ARTEMIS Spacecraft Prepare for Lunar Orbit
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The view from above of the ARTEMIS orbits as they make the transition from the kidney-shaped Lissajous orbits on either side of the moon to orbiting around the moon.
ACredit: NASA/Goddard Space Flight Center
They've almost arrived.
It took one and a half years, over 90 orbit
maneuvers, and – wonderfully – many gravitational boosts and only the barest bit
of fuel to move two spacecraft from their orbit around Earth to their new home
around the moon.
Along their travels, the spacecraft have been through
orbits never before attempted and made lovely curlicue leaps from one orbit to
the next. This summer, the two ARTEMIS spacecraft - which began their lives as
part of the five-craft THEMIS mission studying Earth's aurora – will begin to
orbit the moon instead. THEMIS is an acronym for the Time History of Events and
Macroscale Interaction during Substorms spacecraft.
Even with NASA's
decades of orbital mechanics experience, this journey was no easy feat. The trip
required several maneuvers never before attempted, including several months when
each craft moved in a kidney-shaped path on each side of the moon around, well,
nothing but a gravitational point in space marked by no physical planet or
object.
"No one has ever tried this orbit before, it's an Earth-moon
libration orbit," says David Folta a flight dynamics engineer at NASA's Goddard
Space Flight Center in Greenbelt, Md. "It's a very unstable orbit that requires
daily attention and constant adjustments."
The journey for ARTEMIS -
short for Acceleration, Reconnection, Turbulence and Electrodynamics of the
Moon's Interaction with the Sun - began in 2009, after THEMIS had completed some
two years of science data collection on the magnetic environment around Earth,
the aurora, and how these are affected by the sun.
The spacecraft are
solar-powered, but orbits for the two outermost THEMIS spacecraft had slipped
over time and were going to be subjected to regular eight-hour periods of
darkness. These spacecraft could withstand up to three hours without sunlight,
but this much darkness would soon leave the batteries completely
discharged.
Teams at UC-Berkeley and Goddard handled the day-to-day
control of the THEMIS spacecraft. The Principal Investigator for the mission,
Vassilis Angelopoulos of UCLA talked to the teams about moving the two
spacecraft to the moon to study the magnetic environment there. But quick models
of a conventional boost technique showed that all the remaining fuel would be
used simply in transit. There wouldn't be enough left over for the fuel-hungry
process of adjusting direction and speed to actually begin circling the
moon.
So Angelopoulos pulled together a new, more complex multi-year-long
orbit change plan. The move would rely predominantly on gravity assists from the
moon and Earth to move the spacecraft into place. He brought his idea to two
engineers who had been involved with launching THEMIS in the first place: David
Folta and another flight engineer at Goddard, Mark Woodard. The pair used their
own models to validate this new design, and the plan was on.
First step:
increase the size of the orbits. The original Earth-centric orbits barely
reached half way to the moon. By using small amounts of fuel to adjust speed and
direction at precise moments in the orbit, the spacecraft were catapulted
farther and farther out into space. It took five such adjustments for ARTEMIS P1
and 27 for ARTEMIS P2.
Next step: make the jump from Earth orbit to the
tricky kidney-shaped "Lissajous" orbit, circling what's known as a Lagrangian
point on each side of the moon. These points are the places where the forces of
gravity between Earth and the moon balance each other – the point does not
actually offer a physical entity to circle around. ARTEMIS P1 made the leap – in
a beautiful arc under and around the moon - to the Lagrangian point on the far
side of the moon on August 25, 2010. The second craft made the jump to the near
side of the moon on October 22. This transfer required a complex series of
maneuvers including lunar gravity assists, Earth gravity assists, and deep space
maneuvers. The combination of these maneuvers was needed not only to arrive at
the correct spot near the moon but also at the correct time and speed.
Using a series of Earth and moon gravity assists –
and only the barest bit of fuel – the ARTEMIS spacecraft entered into orbit
around the moon's Lagrangian points in the winter of 2010.
Image Credit:
NASA Goddard Space Flight
Center/SVS
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History was made. Numerous satellites orbit Lagrangian points between Earth
and the sun but, while this orbit had been studied extensively, it had never
before been attempted.
Not only was this an engineering feat in and of
itself, but the spacecraft were now in an ideal spot to study magnetism some
distance from the moon. In this position, they could spot how the solar wind –
made up of ionized gas known as plasma - flows past the moon and tries to fill
in the vacuum on the other side. A task made complicated since the plasma is
forced by the magnetic fields to travel along certain paths.
"It's a
veritable zoo of plasma phenomena," says David Sibeck, the project manager for
THEMIS and ARTEMIS at Goddard. "The moon carves out a cavity in the solar wind,
and then we get to watch how that fills in. It's anything but boring. There's
microphysics and particle physics and wave particle interaction and boundaries
and layers. All things we haven't had a chance to study before in the
plasma."
Life for the flight engineers was anything but boring too.
Keeping something in orbit around a spot that has little to mark it except for
the balance of gravity is no simple task. The spacecraft required regular
corrections to keep it on track and Folta and Woodard watched it
daily.
"We would get updated orbit information around 9 a.m. every day,"
says Woodard. "We'd run that through our software and get an estimate of what
our next maneuver should be. We'd go back and forth with Berkeley and together
we'd validate a maneuver until we knew it was going to work and keep us flying
for another week."
The team learned from experience. Slight adjustments
often had bigger consequences than expected. They eventually found the optimal
places where corrections seemed to require less subsequent fine-tuning. These
sweet spots came whenever the spacecraft crossed an imaginary line joining Earth
and the moon, though nothing in theories had predicted such a thing.
The
daily vigilance turned out to be crucial. On October 14, the P1 spacecraft orbit
and attitude changed unexpectedly. The first thought was that the tracking
system might have failed, but that didn't seem to be the problem. However, the
ARTEMIS team also noticed that the whole craft had begun to spin about 0.001
revolutions per minute faster. One of the instruments that measures electric
fields also stopped working. Best guess? The sphere at the end of that
instrument's 82-foot boom had broken off – perhaps because it was struck by
something. That sphere was just three ounces on a spacecraft that weighed nearly
190 pounds - but it adjusted ARTEMIS P1's speed enough that had they caught the
anomaly even a few days later they would have had to waste a prohibitive amount
of fuel to get back on course.
An artist concept of the ARTEMIS spacecraft in orbit around the
Moon.
Credit: NASA/Conceptual Image Lab
As it is, ARTEMIS will make it to the moon with even more fuel than
originally estimated. There will be enough fuel for orbit corrections for seven
to 10 years and then enough left over to bring the two craft down to the
moon.
"We are thrilled with the work of the mission planners," says Sibeck. "They
are going to get us much closer to the moon than we could have hoped. That's
crucial for providing high quality data about the moon's interior, its surface
composition, and whether there are pockets of magnetism there."
On
January 9, 2011, ARTEMIS P1 jumped over the moon and joined ARTEMIS P2 on the
side of the moon closest to Earth. Now the last steps are about to
begin.
On June 27, P1 will spiral in toward the moon and enter lunar
orbit. On July 17, P2 will follow. P2 will travel in the same direction with the
moon, or in prograde; P1 will travel in the opposite direction, in
retrograde.
"We've been monitoring ARTEMIS every day and developing
maneuvers every week. It's been a challenge, but we've uncovered some great
things," says Folta, who will now focus his attention on other NASA flights such
as the MAVEN mission to Mars that is scheduled to launch in 2013. "But soon
we'll be done with this final maneuvering and, well, we'll be back to just being
ARTEMIS consultants."
Related Links:
Karen C. Fox
NASA's Goddard Space Flight Center,
NASA's Goddard Space Flight Center,
Greenbelt, Md.
WASHINGTON - Two NASA spacecraft have been assigned a new mission after
successfully completing their original science objectives earlier this year. The
duo began making observations this week to study how solar wind electrifies,
alters and erodes the moon's surface. Data could reveal valuable information for
future explorers and give planetary scientists a hint of what's happening on
other worlds around the solar system.
The new mission is called ARTEMIS, or Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun. ARTEMIS uses two of five in-orbit spacecraft from NASA's THEMIS, or Time History of Events and Macroscale Interactions during Substorms, mission.
"Using two repurposed satellites for the ARTEMIS mission highlights NASA's efficient use of the nation's space assets," said Dick Fisher, director of the Heliophysics Division in NASA's Science Mission Directorate at the agency's headquarters in Washington.
ARTEMIS will measure solar wind turbulence on scales never sampled by previous missions. Solar wind is a stream of charged particles emitted from the upper atmosphere of the sun.
"ARTEMIS will provide a unique two-point view of the moon's under-explored space environment," said Vassilis Angelopoulos of the University of California in Los Angeles (UCLA), principal investigator of the THEMIS mission. "These two spacecraft are headed for an incredible new adventure."
One ARTEMIS spacecraft reached what is called the L2 Lagrange point on the far side of the moon on Aug. 25. On Oct. 22, the other spacecraft entered the L1 Lagrange point on the Earth-side of the moon. Lagrange points are places where the gravity of Earth and moon balance, creating a sort of gravitational parking spot for spacecraft. NASA repositioned the two outermost THEMIS spacecraft using spare on-board fuel and a set of complex orbit maneuvers over the course of more than a year.
"ARTEMIS is going where no spacecraft have gone before," said Manfred Bester, Mission Operations manager from the University of California at Berkeley, where the spacecraft are operated. "We are exploring the Earth-Moon Lagrange points for the first time."
After six months at the Lagrange points, ARTEMIS will move closer to the moon. The spacecraft will be approximately 62 miles from the surface at first, but will eventually move closer. From point-blank range, the spacecraft will look to see how the solar wind impacts a rocky world when there's no magnetic field to protect it. Earth is protected from solar wind by its magnetic field. However, the moon is exposed because it has no global magnetism.
The ARTEMIS mission is a joint effort among NASA's Goddard Space Flight Center in Greenbelt, Md.; NASA's Jet Propulsion Laboratory in Pasadena, Calif.; the Space Sciences Laboratory at Berkeley; and UCLA.
Launched in 2007, THEMIS was NASA's first five-satellite mission launched aboard a single rocket. The unique constellation of satellites provided scientists with data to help resolve the mystery of how Earth's magnetosphere stores and releases energy from the sun by triggering geomagnetic substorms. The three remaining THEMIS satellites continue to study substorms that are visible in the Northern Hemisphere as a sudden brightening of the Northern Lights, or aurora borealis.
The mission was one of NASA's series of low-cost, rapidly developed missions in the Explorers Program. ATK, formerly Swales Aerospace, in Beltsville, Md., built the THEMIS probes. Goddard manages the program for the agency's Science Mission Directorate.
For more information about ARTEMIS, visit:
The new mission is called ARTEMIS, or Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun. ARTEMIS uses two of five in-orbit spacecraft from NASA's THEMIS, or Time History of Events and Macroscale Interactions during Substorms, mission.
"Using two repurposed satellites for the ARTEMIS mission highlights NASA's efficient use of the nation's space assets," said Dick Fisher, director of the Heliophysics Division in NASA's Science Mission Directorate at the agency's headquarters in Washington.
ARTEMIS will measure solar wind turbulence on scales never sampled by previous missions. Solar wind is a stream of charged particles emitted from the upper atmosphere of the sun.
"ARTEMIS will provide a unique two-point view of the moon's under-explored space environment," said Vassilis Angelopoulos of the University of California in Los Angeles (UCLA), principal investigator of the THEMIS mission. "These two spacecraft are headed for an incredible new adventure."
One ARTEMIS spacecraft reached what is called the L2 Lagrange point on the far side of the moon on Aug. 25. On Oct. 22, the other spacecraft entered the L1 Lagrange point on the Earth-side of the moon. Lagrange points are places where the gravity of Earth and moon balance, creating a sort of gravitational parking spot for spacecraft. NASA repositioned the two outermost THEMIS spacecraft using spare on-board fuel and a set of complex orbit maneuvers over the course of more than a year.
"ARTEMIS is going where no spacecraft have gone before," said Manfred Bester, Mission Operations manager from the University of California at Berkeley, where the spacecraft are operated. "We are exploring the Earth-Moon Lagrange points for the first time."
After six months at the Lagrange points, ARTEMIS will move closer to the moon. The spacecraft will be approximately 62 miles from the surface at first, but will eventually move closer. From point-blank range, the spacecraft will look to see how the solar wind impacts a rocky world when there's no magnetic field to protect it. Earth is protected from solar wind by its magnetic field. However, the moon is exposed because it has no global magnetism.
The ARTEMIS mission is a joint effort among NASA's Goddard Space Flight Center in Greenbelt, Md.; NASA's Jet Propulsion Laboratory in Pasadena, Calif.; the Space Sciences Laboratory at Berkeley; and UCLA.
Launched in 2007, THEMIS was NASA's first five-satellite mission launched aboard a single rocket. The unique constellation of satellites provided scientists with data to help resolve the mystery of how Earth's magnetosphere stores and releases energy from the sun by triggering geomagnetic substorms. The three remaining THEMIS satellites continue to study substorms that are visible in the Northern Hemisphere as a sudden brightening of the Northern Lights, or aurora borealis.
The mission was one of NASA's series of low-cost, rapidly developed missions in the Explorers Program. ATK, formerly Swales Aerospace, in Beltsville, Md., built the THEMIS probes. Goddard manages the program for the agency's Science Mission Directorate.
For more information about ARTEMIS, visit:
For more information about the THEMIS mission, visit:
The two ARTEMIS spacecraft are healthy and are expected to continue to return science data for several years. The satellites will fly close to the lunar surface once per orbit – approaching anywhere from within 12 to 240 miles of the surface depending on the iteration – in a belt ranging 20 degrees above and below the equator.
Related Links:
› ARTEMIS Spacecraft Prepare for Lunar Orbit
› Additional ARTEMIS orbit images/video
NASA
Guillermo Gonzalo Sánchez Achutegui
ayabaca@gmail.com
ayabaca@hotmail.com
ayabaca@yahoo.com
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