Have to agree with the OP.
A large body in space would more likely become spherical or at least rounded but to see a small asteroid so perfectly formed rings a few alarm bells.

What if this is part of a weaponised cold war relic?!?

See [link to en.wikipedia.org]
for a list of high atmosphere and extraterrestrial nuclear tests.

That's right!!

All done around 1960s!

One Kilotonne to 1.4 Megatonne bombs exploded over our heads. (or our parent's heads!!)

A list of the "now" public tests.

USA – Hardtack I – Johnston Atoll, Pacific Ocean

Yucca 28 April 1958, 1.7 kt, 26.2 km
Teak, 1 August 1958, 3.8 Mt, 76.8 km
Orange, 12 August 1958, 3.8 Mt, 43 km

United States USA – Argus – South Atlantic Ocean

Argus I, 27 August 1958, 1.7 kt, 200 km
Argus II, 30 August 1958, 1.7 kt, 240 km
Argus III, 6 September 1958, 1.7 kt, 540 km (The highest known man made nuclear explosion)

Soviet Union USSR – 1961 tests – Kapustin Yar

Test #88, 6 September 1961, 10.5 kt, 22.7 km
Test #115, 6 October 1961, 40 kt, 41.3 km
Test #127, 27 October 1961, 1.2 kt, 150 km
Test #128, 27 October 1961, 1.2. kt, 300 km

United States USA – Dominic I – Johnson Atoll, Pacific Ocean

Bluegill, 3 June 1962, failed
Bluegill Prime, 25 July 1962, failed
Bluegill Double Prime, 15 October 1962, failed
Bluegill Triple Prime, 26 October 1962, 410 kt, 50 km
Starfish, 20 June 1962, failed
Starfish Prime, 9 July 1962, 1.4 Mt, 400 km(The largest man made nuclear explosion in outer space)
Checkmate, 20 October 1962, 7 kt, 147 km
Kingfish, 1 November 1962, 410 kt, 97 km

Soviet Union USSR – 1962 tests – Kapustin Yar

Test #184, 22 October 1962, 300 kt, 290 km
Test #187, 28 October 1962, 300 kt, 150 km
Test #195, 1 November 1962, 300 kt, 59 km