Why is Hubble better than telescopes?

The most remote object spotted by Hubble is a galaxy called UDFj-39546284, which was spotted as a tiny speck of light in the background of a series of observations made in 2009–2010 (heic1103). While this result awaits spectroscopic confirmation, the astronomers who made this discovery are confident that it is the most remote object ever to be identified. It is at a redshift of around ten, which means that the light from it has taken 13.2 billion years to reach us — about 96% of the way back to the Big spacetelescope.org/images/opo0331b/Note0 the coming years, especially with Hubble’s successor, the NASA/ESA/CSA James Webb Space Telescope, more distant galaxies will undoubtedly be detected, but the limit for our observations will not progress dramatically further back for two reasons.

Firstly, galaxies have to have time to form stars after the Big Bang (this takes several hundred million years) before we can see them. Secondly, the young galaxies will be shrouded in large amounts of gas and dust that obscure our view of the early Universe. Do you think Hubble is still necessary?

Hubble currently provides the best resolution in the optical and ultraviolet (UV) wavelength range in the world. This is extremely important for the identification of interesting objects. Adaptive optics techniques can be used with ground-based telescopes to obtain a similar or even superior resolution to that of Hubble, but only for small fields of view and in the infrared wavelength http://www.spacetelescope.org/images/opo0331b/Note1 mentioned, Hubble provides access to the ultraviolet wavelength range.

The UV light is blocked by the Earth's atmosphere, so observations in the UV can only be performed from space. They are very important for our understanding of the physical processes that take place in astronomical objects. Why is Hubble able to see so much better than telescopes on Earth?

Because it is above the Earth's atmosphere. The atmosphere disturbs the starlight (a bit like looking through water) and blurs the images. So Hubble's images are much sharper than those from other telescopes.

Also, Hubble is able to see in ultraviolet wavelengths that are blocked by the Earth’s atmosphere. I heard that Hubble had problems during its first few years. What really happened?

Yes, it is true. During its first three years Hubble suffered from what is known as spherical aberration. Spherical aberration is an optical defect and Hubble's main mirror is two microns too flat.

The problem was caused by a faulty measuring device used during the process of polishing the mirror. During Servicing Mission 1 the problem was solved by installing an extra optical device in Hubble called COSTAR. This used a series of curved mirrors to correct the light path between the mirror and Hubble’s instruments.In subsequent missions, Hubble’s instruments have all been replaced with ones with built-in optical correction, meaning COSTAR was no longer needed.

COSTAR was removed during Servicing Mission 4 in 2010. Hubble’s mirror still has the flaw, but it is completely corrected for by the design of Hubble’s current instruments. What are the Hubble Deep Fields?

Hubble has made a series of very deep observations taken in very dark parts of the sky. Like using a long exposure on a digital camera, these long exposure shots (up to several weeks) reveal very faint details that are not normally visible in shorter exposures. The Hubble Ultra Deep Field — Infrared is the deepest ever view of the cosmos, and reveals galaxies so distant that their light has taken around 96% of the age of the Universe to reach us.

If the Hubble Space Telescope pointed to Earth, what resolution would the images have? Hubble's so-called angular resolution — or sharpness — is measured as the smallest angle on the sky that it can resolve (i.e. This is 1/10 of an arcsecond (one degree is 3600 arcseconds).

If Hubble looked at the Earth — from its orbit of approximately 600 km above the earth’s surface — this would in theory correspond to 0.3 metres or 30 cm. But Hubble would have to look down through the atmosphere, which would blur the images and make the actual resolution worse. Unfortunately, Hubble will never be turned towards Earth since a) the brightness of the Earth could be damage the telescope and its instruments; b) there is no particularly interesting astronomical research to be done there (this is the province of geophysics); and c) Hubble orbits the Earth at such a rate that any image it took would be blurred by the motion.

What will be better about the James Webb Space Telescope? JWST will not be a like-for-like replacement of Hubble. The biggest difference is that it will be optimised for observing infrared light (with limited visible light capabilities), while Hubble is optimised for visible and UV light (with limited infrared capabilities).

With a bigger mirror and more advanced instruments, JWST will easily exceed Hubble’s ability to image in the infrared. This means it will be better at looking through dust and gas clouds, which is useful for studying star formation. It will also be much better for studying highly redshifted objects, and is therefore expected to make major contributions to the study of the very early Universe.

What has Hubble found out about the beginning of the Universe? This is a bit difficult to explain in just a few sentences. Hubble has measured the age and size of the Universe better than before (by refining the value for the Hubble constant, which is related to the expansion rate of the Universe).

It has also seen details which are not visible from the ground in the first galaxies. Today we know that galaxies were formed earlier than previously thought and most scientists also believe that they evolve by colliding and merging together. In the Hubble FAQ you mention that the telescope will never be aimed at the Earth.

What about the Moon? Could this also damage Hubble's instruments? If feasible, could these observations take a picture of the landing site of the Apollo missions?

There have been a few cases where Hubble has been aimed at the Moon - see here. This has to be done with the greatest care (since the Moon is very bright), and is normally avoided. Even with Hubble's incredible sharpness (resolution) only objects the size of a football field can be seen (~100 metres)... so no Apollo spacecraft would be visible.To see the surface of the Moon in enough detail, you simply need to get nearer to the lunar surface than Hubble is (Hubble is not significantly closer to the Moon than we are here on Earth).

NASA’s Lunar Reconnaissance Orbiter is a robotic probe that is currently making detailed observations of the Moon’s surface from an orbit only a few tens of kilometres from its surface (the Earth is around 400 000 kilometres from the Moon). The Apollo landing sites are visible in these observations.