Ion-Engine Spacecraft On Moon Mission

anactofgod writes "The Times On-line and space.com reports that the European Space Agencies Smart-1 probe has cleared the Lagrangian point between the Earth and its moon and is due to enter lunar orbit on Nov 15th. Smart-1's mission is to make observations related to the moon's formation and composition. What's cool about this mission, other than this is the first European mission to Earth's moon, is that the probe is using solar powered ion engine thrusters. Ion engines are an order of magnitude more efficient than chemical engines. NASA flew the first ion-powered spacecraft, Deep Space 1, in 1998. Smart-1 is the second spacecraft to use the technology, and was designed as a testbed for future ESA missions. The ESA is scheduled to fly the ion-powered BepiColombo on a mission to Mercury in 2009."

Comments

Re:Better description

[azav]

The lagrange point is where the gravity from the moon offsets the gravity from the earth, making a point where there is no gravitational pull towards either body.

Re:Better description

[srstoneb]

The lagrange point is where the gravity from the moon offsets the gravity from the earth, making a point where there is no gravitational pull towards either body.

Not quite -- it also includes the "centrifugal" pseudoforce. The Lagrangian points are fixed points within the rotating reference frame of the Earth/Moon system. Because the frame rotates, it is not an inertial frame, so pseudoforces contribute. If the Earth and Moon were held in place magically, instead of orbiting each other, the balance point between us would be in a different location than it is when the system rotates.

Also, of course, there are five Lagrangian points, only one of which is physically between the Earth and Moon. If only gravity mattered, it would be the only one. The other four arise thanks to the contribution of pseudoforces.

Re:Better description

[Cade144]

A nifty diagram is available here.
As I seem to recall, there are a group of asteroids in the L4 and L5 points (with regards to the Sun and Jupiter) called the Trojan Asteroids. Not shure that this is relevent, but the factoid just popped into my head.

Re:Better description

[centauri]

You can see a picture of the Trojan asteroids here. Jupiter is on the left of the page and revolves counter-clockwise. The trailing Trojans at L5 are 60 degrees behind Jupiter, near the top of the picture and the leading Trojans are, of course, 60 degrees ahead, near the bottom of the page. There are also a smattering of bodies around the L3 point. The Solar-Jovian L3 is closer to Jupiter's orbit than the diagram in the parent post would indicate.

Sometimes, the L4 and L5 Lagrange points of a system are called Trojan points.

This site mentions how the prominent asteroids at the Trojan points are named after characters from the Iliad. One group is named after Greeks, the other after Trojans. Each group also has a "spy" in its midst.

Re:Nuclear Thermal vs. Ion Thruster designs

[anactofgod]

Ion thruster designs have an Isp that start at 3500, and can potentially go much higher. Nuclear thermal designs have an Isp ranging from 825 to 925. By comparison chemical reaction rockets have low specific impulses, ranging from 150-450.

A fisking of TFA

[Engineer-Poet]

Too many newspapers use scientific illiterates to write their science coverage. The Times of London appears to be one of them, if we consider the quality (or lack thereof) of TFA.

a Star Trek-style thruster

Star Trek postulates warp drives (which we have no idea how to build) and "impulse engines". Ion drives are impulse engines, but all rocket motors are impulse engines too.

Had the ion drive fallen just 5 per cent short of maximum thrust, Smart-1 could have collided with the Moon.

It's completely opaque to me how this could be the case. If you don't have enough thrust for one trajectory, you use another.

They work by using electricity from solar panels to charge atoms of the noble gas xenon, which are then fired into space at 1,000mph to power the probe.

The author is obviously innumerate. Impulse of the DS-1 engine peaked at 3100 seconds, for an exhaust velocity of ~30 km/sec. That is not 1000 MPH, it is about 68000 MPH.

This stream of ions accelerates Smart-1 at just 0.2millimetres per second.

Per second squared.

In space, this builds up over time to generate speeds of up to 10miles per second, or 36,000mph.

Except that a mission to the Moon never gets to such speeds; the spacecraft slows down as it spirals outward. Orbital velocity of the Moon around Earth is only about 2200 MPH.

Why newspapers publish drivel like this, I'll never know. If it was hard to get right you wouldn't have amateurs fisking this stuff on Slashdot!