Tuesday, November 13, 2007

Comet Holmes Bigger Than The Sun

Formerly, the Sun was the largest object in the Solar System. Now, comet 17P/Holmes holds that distinction.

Spectacular outbursting comet 17P/Holmes exploded in size and brightness on October 24. It continues to expand and is now the largest single object in the Solar system, being bigger than the Sun (see Figure). The diameter of the tenuous dust atmosphere of the comet was measured at 1.4 million kilometers (0.9 million miles) on 2007 November 9 by Rachel Stevenson, Jan Kleyna and Pedro Lacerda of the University of Hawaii Institute for Astronomy. They used observations from a wide-field camera on the Canada-France-Hawaii Telescope (CFHT), one of the few professional instruments still capable of capturing the whole comet in one image. The present eruption of comet Holmes was first reported on October 24 and has continued at a steady 0.5 km/sec (1100 mph) ever since. The comet is an unprecedented half a million times brighter than before the eruption began. This amazing eruption of the comet is produced by dust ejected from a tiny solid nucleus made of ice and rock, only 3.6 km (roughly 2.2 miles) in diameter.

Caption: (Left) Image of comet Holmes from the 3.6-meter Canada-France-Hawaii telescope on Mauna Kea showing the 1.4 million km diameter coma. The white ''star'' near the center of the coma is in fact the dust-shrouded nucleus. (Right) the Sun and planet Saturn shown at the same scale for comparison. (Sun and Saturn images courtesy of NASA's SOHO and Voyager projects).

The new image also shows the growth of a tail on comet Holmes (the fuzzy region to the lower right in the comet picture), caused by the pressure of sunlight acting on dust grains in the coma. Over the next few weeks and months, the coma and tail are expected to expand even more while the comet will fade as the dust disperses. Comet Holmes showed a double outburst in November 1892 and January 1893. It is not known if the present activity in the comet will follow the pattern from 1892, but continued observations from Mauna Kea are planned to watch for a second outburst. Most comets show small fluctuations in brightness and some have distinct outbursts. The huge event on-going in comet Holmes is unprecedented, however.

The orbit period of comet Holmes is about 6 years, putting it in the class of Jupiter Family Comets whose orbits are strongly influenced by Jupiter. These objects are thought to have spent most of the last 4.5 billion years orbiting the Sun beyond Neptune, in a region known as the Kuiper Belt. Holmes probably was deflected into its present orbit within the last few thousand years and is losing mass as it evaporates in the heat of the Sun. In another few thousand years it is likely either to hit the Sun or a planet, be ejected from the Solar system, or simply die by running out of gas.

Hidden Details of Earth's Atmosphere Revealed By Orbiting Spacecraft

Watching the stars set from the surface of the Earth may be a romantic pastime but when a spacecraft does it from orbit, it can reveal hidden details about a planet’s atmosphere.

The technique is known as stellar occultation. Jean-Loup Bertaux, Service d'Aeronomie du CNRS, France was the first to suggest its use on an ESA mission. It works by watching stars from space, while they drop behind the atmosphere of a planet under investigation, before disappearing from view below the planet’s horizon.

When the stars are shining above the atmosphere, they give off radiation across a wide spread of wavelengths. As the orbit of the spacecraft carries it around the planet, the star appears to sink down, behind the atmosphere of the planet. The atmosphere acts as a filter, blocking out certain wavelengths of the star’s radiation. The key to this technique is that the blocked wavelengths are representative of the molecules and atoms in the planet’s atmosphere.

ESA currently has three spacecraft around three different planets that are using the technique to investigate those atmospheres. Each one is returning unique insights.

Around Earth, ESA’s Envisat mission carries an instrument called GOMOS (Global Ozone Monitoring by Occultation of Stars). As its name suggests, it is designed to study whether the quantity of ozone is increasing now that the use of harmful chemicals has been banned. Since 2002, it has been watching about 400 stars set behind the Earth every day in order to build up a map of the ozone in the Earth’s atmosphere for all latitudes and longitudes.

“It’s still too early to say whether the ozone is recovering or not,” says Bertaux. Nevertheless, as data accumulates, so the instrument is discovering other phenomena that contribute to the amount of ozone in the atmosphere. In January and February of 2004, GOMOS saw a large build up of nitrogen dioxide at an altitude of 65 km.

Nitrogen dioxide is an important gas to trace in the atmosphere because it can destroy ozone. Over the next two months, GOMOS watched as the layer sank to 45 km, clearly destroying ozone as it descended, providing scientists with another piece in the ozone puzzle.

A simplified stellar occultation instrument is onboard ESA’s Mars Express. Since the spacecraft arrived at the Red Planet in 2003, SPICAM (Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars) has observed more than 1000 stellar occultations. This work provides the most detailed description yet of Mars’s upper atmosphere, and reveals persistent haze layers.

Apart from delivering pure science, the data provides practical benefits for future exploration missions. “Atmospheric profiles of Mars are important for designing parachutes for landing craft,” says Bertaux.

The latest addition to this family of instruments is SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus) on Venus Express. Venus has a different atmosphere again from Earth or Mars. It is much denser and SPICAV is revealing the temperature and density profiles of the atmosphere to waiting scientists on Earth, who expect to publish their results soon.

“I think the stellar occultation technique is now ‘combat proven’ and should be useful for further long-term studies,” says Bertaux.

The article reflects results from two publications. 'Stellar Occultations at UV wavelengths by the SPICAM instrument: Retrieval and analysis of Martian haze profile' by F. Montmessin, J.L. Bertaux and P. Rannou was published in the Journal of Geophysical Research in 2006.

The second article 'Large increase of NO2 in the north polar mesosphere in January-February 2004: Evidence of a dynamical origin from GOMOS/Envisat and SABER/TIMED data' by A. Hauchecorne, J.L. Bertaux, F. Dalaudier, J.M. Russell III; M.G. Mlynczak, E. Kyrola, and D. Fussen was published in the Geophysical Research Letters on 16 February 2007.

Antique fridge could keep Venus rover cool

A high-tech refrigeration system could keep a rover functioning for weeks on the searingly hot surface of Venus, say NASA researchers. A long-lived Venus rover could help scientists understand why Venus, with its runaway greenhouse effect, has become so different from Earth.

The surface of Venus broils at a temperature of about 450 °C – hot enough to melt lead. Several probes in the Soviet Venera and Vega series, as well as a NASA Pioneer Venus probe, landed on Venus and returned data from the surface in the 1970s and early 1980s. But they all expired in less than 2 hours because of the tremendous heat.

Now, two NASA researchers have designed a refrigeration system that might be able to keep a robotic rover going for as long as 50 Earth days. The work was carried out by Geoffrey Landis and Kenneth Mellott of NASA's Glenn Research Center in Cleveland, Ohio, US.

The main concern is keeping the electronics cool. The NASA pair plan to do this by packing the electronics in a ceramic-based insulator and placing it inside a metal sphere about the size of a grapefruit.

Stirling cooler

Heat would then be pumped out of the sphere using a Stirling cooler, which works by compressing and then expanding a gas with a piston. When the gas expands, it cools down, absorbing heat from the electronics chamber. Then, as the gas is compressed and its temperature rises, the heat is allowed to dissipate in the atmosphere via a radiator.

Stirling coolers were invented in 1816 by Reverend Robert Stirling, a Scottish clergyman, but were largely ignored until the mid 20th century, when their impressive energy efficiency became better known.

They are already used on Earth to cool equipment in deep shafts drilled in rock for oil exploration, and are being developed for use in energy-saving home refrigerators. Landis and Mellott have now designed one suitable for use in the incredibly hot environment of Venus.

But to dissipate heat, the radiator has to be hotter than the surrounding atmosphere, so the new design can reach 500 °C. The cool end of the Stirling engine would keep the rover's innards at a relatively chilly 200 °C, which should allow commercially available electronics to operate well.

The researchers say the power to run the Stirling cooler, about 240 watts, would be provided by on-board plutonium batteries, which generate power from the heat of radioactive decay.

"The next step is probably going to be trying to build some prototypes and just demonstrate that what we are proposing is something that's going to work," Landis.

Challenging mission

NASA has not committed to a Venus rover mission, but Landis notes that a 2003 National Academies of Science study recommended that high priority be given to a robot mission to investigate the Venusian surface. Landis thinks a Venus rover could become a reality within a decade or so.

But such a mission is not without its challenges, says Venus researcher Mark Bullock of the Southwest Research Institute in Boulder, Colorado, US. "I think that a long-lived rover on Venus is a very, very difficult mission".

However, he thinks the Stirling cooler is a promising approach: "Active cooling is essential, and the Stirling cycle cooler with a radioisotope power source is probably the very best way to do it."

Putting a long-lived rover on the surface of Venus could revolutionise our understanding of the planet, helping to answer such questions as why Venus ended up so different from Earth, he says. Many scientists suspect Venus was much cooler in the past, and was perhaps even covered with oceans of liquid water where conditions could have been friendly to life.