Present Exploration of the Solar System

Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.naic.edu/~pfreire/space/index02.html
Äàòà èçìåíåíèÿ: Wed Feb 20 21:49:25 2008
Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 05:28:16 2012
Êîäèðîâêà:

The best portrait book of the solar system I have ever seen is Beyond: Visions of the Interplanetary Probes, by Michael Benson, but with so many interesting missions going on at the moment, this is fast becoming outdated. The same applies to any book on solar system studies. On a related topic, check Björn Jónsson's site. I really like the kind of stuff he does (realistic computer art, with the solar system as the subject).

Another stunning image from the Cassini spacecraft: The moon Enceladus. The blue cracks near the south pole (at left) is where water vapor is being released, due to cryovolcanic activity. This process is what keeps Enceladus whiter than snow, by coating it uniformly with newly formed ice. This water volcanism has recently been imaged (see below, click on the image for a much closer look). This implies that there is liquid water just a few meters below the surface! Furthermore, this water seems to have substantial amounts of organic compounds! Enceladus might therefore have , within easy reach, environments capable of systaining life!

The seventies were the first golden age of robotic solar system exploration. The two most spectacular missions of the decade were the Voyager (launched in 1977) and Viking (launched in 1975) double missions. The 1990's witnessed a rebirth of this endeavour, with the launch of probes like the Galileo spacecraft to Jupiter, the Magellan spacecraft to Venus, and several probes to Mars and the asteroids. Here are some of the highlights of recent years:

2004

Late 2003, early 2004: ESA's Mars Express orbiter arrives in Martian orbit, starts program or orbit adjustment
January 4, 24: NASA's Mars Exploration Rovers (Spirit and Oportunity) arrive at their destination.
March 2: Launch ofi ESA's Rosetta Mission. This is a lander/orbiter mission to a comet, that will probe it as is approaches the Sun and starts outgassing.
June 30: Arrival at Saturn of NASA's Cassini spacecraft, which was launched in 1997. It has since the arrival time been obtaining stunning results (see pictures above and below), these will continue flowing for several years to come.
August 3: Launch of NASA's Messenger spacecraft, this will be the first probe to orbit Mercury.

2005

Januray 14: Landing of ESA's Huygens probe on Titan.
July 4: Crash of NASA's Deep Impact probe into comet Tempel 1.
August 12: Launch of NASA's Mars Reconnaissance Orbiter
November 9: Launch of ESA's Venus Express spacecraft.

2006:

January 19: Launch of NASA's New Horizons mission, the first ever sent to the Kuiper Belt. First stop: Pluto and its satellite system.
March 10: Arrival of the Mars Reconnaissance Orbiter to its destination.
May 7: Arrival of the Venus Express probe to its destination.

2007:

February 28: New Horizons flies by Jupiter.
August 4: Launch of NASA's Phoenix spacecraft towards Mars.
September 27: Launch of NASA's Dawn spacecraft towards Ceres and Vesta.

2008:

January 14 and October 6: The Messenger spacecraft flies by Mercury, and snaps the first pictures of that planet in 33 years.
May 25: Arrival of the Phoenix spacecraft at Mars.
June 30: End of Cassini's 4-year prime mission. Start of the first 2-year extended mission.

Dione, Saturn and Saturn's rings.

Robotic Mars Exploration

After 20 years of absence, with a few failed missions in between, we have finally returned to Mars. The Pathfinder mission arrived to Mars in 1997, and gave us a lot of nice pictures, the first since the demise of the last Viking Lander in 1982. In 1997, the Mars Global Surveyor also arrived at the Red Planet. This mission, no longer in operation, provided high resolution images of Mars from low orbit, and also MOLA's global topographic map of unprecedented accuracy.

After two failed attempts to reach Mars in 1999, NASA has returned in 2001 with 2001 : Mars Odyssey. This spacecraft has made measurements of the amounts of subsurface ice of Mars which are of extreme importance for the future of this planet's exploration. In late 2003 and early 2004, two more probes have arrived: ESA's Mars Express orbiter, and NASA's 2003 Mars Exploration Rovers (Spirit and Oportunity). With data from the Opportunity rover, it has been determined that the region of Mars where it landed (Sinus Meridianii) was once covered in water, which was part of a salty, acidic sea that seems to have persisted for a long time. This essentially confirms the idea of a wet early Mars, and has fundamental implications for the search for past life in that planet. Meanwhile, the Mars Express Orbiter has obtained fantastically detailed 3-D color pictures of the whole Martian surface, and has detected the presence of methane in the Martian atmosphere, which might indicate the existence of present life. Since March 2006, these spacecraft were joined by the Mars Reconnaissance Orbiter. This has provided the best, most detailed pictures ever returned from Mars orbit, and it will return ten times more data than all other previous missions combined.

In May 2008 these spacecraft will be joined by the Phoenix lander, the first to explore the high latitudes of Mars. In the Fall of 2009, the Mars Science Laboratory will be launched, arriving at Mars in October 2010. These missions are part of NASA's Mars Exploration Program.

The picture above was taken by the Spirit rover in the "Home Plate" geological formation.

Kuiper Belt

Before the Space Age, astronomical observations via ground-based telescopes were the only way of exploring the solar system. Such studies are still extremely useful, in fact, for the very edge of the solar system, distances are so large that exploration with space probes is not a practical proposition: ground-based observations are still the only alternative. This does not mean that the results are not important, quite the contrary.

The discovery of the long-sought source of the short-period comets, the Kuiper Belt, has been the single most important advance in the study of the solar system since the discovery of Pluto. This started in 1992 with the discovery of the first Kuiper Belt object after Pluto, 1992 QB1, by David Jewitt and Jane Luu. Since then more than 1000 other objects have been found beyond the orbit of Neptune. These objects are remnants of the formation of the outer solar system, and their dynamics are a direct consequence of the formation and history of the outer giant planets. As such, they contain fundamental clues to the understanding of the formation of the solar system. Another clue is the recent finding that a surprisingly large fraction of these objects are in binary systems (three of the four largest objects in the Kuiper Belt have satellites) which suggests, if we accept the idea that these satellites were formed from collisions, that the region where they formed had a much larger density of objects in the distant past.

In July 2005, it was announced that one of these objects, Eris, is apparently larger than Pluto. The discovery of the moon of Eris, Dysnomia, will allow the measurement of the mass of this object.

Before the discovery of 1992 QB1, it was thought by most people that Pluto was the oddity among the planets. The discovery of all the new Kuiper Belt objects, and the redefinition of the term "planet" has put Pluto in context. The discovery of Eris and other objects similar in size to Pluto probably means that the vast majority of the planetary objects in the solar system are icy dwarf planets, and that the eight inner planets are the odd ones out! That is a very significant change in the way we view the solar system.

We should expect exponential growth in the number of members of the Kuiper Belt in the next few years, the number of objects larger than 100 km between 30 and 50 a.u. from the Sun is presently estimated to be at least 100,000. Many, if not most of these, will probably be found with the Large Synoptic Telescope.

The three main categories of Kuiper belt objects known are:

The classical Trans-Neptunian objects. These lie beyond Neptune. One sub-class of these objects (the Plutinos) complete 2 orbits around the Sun while Neptune completes 3, the orbital configurations are such that these objects never come close to Neptune. Because Pluto does exactly that, these new objects were called "Plutinos", meaning literally "Little Plutos". For an updated list of the classical KBOs, look at the IAU KBO Table.
A second group consists of the "Scattered" Trans-Neptunians. These have perihelia between 30 and 40 a.u., but the aphelia can be considerably more distant. They were clearly scattered by Neptune
Finally, the objects that lie between the orbits of Saturn and Uranus are called "Centaurs". The first such object was discovered in 1977, and it was called Chiron, one of the Centaurs. These objects are thought to originate in the Kuiper Belt, after being perturbed by Neptune, and are dynamically similar to the Scattered Trans-Neptunians. For this reason, both groups are listed together in the IAU Centaur & SKBO Table.

Another recent, exciting result is the discovery of Sedna. Not only is the object very large, but the perihelion of this object (at 75 a.u. from the Sun) is far outside the outer edge of the Kuiper Belt, which is at about 50 a.u. It follows that, if it was ever associated with the inner solar system, it had to be perturbed by Neptune and then another object further away. The discovery of this object implies the existence of hundreds of other objects with similar sizes and similar orbits. This is, beyond the Kuiper Belt, a whole new region of the solar system!

For news on the study of the Kuiper Belt, have a look at the Distant EKOs newspage. For an updated plot of the positions of all these objects, click here. For a comparison with the number of objects known in the inner solar system, click HERE.

Trojans, quasi-satellites and irregular satellites

Apart from the Asteroid and the Kuiper Belts, small bodies are also found in very large numbers in two other large, stable regions of the Solar System:

Jupiter is known to have a large number of asteroids sitting in or near its Lagrangian points. These are called Trojans, and they are as numerous as the main asteroid belt!
Neptune is also known to have is own Trojans. Taking into account the fact that these are much further from the Sun, it is now known that this region has even more objects and more mass than the Jupiter Trojan belt!

Curiously, the only other planet in the Solar System that has Trojans is Mars, but here the number of objects is much smaller. Surprisingly, the two massive planets Saturn and Uranus have no known Trojans, nor are these Trojans expected to be stable over the age of the Solar System.

Although the Earth and Venus do not have any known Trojans yet, they have several "attendants".

Cruithne is an asteroid whose orbit has a 1:1 resonance with the Earth. As such, its annual trajectory in the sky changes only very slowly. Its orbit is known as a spiralling horseshoe orbit, in which, apart from the asteroid's 1-year orbit around the Sun, its position changes slowly between two extrema (the Earth-Sun Lagrangian points) every 385 years.
An even more interesting case is that of 2002 AA29. Most of the time, this asteroid is in a spiralling horseshoe orbit as well. However, once in a while it becomes a quasi-satellite of the Earth, apparently orbiting our planet with an orbital period of 1 year!
In 2004, the first present quasi-satellite was found orbiting Venus, 2002 VE68.
In the same year, the first and also present quasi-satellite of the Earth, 2003 YN107, was also found!

The other very cool search has, since 1997, found more than 100 new small objects in the other stable regions of the solar system, the Hill spheres of the planets, where they are trapped as true satellites (see my updated satellite list). The vast majority of these objects have irregular orbits around their parent planets, i.e., orbits so distant from the parent planet that they are very strongly perturbed by the Sun. This number is larger than the total number of satellites previously known.

Forty-seven irregular satellites were found orbiting Jupiter. This planet has now a total of 63 known moons, of which 55 have irregular orbits.
The Cassini spacecraft has found 4 new regular satellites orbiting Saturn, (Daphnis, Methone, Pallene and Polydeuces). Ground-based projects have found 37 small irregular satellites. This planet now has a total of 59 known satellites.
Three new inner satellites were confirmed orbiting Uranus using the HST (Perdita, Cupid and Mab), and 9 new outer irregular satellites. This planet now has a total of 27 known satellites.
Five new irregular outer satellites were found orbiting Neptune. This planet now has a total of 13 known satellites.

It is estimated that if we could find all the outer satellites to a size of about 1 km, all the outer planets would have about 100 irregular moons!
I find these discoveries fascinating, mostly because of their orbits. These form groups that suggest that these little moons result from the breakup of larger objects. The origin of these satellites is still not understood, the best hypothesis is that they are captured asteroids. How the capture proceeded no one knows, but clearly these satellites hold clues to understanding how the giant planets formed.