diameter and 1.6 meters tall that spun at a rate of 20 rpm. Its outer surface was covered with solar cells that supplied the craft with 160 watts of power. It had two 3-meter booms that carried magnetometer and plasma wave sensors, and four antennas spanning 91 meters for radio and plasma wave studies. Sensors to determine plasma composition as well as measure protons, electrons, cosmic rays, X-rays, and gamma rays rounded out the suite of 13 instruments on ISEE 3. The spacecraft was efficiently tailored for its mission which, in concert with its sister satellites, was officially completed in 1981.

A Comet Mission

In the mean time, NASA had been considering a series of proposals for a spacecraft to be sent to the most famous comet of all, Comet Halley, during its long-anticipated swing through the inner solar system in 1986. Highly advanced and very expensive concepts involving novel propulsion methods such as solar sails and ion engines were considered, along with some more conventional approaches. In the end the expense of these proposals and the skyrocketing costs of NASA’s other programs, including the then-new Space Shuttle, combined with the cost-cutting philosophy of the Reagan Administration, would doom a dedicated NASA mission to Comet Halley. Instead, an armada of spacecraft from the Soviet Union, ESA, and Japan would make the once-in-a-lifetime trip to the famous comet without a dedicated, NASA-sponsored mission (see “The Missions to Comet Halley“).

A diagram showing the major components of NASA’s ISEE 3/ICE spacecraft. Click on image to enlarge. (NASA)

But there were still other options available. Since the early 1970s a team led by GSFC scientist Robert Farquhar had been studying the possibility of redirecting ISEE 3 towards a comet using a novel and fuel-saving series of maneuvers involving multiple flybys of the Moon. During the course of these long loops through the Earth-Moon system, it would also be possible for ISEE 3 to study the geotail, the largely unexplored tail of Earth’s magnetosphere. Although by the end of its primary mission the battery on ISEE 3 had died, most of its instruments were still fully functional and it even had 75 percent of its original 91-kilogram hydrazine propellant supply left. The probe had plenty of useful life left in it and, even though it was not equipped with cameras and dust detectors, it could still make very useful measurements of the environment around a comet.

Unfortunately it would prove to be impossible to send ISEE 3 close to Comet Halley while still keeping the probe near enough to the Earth so that usable data could be received via its relatively weak 5-watt transmitter. But another mission to a closer target was identified: A mission to the smaller and less active short-period comet 21P/Giacobini-Zinner which would cost a very modest $3 million dollars (about $7 million in today’s dollars). Comet Giacobini-Zinner, which was discovered in 1900 by French astronomer Michel Giacobini and subsequently recovered in 1913 by German astronomer Ernst Zinner, could be reached during its passage through the ecliptic plane when it was 70 million kilometers from Earth which was still within reasonable communication range. By chance, ISEE 3 would reach the comet a half a year before the international armada of spacecraft was scheduled to encounter Comet Halley in March 1986 so the new mission would make the world’s first comet flyby. As an added bonus, the probe could still make a distant (0.21 AU, or 31 million kilometers) pass of Comet Halley while the international armada was in its vicinity, allowing some useful data to be gathered from a unique vantage point.

This diagram shows the complicated trajectory of ICE in the Earth-Moon system from its initial orbit at the L1 point to Comet Giacobini-Zinner. Click on image to enlarge. (NASA)

The new target could be reached after 18 months of long, looping orbits around the Earth-Moon system combined with 37 propulsive maneuvers and five flybys of the Moon, including a potentially dangerous close flyby of the Moon through its shadow. Because of its dead battery, ISEE 3 would have no power for 28 minutes during this final lunar encounter and there was some danger that its hydrazine propellant supply would freeze, causing a mission-ending rupture in the probe’s plumbing.

On June 10, 1982, controllers at GSFC fired the thrusters on ISEE 3 to leave the L1 point and head to its new mission in the geotail. In October NASA officially approved the subsequent mission to Comet Giacobini-Zinner and the probe was rechristened the International Cometary Explorer (ICE). On December 22, 1983 ICE, flew to within 120 kilometers of the Moon and survived unscathed. ICE was now on its way into interplanetary space and towards Comet Giacobini-Zinner.

An artist’s depiction of ICE during its last lunar flyby on December 22, 1983 that sent it into solar orbit. (NASA)

The Comet Encounter and Beyond

As ICE approached its target, there was concern that the probe would not survive the 20.6-kilometer-per-second encounter. Like any comet, Giacobini-Zinner emitted dust and is the source of the Giacobinid (also known as the Draconid) meteor shower which peaks around October 8 each year. This stream of dusty debris is quite clumpy and was responsible for spectacular meteor storms observed in 1933 and 1946. Unlike a purpose-built comet probe, ICE had no protection against this hazard. Since ICE had no tape recorder and had to transmit all of its data live, a modest-sized dust strike could knock the spinning probe’s antenna out of alignment, resulting in the loss of data. It was also feared that dust could coat or otherwise affect the solar panels, robbing ICE of power. To decrease the likelihood of a potential power supply undervoltage, controllers would shut down the hydrazine propellant heater and four of the 13 instruments to conserve power. In the worst case, a dust strike of sufficient size could disable the spacecraft entirely. Going into the encounter the mission’s flight director, Robert Farquhar, put the chances of success at just 50 percent.

ICE made a course correction in early August 1985 to ensure that it would pass through the tail of Comet Giacobini-Zinner about 10,000 kilometers downstream of the nucleus. A final course correction had been scheduled for September 4 but ground controllers postponed it by a few days to get a better handle on the comet’s orbit. As Comet Giacobini-Zinner approached perihelion on September 5 1.038 AU from the Sun, increased activity had changed its orbit so that ICE would now pass 7,800 kilometers from the nucleus missing the tail’s center line. Since a large maneuver to restore the originally planned aim point was deemed too risky, on September 8 controllers commanded ICE to perform a smaller 2.3 meter-per-second maneuver to correct only the cross-tail aiming error while maintaining the closer 7,800 kilometer miss distance with an estimated 600 kilometer navigation error.

An Earth-based telescopic image of Comet Giacobini-Zinner. The streaks are background stars smeared by the long exposure tracking the moving comet. (NASA)

In the end Giacobini-Zinner proved to be less dusty at the time of the encounter than feared and the mission was a success. ICE passed 7,862 kilometers from the two-kilometer in diameter nucleus of Comet Giacobini-Zinner at 11:02 GMT on September 11, 1985. Its instruments had detected the comet’s bow shock at a distance of 127,000 kilometers about two hour and a half hours before this and spent about 20 minutes traversing the comet’s 25,000-kilometer wide plasma tail. These first in-situ measurements of a comet gave scientists the information they needed to fine-tune their models in time to tweak their plans for the upcoming encounters with Comet Halley.

After its distant encounter upstream of Comet Halley on March 28, 1986, ICE continued making scientific observations in its independent 355-day solar orbit in which it gradually drifted farther from the Earth. Expected to return to the vicinity of Earth in 2014, when it could potentially be recovered, NASA donated ICE to the Smithsonian Institution. In 1991 NASA approved an updated heliospheric mission for ICE in coordination with ground-based observations and the ESA Ulysses probe. Because of its increasing distance, which severely limited how much data could be returned, on May 5, 1997, NASA officially ended the ICE mission and shut down the spacecraft leaving only its transmitter carrier signal beacon operating. In 1999 contact was briefly made with ICE to verify that it was still transmitting.

This diagram shows the orbit of ICE relative to a fixed-Earth-Sun frame of reference. ICE slowly looped counterclockwise away from the Earth after leaving in December 1983 and eventually approached it once again in August 2014. Click on image to enlarge.

On September 18, 2008, ICE, which had finally begun drifting closer to the Earth, was located and successfully reactivated. It was found that all but one of its 13 instruments were still functioning and enough propellant appeared to remain for a velocity change of 150 meters per second. There seemed to be enough life left in the old probe to perform more useful science. Expected to return to the Earth-Moon system in August 2014, a NASA team led by Robert Farquhar started considering several options for the future of ICE, including redirecting it towards additional comet encounters in 2017 or 2018. There were even hopes that still other missions would be possible for this robust, reused spacecraft before it once again drifted back into interplanetary space and subsequently returned to the vicinity of the Earth perhaps in the 2040s.

Unfortunately, these plans to reuse ICE were not meant to be. The ground-based hardware needed to communicate with ICE was no longer available at NASA’s Deep Space Network and it was deemed too expensive to replace. With no official interest at NASA to revive the aging spacecraft, in April 2014 a privately funded ISEE 3 Reboot Project was started. Using online crowdfunding, the group raised about $160,000 for their project and were able to recontact ICE on May 29 – eight days after NASA officially handed over control of the spacecraft to the Reboot Team. While initial tests showed the 36-year old spacecraft to be in good shape, a failed attempt to test the propulsion system on July 8 revealed that there was no longer sufficient nitrogen gas left to keep the propellant tanks pressurized. This ended all hopes of redirecting ICE into a new trajectory for any new mission.

ISEE 3 (later known as ICE) undergoing ground testing prior to its launch in 1978. It is now in solar orbit and unlikely to be reactivated. (NASA)

ICE flew 15,600 kilometers above the lunar surface at 18:16 GMT on August 10, 2014 and back into solar orbit. The Reboot Team lost contact with ICE on September 16 with the spacecraft expected to return to the vicinity of the Earth around 2030. Unfortunately, the new orbit for ICE after its encounter with the Earth and Moon is too uncertain to predict precisely where it will be after 17 years. In addition, ICE’s new solar orbit will carry it farther from the Sun than before reducing the power levels to the point where it is expected to enter a safe mode. It now seems unlikely that ICE will ever be heard from again. Despite this apparent set back, ICE was still a highly successful mission that far exceeded expectations.

Follow Drew Ex Machina on Facebook.

General References

J. Kelly Beatty, “Comet G-Z: The Inside Story”, Sky & Telescope, Vol. 70, No. 5, pp. 426-427, November 1985

Stephen P. Marin, “On the Trail of Comet G-Z”, Sky & Telescope, Vol. 70, No. 3, pp. 198-203, September 1985

Paolo Ulivi with David Harland, Robotic Exploration of the Solar System Part 2: Hiatus and Renewal 1983-1996, Springer-Praxis, 2009

Tycho T. von Rosenvinge, John C. Brandt and Robert W. Farquhar, “The International Cometary Explorer Mission to Comet Giacobini-Zinner”, Science, Vol. 232, No. 4748, pp. 353-356, April 18, 1986