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12140	13640	Hello, and welcome back.
13640	16240	Live with another competition update for HackASAP.
16240	17980	I'm your host, Jordan Wines.
17980	20800	First, let's take a quick look at our current scoreboard,
20800	23780	which at this point should be pretty familiar to everyone.
23920	25960	Since our last update an hour ago,
25960	28840	we've actually seen no movement on the scoreboard yet.
28840	31280	Challenge 4 is proving to be difficult for the teams,
31280	32920	as we predicted it would be.
32920	35360	But, from the sound of it on the team chats,
35360	37220	they might be getting a little closer.
37220	40140	So, hopefully we'll see some movement soon there.
40500	42740	While we wait a little bit more game action,
42740	45900	let's learn about the complexities of navigation in space
45900	48360	from our resident expert, Jess Coffey,
48360	50740	in another episode of Space Talk.
50940	53820	Now that we've seen how precession takes place,
53820	56620	let's try to predict its direction in this case.
66120	69060	So, how do satellites know where they are?
69180	71560	There are actually two completely separate dimensions
71560	73520	that we need to consider with satellites.
73520	75920	The first being orbital parameters.
75920	78660	This describes the movement of the satellite over time,
78660	82260	kind of like referencing a flight plan to tell where a plane is.
82260	84040	The second is the attitude.
84040	86040	Basically, what direction am I facing?
86040	88980	This is entirely independent of location for satellites,
88980	92860	because there's no natural state that it's expected to be in.
92860	96140	On Earth, that natural state is aligned with gravity,
96140	97740	basically upright,
97740	103140	and satellites have no force or no natural state to adhere to.
103140	105700	Figuring out the orbital parameters of a satellite
105700	109040	requires taking several observations and deriving them.
109040	110580	One way to obtain these observations
110580	113400	is to use the same technology we use here on Earth,
113400	115420	an onboard GPS receiver.
115480	119060	Additionally, we can look for them through external sensors.
119060	120940	NORAD tracks all objects in space
120940	124120	through the United States Space Surveillance Network,
124120	125680	primarily using radar.
125680	128580	Each object in space gets a unique ID assigned to it,
128580	129740	called a NORAD ID,
129740	133840	which is universally used by satellite operators to reference it.
133840	137400	NORAD uses these observations to derive the orbital parameters
137400	140420	and uses two-line elements, or TLEs,
140420	143740	as a standard for providing all the orbital information.
143740	147120	Here's an example of one for the International Space Station.
147120	152360	You can see the ISS's NORAD ID at the beginning of each line, 25544.
152360	155180	The rest is used to describe the orbital motion.
155180	158460	This TLE is simply an estimation of the orbital trajectory,
158460	160500	and there are inherent inaccuracies.
160500	162620	The way we propagate orbital trajectories
162620	166020	introduces an error that becomes significant over time.
166020	169860	The TLE format accounts for this by having a timestamp, or epoch,
169860	171940	built into it to indicate when it was taken.
171940	175460	It is then valid for only about a week or two around that epoch,
175460	178980	depending on how important accuracy is for that specific satellite.
178980	183120	As a result, they must be constantly updated to keep from going stale.
183240	184960	Now we know our orbital parameters,
184960	187440	what about the second category, attitude?
187440	189620	Space is full of a lot of nothing,
189620	192340	but there are still a few somethings that you can sense.
192340	196660	The most common sensors are magnetometers, sun sensors, and star cameras.
196700	200800	Magnetometers are used by satellites when they are within Earth's magnetic field.
200800	205340	This is a very low-fidelity measurement, though, so it's typically not used by itself.
205340	208720	Sun sensors are photodiodes that measure light intensity.
208720	212440	By putting these sensors in several places surrounding the satellite,
212440	217320	we can measure sunlight on each sensor, giving us the direction of the sun.
217320	222340	That, combined with location data, can give us a pointing accuracy of 10 degrees or less.
222440	227440	And finally, star cameras are the primary means of precise attitude measurement.
227440	230480	Stars are far enough away from us and moving slowly enough
230480	234740	that we can treat them as constant for all but the highest precision tasks.
234740	238740	This means we can create a map of the star field that surrounds Earth,
238740	243960	and we can reference it to calculate exactly where a camera mounted on the satellite is pointing.
243960	247260	Now that we have determined our orbital parameters and orientation,
247260	250520	we have a natural follow-on question. How do I change them?
250520	254760	Getting around in space, meaning both changing your direction and changing your orientation,
254760	259360	are difficult primarily because there is simply very, very little to work with.
259360	265220	All the modes of transportation here on Earth require exerting a force on your external environment.
265220	267940	Cars push their tires against the friction of the roadway.
267940	272740	Planes pull air through their engines, and ships push water with their propellers.
272740	278700	Again, satellite orbit and satellite attitude are generally disconnected, so we have to control them separately.
278700	281040	First, we'll cover how do I change my orbit.
281040	285600	In orbital motion, we don't actually point to where we want to go and start moving there.
285600	291520	Satellites are already moving at a tremendous speed, around 17,000 miles per hour in low-Earth orbit.
291520	297480	Changing orbit in space actually consists of carefully adding and subtracting energy at the right times.
297480	302360	Not electrical energy, but rather the satellites combined kinetic and potential energy.
302360	307100	Adding energy raises the orbit farther away from Earth, and subtracting brings you closer.
307100	311200	Notice I didn't say going faster raises the orbit, because as you get farther away from Earth,
311200	315540	you actually slow down due to the properties of orbital dynamics.
315540	317680	So we'll stick with the term energy.
317840	322780	Since the satellite already has so much energy that it acquired from launch to stay in orbit,
322780	325440	most changes are very small by comparison.
325440	330680	We're raising or lowering orbits to maintain or change certain orbital characteristics a small amount.
330680	336140	There are all kinds of technologies being developed and in use to add and subtract energy from satellites,
336140	338220	but the most common are thrusters.
338360	343620	Thrusters exist to eject fuel out of a spacecraft fast enough to change its momentum significantly.
343620	349760	This is the same way rockets work, and it comes from one of the fundamental laws of physics, conservation of momentum.
349760	353720	There are several different types of thrusters for spacecraft that each have different trade-offs,
353720	358500	but all of them operate on the core principle, throw stuff out the back really fast.
358540	361900	There are cold gas thrusters, which are the simplest ones.
361900	365360	They release compressed air to generate a small amount of thrust.
365360	368620	There are chemical thrusters, or miniaturized rockets,
368620	373920	that are harnessing a chemical reaction to energize and release fuel to generate thrust.
373920	378300	And finally, there are ion thrusters, which use electromagnetic force to control,
378300	381720	accelerate, and release charged particles to generate thrust.
381720	383600	Now, how do I control the attitude?
383700	386980	Satellites again have the problem that they exist in a literal vacuum.
386980	391620	You can't change your direction with a steering wheel, rudder, or flaps.
391620	398220	To tackle this, they actually make use of the same law of physics, but with a slight twist, conservation of angular momentum.
398280	401260	This one is easier to demonstrate in your daily life.
401260	407540	If you ever spun yourself on a chair, you probably realized that pulling your arms in will make you go slightly faster.
407540	409600	This is a demonstration of that law.
409600	413920	Satellites make use of this through a technology called reaction wheels.
413960	417770	The core concept is there are very small wheels contained within the spacecraft,
418160	424980	and if you spin that wheel very, very fast, you'll cause the spacecraft to react and spin in the opposite direction.
424980	428920	The motors that spin that wheel need to exert a torque to make that happen,
428920	434980	and the same opposing torque is then exerted back on the spacecraft, causing it to spin as well.
434980	439120	All of these technologies put together that allow you to determine the orbit,
439120	443040	determine the attitude, and control the orbit and control the attitude,
443040	447420	make up the satellite's Attitude and Orbital Control System, or AOCS,
447420	450640	which is the most complex subsystem on most satellites.
462930	464790	Thanks, Jess, for that update.
464850	469090	Before we sign off here, I want to give another update on the progress of the on-orbit mission.
469090	473990	Remember, we chose a single-team solution to send to our orbiting satellite.
473990	477450	The ground control team is telling me that we have those commands scheduled to be sent
477450	482590	between 1 p.m. and 4.30 p.m. Pacific, and we are just now starting that comms window.
482910	485210	Given the orbit predictions of our satellite and the moon,
485210	489490	we expect the actual photo to be taken around 6.30 Pacific this evening.
489550	494670	With any luck, we expect to receive that moonshot back here at Earth at 1 a.m. Pacific tonight,
494670	497230	and we'll have it for you tomorrow at the closing awards.
497350	498990	That's it for this hour's update.
498990	505230	We'll see you back here at 4.45 Eastern, 1 p.m. Pacific,
505230	508530	for a final hourly update before our end-of-day recap tonight.
508650	509810	Thanks, and have a good one.