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Frequently Asked Questions

Below are answers to frequently asked questions we receive from interested folks like you! Click on each question to reveal the answer.


1. How long does it take spacecraft to get to a planet?

When we send a spacecraft to a planet or other object (asteroid or comet), the spacecraft is actually just put into an orbit around the Sun that will bring it close to the planet, etc. You have to deal with the gravity of the Sun. Here are some numbers I looked up: Mars: 7 months, Venus: 5 months, and Mercury: 5 months.

Warning: it gets complicated

I mentioned above that the closer a planet is to the Sun the faster it moves around the Sun. However, when you leave the Earth to go to another planet, you have to fight the gravity of the Earth and the Sun (like throwing something into the air). If you just want to fly by something, you are not too worried about how fast you go by. But, if you want to orbit or land, you want to be able to match the speed of the planet. For Mars, you have to give a spacecraft extra energy to move it away from the Earth and the Sun (if you throw something into the air, it slows down and stops, but to throw it higher, you have to give it more energy to begin with). When going closer to the Sun, however, you are still working against the gravity of the Sun. Drop a plate out of your hand and it may be moving fast enough to break it when it hits the ground. Drop it out of a second story window and it will hit the ground much faster thanks to gravity. The same is true for sending a spacecraft to Mercury. If you send something from the Earth to Mercury (the MESSENGER spacecraft), you have to fight against gravity to make sure it is not going too fast if you want to orbit around Mercury. You can use a planet to speed you up or slow you down. This is called a gravity assist. You use the planet instead of using a lot of rocket fuel. To slow it down, MESSENGER flew by Earth once and Venus twice before it headed for Mercury. It flew by Mercury three times, slowing it down each time. It will finally “match” Mercury’s orbital speed and be able to go into orbit in March, 2011. Its first flyby was 3.5 years after launch and it will be 6.5 years from launch until it goes into orbit!

Spacecraft Target Time Comment
Apollo Moon 3 days Slow down to orbit
Magellan Venus 15 months Slow down to orbit
Phoenix Mars 11 months Slow down to orbit


4 years Mars flyby gravity assist, using efficient fuel source so, slow but steady
Galileo Jupiter 6 years Two Earth flyby gravity assists
Messenger Mercury 6.5 years Earth flyby, two Venus flybys, three Mercury flybys before it will eventually orbit Mercury
Cassini Saturn 7 years Jupiter flyby gravity assist, orbit Saturn
1 & 2
13, 23 months
3,4 years
8.5 years
12 years
Voyager 1 was on a faster orbit that went only to Jupiter and Saturn. Both are now far out of the Solar System. Voyager 2 is about 90 au from the Sun and Voyager 1 about 110 au (1 au = Earth distance from Sun or astronomical unit)
New Horizons Pluto 9.5 years Jupiter flyby gravity assist

2. Can carbon isotopes used for age estimates of meteorite samples?

We have an activity that helps explain this in one of our professional development workshops, Exploring the Terrestrial Planets. You can use the radioactive elements to measure the age of rocks and minerals. Below is a list. Their useful range is from about 1/10 their half-life (the time it takes for half of the radioactive element/isotope, the parent, to convert into a non-radioactive element/isotope, the daughter) to 10 times their half-life. For example, 40Potassium decays to 40Argon. You can use this to measure the age of a rock from about 128 million years to more than 10 billion years (the Solar System is 4.56 billion years old). To answer your question, 14Carbon can only measure things up to just over 50,000 years old. So its great for determining when someone built a wood fire, but not good for determining the age of a meteorite.
Parent Daughter Half-Life (billions of years)
238Uranium 4Helium, 206Lead
235Uranium 4Helium, 207Lead 0.704
232Thorium 4Helium, 208Lead 14.0


147Samarium 143Neodymium 106
87Rubidium 87Strontium 48
40Potassium 40Argon 1.28
14Carbon 14Nitrogen 0.0000057

3. How do scientists estimate the age of planets (date samples) or quantify planetary time (relative and absolute age)?

We have rocks from the Moon (brought back from Apollo missions and also from meteorite finds) and we have rocks that we now know came from Mars. We can then use radioactive age dating in order to date the ages of the surfaces (when the rocks first formed, i.e. lava from which cooled from a molten state). We also have pieces of asteroids and can date them, too. These are the surfaces that we can get absolute ages for. For the others, one can only use relative age dating (such as counting craters) in order to estimate the age of the surface and the history of the surface. The biggest assumption is that, to first order, the number of asteroids and comets hitting the Earth and the Moon was the same as for Mercury, Venus, and Mars. There is a lot of evidence that this is true. The bottom line is that the more craters one sees, the older the surface is.

4. What are the chances of a meteor hitting the Earth?

First of all, the chances of any meteor of hitting Earth are… 1! This is because a meteor is the visible display of a meteoroid (a small object, ranging from a dust grain to pebble-size) entering the Earth’s atmosphere and "burning up" while traversing it (usually never reaching the Earth’s surface). Therefore, a meteor, by definition, is an object already hitting the Earth!

However, if the question is related to any object, small (in which case we usually refer to them as "meteoroids") or large (in which case we usually refer to them as "asteroids" or "comets"), then we need to look at the probability of different sized objects hitting the Earth. In the "Chances of Impact" activity from one of our professional development workshops, we discuss this probability, which is summarized in the impact hazard figure. The chance of any object hitting the Earth varies with object size: pebble-sized objects hit the Earth everyday; Tunguska-sized objects (equivalent to a small house) hit the Earth every few centuries; Meteor Crater-sized objects (medium house) hit the Earth every millennium or two; civilization-threatening objects (roughly the size of our “A” mountain) will hit Earth every million years or so).

Obviously, we are not particularly afraid of pebble-sized objects, mainly because they never make it through the Earth’s atmosphere, but even a Tunguska-sized object may create havoc if it ever hits or explodes over a city.

5. Do all terrestrial planets have an equal chance of being hit by objects?

The short answer is no. If you look at the distribution of objects that could potentially hit the terrestrial planets— Mercury, Venus, Earth, and Mars— the closer you are to the asteroid belt, the ultimate source of the Near Earth Objects (NEOs), the more often that you will get hit by one of them. It gets more complicated when one looks at the satellites of the outer planets. Beyond the asteroid belt, there are fewer asteroids, but there are more comets. So, it is believed that comets are the dominant impactors of these satellites.

6. Are there any asteroids/meteorites headed toward Earth in the next 100 years?

Small asteroids and meteoroids are always coming close to the Earth and sometimes along a direct path to striking the Earth (there are no stop signs or yield signs in space!). Surveys such as the Catalina Sky Survey are doing a good job of find most of the larger asteroids, the ones that can cause severe damage. But, there are more out there, mostly in the size range that could eliminate a city. However, unlike in hollywood movies, there is nothing very large that is predicted, in the near future, to have any chance of hitting the Earth.

The largest possible threat is from the asteroid 99942 Apophis. It has one chance in 250,000 of hitting the Earth in 2029 and about one chance in 300,000 of hitting the Earth in 2036 (due to the uncertainty of its orbit). It has a diameter of about 250 meters, so could create an impact crater about 2 or 3 kilometers in diameter. This would be bad for a city and for tens of kilometers around it, but not much beyond that. There is nothing that is big enough to do anything in the near future. Space is big and if there were a new comet heading toward us right now and big enough to cause planet-wide damage, we would already have seen it with our telescopes. However, it is likely that something could in the future hit the Earth. For that reason, there are people who are looking at ways to prevent this from happening and do have meetings to discuss plans for such an event.

7. Why did the asteroid belt between Mars and Jupiter not form into a planet?

In a single word, Jupiter. Jupiter probably grew very large very quickly, probably in about 3 million years, early in the history of the Solar System. Much of the material that went to form Jupiter would have come from the region of the present asteroid belt and the remaining material was moved around enough by the gravity of Jupiter to prevent it from forming a planet.

8. What makes an atmosphere? How can gases escape from a planet?

There was recently an article that discussed this very topic. Basically, if you are big enough, like Jupiter or Saturn, you kept your atmosphere that was the remnants of the gas in the solar nebula when the planets were formed (mostly hydrogen and helium). The smaller, inner planets probably got their atmospheres from the outgassing that occurred as they cooled down (spewed volcanic gases). Some scientists think that much of the Earth’s atmosphere came late in Earth’s formation history and was brought in by the last of the impacts that formed the planet (comets and "wet" asteroids). In the case of Venus, it had what is called a "runaway greenhouse". It got hot enough that surface water (if it ever had any) evaporated and greenhouse gasses went into the atmosphere. As the temperature kept rising, surface rocks that contained carbonates heated up and put more carbon dioxide into the atmosphere, another greenhouse gas.

To answer how planets lose their atmosphere, there are several ways: a large impact can literally blow off the atmosphere (which probably happened to Mars). When you heat up the molecules in an atmosphere, the molecules move faster and some of them can escape the gravitational forces keeping them in. The hotter the atmosphere, the more molecules can escape. The smaller the object, the lower the gravity, so the escape velocity is lower and it is harder to retain an atmosphere (e.g., Moon and Mercury). Finally you can lose atmosphere to the surface. How can this be, you may ask? Basically, the atmosphere can condense onto the surface in the form of rain and/or snow. On the Earth, much of the carbon dioxide that is generated is absorbed by the oceans.

9. Would the Earth be different if something had not flung off the material that made the Moon, if that is in fact what happened?

There are two ways to look at this. First, the mass of the Moon is only 1/81 of the Earth and the impactor was probably only about 1/10 the mass of the Earth. So, this would make very little difference to what we have now. Second, the other way to look at it is from the point of how the Moon has affected the Earth. Its been theorized that the tilt of Mars (i.e., obliquity) may have changed significantly and many times over its history. The same could have been true for the Earth, but the Moon is thought to have acted as a stabilizing effect, helping keep the Earth's tilt about the same. Another thing that might have been more important is life on Earth. It is thought that life on Earth may have started in tidal pools. Without a Moon, the evolution of life on Earth may have been different.

10. So what is Pluto? A planet? A dwarf planet?

First off, scientists tend to make definitions in order to classify/group/categorize things. This helps us understand the world around us and to form a picture in our mind of what we are talking about (plant vs. animal, mammal vs. bird, dog vs. cat). Thanks to the IAU (International Astronomical Union), whether we like it or not, we now have a definition for objects such as Pluto— they are dwarf planets. So what is a dwarf planet? It is:

An object in orbit around the Sun that is large enough (massive enough) to have its own gravity pull itself into a round (or nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects.

If we wanted to test this definition, then we need to look at what we know about Pluto and decide if Pluto is more like a planet or a dwarf planet. The decision to define Pluto as a dwarf planet was done by a group of IAU voters.

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