Astronomers Prepare To Obtain Close Images Of A Near-Earth Asteroid

Daily Science Journal (Jan. 27, 2008) — The Arecibo Observatory in Arecibo, Puerto Rico will observe a newly discovered asteroid on Jan. 27-28, as the object called 2007 TU24 passes within 1.4 lunar distances, or 334,000 miles, from Earth.

The asteroid, estimated at between 150 and 600 meters in diameter -- about 500 feet to 1,900 feet, or the size of a football field, at 360 feet, to the size of Chicago's 110-story Sears Tower, at 1,454 feet -- was discovered by the University of Arizona's Catalina Sky Survey in October 2007. It poses no threat to Earth, but its near approach gives Arecibo astronomers a golden opportunity to learn more about potentially hazardous near-Earth objects.

"We don't yet know anything about this asteroid," said Mike Nolan, head of radar astronomy at the Puerto Rico observatory. Such objects pass near Earth with relative frequency, he said -- approximately one every five years or so -- but it's rare that astronomers have enough advance notice to plan for rigorous observing.

"Because it's coming so close, we'll get our highest quality imaging," said Nolan.

Using Arecibo's powerful radar, which is the most sensitive in the world, researchers will gauge the object's size, observe its speed and measure its spin. Switching then to imaging mode, which will offer resolution to 7.5 meters -- three times more precise than NASA's Goldstone telescope, the only other radar telescope in the world -- the researchers hope to map the object's surface in detail. The Robert C. Byrd Green Bank Telescope, Green Bank, W.Va., will receive Arecibo's echo from the asteroid and transmit its data back to Arecibo.

TU2 is one of an estimated 7,000 near-Earth objects, its size or larger -- most have never been closely studied.

"We have good images of a couple dozen objects like this, and for about one in 10, we see something we've never seen before," said Nolan. "We really haven't sampled the population enough to know what's out there."

Arecibo's radar is vital for continuing to classify and understand such objects, said Cornell University assistant professor of astronomy Jean-Luc Margot. "Arecibo does a fantastic job at getting images, discovering the shape, spin and reflection properties of such an object . . . all these things that are important to know."

The telescope will be trained on TU24 Jan. 27-28 and again Feb. 1-4. Goldstone's planetary radar observed it Jan. 23-24.

Steven Ostro, astronomer at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., is principal investigator for the project; also contributing are Lance Benner and Jon Giorgini at JPL and Greg Black of the University of Virginia. Their research is funded by NASA.

The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, a national research center operated by Cornell for the National Science Foundation. The Green Bank Telescope is operated by National Radio Astronomy Observatory for the National Science Foundation.

Adapted from materials provided by Cornell University.

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Asteroid To Make Rare Close Flyby Of Earth January 29

Daily Science Journal (Jan. 25, 2008) — Scientists are monitoring the orbit of asteroid 2007 TU24. The asteroid, believed to be between 150 meters (500 feet) and 610 meters (2,000 feet) in size, is expected to fly past Earth on Jan. 29, with its closest distance being about 537,500 kilometers (334,000 miles) at 12:33 a.m. Pacific time (3:33 a.m. Eastern time). It should be observable that night by amateur astronomers with modest-sized telescopes.

The illustration from an amateur astronomer shows the asteroid's track on the sky for 3 days near the time of the close Earth approach as seen from the city of Philadelphia. (Credit: Dr. Dale Ireland, Silverdale, WA)

Asteroid 2007 TU24 was discovered by the NASA-sponsored Catalina Sky Survey on Oct. 11, 2007. Scientists at NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif., have determined that there is no possibility of an impact with Earth in the foreseeable future.

"This will be the closest approach by a known asteroid of this size or larger until 2027," said Don Yeomans, manager of the Near Earth Object Program Office at JPL. "As its closest approach is about one-and-a-half times the distance of Earth to the moon, there is no reason for concern. On the contrary, Mother Nature is providing us an excellent opportunity to perform scientific observations."

Asteroid 2007 TU24 will reach an approximate apparent magnitude 10.3 on Jan. 29-30 before quickly becoming fainter as it moves farther from Earth. On that night, the asteroid will be observable in dark and clear skies through amateur telescopes with apertures of at least 7.6 centimeters (3 inches). An object with a magnitude of 10.3 is about 50 times fainter than an object just visible to the naked eye in a clear, dark sky.

NASA detects and tracks asteroids and comets passing close to Earth. The Near Earth Object Observation Program, commonly called "Spaceguard," discovers, characterizes and computes trajectories for these objects to determine if any could be potentially hazardous to our planet.

Adapted from materials provided by NASA/Jet Propulsion Laboratory.

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Double Star TC-1 Completes Its Mission ; Details Of Solar Particles Penetrating The Earth's Environment Revealed

Daily Science Journal (Oct. 19, 2007) — TC-1, one of the two satellites of the CNSA/ESA Double Star mission, was decommissioned on 14 October as its designed orbit lifetime came to an end. The satellite re-entered Earth’s atmosphere and turned to dust during its descent.

Flying in formation around the Earth, they relay detailed information about how solar wind affects our planet in 3D. The satellites are called Rumba, Salsa, Samba and Tango. (Credit: ESA)

Along with its twin TC-2, TC-1 is the first satellite built and operated by the Chinese National Space Administration (CNSA) in cooperation with ESA. Along with its twin and the four Cluster satellites, TC-1 has helped accomplish much during its lifetime.

The four years during which Double Star was operational brought in new perspectives concerning the boundaries of the magnetosphere and the fundamental processes that are playing a role in the transport of mass, momentum and energy into the magnetosphere. Thanks to the measurements of TC-1, there was a chance to observe the evolution of structures and physical processes at small scales with Cluster, and then on large scales with Double Star.

Here we list some of the most interesting results where TC-1 played a crucial role.

Space is fizzy

Above our heads, at the bow shock, where the Earth’s magnetic field meets the constant stream of gas from the Sun, thousands of bubbles of superheated gas, or ion density holes, are constantly growing and popping. These bubbles were discovered by Cluster and Double Star together, and the discovery allowed scientists to better understand the interaction between the solar wind and the Earth’s magnetic field.

Celestial chorus further away

Chorus emissions are waves naturally generated in space close to the magnetic equator. They play an important role in creating killer electrons that can damage solar panels and electronic equipments of satellites and are a hazard for astronauts. It was found that these waves are created further away from Earth during high geomagnetic activity. This information is crucial to be able to forecast their impact.

Oscillations of Earth’s natural cloak of magnetism

The four Cluster satellites and TC-1 unexpectedly found themselves engulfed by waves of electrical and magnetic energy as they travelled through Earth’s night-time shadow. Something had set the tail of Earth’s natural cloak of magnetism oscillating, like waves created by a boat travelling across a lake. The data collected gave scientists an important clue to the effects of space weather on Earth’s magnetic field.

"Double Star has demonstrated mutual benefit and fostered scientific cooperation in space research between China and Europe. But there is still much more to come as the full, high-resolution data archive becomes available," says Philippe Escoubet, ESA’s Cluster and Double Star Project Scientist.

Adapted from materials provided by European Space Agency.

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Details Of Solar Particles Penetrating The Earth's Environment Revealed

Co-ordinated efforts by China/ESA’s Double Star and ESA’s Cluster spacecraft have allowed scientists to zero in on an area where energetic particles from the Sun are blasting their way through the Earth’s magnetic shield. Solar material penetrating the Earth's magnetic shield can represent a hazard to both astronauts and satellites.

This sketch shows the orbits of the Cluster and Double Star (TC-1) spacecraft on 8 May 2004, when the five satellites observed magnetic channels created by the merging of the Sun and the Earth's magnetic fields. Such events, called 'Flux Transfer Events,' allow solar particles to break through the Earth's magnetic shield and penetrate the Earth's environment. (Image courtesy of European Space Agency)

On 8 May 2004, one of the two Double Star satellites (TC-1) and all four Cluster spacecraft found themselves in the firing line. For about 6 hours, the Cluster spacecraft were buffeted every 8 minutes by intense flows of electrically charged particles released by the Sun. The Double Star TC-1 spacecraft had it even rougher, being blasted every four minutes for eight hours.

During such events, magnetic channels created by the merging of the Sun and the Earth’s magnetic fields allow solar particles to break through the Earth’s magnetic shield and penetrate the Earth’s environment. Physicists call the occurrence of these magnetic channels Flux Transfer Events. Each magnetic channel appears like a curve shaped tube that can be anything from 5000 to 25000 kilometres in diameter. One end of the magnetic flux tube is connected to Earth while the other end is connected to the solar wind.

The basic physical mechanism responsible for the occurrence of flux transfer events is called magnetic reconnection. In the 1950s, space physicists believed that magnetic reconnection let solar particles break through at a steady rate. That view changed in the late 1970s, when several studies showed that the magnetic reconnection could also be intermittent and take place in pulses, lasting a few minutes. Each pulse produces a magnetic flux tube (a Flux Transfer Event).

On 8 May 2004, these magnetic flux tubes swept over Cluster and Double Star again and again. As the Cluster and Double Star data clearly showed, the same location underwent magnetic reconnection several times, creating new successive magnetic flux tubes to channel more charged particles towards the Earth. The observations stopped probably because the spacecraft moved out of range and not because the reconnection region weakened in any way.

The data from the five spacecraft allowed scientists led by Aurélie Marchaudon of the Laboratoire de Physique et Chimie de l’Environnement, Centre Nationale de la Recherche Scientifique (CNRS) and Université d’Orléans, Orléans, France to triangulate the location of the magnetic reconnection region, and to deduce its size. They found that the reconnection site was located on the daylight west side of the Earth’s magnetic shield and was around 25000 kilometres across. A computer simulation of the event, conducted by Jean Berchem of the University of California Los Angeles (UCLA) and his team, confirmed the possibility of magnetic reconnection occurring at that location.

Although intermittent reconnection has been observed in the past, this was one of the longest series of continuous observations ever taken of a magnetic reconnection region in the Earth’s magnetosphere. Perhaps most surprising is that 8 May 2004 was just relatively a normal day for the Earth’s magnetic field. There were no large magnetic storms on Earth, or spectacular aurorae to fill the night sky. However, Cluster and Double Star revealed that energetic particles from the Sun were blasting their way through the Earth’s magnetic shield and penetrating the Earth’s environment.

Each day, Cluster and Double Star return more observations that allow scientist to understand the invisible magnetic turbulence high above our heads.

Adapted from materials provided by European Space Agency.

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Cluster And Double Star Uncover More On Bright Aurorae

Daily Science Journal (Sep. 12, 2007) — Cluster data has helped provide scientists with a new view of magnetospheric processes, challenging existing theories about magnetic substorms that cause aurorae and perturbations in GPS signals.

The diagram shows all 11 spacecraft of the Cluster, Double Star and THEMIS missions orbiting the Earth. All three missions are studying the environment of Earth's magnetosphere. (Credit: ESA)

The onset of magnetic substorms that originate in Earth’s magnetosphere has been explained by two competing models: current disruption and near-earth reconnection. Current beliefs have been challenged using data from ESA’s Cluster satellites, and CNSA’s Double Star, a mission with ESA participation. Recent research suggests a third type of substorm onset.

Magnetic substorms often cause bright and colourful aurorae at high latitudes, in places such as Scandinavia or Canada. These aurorae are caused by energetic electrons that spiral down Earth’s magnetic field lines and collide with atmospheric atoms at an altitude of about 100 km. The energetic electrons come from the magnetotail, located on the nightside of Earth where the solar wind stretches Earth’s magnetic field lines into a long tail.

At the centre of the magnetotail is a denser region known as the plasma sheet. Plasma is a gas composed of ions and electrons which is electrically neutral. It is spread over large distances in space and guided by the action of magnetic and electric fields. A substorm induces violent changes in the plasma sheet. It energises ions and electrons and hurls them Earthward. The substorm itself can occur as a series or in isolation.

Apart from producing the beautiful show of light, substorms also excite a large portion of Earth's ionosphere, perturbing the reception of GPS signals and communication between Earth and orbiting satellites. Despite decades of space research, the mechanism causing the onset of substorms remains a mystery.

There are three events associated with the onset of a substorm: auroral brightening, current disruption, and magnetic reconnection.

Auroral brightening is a sudden change of the aurora from light grey to very bright and colourful auroras at an altitude of about 100 km, visible from ground. Current disruption occurs at a height of roughly 60 000 km on the nightside or at a sixth of the distance to the Moon and is associated with turbulent fluctuations in the magnetic field.

Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains collide and reconnect, heating and accelerating plasma. It occurs at around a third of the distance to the Moon or at a height of 120 000 km, in a thin plane close to the magnetic equator of the magnetotail.

The difference between the two existing theories on magnetic substorms is that they differ on the order in which the events take place.

Prof Sergeev (St Petersburg State University, Russia) and colleagues from Europe, the USA and China studied three consecutive substorm onsets, from data collected on board Cluster and Double Star on 26 September 2005. For the first time, data indicate that the current disruption process and magnetic reconnection can coincide in space and time showing, possibly, two sides of the same process.

They also found that in this case, magnetic reconnection occurred closer to the Earth than usual, almost co-located with the current disruption process, between 60 000 and 90 000 km. Related localised auroral brightenings were captured few tens of seconds later by an ultra violet imager onboard the NASA’s IMAGE satellite.

“Cluster’s multipoint measurements and the spatial coverage possible together with Double Star have been instrumental in making these unique observations possible,” commented Sergeev.

In February 2007, NASA launched THEMIS, a five-satellite mission dedicated to the study of the onset of substorms. "With the many scientific satellites in orbit, we have a never-before opportunity to study the global solar-magnetospheric environment and the physical processes involved," said Philippe Escoubet, Cluster and Double Star project scientist of the European Space Agency.

The results appear in ‘Observation of repeated intense near-Earth reconnection on closed field lines with Cluster, Double Star and other spacecraft’ by V. Sergeev, V. Semenov, M. Kubyshkina, V. Ivanova, W. Baumjohann, R. Nakamura, T. Penz, A. Runov, T. L. Zhang, K. Glassmeier, V. Angelopoulos, H. Frey, J. Sauvaud, P. Daly, J. Cao, H. Singer, and E. Lucek. The paper is published in the 20 January 2007 issue of the Geophysical Research Letters.

Adapted from materials provided by European Space Agency.

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Pioneering 3D View Of Near-Earth Magnetic 'Dance'

Daily Science Journal (Jul. 1, 2007) — Scientists have obtained the first-ever 3D picture of interconnected magnetic ‘dances’ in near-Earth space, known as magnetic reconnection events.

A diagram illustrating the null-null line observed by Cluster's four spacecraft in the Earth's magnetotail on 1 October 2001. (Credit: Inset: Chinese Academy of Sciences (C. Xiao), background: NASA)

The data from ESA’s Cluster satellites will help to understand better magnetic reconnection, a process related to star formation, solar explosions and the entry of solar wind energy into the near-Earth environment.

Magnetic reconnection is the process whereby magnetic field lines from different magnetic domains collide and reconnect, mixing previously separated plasma. Plasma is a gas composed of ions and electrons but is electrically neutral, spread over large distances in space and guided by the action of magnetic and electric fields.

Magnetic reconnection converts the energy of the magnetic field into particle energy, generating jets and heating the plasma.

This affects us here on Earth because it can possibly affect telecom satellites and prevent the efficient production of electricity in controlled fusion reactors, potential sources of electricity for the future. On Earth, we can also see the effect in more intense displays of the Northern lights.

On 1 October 2001, the four Cluster spacecraft were flying in formation at approximately 110 000 km from Earth in the magnetotail, a long-tail-like structure on the night-side of Earth’s magnetic field. The satellites meandered around a reconnection region over a period of nearly 15 minutes.

During reconnection, the geometry of the magnetic field forms an X-shape, also called a ‘magnetic null’. Analysed in 2D, the magnetic field, plasma density and flow velocity data collected during this event showed that only one reconnection region with an X-shape, or a magnetic null, was seen by the satellites.

An international team of scientists led by Chinese researchers challenged this result, suggesting that it could be seen in 3D.

The researchers tested theoretical results published 20 years ago, which predicted that any small perturbation to such a reconnection site would produce not one, but two magnetically linked reconnection sites, a pair of magnetic nulls and magnetically linked reconnection geometry.

By analysing a subset of the same data in 3D with a higher temporal resolution, they found what they were looking for. Two magnetic reconnection sites jumped out, along with the null-null line which connects two magnetic nulls, a previously unobserved phenomenon.

The situation can be compared to viewing a cross section of a 3D volume in space. It is not possible, when looking into only one plane, to tell whether or not something is going on in another plane, in the same volume of space.

Only when seen in 3D, with Cluster’s multi-satellite viewpoints, could scientists determine that there actually were two, interlinked events occurring simultaneously.

When reduced to two dimensions, this complex 3D magnetic geometry is still consistent with past results obtained under the 2D assumption where the null-null line is seen as one X-point. It is also in agreement with past results found in the laboratory and by Cluster in space.

"For the first time, the link between two sites of magnetic reconnection has been observed in-situ, in 3-D. This result is another major scientific achievement of Cluster obtained owing to fruitful scientific collaborations between Chinese, American and European scientists", said Philippe Escoubet, Cluster and Double Star project scientist of the European Space Agency.

Notes for editors:

The article “Satellite Observations of Separator Line Geometry of Three-Dimensional Magnetic Reconnection” by C. Xiao, X. Wang, Z. Pu, Z. Ma, H. Zhao, G. Zhou, J. Wang, M. Kivelson, S. Fu, Z. Liu, Q. Zong, M. Dunlop, K-H. Glassmeier, E. Lucek, H. Rème, I. Dandouras, C. Escoubet appeared on 24 June 2007 in the advance online publication of Nature Physics.

Adapted from materials provided by European Space Agency.

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