Spaceship chasing an asteroid to slingshot past Earth

[Buster's Uncle]

Spaceship chasing an asteroid to slingshot past Earth
CNN
By Amanda Barnett,Updated 1217 GMT (2017 HKT) September 22, 2017


Story highlights
OSIRIS-REx will swing by Earth on Friday
Spaceship is on its way to sample an asteroid



An artist's concept of what the OSIRIS-REx spacecraft will look like as it orbits asteroid Bennu.



 (CNN) — A spaceship will swing by Earth on Friday and use our planet's gravity to help it pick up speed on its way to explore an asteroid.

NASA's OSIRIS-REx will fly about 11,000 miles (17,000 kilometers) above Antarctica at 12:52 p.m. ET.

The maneuver, called an Earth gravity assist, will point the spacecraft in the right direction to match Asteroid Bennu's path and speed, NASA said in a statement.

"The Earth gravity assist is a clever way to move the spacecraft onto Bennu's orbital plane using Earth's own gravity instead of expending fuel," the mission's principal investigator, Dante Lauretta, said in the statement.

While OSIRIS-REx is making its flyby, mission scientists at the University of Arizona will test its instruments and use the spacecraft's camera to take pictures of the Earth and moon.

It won't be the last we see of this spacecraft. It will be back in 2023 -- and it will come bearing gifts. NASA launched OSIRIS-REx from Cape Canaveral, Florida, in September 2016 to chase down Bennu, a dark asteroid that could one day threaten Earth.

The probe is scheduled to arrive at Bennu in August 2018 and will survey the asteroid for several months. Then, in July 2020, it will use its robot arm to blast the asteroid with nitrogen, causing it to kick up rocks and dust. It will try to snag a sample of the dust to bring back to Earth in 2023.


Can I see it?

On September 2, a telescope in Arizona snapped the first images of OSIRIS-REx taken from Earth since it was launched a year ago. The Large Binocular Telescope Observatory on Mount Graham captured grainy images of the spacecraft while it was about 7 million miles (12 million kilometers) away.

But can you see it with your own eyes? Yes, but you'll need some equipment. NASA is encouraging amateur astronomers with specialized gear to photograph OSIRIS-REx as it swings by and to share their photos with the space agency.

"The opportunity to capture images of the OSIRIS-REx spacecraft as it approaches Earth provides a unique challenge for observers to hone their skills during this historic flyby," Lauretta said.


Wave to the spaceship

If you don't get a chance to see OSIRIS-REx, don't fret. Just wave. The mission team is asking the public to celebrate the Earth gravity assist by joining in the "Wave to OSIRIS-REx" social media campaign.

No matter where you are on Earth, take a selfie (or a group photo) waving to OSIRIS-REx. Share your photos at hashtag #HelloOSIRISREx.

NASA says you can share your photos at any time -- or wait until the OSIRIS-REx spacecraft makes its closest approach to Earth.

On Tuesday, the OSIRIS-REx team will let us know how everything worked out with the flyby. They'll discuss the results and release images taken by the spacecraft's cameras.


http://edition.cnn.com/2017/09/22/us/nasa-osiris-rex-slingshot-past-earth/index.html

[Geo]

"The Earth gravity assist is a clever way to move the spacecraft onto Bennu's orbital plane using Earth's own gravity instead of expending fuel," the mission's principal investigator, Dante Lauretta, said in the statement.

That's not entirely correct.
You need to expend fuel to break out of Earth's gravity field at the right time (when the probe's trajectory is "bend" into Bennu's orbital plane).

[Buster's Uncle]

A point I've never been entirely clear on - flyby boosting maneuvers work by overtaking a body from behind in its orbit, don't they?  I've never seen that explained adequately, even in the kind of science fiction that should have, and certainly never in science journalism.  I imagine there's also a tidal component possible, the same effect that continually widens the Moon's orbit, but I have trouble imagining it being very significant for a craft that hasn't been doing it for four billion years, though most characterizations of flyby boosts seem to imply that, if anything, not picking up some orbital speed...

[Geo]

A point I've never been entirely clear on - flyby boosting maneuvers work by overtaking a body from behind in its orbit, don't they?

Actually, I don't know. Haven't looked that deep into the manoeuver.

And on second thought, it should be possible to whip through Earth's gravity field and have your trajectory slightly turned if you come in above Earth's escape velocity without expending fuel.

[Buster's Uncle]

Oh, the part about using a gravity well to change direction without spending much fuel is pretty intuitive.

I 'spose I should google "gravity assist" and do some reading...

[Geo]

There's a nice gif for possible "assist" manoeuvers on the wiki page.

https://en.wikipedia.org/wiki/Gravity_assist#/media/File:GravPoss.gif

[Buster's Uncle]

Yeah; I've seen stuff like that lots of times, but it doesn't tell me how it works.

[Geo]

Sufficiently advanced physics are...

[Buster's Uncle]

More something Lorizael might be able to explain.  Gotcha.

[Buster's Uncle]

-The Wikipedia article attached to that .gif DID answer my question, BTW.  It is orbital speed being picked up.  Rotational momentum was not mentioned at all, though I really thought that was what Arthur C. Clarke was getting at in something I read long ago - but I must have misunderstood; he knew his orbital mechanics too well to blow that one...

[Geo]

-The Wikipedia article attached to that .gif DID answer my question, BTW.  It is orbital speed being picked up.  Rotational momentum was not mentioned at all, though I really thought that was what Arthur C. Clarke was getting at in something I read long ago - but I must have misunderstood; he knew his orbital mechanics too well to blow that one...

I think it is rotational momentum that aids rockets being sent in orbit.

[Buster's Uncle]

Yes, of course.  That's why when you here about a launch in the far north -Alaska and such- you assume it's aiming at a polar orbit where Earth's rotational momentum at lower altitudes would actually get in the way.

[Lorizael]

Are my services still required?

[Buster's Uncle]

Dunno.  Are we still getting anything grossly wrong?

(Besides me saying "overtake" that is - something more like "intercept, swing 'round and leave in the general direction of the body's orbit" is closer....)

[Lorizael]

I see Geo corrected his earlier misstatement that fuel would be required for the maneuver. You can think of the probe as being in a hyperbolic (unbound) orbit around the Earth, which means it's never moving slowly enough (or getting close enough) to be trapped by Earth's gravity.

As far as what's happening, the probe steals a bit of the Earth's orbital angular momentum, which speeds it up and imperceptibly slows down the Earth in its passage around the sun. If the probe were going the opposite direction (against the Earth's orbit), it would slow down (and imperceptibly speed up the Earth).

And in this particular case, the delta-v is being used to change the direction of the probe rather than the magnitude of its velocity.