Asteroids and comets are both remnants of the early formation of the solar system 4.5 billion years ago. As of August 2011, there were less than 4,500 known comets in the solar system, compared to over 550,000 known asteroids (although there are thought to be many millions more).

Asteroids are composed of rocky material and metals, while comets are made of ice. As a result, asteroids formed nearer the Sun than comets, because ice could not remain solid at a close distance. Comets that formed further out and later approached the Sun lose material with each orbit because the ice melts, forming a tail behind the body. Asteroids, on the other hand, do not lose material, and thus do not have a tail. Comets are often found in large elongated orbits extending outwards up to 50,000 times the distance from the Earth to the Sun. By comparison, Neptune – the furthest planet of the solar system – is just 30 times further from the Sun than the Earth.

Concurrently, asteroids are usually found following a circular orbit around the Sun and they tend to group together in belts, such as the asteroid belt found between Jupiter and Mars, which was formed when the gravitational pull of Jupiter prevented the asteroids from forming into another planet.

Asked by Jane Parker

The Voyager 1 space probe was launched by NASA on the 5 September 1977, on a mission to study the furthest reaches of our solar system – and it’s still working! Responsible for sending back some our best images of Saturn and Jupiter and their Moons, Voyager 1 is travelling at 61,425km/h (38,168mph) and is just over 17.5 trillion kilometres (10.9 trillion miles) from Earth. It’s so far away that data transmitted by radio waves takes over 11 hours to reach Earth and it’s about to leave our solar system and go into interstellar space (the space between our solar system and the nearest star). Voyager 1 is running out of power and will shut down around the year 2025-2030, but it will carry on speeding through space. In case Voyager 1 is discovered by intelligent life forms, it carries a gold-plated audio-visual disc which contains photos and messages from Earth as well as a mix of Earth sounds, such as a baby crying. We’ll have a long time to wait until we get a reply, because it’ll be just under 40,000,000 years before Voyager 1 passes within 1.6 light years (9.6 trillion miles) of the star AC+79 3888.

David Houston, Science Museum

Source SPACE.com: All about our solar system, outer space and exploration

When travelling in a car at a constant speed, we do not feel the car’s forward motion as our bodies are travelling at exactly the same speed as the vehicle. We become aware of the vehicle’s movement if it accelerates or decelerates, because our bodies are suddenly travelling at a different speed to our surroundings.

Although our galaxy is travelling at 190 miles (300 kilometres) per second, so are we. As the galaxy’s speed is not accelerating or decelerating by any noticeable amount, we are not aware of the motion.

Nathaniel Marten, Science Museum

On Thursday 3 December NASA will hold a press conference, the subject of which has caused worldwide interest. Forums are abuzz with with the possibility that NASA might announce the discovery of extraterrestrial life in our Solar System. However, despite some far-fetched conspiracy claims, the signs are increasingly pointing to “no”. Some media sources claim to have revealed that the announcement will be nothing of the sort, but we can still hope can’t we?

The whole incident is similar to a news story that broke in 2009 when the image of Mercury below, taken by NASA’s Messenger Spacecraft, sparked worldwide interest. Some people claimed the black circle like object in the lower right corner was some sort of UFO, later debunked by NASA as merely being a rock or crater with a different mineral compound to the surrounding area.

The latest hunt for E.T. started a few days ago when NASA released a short summary of what would be in its latest press conference:

An astrobiology finding that will impact the search for evidence of extraterrestrial life. Astrobiology is the study of the origin, evolution, distribution and future of life in the universe

Today, however, it looks like NASA will be denting the hopes of UFO-enthusiasts the world over come tomorrow. It is highly unlikely that there be any announcement regarding the discovery of microbial life on another world. Instead, it is almost certain that NASA will release details of a bacteria which is able to survive in arsenic, an element which was previously thought to be too toxic for life to inhabit.

While disappointing to some, the announcement could still potentially pave the way for the discovery of bacterial life on another celestial body in the future. The bacteria in this instance, found in Mono Lake in California’s Yosemite National Park, raises the possibility that similar microbial life could be found on moons or planets such as Saturn’s moons Titan and Rhea.

You can watch the press conference from 1900 GMT at http://www.nasa.gov but don’t get your hopes of some Klingons walking out on stage too high. However, the announcement will still prove to be very interesting, so we would recommend tuning in.

The Sun is an amazing source of energy and people have long wondered how all that energy is produced. The word burning usually means combustion and if the Sun was burning this way, it would indeed need oxygen. We’re certain this isn’t the case because the Sun wouldn’t last for long.

Nuclear reactions are the only source of energy able to keep the Sun shining for at least the age of the solar system and keeping us warm on a good day.

When physicists say a star is burning hydrogen, they mean it’s joining hydrogen atoms together to obtain helium and release energy. The exact name for this process is nuclear fusion. There is no need for oxygen in these nuclear reactions but with respect to tradition, and as it sounds better, astrophysicists use the word burn. This means that you can use it as well if you want to sound like a pro… just remember the explanation to back it up.

Answer by José Monteiro.

We thought we’d treat the regular How It Works Daily users to an exclusive sneaky peek at one of the questions from next issue’s exciting interview with Professor Brian Cox. We asked him to tell us about his latest series for BBC 2, Wonders Of The Solar System…

Professor Brian Cox: Wonders is a documentary about astronomy and physics. Since the BBC last made a documentary series about the solar system a decade ago, we have lived through what I believe is something of a golden age of exploration. We’ve confirmed that there was once water on Mars, and have seen evidence that there may still be liquid water beneath the Martian surface today. We are now virtually certain that Jupiter’s moon Europa has a liquid water ocean beneath its icy crust that is perhaps 100km deep, and therefore contains more water than all the oceans of Earth. We’ve seen fountains of ice erupting from Saturn’s tiny moon Enceladus, and parachuted to the surface of the giant moon Titan, on which we have discovered lakes of liquid methane, methane snow and methane rain.

To find out what else everyone’s favourite scientist had to say, including the rest of this answer, remember to get your copy of How It Works issue 7, which hits the shops on 22 April – earlier if you subscribe.

Images courtesy of the BBC

They may not be as interesting as the moons around Uranus but Saturn’s rings do bear further investigation…

Saturn – The Roche lobe causes gravitational forces around Saturn to hold rocky particles

Inner rings – Inner rings are made up of rock particles that never formed into a moon

Outer rings – Outer rings are caused by geysers in the south pole of Saturn

Why does Saturn have rings but other planets do not? The answer has to do with something called the Roche lobe, named after a French astronomer. It seems when a planet orbits around a star (eg our Sun) and that planet has its own orbiting objects (eg a moon), a gravitational pull occurs between the objects. Around Earth, orbiting rocks formed into the moon. On Saturn, the rocks never coalesced and are still orbiting.

Interestingly, the rings are only a few miles in thickness because of the highly localised effects from the Roche lobe. Dr Steve Maran, a noted astronomer, says Galileo was the first to discover the rings, but could not explain them. Today, viewing angles from the Hubble Space Telescope reveal an enormous region extending widely around the planet. There’s also one distinct outer ring, which Maran attributes to geysers emitting from the icy southern polar region on Saturn, leaving a more distinct trail.