Universe Today

Image credit: NASA/JPL

NASA’s Stardust spacecraft took this photograph of its target, Comet Wild 2, while it was still 25 million kilometers away. The spacecraft is on track to reach the comet on January 2, 2004 when it will pass only 300 km away and capture particles of its tail to return to Earth for analysis – the best photographs are still to come. Mission planners will use these early images to help fine-tune Startdust’s trajectory to give it the closest possible approach to Wild 2’s centre.

Forty-nine days before its historic rendezvous with a comet, NASA’s Stardust spacecraft successfully photographed its quarry, comet Wild 2 (pronounced Vilt-2), from 25 million kilometers (15.5 million miles) away. The image, the first of many comet portraits it will take over the next four weeks, will aid Stardust?s navigators and scientists as they plot their final trajectory toward a Jan. 2, 2004 flyby and collection of samples from Wild 2.

?Christmas came early this year,? said Project Manager Tom Duxbury at NASA?s Jet Propulsion Laboratory, Pasadena, Calif. ?Our job is to aim a 5 meter (16 foot) long spacecraft at a 5.4 kilometer (3.3 mile) wide comet that is closing on it at six times the speed of a bullet. We plan to ?miss the comet? by all of 300 kilometers (188 miles), and all this will be happening 389 million kilometers (242 million miles) away from home. By finding the comet as early and as far away as we did, the complexity of our operations leading up to encounter just dropped drastically.?

The ball of dirty ice and rock, about as big as three Brooklyn Bridges laid end-to-end, was detected on November 13 by the spacecraft?s optical navigation camera on the very first attempt. The set of images was stored in Stardust?s onboard computer and downloaded the next day where mission navigator Dr. Shyam Bhaskaran processed them and noticed a white blob of light bisecting the base of a triangle made by three stars Stardust uses for deep space navigation.

?When I first looked at the picture I didn?t believe it,? said Bhaskaran. ?We were not expecting to observe the comet for at least another two weeks. But there it was, very close to where we thought it would be.?

The Wild 2 sighting was verified on November 18 using the second set of optical navigation images downloaded from Stardust. To make this detection, the spacecraft?s camera saw stars as dim as 11th visual magnitude, more than 1,500 times dimmer than a human can see on a clear night.

The early detection of Wild 2 provides mission navigators critical information on the comet?s position and orbital path. Future optical navigation images will allow them to do more fine-tuning. In turn, these new orbital plots will be used to plan the spacecraft?s approach trajectory correction maneuver. Stardust?s first such maneuver is planned for December 3.

Unlike other orbiting bodies, the paths of comets cannot be precisely predicted because their orbits about the Sun are not solely determined by gravity. The escape of gas, dust and rock from comets provides a “rocket effect” that causes them to stray from a predictable orbital path. The actual orbital path cannot be precisely determined from Earth-based telescopes because the comet is shrouded in a cloud of escaping gas and dust. What is seen from Earth is not the actual 5.4 kilometer (3.3 mile) wide body composed of rock and ice, but the cloud of debris and gas that envelops it.

?With these images we anticipate we will flyby comet Wild 2 at an altitude of 300 kilometers, give or take about 16 kilometers,? added Bhaskaran. ?Without them, we wouldn?t be able to safely get any closer to the comet than several thousand kilometers.?

Stardust will return to Earth in Jan. 2006 to make a soft landing at the U.S. Air Force Utah Test and Training Range. Its sample return capsule, holding microscopic particles of comet and interstellar dust, will be taken to the planetary material curatorial facility at NASA’s Johnson Space Center, Houston, where the samples will be carefully stored and examined.

Stardust?s cometary and interstellar dust samples will help provide answers to fundamental questions about the origins of the solar system. More information on the Stardust mission is available at http://stardust.jpl.nasa.gov .

Stardust, a part of NASA’s Discovery Program of low-cost, highly focused science missions, was built by Lockheed Martin Astronautics and Operations, Denver, Colo., and is managed by JPL for NASA’s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena. The principal investigator is astronomy professor Donald E. Brownlee of the University of Washington in Seattle.

Original Source: NASA/JPL News Release

Image credit: ESO

The European Southern Observatory has released new images of a relatively nearby star, Eta Carina, which could be in the final stages of its life and could explode as a supernova in the near future (astronomically-speaking) – within the next 10-20,000 years or so. The star is 7,500 light years away, 100 times the mass of the Sun, and the most luminous object in the Milky Way. Since 1841, it has created a beautiful nebula around itself by continuously shedding outside layers while it spins quickly. By watching how Eta Carina changes, astronomers will gain valuable insights into the final stages of a supermassive star’s life.

Ever since 1841, when the until then inconspicuous southern star Eta Carinae underwent a spectacular outburst, astronomers have wondered what exactly is going on in this unstable giant star. However, due to its considerable distance – 7,500 light-years – details of the star itself were beyond observation.

This star is known to be surrounded by the Homunculus Nebula, two mushroom-shaped clouds ejected by the star, each of which is hundreds of times larger than our solar system.

Now, for the first time, infrared interferometry with the VINCI instrument on ESO’s Very Large Telescope Interferometer (VLTI) enabled an international team of astronomers [1] to zoom-in on the inner part of its stellar wind. For Roy van Boekel, leader of the team, these results indicate that “the wind of Eta Carinae turns out to be extremely elongated and the star itself is highly unstable because of its fast rotation.”

A monster in the southern sky
Eta Carinae, the most luminous star known in our Galaxy, is by all standards a real monster: it is 100 times more massive than our Sun and 5 million times as luminous. This star has now entered the final stage of its life and is highly unstable. It undergoes giant outbursts from time to time; one of the most recent happened in 1841 and created the beautiful bipolar nebula known as the Homunculus Nebula (see ESO PR Photo 32a/03). At that time, and despite the comparatively large distance – 7,500 light-years – Eta Carinae briefly became the second brightest star in the night sky, surpassed only by Sirius.

Eta Carinae is so big that, if placed in our solar system, it would extend beyond the orbit of Jupiter. This large size, though, is somewhat arbitrary. Its outer layers are continually being blown into space by radiation pressure – the impact of photons on atoms of gas. Many stars, including our Sun, lose mass because of such “stellar winds”, but in the case of Eta Carinae, the resulting mass loss is enormous (about 500 Earth-masses a year) and it is difficult to define the border between the outer layers of the star and the surrounding stellar wind region.

Now, VINCI and NAOS-CONICA, two infrared-sensitive instuments on ESO’s Very Large Telescope (VLT) at the Paranal Observatory (Chile), have probed the shape of the stellar wind region for the first time. Looking down into the stellar wind as far as possible, the astronomers could infer some of the structure of this enigmatic object.

The astronomer team [1] first used the NAOS-CONICA adaptive optics camera [2], attached to the 8.2-m VLT YEPUN telescope, to image the hazy surroundings of Eta Carinae, with a spatial resolution comparable to the size of the solar system, cf. PR Photo 32a/03.

This image shows that the central region of the Homunculus nebula is dominated by an object that is seen as a point-like light source with many luminous “blobs” in the immediate vicinity.

Towards the limit
In order to obtain an even sharper view, the astronomers then turned to interferometry. This technique combines two or more telescopes to achieve an angular resolution [3] equal to that of a telescope as large as the separation of the individual telescopes (cf. ESO PR 06/01 and ESO PR 23/01).

For the study of the rather bright star Eta Carinae the full power of the 8.2-m VLT telescopes is not required. The astronomers thus used VINCI, the VLT INterferometer Commissioning Instrument [4], together with two 35-cm siderostat test telescopes that served to obtain “First Light” with the VLT Interferometer in March 2001 (see ESO PR 06/01).

The siderostats were placed at selected positions on the VLT Observing Platform at the top of Paranal to provide different configurations and a maximum baseline of 62 meters. During several nights, the two small telescopes were pointed towards Eta Carinae and the two light beams were directed towards a common focus in the VINCI test instrument in the centrally located VLT Interferometric Laboratory. It was then possible to measure the angular size of the star (as seen in the sky) in different directions.

Pushing the spatial resolution of this configuration to the limit, the astronomers succeeded in resolving the shape of the outer layer of Eta Carinae. They were able to provide spatial information on a scale of 0.005 arcsec, that is about 11 AU (1650 million km) at the distance of Eta Carinae, corresponding to the full size of the orbit of Jupiter.

Scaled down to terrestial dimensions, this achievement compares to making the distinction between an egg and a billiard ball at a distance of 2,000 kilometers.

A most unusual shape
The VLTI observations brought the astronomers a surprise. They indicate that the wind around Eta Carinae is amazingly elongated: one axis is one-and-a-half times longer than the other! Moreover, the longer axis is found to be aligned with the direction in which the much larger mushroom-shaped clouds (seen on less sharp images) were ejected.

Spanning a scale from 10 to 20-30,000 AU, the star itself and the Homunculus Nebula are thus closely aligned in space.

VINCI was able to detect the boundary where the stellar wind from Eta Carinae becomes so dense that it is no longer transparent. Apparently, this stellar wind is much stronger in the direction of the long axis than of the short axis.

According to mainstream theories, stars lose most mass around their equator. This is because this is where the stellar wind gets “lifting” assistance from the centrifugal force caused by the star’s rotation. However, if this were so in the case of Eta Carinae, the axis of rotation (through the star’s poles) would then be perpendicular to both mushroom-shaped clouds. But it is virtually impossible that the mushroom clouds are positioned like spokes in a wheel, relative to the rotating star. The matter ejected in 1841 would then have been stretched into a ring or torus.

For Roy van Boekel, “the current overall picture only makes sense if the stellar wind of Eta Carinae is elongated in the direction of its poles. This is a surprising reversal of the usual situation, where stars (and planets) are flattened at the poles due to the centrifugal force.
The next supernova?

Such an exotic shape for Eta Carinae-type stars was predicted by theoreticians. The main assumption is that the star itself, which is located deep inside its stellar wind, is flattened at the poles for the usual reason. However, as the polar areas of this central zone are then closer to the centre where nuclear fusion processes take place, they will be hotter. Consequently, the radiation pressure in the polar directions will be higher and the outer layers above the polar regions of the central zone will get more “puffed up” than the outer layers at the equator.

Assuming this model is correct, the rotation of Eta Carinae can be calculated. It turns out that it should spin at over 90 percent of the maximum speed possible (before break-up).

Eta Carinae has experienced large outbursts other than the one in 1841, most recently around 1890. Whether another outburst will happen again in the near future is unknown, but it is certain that this unstable giant star will not settle down.

At the present, it is losing so much mass so rapidly that nothing will be left of it after less than 100,000 years. More likely, though, Eta Carinae will destroy itself long before that in a supernova blast that could possibly become visible in the daytime sky with the naked eye. This may happen “soon” on the astronomical time-scale, perhaps already within the next 10-20,000 years.

Original Source: ESO News Release