Dawn Spacecraft Closing In on Vesta

[lpetrich]

Dawn Closing In on Vesta: The Movie : Greg Laden's Blog
Dawn Mission: Dawn - Home Page

The Dawn team has made a short movie from some pictures the spacecraft took of the asteroid recently:
Vesta's surface comes into view (QuickTime)

Not much detail, but its surface already looks rather lumpy.
The spacecraft will arrive at the asteroid on July 16, and will orbit it for a year. It will then depart and head for the asteroid Ceres, arriving on February 2015. It had been launched Sep 2007, and it flew by Mars Feb 2009.

It is propelled by ion engines, which have an exhaust velocity of 30 km/s. This is much better than combustion-fueled rocket engines:

Propellant	Temperature	Exh Vel	Dawn / ()
Hydrogen, Oxygen	20 K	4.4 km/s	6.8
Kerosene, Oxygen	90 K	3.5 km/s	8.6
Various	~300 K	2.7 km/s	11

The last one refers to various room-temperature liquid fuels, and also solid fuels.

Exh Vel = exhaust velocity -- the more then better

It would be impractical for the Dawn spacecraft to carry around a refrigerator to keep oxygen liquid, let alone hydrogen. So for the velocity changes it has been doing, it requires over 10 times less propellant than the most feasible chemical propellants.

The main downside of ion engines is that their thrust is teeny teeny tiny -- Dawn's have a thrust of about 0.09 newtons. The spacecraft's weight is 1250 kg, giving an acceleration of 7.2*10^-5 m/s². This is about 140,000 times less than the Earth's acceleration of gravity.

But the engines are run almost continuously, and the acceleration adds up.


Not surprisingly, the spacecraft is powered by solar panels. Those panels even power the engines.

[Quath]

Pretty cool!

[lpetrich]

Dawn Mission: News & Events > Dawn Team Members Check out Spacecraft
Dawn Mission: Mission > Mission Status
Had some trouble with a control system, trouble which shut down its engines for a few days. But the spacecraft is back in operation, and its due to go into orbit some time in the next few weeks.

Dawn Mission: News & Events > Dawn's Approach to Vesta -- some nice video (QuickTime)


I forgot to mention it earlier, but the MESSENGER spacecraft is now in orbit around Mercury. It went into orbit on March 18 this year, the first spacecraft ever to do so. It was launched in 2004, and it flew by the Earth once, Venus twice, and Mercury three times before going into orbit around that planet. All to slow down and move inward without using a LOT of rocket fuel.

It's functioning well, and returning lots of pictures from Mercury orbit.
MESSENGER Web Site

The New Horizons spacecraft is still on its way to Pluto. It passed Uranus's orbit on March 14, and it's due to arrive at Pluto in 2015. It was launched in 2006, and it flew by Jupiter in 2007.
New Horizons Web Site

[lpetrich]

On July 14, JPL issued a press release: Dawn Mission: News & Events > NASA Spacecraft to Enter Asteroid's Orbit on July 15

That article has a nice picture of that asteroid:

Dawn Mission: News & Events > Image of Vesta Captured by Dawn on July 9, 2011

July 14, 2011 - PASADENA, Calif. -- NASA's Dawn spacecraft obtained this image with its framing camera on July 9, 2011. It was taken from a distance of about 26,000 miles (41,000 kilometers) away from the protoplanet Vesta. Each pixel in the image corresponds to roughly 2.4 miles (3.8 kilometers).
Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

[lpetrich]

NASA - NASA's Dawn Spacecraft Enters Orbit Around Asteroid Vesta

PASADENA, Calif. -- NASA's Dawn spacecraft on Saturday became the first probe ever to enter orbit around an object in the main asteroid belt between Mars and Jupiter. ...

The spacecraft relayed information to confirm it entered Vesta's orbit, but the precise time this milestone occurred is unknown at this time. The time of Dawn's capture depended on Vesta's mass and gravity, which only has been estimated until now. The asteroid's mass determines the strength of its gravitational pull. If Vesta is more massive, its gravity is stronger, meaning it pulled Dawn into orbit sooner. If the asteroid is less massive, its gravity is weaker and it would have taken the spacecraft longer to achieve orbit. With Dawn now in orbit, the science team can take more accurate measurements of Vesta's gravity and gather more accurate timeline information.

Vesta's mass was previously measured by watching other asteroids pass close to it and then finding out how much Vesta has pulled them.

The Prius of Space - NASA Jet Propulsion Laboratory

The engine is called NASA Solar Electric Propulsion Technology Applications Readiness. Most people in the deep space exploration business just refer to it as "ion propulsion." The juice is, of course, electricity, courtesy of 54 feet of electricity-producing solar array. The gas is xenon, an inert, colorless gas that is four times heavier than air and is the propellant of choice for asteroid explorers everywhere.

"Each of our three ion engines weighs in at 20 pounds and is about the size of a basketball," said Patel. "From such a little engine you can get this blue beam of rocket exhaust that shoots out at 89,000 miles per hour. The fuel efficiency of an ion engine is an order of a magnitude higher than chemical rockets and can reduce the mass of fuel onboard a spacecraft up to 90 percent. It is a remarkable system."

89,000 mph = 143,000 km/h = 25 miles / s = 40 km/s

But why use other kinds of engines when one has ion engines?

"We have a powerful rocket to cover those initial 175 miles," added Brophy. "Our Delta II Heavy will give the Dawn spacecraft enough energy to leave Earth's atmosphere and its gravitational sphere of influence. But getting into space is just the beginning. There will still be a lot of motoring ahead."

By comparison, ion engines have VERY tiny thrust; they are unable to lift themselves off our planet's surface.

"Hold a piece of notebook paper in your hand. The weight of that paper pushing against your hand is the same as the thrust provided by one of Dawn's ion engines -- at full throttle I might add," said Brophy. "If you had an ion engine firing here on Earth, it would not be able to push a skateboard across a sidewalk!" ...

At first glance, Dawn's full throttle, pedal-to-the-metal, performance is a not-so-inspiring 0-to-60 mph in 4 days.

By comparison, its Delta II booster rocket starts off with 0-to-60-mph in 2 seconds, and it gets to 0-to-60-mph in 0.4 seconds when it runs out of first-stage fuel.

But consider this - because of its incredible efficiency, it expends only 40 ounces of xenon propellant during that time. And then take into consideration that after those four days of full-throttle thrusting, it will do another four days - and then another four. By the end of 12 days the spacecraft will have increased its velocity by over 180 miles per hour, with more days and weeks and months of continuous thrusting to come. After a year Dawn's ion propulsion system will have increased the spacecraft's speed by 5,500 mph while consuming the equivalent of only 15 gallons of fuel. By the end of its mission Dawn will have accumulated more than 5 years of total thrust time, giving it an effective change in speed of about 23,000 mph.

So after a year, it's velocity change is 5500 mph = 8700 km/h = 1.5 mi/s = 2.5 km/s.

Its total change will be 23,000 mph = 37,000 km/h = 6.4 mi/s = 10.3 km/s.

However, all the Delta II's engines have an exhaust velocity of about 3 km/s, which is about 13 times less than the Dawn engines' exhaust velocity, meaning 13 times as much fuel for the same velocity change.

[Quath]

Very cool!

[lpetrich]

Dawn is now in orbit around Vesta, and as I write this, it has spiraled down to a survey orbit. It continues to take pictures, like Dawn Mission: News & Events > Image of Vesta Captured by Dawn on July 31, 2011

In other spacecraft news, Juno was successfully launched, and that spacecraft is now on its way to Jupiter. Two years from now, it will fly by the Earth to get a gravity assist, and after three more years, it will reach Jupiter.

[BigBlue2]

This is one thing that the Yanks are very good at.

[lpetrich]

Dawn Mission: News & Events > NASA's Asteroid Photographer Beams Back Science Data

Vesta is now in a survey orbit, about 1700 mi / 2700 km above Vesta's surface, where it will take pictures of nearly all of Vesta's surface. After about 20 days, it will go into another orbit, presumably for a more detailed look.

Vesta's equatorial radius is about 165 mi / 265 km, so Dawn is still some distance away.

Vesta's circumference is about 1000 mi, about the distance from Boston to Atlanta or Portland OR to Los Angeles.

I'm also reminded of Marooned Off Vesta (Wikipedia), Isaac Asimov's third short story and first published one. Think of the F&SF science essay he would write if he could.