Friday, January 29, 2010

I like ICE

The International Cometary Explorer, that is.

I want to celebrate this remarkable mission that IMHO deserves more attention than it gets for the many firsts it achieved:

  • First spacecraft in a halo orbit at a Lagrange point (Earth-Sun L1).
  • First to detect the solar wind approaching Earth
  • First spacecraft to utilize the instability of the Earth-Moon and Earth-Sun Lagrange points for orbital maneuvering.
  • First comet flyby.
  • Maybe the first interplanetary spacecraft that returns to Earth without that being the plan from the outset.
and for the simple fact that it was launched in 1978 and is still alive!

ICE started out as ISEE-3, or International Sun-Earth Explorer 3, one of a set of three missions launched in the late 1970's to investigate the Sun/Earth interactions at the boundary of Earth's magnetosphere and the solar wind.

The other two satellites ISEE-1, build by NASA, and ISEE-2, build by ESA, were launched in 1977 and orbited the Earth in almost identical eccentric orbits with a period of 57 hours. ICEE-3, however, was launched into an elliptical halo orbit around the Earth-Sun L1 Lagrange point about 1.5 million kilometer or 235 Earth-radii from Earth towards the Sun and was the first ever spacecraft to do so. The observations of the three spacecraft then allowed to map the effects of the solar wind on Earth's magnetosphere.

The spacecraft carried a set of complimentary instruments to observe plasmas, energetic particles, waves, and fields emitted by the sun and interacting with the terrestial magnetosphere. This made ISEE-3 the first explorer to observe solar weather as it approached Earth in near interplanetary space. The instruments onboard included an x-ray spectrometer from the University of California and the Goddard gamma-ray burst spectrometer, which represented the first successful flight of a high purity germanium detector on a satellite (see the mission homepage for details, I'm just quoting this verbatim here). It provided an order of magnitude improvement in the measurement of spectral properties of gamma-ray bursts than any previously flown detector. The mission did not, however, require an imaging instrument (what laypersons like me would call "camera").

With these capable high-energy spectrometers ISEE-3 also served as part of the Interplanetary Network (IPN), an effort to locate the origins of gamma-ray bursts by triangulation trough long baseline interferometry. This network consisted of multiple spacecraft equipped with gamma ray detectors widely spaced across the solar system to provide the necessary baseline for precisely measurement of the point of origin of each captured gamma-ray burst. The first such network included apart from ISEE-3 the solar probe Helios-2, the Venus probes Pioneer Venus and it's Soviet counterparts Venera 11 and 12, as well as Prognoz-7 in Earth orbit, thus completing the Sun-Venus-Earth triangle necessary for directional measurements. This network detected 83 bursts until its end in 1980. This represents the first time a scientific instrument was implemented that reached across the whole inner solar system, and so could be chalked up as another first for ISEE-3.

The latest incarnation of the IPN, the third of its kind and therefore called IPN3, began with the launch of the Ulysses spacecraft with an instrument suite similar to that of ISEE-3, but in a polar orbit around the sun. It remained the centerpiece of the IPN until its mission ended in 2009, in itself quite an amazing story. Today this role is filled by the Swift satellite that provides a 24/7 360° gamma-ray burst watch.

ISEE-3/ICE trajectory from launch to comet flybys.

After completion of the ISEE mission in 1982 NASA decided to utilize the spacecraft as a cometary explorer. To do so required getting the spacecraft from its L1 halo orbit into a heliocentric orbit. This was only possible by maneuvering through the Earth-Moon and Earth-Sun Lagrange points, which due to the balance of gravitational forces allow the change of orbits with only small velocity changes. This is bought with lots of time; as you can see in the trajectory plot above, a complex set of maneuvers around the Earth-Moon system with five lunar flybys was necessary to do the job. These maneuvers also enabled several passes through Earth's magnetotail. The last Moon flyby at an altitude of 119.4 km above the lunar surface in December 1983 ejected the spacecraft from Earth's gravity and sent it on its way to become the first ever comet interceptor. At this time it was renamed ICE or the International Cometary Explorer.

The spacecraft passed through the plasma tail of Comet Giacobini-Zinner in September 1985, which constituted the first in-situ investigation of cometary matter, for which ICE - equipped with a suite of instruments to investigate fields and plasma - was very well suited. It repeated the feat at the plasma tail of Comet Halley in March 1986 at a distance of 28 million km (about 0.2 AU). At the same time many other spacecraft — Giotto, Vega 1, Vega 2, Suisei and Sakigake — were also close to Comet Halley. See the International Halley Watch archive for details of those encounters.

In 1991 ICE began yet another phase of its long life with investigations of solar coronal mass ejections in coordination with ground-based observations, continued cosmic ray studies, and special period observations such as when ICE and Ulysses were on the same solar radial line. This mission continued until 1997, when NASA decided to end it and shut the spacecraft down, apart from a simple carrier signal.

But then in 2008, NASA locked onto the signal via the Deep Space Network and determined that all but one of its instruments are still working and so the spacecraft could still be used for yet another comet flyby in 2017 or 2018.

The plot below shows ICE' orbit around the sun since 1983 relative to Earth. It is in a slightly elliptical orbit at 0.93-1.03 AU with a period of 355 days. The elliptical shape means that it is sometimes slower and sometimes faster than Earth, resulting in the spiral patterns marking its yearly progress. As you can see, ICE is slowly catching up to Earth and will be returning home in 2014.

ICE trajectory since leaving the Earth/Moon system in 1983, plotted relative to a fixed Sun/Earth-line (from here).

So in conclusion, we have a spacecraft that is almost as old as the Voyagers, still alive, and it's coming back. NASA has still to decide what to do with it; the options are recapture in 2014 into a parking orbit or use a Moon-flyby to send it on yet another comet mission, pushing the opportunity for capture to the 2040s. I for one feel that it would be awesome to see ICE continue its long and amazing life and do more science, even at 64 bits/s (last measured data rate in 1991).

And here's my own graphical contribution to this post, showing the current position of ICE following Earth about 55° behind, along with all other active missions currently on heliocentric orbits in the inner solar system. Note that SOHO and ACE occupy the same halo orbit at the Sun/Earth L1 that ICE had in its first life as ISEE-3. See this earlier post for the extended version of the chart.

Probes in the inner solar system, current locations. See here for details. [larger image]

Mission summary

Phase Dates Duration Main Objectives
Transfer to halo insertion 1978-08-12 to 1978-11-20 100 days
Halo Orbit Phase 1978-11-20 to 1982-09-01 3 years, 9 months Sun and Earth-magnetosphere observation
Gamma-ray burst detection
Cosmic ray detection
Extended Mission 1982-09-01 to 1983-12-22 1 year, 3.7 months
Comet Intercept Mission 1983-12-22 to 1985-09-11 1 year, 8.7 months Flybys of comets Giacobini-Zinner and Halley
Heliocentric Cruise 1985-09-11 to 1997-05-05 11 years, 8 months Solar coronal mass ejection observation
coordinated obsevations with the Ulysses probe
Hibernation 1997-05-05 to 2014-08-10 17 years, 3 months
The Future™ 2014-08-10 to ? Return to Earth
Capture or further comet flybys

"It's Alive!" Planetary Society Blog post about ICE
ISEE project page NASA NSSDC
ISEE3/ICE mission profile NASA Solar System Exploration HP
HEASARC ISEE3/ICE mission profile
International Halley Watch (IHW)
The Third Interplanetary Network (IPN3) Current IPN Status

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