Herschel’s Final Destination: The Moon?

Artist’s impression of Herschel

Herschel will run out of its helium coolant at some point in the first half of 2013, after which the mission will be over. There is much debate about what to do with the spacecraft after that. One suggestion is to fly Herschel into the Moon, creating a fresh impact crater and allowing astronomers to search for further signs of water.

Once the helium coolant has run out, none of the the instruments on board will work, but the spacecraft itself will still be fully functional. There will be a few engineering tasks to complete, but after that the scientific mission of the spacecraft will be done. It won’t be a complete derelict however: there will still be the on-board computers, the thrusters, the communications equipment, and the “Space Radiation Environment Monitor” – which measures high-energy particles coming from the Sun and other astrophysical phenomena.

Herschel has on board a number of thrusters, which it fires every month or so to keep it in its orbit around L2, a point in space 1.5 million km from Earth in the opposite direction to the Sun. The thrusters are not particularly powerful, but they are crucial – if the orbital maintenance manoeuvres weren’t done, Herschel’s orbit would become unstable and it would drift onto an unknown trajectory. While the risk is low, this unknown trajectory could cause it to return to the Earth in the future, possibly causing damage either on the ground or to orbiting satellites and spacecraft.

It’s a problem that the European Space Agency are well aware of. Göran Pilbratt, Herschel’s Project Scientist, told Spaceflight Now “When the coolant is gone, then Herschel is useless as an astronomical facility. The spacecraft needs to be put somewhere for posterity – you don’t want it to come hitting our heads”.

Herschel trajectory to its orbit around the L2 point, with the Earth, Sun and Moon also shown

The solution envisaged at the start of the mission was to fire Herschel’s thrusters to place in a stable orbit around the Sun, which would remove the risk of it causing damage for at least a few hundred years. This would allow the spacecraft to be retrieved many years in the future, though some argue that it would be so out of date by then that. A research team, coordinated by Neil Bowles from Oxford University, has proposed a more permanent solution – to put Herschel on a collision course with the Moon.

This might sound more risky than the current plan, but in fact the spacecraft could be targeted at an area the size of a football stadium. Herschel’s thrusters would not cause it to travel very quickly, and it would take severl months to reach the Moon. As it nears the Moon, the spacecraft would be accelerated by the pull of the Moon’s gravity, and the 3-tonne spacecraft would impact at a speed of around 5500 mph. This would give it enough speed to excavate a crater 30 metres wide and 5 metres deep – large enough to be seen by spacecraft currently in orbit around the Moon.
The purpose of doing this would be to try to find water under the Moon’s surface. The Moon used be thought to be a completely dry place. But parts of the lunar surface, particulary in deep craters near the Moon’s poles, are never exposed to sunlight, and it is thought that water ice could be trapped beneath the surface, at depths up to a few metres. It’s something that has be tried before, notably by NASA’s LCROSS mission which crashed a rocket stage – which had a slightly lower mass than Herschel – into the crater Cabeus near the Lunar south pole. The trailing spacecraft took images of a plume of material that rose several miles high, and identified the presence of water – though only in this one location on the Moon.

The image of the impact of a rocket stage, taken by a trailing LCROSS spacecraft

The presence of water ice on the lunar surface could have an impact on future manned bases, not least because water can be used to generate fuel for rockets and spacecraft. Neil Bowles, a planetary scientist at Oxford University who is coordinating lunar impact proposal, pointed out that “It is certainly lower than the cost of a purpose built mission with similar science goals.”

There are mixed reactions amongst the Herschel team. Some object on emotional grounds, while others are all for it. Matt Griffin, from Cardiff University and lead scientist of the SPIRE Instrument, commented that it’s “better to go out in a blaze of glory and do some more science as we do so!”

The decision will be made by ESA in the coming months, but the research team are already picking possible locations. One question will still remain however: there’s already a crater on the Moon called Herschel, so what will we call the new one?
Raed more: herschel.cf.ac.uk

Herschel Finds Oceans of Water in Disk of Nearby Star

This artist's concept illustrates an icy planet-forming disk around a young star called TW Hydrae, located about 175 light-years away in the Hydra, or Sea Serpent, constellation

Using data from the Herschel Space Observatory, astronomers have detected for the first time cold water vapor enveloping a dusty disk around a young star. The findings suggest that this disk, which is poised to develop into a solar system, contains great quantities of water, suggesting that water-covered planets like Earth may be common in the universe. Herschel is a European Space Agency mission with important NASA contributions.

Scientists previously found warm water vapor in planet-forming disks close to a central star. Evidence for vast quantities of water extending out into the cooler, far reaches of disks where comets take shape had not been seen until now. The more water available in disks for icy comets to form, the greater the chances that large amounts eventually will reach new planets through impacts.

“Our observations of this cold vapor indicate enough water exists in the disk to fill thousands of Earth oceans,” said astronomer Michiel Hogerheijde of Leiden Observatory in The Netherlands. Hogerheijde is the lead author of a paper describing these findings in the Oct. 21 issue of the journal Science.

This graph of data from Herschel shows how the cool water vapor was detected.

The star with this waterlogged disk, called TW Hydrae, is 10 million years old and located about 175 light-years away from Earth, in the constellation Hydra. The frigid, watery haze detected by Hogerheijde and his team is thought to originate from ice-coated grains of dust near the disk’s surface. Ultraviolet light from the star causes some water molecules to break free of this ice, creating a thin layer of gas with a light signature detected by Herschel’s Heterodyne Instrument for the Far-Infrared, or HIFI.

“These are the most sensitive HIFI observations to date,” said Paul Goldsmith, NASA project scientist for the Herschel Space Observatory at the agency’s Jet Propulsion Laboratory in Pasadena, Calif. “It is a testament to the instrument builders that such weak signals can be detected.”

TW Hydrae is an orange dwarf star, somewhat smaller and cooler than our yellow-white sun. The giant disk of material that encircles the star has a size nearly 200 times the distance between Earth and the sun. Over the next few million years, astronomers believe matter within the disk will collide and grow into planets, asteroids and other cosmic bodies. Dust and ice particles will assemble as comets.

As the new solar system evolves, icy comets are likely to deposit much of the water they contain on freshly created worlds through impacts, giving rise to oceans. Astronomers believe TW Hydrae and its icy disk may be representative of many other young star systems, providing new insights on how planets with abundant water could form throughout the universe.

Herschel is a European Space Agency cornerstone mission launched in 2009, carrying science instruments provided by consortia of European institutes. NASA’s Herschel Project Office based at JPL contributed mission-enabling technology for two of Herschel’s three science instruments. The NASA Herschel Science Center, part of the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena, supports the U.S. astronomical community. Caltech manages JPL for NASA.

Two Views of a Star’s Exploded Remains

This layout compares two pictures of a supernova remnant called SN 1987A — the left image was taken by the Herschel Space Observatory, and the right is an enlarged view of the circled region at left, taken with NASA’s Hubble Space Telescope. SN 1987A is the result of a stellar blast that occurred 170,000 light-years away and was seen on Earth in 1987. The tiny pink ring in the Hubble image shows where a shock wave from the blast is hitting the surrounding material expelled from the star before the explosion. The cause for the outer, faint rings is unknown….. Continue reading Two Views of a Star’s Exploded Remains