One of NASA's orbiting sentinels is expected to return to Earth in a
few days. The agency's Ice, Cloud, and land Elevation (ICESat)
satellite completed a very productive scientific mission earlier this
year. NASA lowered the satellite's orbit last month and then
decommissioned the spacecraft in preparation for re-entry. It is
estimated that the satellite will re-enter the Earth's atmosphere and
largely burn up on or about August 29.
ICESat was launched in January 2003, as a three-year mission with a
goal of returning science data for five years. It was the first mission
of its kind –specifically designed to study Earth's polar regions with
a space-based laser altimeter called the Geoscience Laser Altimeter
System, or GLAS...
ICESat's lasting legacy will be its impact on the understanding of ice
sheet and sea ice dynamics. The mission has led to scientific advances
in measuring changes in the mass of the Greenland and Antarctic ice
sheets, polar sea ice thickness, vegetation-canopy heights, and the
heights of clouds and aerosols. Using ICESat data, scientists
identified a network of lakes beneath the Antarctic ice sheet. ICESat
introduced new capabilities, technology and methods such as the
measurement of sea ice freeboard – or the amount of ice and snow that
protrudes above the ocean surface - for estimating sea ice thickness.
"ICESat has been a tremendous scientific success," said Jay Zwally,
ICESat's project scientist at NASA's Goddard Space Flight Center in
Greenbelt, Md. "It has provided detailed information on how the Earth's
polar ice masses are changing with climate warming, as needed for
government policy decisions. In particular, ICESat data showed that the
Arctic sea ice has been rapidly thinning, which is critical information
for revising predictions of how soon the Arctic Ocean might be mostly
ice free in summer. It has also shown how much ice is being lost from
Greenland and contributing to sea level rise. Thanks to ICESat we now
also know that the Antarctic ice sheet is not losing as much ice as
some other studies have shown."
The End of an Era
After seven years in orbit and 15 laser-operations campaigns, ICESat's
science mission ended in February 2010 with the failure of its primary
instrument. Because the spacecraft remained in operating condition,
NASA's Science Mission Directorate accepted proposals for engineering
tests to be performed using ICESat. These tests were completed on June
20. NASA's Earth Science Division then authorized the decommissioning
of ICESat. After completing a review of decommissioning activities, the
agency directed that ICESat be decommissioned by this August.
Mission flight controllers began firing ICESat's propulsion system
thrusters on June 23 to lower its orbit. Thruster firings ended on July
14, safely reducing the lowest point of the spacecraft's orbit to 125
miles (200 km) above Earth's surface. The orbit has since naturally
decayed. ICESat was successfully decommissioned from operations on Aug.
14. All remaining fuel on the spacecraft is now depleted, and
atmospheric drag is slowly lowering ICESat's orbit until the spacecraft
re-enters the Earth's atmosphere.
A statement from the Earth Science Mission Operations office summarized the achievement:
"The ICESat mission operations team is commended for its exceptional
performance, working tirelessly for the past eleven years (four years
of preparation and seven years of operations), overcoming several
obstacles in the early years of the mission, and closing out the
mission with a flawless series of orbital maneuvers before final
decommissioning. The positive control maintained over the mission right
to the end shows the quality and effort that went into designing,
building, qualifying, launching, and operating a tremendously
successful mission such as ICESat."
The Return to Planet Earth
The vast majority of ICESat
will burn up in the atmosphere during re-entry. Of the spacecraft's
total mass (about 2000 lbs.), only a small percent will reach the
surface of Earth. Some pieces of the spacecraft, weighing collectively
about 200 pounds, are expected to survive re-entry. The risk of harm
coming to anyone on Earth from this debris is estimated to be very low.
ICESat was not designed to perform a controlled re-entry and is unable
to provide targeting to a particular location on Earth. ICESat circles
the Earth from pole to pole, so surviving debris could land almost
anywhere on the planet. Due to natural variability in the near-Earth
environment, a precise location of where spacecraft debris will
re-enter cannot be forecast. The U.S. Space Surveillance Network is
closely monitoring the orbit of ICESat during its final days and will
continue to issue periodic predictions of re-entry time and location.
The NASA Orbital Debris Program Office will issue re-entry information
based on these predictions.
NASA and international standards for space objects re-entering Earth's
atmosphere do not require controlled re-entry but do have requirements
and guidelines for the maximum risk posed by debris surviving re-entry.
"The ICESat team has done a marvelous job to ensure that the spacecraft
is removed as a hazard to other spacecraft and as a potential source of
future orbital debris," said Nicholas L. Johnson, NASA Chief Scientist
for Orbital Debris at NASA's Johnson Space Center in Houston.
The Future Looks Bright
Despite the end of ICESat's mission, NASA's observations of Earth's
polar regions continue. In anticipation of the ICESat mission coming to
an end, and in accordance with the National Research Council's Decadal
Survey of future NASA Earth science missions, NASA has begun
development of ICESat-2, planned for launch in 2015. ICESat-2 will
continue the science legacy of its predecessor, and improve our
understanding of Earth's dynamic polar regions with new and advanced
technology.
The Operation Ice Bridge airborne mission, started in 2009, is the
largest airborne survey of Earth's polar ice ever flown. The mission is
designed to partially fill the data gap between the ICESat and ICESat-2
satellite missions. For the next five years, instruments on NASA
aircraft will target areas of rapid change to yield an unprecedented
3-D view of Arctic and Antarctic ice sheets, ice shelves, and sea ice.
Targeted information from aircraft combined with the broad and
consistent coverage from satellites contribute to a more complete
understanding of Earth's response to climate change, helping scientists
make better predictions of what the future might hold.
