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Science & Exploration

Frequently asked questions

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ESA / Science & Exploration / Space Science

Thank you for your interest in the ESA Science web site. We appreciate your thoughts and comments. Before e-mailing us, please browse the Frequently Asked Questions below to see if your query has already been answered.

We cannot guarantee to respond to all your comments, but we appreciate your feedback.


What is space science?
Science gives man inspiration and aspiration. Space science makes man look outwards from his planet, towards the stars. Space science tries to answer the ultimate questions…

  • How did our Earth and our Solar System evolve?
  • Where are we in the Universe?
  • Where are we going?
  • Where did life come from, and are we alone in the Universe?


Why does ESA do space science?
Space science is a strategic asset. With it, ESA ensures technological independence, it safeguards a European cultural identity, it supports a science-based society and clearly demonstrates European capability and vision.

ESA does what individual European nations cannot do on their own. Scientists from European nations can function at world-class level in their specialist fields. Working in this way gives a framework for national programmes, and allows integration of the best of national approaches into one joint European approach.

What is the ESA Science Programme for?
The Science Programme underpins ESA. It is the only mandatory element of the ESA programme, and it is therefore both a flagship and symbol for the Agency. It enhances European capability in space science and applications, builds European industrial technical capacity, and brings together European national space programmes.

Where can I find more technical information about ESA's scientific satellites?
Visit the Science & Technology web site for in-depth updates on past, present and future missions.

Where does ESA space science happen?
At ESA’s European Space Technology and Research Centre (ESTEC) in Noordwijk in the Netherlands, scientists create and operate new ESA scientific spacecraft. In addition, space science staff work in Spain, Germany and the United States. No major rocket launches are done from Europe itself.

Why do the Earth and other planets orbit around the Sun?
The motion of something in orbit (like a planet around a star) is a combination of the effect of inertia and of gravity. 'Inertia' means that something moving will continue going in a straight line at a constant velocity unless it is stopped (this is Newton's first law). If there were no gravity, for example, something moving past the Sun would just keep going in the same direction. The force of gravity from the Sun pulls the planet towards the Sun.

However, the material which formed the planets was originally moving too and the inertia from the original motion does not go away; instead you get a sum of the two. That makes for a diagonal motion at any one instant. However, the direction the force of gravity and therefore the planet's direction of motion changes as the planets changes its position relative to the Sun. The result is an orbit around the Sun.

Does the Sun itself spin? If so, how fast?
The rotation period of the Sun varies from about 25 days at the equator to over 30 days at the poles, so it takes roughly two weeks for a feature to cross the solar disc.

How would we know if a distant planet had an Earth-like atmosphere?
If a planet’s surface is similar to Earth, dominated by plants using photosynthesis, atmospheric carbon dioxide will be steadily converted into oxygen. These oxygen atoms will produce an ozone layer that emits particular, unique spectral lines in the light spectrum of that planet. We should be able to see the 'ozone line' in the spectra of planets orbiting nearby stars. We can also detect carbon dioxide absorption features and even methane and ammonia.

Together, these lines will tell us the principal atmospheric constituents, and whether the planet is Earth-like or not. If you detect ozone, there must be an Earth-like surface and atmosphere because free oxygen cannot exist in an atmosphere without constant replenishment. The only chemical process we know to replenish oxygen on a planetary scale is photosynthesis.

Why can't we get pictures of planets outside the Solar System?
Direct imaging is possible, but the planets are too far away, too small, and not bright enough to make detailed observations of the surface, for example. Also, most of them are so close to their stars that the glare from the nearby star overwhelms the image and hides the much fainter planet. A planet like Jupiter at the same distance from its star is about one million times fainter, optically, than the star. This kind of contrast is very hard to detect so close to the main star.

What are the risks of nuclear power sources for space probes?
There has been extensive discussion on this topic sparked by attempts to block the Galileo and Ulysses launches on grounds of the plutonium thermal sources being dangerous. Numerous studies claim that even in worst-case scenarios (explosion during launch, or accidental re-entry at interplanetary velocities), the risks are extremely small.

There's a vacuum in space, so how do we know if the Sun makes a noise?
The Sun is full of all sorts of sound waves travelling through the interior. Some of them resonate because they are particular frequencies. However, you cannot hear the Sun using a microphone. Instead, we see the sound waves because of the effect they have on the Sun.

These sound waves are reflected at the surface, although it is not solid. However, there is an abrupt change in density, which allows the reflection to happen. As they reflect at the surface, the sound waves cause a change in brightness and push up some material too. With the kind of instruments as carried on board SOHO, we can measure the changes in brightness and the velocity changes at the surface. From those measurements, we can work out the sound of the Sun.

How big are the largest sunspot groups?
In 1947, experts measured a sunspot group that was 18 times the surface area of Earth (or 6132 millionths of the solar disc). Sunspot groups are numbered sequentially as they are identified. If a group rotates around the far side of the Sun and then back into view, it is given a new number when it is spotted again at the east limb of the Sun.

Do sunspots affect the climate here on Earth?
Sunspots are slightly cooler areas on the surface of the Sun, due to the intense magnetic fields. There is a little bit more radiation coming from the Sun when it has more sunspots, but the effect is so small that it has very little (direct) impact on the weather and climate on Earth.

However, there are many important indirect effects. Sunspots are associated with so-called 'active regions', which are large magnetic structures containing very hot material. They cause more ultraviolet radiation (the rays that give you a suntan or sunburn) and extreme ultraviolet radiation (EUV). These types of radiation affect the chemistry of the upper atmosphere (for example, producing ozone). Many active regions produce giant outflows of material, called Coronal Mass Ejections (CMEs). These drag with them some of the more intense magnetic fields found in the active regions. The magnetic fields act as a shield for high-energy particles coming from outside the Solar System. These 'cosmic rays' cause ionisation of molecules in the atmosphere. The ionised molecules or dust particles can act as 'seeds' for drop formation, thereby causing clouds to form. If clouds are formed very high in the atmosphere, the net result is a heating of Earth - it acts as a 'blanket' that keeps warmth in. If clouds are formed lower down in the atmosphere, they reflect sunlight better than they keep heat inside, so the net result is cooling.

What is a comet tail made of?
If you condensed the entire 160 000 km long tail of a comet such as Hale-Bopp to the density of water, it would not even fill a swimming pool! Comet tails are made up gas and dust. These evaporate off the comet itself, which is made of ice and rock. The ice is both regular water-ice and other kinds, like carbon dioxide ice (also known as 'dry ice'). Comets often have two tails: a 'plasma tail', which is made up of charged ions, and a 'dust tail', which is made up of dust. The lighter ions are 'blown' directly away from the Sun by the solar wind. The heavier dust particles form a tail that points at an angle closer to the path of the comet's motion.

Do you have to wear safety goggles to observe the Sun?
Yes. Never look at the Sun with your naked eyes, not even for a quick glance. Obey this rule and your retinas will thank you. Astronomers using telescopes on the ground observe with special filters that block out much of the Sun's light. There are kinds of 'safety glasses' you can use to look at the Sun briefly. However, unless you are very sure you have the right kind, it is better to project a light from the Sun on a wall or piece of paper and look at that.

Do rocket launches harm the ozone layer?
From time to time, people claim that chemicals released from some rockets, in particular NASA's Space Shuttle's Solid Rocket Boosters (SRBs), are responsible for a significant amount of damage to the ozone layer. Studies indicate that they have only a minute impact (less than seven-thousandths of one percent), both in absolute terms and relative to other chemical sources. It is much less than total ozone variability associated with volcanic activity and solar flares.

What is ‘space weather’?
‘Space weather’ is a concept that is growing in importance as mankind pushes outwards from limits within which we live. The Sun is the source of the solar wind; a flow of gases from the Sun that streams past Earth at speeds of more than 500 kilometres per second. Disturbances in the solar wind shake Earth's magnetic field and pump energy into the radiation belts. Regions on the surface of the Sun often flare and give off ultraviolet light and X-rays that heat up the Earth's upper atmosphere.

This is ‘space weather’ and it can change the orbits of satellites and shorten mission lifetimes. The excess radiation can physically damage satellites and pose a threat to astronauts.

Shaking Earth's magnetic field can also cause current surges in power lines that destroy equipment and knock out power over large areas. As we become more dependent upon satellites in space, we will increasingly feel the effects of space weather and need to predict it.

Is there really a face on Mars?
There is a big formation on Mars that looks remarkably like a humanoid face in some images. It appears in two different frames of Viking Orbiter imagery. The feature, about 2.5 kilometres across, is located near the border between region Arabia Terra and region Acidalia Planitia.

Although most analyst agreed that the resemblance is most likely coincidental and caused by lighting effects, science writer Richard Hoagland has championed the idea that the ‘face’ is artificial, intended to resemble a human, and erected by an extraterrestrial civilisation.

In April 1998 NASA's Mars Global Surveyor spacecraft ended all speculation by photographing the site of the allured 'face' in high resolution. The photos revealed it to be a natural landform.

If you find a trace of life (present or past) on Mars or Titan, how can you be sure you didn’t carry it with you from Earth?
The international scientific organisation COSPAR (Committee on Space Research) has established strict requirements for an adequate ‘planetary protection’. Accordingly, Beagle 2 for example was sterilised so that it contained less than 300 microorganisms per square metre at launch, and no more than 300 000 inside the whole launcher. By comparison, the cleanest kitchen on Earth has several thousands of millions of microorganisms on it. With such level of sterilisation, it has been demonstrated that instruments will not detect anything coming from Earth.

Do you think it will be feasible to send people to Mars in the short-term?
Perhaps in 20-25 years time. There is still a lot for us to learn before we send people there.

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