Trojan rock seen in Earth's orbit

28 July 2011 Last updated at 08:57 GMT By Jonathan Amos Science correspondent, BBC News 2010 TK7 traces a complex path at its orbital point, moving above and below the plane of the Earth's orbit Astronomers have detected an asteroid not far from Earth, moving in the same orbit around the Sun.

The 200-300m-wide rock sits in front of our planet at a gravitational "sweet spot", and poses no danger.

Its position in the sky makes it a so-called Trojan asteroid - a type previously detected only at Jupiter, Neptune and Mars.

2010 TK7, as it is known, was found by Nasa's Wise telescope. The discovery is reported in this week's Nature journal.

It is a fascinating observation because the relative stability and proximity of Trojans would make possible targets for astronaut missions when we eventually go beyond the space station.

2010 TK7 is probably not the rock of choice, simply because it travels too far above and below the plane of Earth's orbit, which would require a lot of fuel to reach it.

Nonetheless, its detection means it is highly likely there are other, more suitable Trojans out there waiting to be found.

The difficulty is the viewing geometry that puts any Trojan, from the perspective of an Earth-based telescope, in bright skies.

Trojan asteroids are considered possible targets for astronaut missions

It took an orbiting telescope sensitive to infrared light to pick up 2010 TK7.

Wise, the Wide-field Infrared Survey Explorer launched in 2009, examined more than 500 Near-Earth Objects (NEOs), 123 of which were new to science.

The authors of the Nature paper sifted through data on these rocks, looking for the candidates that might be Trojans.

Follow-up work on the Canada-France-Hawaii Telescope confirmed the status of 2010 TK7.

It traces quite a complex path at its orbital point. Currently, it is about 80 million km from Earth, and should come no closer than about 25 million km.

The team says its orbit appears stable at least for the next 10,000 years.

2010 TK7's existence should not really be a surprise. Jupiter, Neptune and Mars all have collections of rocks sitting in the so-called Lagrange points 60 degrees ahead of or behind the planets in their orbits.

In the case of Jupiter, the number of Trojans now tops 1,000 rocks.

"These objects are difficult to find from Earth, simply because they're not very big and they're pretty faint, and they're close to the Sun as seen from Earth," explained Christian Veillet from the Canada-France-Hawaii Telescope and a co-author on the Nature study.

"But we can find them from space, and future satellites will likely find some more. We think that there are others which will be very close to the Earth and have motions that make them relatively easy to reach. So, they could be potential targets to go to with spacecraft," he told BBC News.

Jonathan.Amos-INTERNET@bbc.co.uk

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'Salt mapper' goes into orbit

10 June 2011 Last updated at 16:52 GMT By Jonathan Amos Science correspondent, BBC News Nasa studies many ocean properties from orbit but salinity has been a major omission A satellite that will map the saltiness of seawater has gone in orbit.

Data from the Aquarius/SAC-D spacecraft will help scientists understand better the processes that drive ocean circulation and the movement of freshwater around the planet.

A Delta rocket carried the satellite aloft, launching from California, US, at 0720 local time (1420 GMT).

The mission is led by the space agencies of America and Argentina (Nasa and Conae).

There are inputs also from Brazil, Canada, France and Italy.

Scientists have been able to measure ocean salinity for decades by lowering instruments from ships or by deploying robotic floats, but the technology to sense this property from orbit is a recent innovation.

Salinity is of interest to researchers because it is both a determinant of ocean behaviour and a tool to diagnose what might be happening in the climate system.

Aquarius carries three high-precision radio receivers that will record the natural microwave emissions coming up off the water's surface. These emissions vary with the electrical conductivity of the water - a property directly related to how much dissolved salt it is carrying.

Together with temperature, salinity will define water density, and density and wind are the wheels of ocean circulation - the means by which Earth moves much of the energy it receives from the Sun around the globe.

By monitoring changes in the amount of dissolved salts at the surface, scientists can see also where water is being evaporated and precipitated.

Continue reading the main story Salinity levels in the open ocean vary by about five parts per thousand - a very narrow rangeTraditionally, data has come from instruments lowered from ships of from robotic floats (above)Aquarius will make monthly maps of the entire Earth ocean system at resolutions of 150kmSurface salinity can help scientists understand better ocean currents and circulationThe property is a tracer for evaporation and precipitation and the global water cycleEvaporation at the surface increases salinity by leaving behind salts as the water moves into the atmosphere. When it rains, the surface is diluted and salinity drops. In this sense, the amount of salt present is a tracer for the global water cycle. It is thought something like 80% of this cycle - which moves fresh water from the ocean to the atmosphere, to the land, and back to the ocean again - occurs out over the sea.

Eric Lindstrom, an Aquarius programme scientist at Nasa, said the new satellite would fill an important gap in the remote sensing observations made by the American agency.

"In the Earth science division of Nasa, we have 13 missions in orbit right now and about half of them measure ocean quantities - we get sea-surface temperature, ocean winds, sea level, ocean colour, and the changing mass of the oceans," he explained.

"A key missing piece that's really in demand by the ocean science community is ocean salinity."

Getting this data from orbit is not a simple matter, however.

The range of salinity that can be measured is a small one - generally between about 32 and 37 parts per thousand out over the open ocean.

Europe has already begun to make space-based maps of salinity; soon, scientists will have double the data and an even better view of the subject

"Our goal is to retrieve salinity at 0.2 parts per thousand," said Aquarius Principal Investigator Gary Lagerloef of Earth & Space Research in Seattle. "That's the equivalent concentration if you were to take a dash of salt about the same as one-eighth of teaspoon and put it in a gallon of water.

That works out to about one millilitre of salt in six litres of water.

The Delta is seen climbing high into the sky above the Vandenberg Air Force Base

"That's the amount of salinity change that Aquarius will be able to observe from month to month over any part of the ocean."

The Nasa-Conae spacecraft will not be the first ocean salinity mission in orbit. Europe already has a satellite in operation called Smos. This was launched in 2009 and produced the first-ever global maps of salinity built from space data.

Before it achieved this milestone, Smos had a torrid time coping with interference from radars, TV and radio links, and even wi-fi networks whose emissions were bleeding across the microwave signal. But Gary Lagerloef says the different radiometer design on Aquarius should fare much better, and he looks forward to getting double the data.

"The two teams have worked in parallel for a decade now, developing their respective missions; we have shared information and it has been a strong cooperative effort," he told BBC News.

"Once we have both of these missions in orbit, we will compare results - we'll inter-calibrate them, we'll combine the measurements together. We'll do all we can to provide the best information about ocean surface salinity to the scientific community."

Europe's Smos spacecraft went into orbit in 2009

The Aquarius/SAC-D spacecraft weighed about 1.3 tonnes at lift-off.

Its radiometers are complemented by a radar instrument which studies how wind is roughening the sea surface and potentially skewing the microwave signal. All this equipment is provided by Nasa.

Conae has been responsible for the spacecraft bus, or chassis. This bus also hosts seven instruments - most built by the Argentinians.

These systems will be making a wide range of observations - from the imaging of urban lighting and monitoring forest fires to looking for space debris and making measurements of sea-ice.

Nasa had been under some pressure with this launch. Its previous attempt to get an Earth observation spacecraft into orbit ended in failure when the carrier rocket malfunctioned. The Glory satellite fell into the Southern Ocean off Antarctica and was destroyed.

Nasa has provided the ocean salinity instrument; Conae has provided the main satellite bus and a number of other instruments. The spacecraft will circle the Earth, travelling from pole to pole Jonathan.Amos-INTERNET@bbc.co.uk

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