How does a spacecraft such as Soyuz detect when it's on collision course with an object?

Question

Today Roscosmos reported (in Russian, here's a machine translation into English) that the Soyuz vehicle on its way to the ISS avoided collision with a piece of Japanese rocket body, allegedly launched back in 1989.

How does Soyuz detect such objects? Does it have some sort of radar like aircraft have, or receives information about it from ground-based debris tracking stations (e. g. the NORAD observing the objects flying in the orbit)?

Answer 1

Spacecraft rely on information from earth to avoid space debris, they don't have instruments for scanning and detecting debris. There's a few reasons for this:

• Power: most spacecraft don't have enough electrical power to run a radar powerful enough to detect small debris far enough away to matter
• Weight: in order to be useful for detecting debris the radar would have to have a very wide coverage as debris could be from any direction, this would be a heavy thing to haul around
• Cost: not only would the radar be expensive in itself, the weight of launching it would make the required rocket much bigger and expensive

Even if you did put a radar on a spacecraft for detecting debris it's utility would be limited because orbital debris is moving so fast. Even if a radar was able to detect debris at 40 miles away there would be mere seconds to react. First a radar would need to detect an object, then track it long enough to project its course with the accuracy needed to determine it's a threat, and by that time it's probably either past or your spacecraft is in tiny little pieces.

In order to maneuver a spacecraft out of the way of debris you have to know about it while it's still over the horizon, preferably several orbits away, this is why debris detection is ground based.

Answer 2

Let me tell you how a typical satellite avoids debris, which I believe will answer your question about how Soyuz does it.

1. All space objects are tracked by a US military organization called JSPOC. They also release data to those operating satellites via https://www.space-track.org/ . Note that they will not release information about US classified payloads via this site. Also note that the Russians may have their own method of determining the orbit of such objects.
2. JSPOC determines the orbit of these objects by a worldwide array of sensors, which ping the objects and find where they are. The position and velocity of each object is determined, and updates are continually made.
3. JSPOC periodically determines if any two objects are going to be very close to each other. This requires knowing precise orbits of all of the potential candidates.
4. When a close approach is determined to be likely, JSPOC issues a notice to the team responsible for managing said satellites (If the satellites are alive).
5. The team uses the information from JSPOC to determine if they are going to attempt a collision avoidance maneuver. If they do, they typically want to do this as early as possible, to use little fuel. 24 hours in advance is typically the timeframe for such decisions, if possible, otherwise they do it as quickly as they can.

A few other interesting tidbits:

1. The close approach velocity of two objects can have speeds above Orbital Velocity. It's not uncommon for the velocity to be 10 km/s!
2. It is very difficult to determine if these objects will be a threat.
3. Even if you could detect objects in your area, the typical thrust of a satellite is so small that a few seconds wouldn't be enough to get it out of the way.