ESA’s Mars Express provides video of the week!

On June 1st of this year, the European Space Agency’s (ESA) Mars Express spacecraft was able to capture images of an unusual alignment as Mars’ moon Phobos passed in front of Jupiter (seen in background). The images were put together to form this amazing animation.

Mars has two moons – Phobos and Deimos. The origins of these names are a bit gloomy : Phobos, named after a Greek God, means “fear” and Deimos is a figure representing “dread” in Greek mythology. Phobos is the largest of the two, and the closest moon to Mars.

You can see quite clearly that Phobos has an irregular shape – it’s mean radius is only  of 11.1 km (6.9 mi). Compare that with our Moon with its mean radius of approximately 1737.5 km (1079 miles). And that seemingly small object behind Phobos is of course Jupiter with a mean radius of (a not so small) 69,911 km (or 43440 miles).

This was a unique opportunity for the Mars Express spacecraft which performed a special maneuver to capture the alignment.  At the time these images were taken, there was a distance of 11,389 km (7076 miles) between the spacecraft and Phobos with Jupiter a further 529 million km away.

The Mars Express spacecraft was launched in 2003 and consisted of two parts – the Mars Express orbiter reponsible for the images in the animation above and the Beagle 2 – which was to land on the Martian surface and study the planet’s geochemistry.  Sadly the ill-fated Beagle 2 failed to make a safe landing (reminding us that planetary exploration is not easy!).  The Mars Express, however, has been collecting valuable data since 2004.  In particular, experiments are looking at the atmospheric environment, studying the distribution of water vapour, imaging and analysing the surface composition of Mars and searching for possible ice below the planet’s surface.

The science return and flexibility of the Mars Express  has earned it significant mission extensions – in fact it was initially planned to have a mission length of one Martian year (that’s 687 days for we earthlings).  It is now expected to continue its operations until December 2012.  Find out more about the mission at ESA’s site http://www.esa.int/SPECIALS/Mars_Express/index.html.

 


A Blast from the Past

Late last month, space and astronomy blogs and news sources were abuzz over the discovery of what may be the most distant event ever detected in the Universe.

NASA Swift Satellite

The event was something referred to as a Gamma Ray Burst officially designated as GRB 090429B, and it was detected by the ‘Burst Alert Telescope’ which is part of NASA’s ‘Swift’ space satellite. The satellite, launched in 2004, orbits at an altitude of 600 km above the Earth.  Here, I take a closer look at what a “gamma ray burst” really is and why this observation is interesting.

A whole lot of “bursting” going on?

First of all, what is all this “bursting” about and why is it important to astronomers?  Gamma ray bursts (or GRBs) are really huge energetic explosions, which as the name suggests produce gamma rays. Gamma rays have the smallest wavelengths and highest energies of any other wave in the electromagnetic spectrum.  On Earth, they are produced in nuclear explosions as well as by radioactive atoms.  In fact, we use gamma rays on Earth because their high energy means that they can kill living cells.  In cancer treatment, radiotherapy involves using targeted gamma rays to destroy cancerous cells.

The discovery of Gamma Ray Bursts coming from space was actually more to do with chance than anything else.  During the 1960s, the United States Air Force launched a series of satellites, (known as the Vela satellites) to monitor compliance with the Nuclear Test Ban Treaty.  These satellites were sensitive to gamma radiation, since a nuclear blast in the Earth’s atmosphere would produce a large amount of gamma-rays.  However a strange thing happened: the detectors onboard the satellites began to pick up intense bursts of gamma rays which were very different to those expected– rather like a flashlight shining brightly for a short time, coming from random directions in the sky. By the early 1970’s GRB’s had been discovered.  It wasn’t until the early 1990’s though that scientists were able to conclude that these burst events were coming from deep space – with cosmological distances i.e distances ranging from hundreds of millions to billions of light-years away. Despite how very luminous these explosions are, don’t expect to witness one in the night sky. Most of the energy reaches us as gamma rays and our human eyes simply aren’t sensitive to this form of radiation: in short – we can’t see them.  In addition, the Earth’s atmosphere does not allow gamma rays to penetrate so we have to rely on space-borne satellites to make observations.

What do we know about Gamma Ray Bursts now?

We now know a lot more about these bursts of light.  They can last anywhere from a few milliseconds to a few minutes with most lasting in the 20-40 second range.  The initial burst is usually followed by a longer-lived “afterglow” emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared ,etc.)  The light itself is extremely bright – hundreds of times brighter than an average supernova and about a million trillion times as bright as the Sun.  In fact, GRBs can release more energy in 10 seconds than the Sun will emit in its entire 10 billion-year lifetime.

Gamma Ray Burst
Gamma Ray Bursts produce twin beams of radiation. Credit: Swift / NASA

Because they are the most energetic form of light in the universe, they are only produced in the very hottest regions.  Most GRBs occur when a massive star at least 30 times the mass of our Sun, dies (runs out of fuel).  These stars collapse dramatically to form black holes or neutron stars and when they do, a huge burst of high energy gamma rays and particles gets ejected in an explosion.  Most of this energy is focussed into two distinct ‘beams’ which race out through space heating everything in their  path, producing the aforementioned “afterglow effect”.  The fact that GRBs (unlike supernovae) shoot out radiation in these narrow beams like a laser make them more difficult to observe – depending on which direction the beams are pointed.  These gamma-ray bursts,  appear like cosmic flashbulbs from any direction, flicker, and then fade after briefly dominating the gamma-ray sky:

Gamma Ray Burst
Gamma Ray Bursts (GRBs) may last only seconds. Credit: NASA Goddard

To put it in perspective, GRB’s are the brightest events in our Universe since the Big Bang.  Currently orbiting satellites detect an average of about one gamma-ray burst per day. Because gamma-ray bursts are visible to distances encompassing most of the observable universe, which is home to many billions of galaxies, this suggests that gamma-ray bursts must be very rare events per galaxy.   Perhaps occurring at a rate of a few, per galaxy per million years.  So far, astronomers have found most GRBS detected originate in distant galaxies. And this is precisely why this new detection of a GRB is important.  When something is so bright – we can observe it even when we may not even be able to observe light from the galaxy containing the GRB. So GRB’s allow us to measure distances in space greater than we have ever been able to before. Because light travels at a constant, finite speed – this also means that the farther away we see something in the Universe, the further back in time we are looking.  If the Universe is assumed to be 13.7 billion years old then the light from the very first stars will take approximately 13.7 billion years to reach us. So the very farthest object we could ever see would be 13.7 billion light years away.

Bring in GRB 090429B

And so we get to last month’s announcement at the American Astronomical Society meeting that astronomers may have discovered the most distant object in the Universe: GRB 090429B.  Okay, so perhaps not a name that encompasses how important this may be.  Perhaps a picture will help:

NASA Swift image of GRB090429B
NASA Swift image of GRB 090429B: candidate for the most distant object ever seen in space Credit: NASA / Swift / Stefan Immler

Again, perhaps not what we expect as the image of a massive exploding star in the throes of death.  However, this is the actual image that NASA’s Swift satellite snapped of the Gamma Ray Burst.  Given that this GRB lasted a mere 10 seconds, it is clear how important it is to have a satellite like Swift which slews quickly to point at the source of a GRB within seconds of detecting the burst.  Unlike many NASA satellites, “Swift” is not an acronym but instead refers to this ability to quickly change angle and position. Swift is also equipped with X-ray, UV and optical telescopes which measure the “afterglow” of the GRB.  What we actually see in the image above is this “afterglow”.

How far away is this thing?

Calculations have estimated the distance to GRB 090429B at 13.14 billion light years.  Yes, billion.  I mentioned earlier that the age of the Universe is an estimated 13.7 billion years, and so the furthest object we could see would be 13.7 billion light years away.  This also means the universe had only been around for a mere 600 million years when this massive explosion took place.  The galaxy which contained the GRB was possibly one of the very first formed after the Big Bang- in fact it may still have been in the process of forming when this burst occurred!  And on April 29, 2009, courtesy of the Swift satellite – we saw it! (The numerical designation 090429B actually tells us when the GRB was observed.)  You can tell by the lapse in time between observation and the announcement last month that painstaking analysis was required to give scientists enough confidence in the data that they believe that this GRB event did in fact happen 13.14 billion years ago,  at a distance 96% of the way to the furthest we could ever hope to observe.

How did scientists calculate this distance?

How on earth are we able to say that this could be the most distant object or event in the Universe to date? Astronomers commonly use the term “redshift” to give an idea of how far away an object in space is from us. This is based on the more familiar Doppler effect where wavelengths are stretched out as an object moves away from us. When light travels from the vast distance we are talking about here, we refer to a different type of redshift – the cosmological redshift.  This redshift is actually caused by the expansion of space.  The wavelength of light increases as it traverses the expanding universe between its point of emission and its point of detection by the same amount that space has expanded during the crossing time. At huge distances, far ultraviolet light for example gets stretched or redshifted all the way into the visible part of the spectrum.  We can observe this light that reaches us with ground based telescopes, and by looking at how the wavelength of the light has changed we can calculate how far it has travelled!

In this case, because the light travelled so far much of the UV radiation is absorbed by gas in the universe which the visible light from the burst passed through.  The visible light gets redshifted all the way to the infrared.  So if the light really had travelled this enormous distance we’d expect to see no visible light (formerly UV light) from the GRB at the earth, but we would see the infrared light that started off its journey as visble light. It turns out this is exactly what we saw, which suggests we are looking at something very distant that happened very early on in the life of our universe.

Ideally, astronomers would measure a complete spectrum of the afterglow of the GRB using several ground based telescopes but thanks to bad weather – this wasn’t possible in this case, making a precise distance measurement difficult.  Because GRB’s are so short lived, time is of the essence.  However the research team (led by former Penn State University graduate student Antonino Cucchiara) did gather data from the Gemini telescope in Hawai’i and were able to combine this with data from other telescopes which they feel has given them a good level of confidence in estimating the distance to GRB090429B.

Gemini Telescope images of GRB090429B
Observations made of infrared light from GRB090429B with the Gemini telescope (Hawai’i) . The different letters represent different filters of the Gemini Near-Infrared Imager (NIRI). Credit: Credit: Gemini Observatory/AURA/Penn State/UC Berkeley/University of Warwick, UK.

Another key piece of information is the fact that astronomers were never able to detect anything in the spot where they saw the afterglow . “We looked with Gemini, the Hubble Space Telescope and also with the Very Large Telescope in Chile and never saw anything once the afterglow faded.” said Cucchiara. “This means that this GRB’s host galaxy is so distant that it couldn’t be seen with any existing telescopes. Because of this, and the information provided by the Swift satellite, our confidence is extremely high that this event happened very, very early in the history of our universe.” Astronomers tend to quantify large distances in terms of this redshift, ‘z’, where higher values of z indicate greater distance and greater lookback time into the early universe.  The previous GRB record holder has an estimated z value of around 8.2, with GRB 090429B estimated at 9.4.

So is this REALLY, DEFINITELY, ABSOLUTELY the most distant object ever observed?

Well, maybe.

It is definitely a convincing candidate but ask some of the Hubble Space telescope astronomers and cosmologists and they may have a different answer. As with all things astronomy, there is a lot of error and uncertainty. Swift’s principal Investigator, Antonino Cucchiara puts it like this: “Like any finding of this sort there are uncertainties.  However, if I were in Vegas, I would never bet against the odds that this is the most distant GRB ever seen and we estimate that there is even a 23% chance that it is the most distant object ever observed in the universe.”

Whether or not GRB 090429B is the most distant object we’ve observed to date, the observation itself is a testament to the sensitivity and advancement of both space satellite and ground based telescope technology.  Who knows how much further back in time and out in space we’ll be looking in the coming years?

I, for one, am excited at the prospect.

How to Catch a Comet

There is plenty of excitement for NASA this week with both manned and unmanned missions sharing the limelight.  Avid shuttle watchers are eagerly awaiting this week’s scheduled launch of Space Shuttle Discovery’s final mission to the International Space Station now scheduled for Nov.5th at the earliest.

Nov. 4th held a real treat: NASA’s  EPOXI mission made a very successful close encounter with a comet known as Hartley 2.  In fact this encounter is the closest a  man-made object has ever come to any comet – coming within 435 miles/700 km.  This is only the fifth time a spacecraft has “visited” a comet in this way.  Many may recall the “Deep Impact” mission which launched in 2005 and aimed to rendezvouz with comet Tempel 1. It did just that on July 4th, 2005. That mission involved the spectacular release of  a washing machine-size probe, known as the “impactor,” which collided with the comet, releasing material which was imaged by the Deep Impact spacecraft (which is, in comparison approx the size of a VW beetle).

Scientists used the data and spectra they observed as a means of better understanding the nature and composition of the comet.  Although the Deep Impact spacecraft had completed its mission, NASA scientists saw potential for continuing to use the still functioning craft and set about determining a new scientific adventure for the probe.  After realizing that a new mission could be accomplished using the same imaging equipment, scientists decided on a new target – the comet Hartley 2.

Hence the EPOXI mission was born. Same spacecraft, different set of of targets – which is why you may be hearing the term “Deep Impact” frequently when listening to coverage of the mission.  EPOXI is actually a combination of two scientific investigations.   The new mission name “EPOXI” comes from combining two scientific investigations being undertaken by the spacecraft:

EPOCH: Extrasolar Planet Observation and CHaracterization

Deep Impact spacecraft

DIXI: the Deep Impact eXtended Investigation of comets

So it came to be that the original $252 million dollar spacecraft was to fly an additional 2.9 billion miles (4.6 billion Km) to hunt down Hartley 2 arriving 5 years later on Nov. 4th 2010. During it’s time cruising between these comets, the Deep Impact spacecraft completed the  Extrasolar Planet Observation and Characterization part of it’s mission objective: primarily a search and study of extrasolar planets and moons.

Catching comet Hartley 2 was to be significantly more challenging than the approach to Tempel-1 because it is about a seventh of the size at roughly 1.25 miles across (2 km) and yet still releases about the same amount of material into space. This makes the comet “flit around the sky” according to mission navigator Shyam Bhaskaran of NASA’s Jet Propulsion  Laboratory.  In fact the comet moves so much that three maneuvers were needed to adjust the spacecraft’s course – the latest last minute maneuver was 2 days ago!

Finally, yesterday at 10:10am EDT, the EPOXI spacecraft reached it’s goal, flying past Hartley 2 at a distance of 435 miles.  The comet came by the EPOXI craft at 12.3 km/s or more than 27, 000 miles per hour.

The flyby went off without a hitch and within an hour 5 spectacular high resolution images arrived at Earth:

The initial data suggests the comet's nucleus, or main body, approximately 2 kilometers (1.2 miles) long and .4 kilometers (.25 miles) at the "neck," or most narrow portion. Jets can be seen streaming out of the nucleus
Scientists plan to use the extensive data they will receive from its imagers ( two operating at visible wavelengths and one in the infrared) to study the structure of the nucleus and compare it with observations of other comets. Other important questions include what makes this comet so active? Which parts of the comet are emitting gas and what is the nature of these chemicals? With such detailed imagery we may be able to link the activity we observe (jets of gas being emitted) to distinct structures of the nucleus.

The excitement in being able to answer such questions relates to our desire to better understand our Solar System.  Since comets are leftovers from the solar system’s early days, this knowledge could reveal a great deal about how our cosmic neighborhood came to be.
Hartley 2 nucleus
The unusual rough, peanut shaped nucleus with "jets" emitting material

Expect more- much more in the coming months. Today’s flyby and the approach leading up to it have already provided a mountain of data for scientists and by Thanksgiving when EPOXI will shift it’s gaze from Hartley 2, scientists expect around 120,000 comet images to have been downloaded to the scientists computers.

NASA scientists have said that they feel that by reusing the Deep Impact spacecraft in this extended mission they have succeeded in getting a very good deal.  Although extending the mission into EPOXI has cost an additional $45 million,  Ed Weiler, associate administrator at NASA’s Science Mission Directorate has said that this amounts to about 10 percent of what it would have cost to launch a whole new mission. In his words:  “The spacecraft has provided the most extensive observations of a comet in history.” “Scientists and engineers have successfully squeezed world class science from a re-purposed spacecraft at a fraction of the cost to taxpayers of a new science project.”

So what is next for Deep Impact and EPOXI? Sadly the Deep Impact spacecraft is running out of fuel, so whether it will remain as a stationary observing platform or set it’s sights on another comet is unknown at this time.  As for comet Hartley 2, its days are numbered too. Although it will continue to zip around the Sun for a while longer, (it orbits the Sun once every 6.5 years) it seems the sun is “cooking”  3 to 5 feet (1 to 1.5 meters) of material off the comet’s surface on each orbit.   With its smallest side measuring about 1650 feet (500 m), Hartley 2 will not be around for very much longer.

Whatever the future holds the Deep Space/EPOXI mission has certainly shown us a new way to think about “recycling”, NASA style. Rethinking and repurposing missions to get the most science for our buck – that’s something I think we can all get behind.

Read more about past spacecraft-comet rendezvous:  http://www.space.com/scienceastronomy/comet-close-encounters-history-101103.html

What’s NASA crashing into next?

Last year the infamous NASA LCROSS mission gained attention as the unmanned space probe was set on a collision course with the lunar surface.  On October 9 2009, viewers watched as footage of the crash event was streamed back to Earth.  The mission crashed a rocket into the moon’s southern pole while the LCROSS craft with all the sensors and recording equipment followed behind, analyzing the cloud of material kicked up by the impact, looking for water.  And water was found: on November 13 2009, scientists confirmed the presence of water in data collected from the mission.

It seems that sometimes you just have to be there. Being in direct contact with the subject of research (termed “in situ” observations) has its benefits. 

Amateur astronomers lined up telescopes and hundreds of people watched the LCROSS impact on the internet. Although not visually as exciting as many hoped for, the data obtained seems to have justified NASA’s methods of “bombing the Moon” as so many media outlets described. If you were underwhelmed by LCROSS, hold onto your hats because NASA is at it again. And this time it’s shooting for the stars. Or star, actually: our Sun. 


Artist's impression of SPP. Credit: JHU/APL

NASA’s “Solar Probe Plus” (SPP) is slated for launch in 2018 and involves sending an unmanned probe into the Sun’s atmosphere (the corona) in order to better study its properties. The probe itself will be roughly the size of a small car and as you might imagine, will face scorching temperatures during the planned 6 year, 321 day mission. 

At its closest approach the SPP will come within 4 million miles from the Sun’s surface where it will be subject to intense bursts of radiation and temperatures in the range of 2550 degrees Farenheit (1399 °C,  1672 K).  Compare this with the previous record-setters for closest solar approach, the Helios probes, which came within roughly 27 million miles (43.5 million km) of the Sun in 1976.  At SPP’s closest approach, the intensity of solar radiation that it will experience is over 500 times greater than at the Earth’s distance from the Sun.

To protect the SPP from such intense temperatures, the probe will be equipped with a revolutionary carbon-composite heat shield to protect it during its fiery orbits. This material will be similar to that which protects the Space Shuttle from the heat of atmosphere reentry. The probe itself will be solar powered, getting its electricity from liquid-cooled solar panels that can retract behind the heat shield whenthe sunlight becomes too intense. 


The sun's wispy corona can be seen during a solar eclipse.

Before the science can begin, the Solar Probe Plus has a journey ahead to bring it into an oval orbit around the Sun. This involves 7 “fly-by” approaches to Venus to adjust its orbit.  This can be seen as the opposite of the traditional “gravity assist” where probes like Cassini have orbited planets to increase their speed so as to reach further out into the Solar System.  When we send spaceprobes towards the Sun, they are greatly accelerated by the Sun’s gravitational well and so they must be slowed down to enter into a useful orbit.  As the schedule stands, the SPP would make its closest approach to the Sun in late 2024. During its 24 orbits of the Sun the probe comes ever closer to the Sun so that at its closest position it will be well within Mercury’s orbit, about 8.5 solar radii or 3.7 million miles from the Sun’s surface. This is 7 times closer than any previous spacecraft. 

Another first will be its speed of orbit: on its closest approach the SPP will be flying past the sun at a speedy 125 miles per second (450 000 mph, 724000 km/h). 

That’s fast. Roughly 3 times as fast as any other space probe has travelled. Compare with the Space shuttle and International Space Station which orbit the Earth at approximately 17,500 mph (28000Km/h). 

Five scientific experiments have been selected for the mission and range from counting the most abundant particles present in the solar wind to a telescope that will make 3D images of the Sun’s corona. 

So why the Sun? Don’t we already know a lot about our closest star? The answer is yes – and no. Some important questions which scientists studying the Sun (heliophysics) have been searching for answers to for decades remain. 

Two of the most important of these questions may be answered by NASA’s Solar Probe Plus mission: 

  1. Why is the sun’s outer atmosphere, the corona,  so much hotter than the sun’s visible surface? (the coronal heating problem)
  2. What drives the solar wind – the streams of highly energetic particles that blow out at speeds of a million miles an hour, affecting the Earth and entire Solar System

In addition, during its time orbiting the Sun, the SPP is likely to encounter several “solar storms”.  Researchers suspect that many of the most dangerous particles produced by solar storms are energized in the corona—just where Solar Probe Plus will be. The spacecraft will have an up close and personal view of these Solar Energy Particle (SEP) events, enabling scientists to better understand, characterize and forecast the radiation levels that might threaten the heath and safety of future space explorers. 

I go into some of the science involved below. This is without doubt an exciting mission and the first time humankind has dared to rendezvous with a star.  In itself that makes this adventure quite remakable. 

The anatomy of our sun. (image: original by C Qualtrough)

Some of the science behind the mission 

The first  of the two questions stated above arises when we look at the Sun’s composition: 

The Sun’s Core is at a temperature of approximately 13.6 million Kelvin (~25 million degrees Farenheit). The optical surface of the sun (the photosphere) is known to have a temperature of approximately 6,000 K ( 10340 degrees Farenheit, 5700°C).  Above it lies the solar corona, rising to a temperature of 1,000,000–2,000,000 K. 

Herein lies the problem: how can the corona of the sun be millions of kelvin hotter than the lower surface of the sun (photosphere)? 

The second law of thermodynamics can be stated in the form attributed to Rudolf Clausius: 

“Heat generally cannot flow spontaneously from a material at lower temperature to a material at higher temperature.”

In other words, heat would normally be unable to flow from the solar photosphere to the hotter corona, so we must conclude that something other than direct heat conduction must be responsible for the high temperatures in the corona. 

At present the two theories considered the most likely involve 

1)    Wave heating 

Several different types of waves can exist in the Sun’s plasma environment and are similar to sound waves in air. Two specific types of waves: Alfvén waves and magneto-acoustic waves could potentially carry energy from the Sun’s photosphere up into the corona. However, magneto-acoustic waves seem unable to carry enough energy upwards into the corona without  being reflected back to where they came from due to low pressure. 

And Alfvén waves may reach the corona but dissipate energy far too slowly and insufficiently to account for how much heating we observe. 

We’ve also seen quite a lack of direct observations of uniformly energetic waves in the Sun’s corona so far — which we’d expect if the theory holds true. 

2)    Magnetic reconnection (nanoflares) 

This theory relies on the Sun’s magnetic field present in the Corona. In fact, as well as the magnetic field loops we associate linking active parts of the Sun, we now know that the Sun’s entire surface contains small “patches” of magnetic fields. A patchwork of small magnetic fields and loops, now dubbed the “magnetic carpet”, can appear and disappear again on timescales on the order of ~40 hours (see figure below). 

Credit: SOHO consortium, ESA, NASA
The "magnetic carpet" of the sun's surface. Black and white spots represent magnetic field concentrations with opposite orientations, called polarity. Each spot is roughly 5,000 miles across. The loop joining these regions extend from surface into the corona.

A magnetic field induces an electric current (as we see in generators). Sometimes in a plasma, their electric currents “collapse” and the magnetic field lines which were maintained by those currents “reconnect” to other poles and energy is released as waves and heat. 

No one really knows what could produce these magnetic patches, which vary on such short timescales. Professor Edward Spiegel from Columbia University has suggested that the patches are produced by small dynamos located just beneath the surface of the Sun. But there is no proof so far. 

It is also possible that a combination of these theories provides the answer. 

The second question that the Solar Probe Plus hopes to shed light on is that of how the solar wind is driven. The solar wind streams  off of the Sun in all directions at speeds of about 1 million miles per hour (400 km/s). Originating in the corona, the temperature of the solar wind helps it escape the sun’s gravity. Determining how the corona is heated is important in also understanding how and where the coronal gas is accelerated to such high velocities. Additionally the solar wind is NOT uniform and has both slower and faster components. Determining the mechanisms for both the slow and fast components of the solar wind is of great scientific interest. 

In both problems, the fundamental role of the sun’s magnetic field in shaping dynamical processes on all scales has become very apparent. 

Want to know more about spacecraft that have crashed into or landed on solar system bodies? Check out a list here